INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 22
ULTRASOUND
This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.
Published under the joint sponsorship of
the United Nations Environment Programme,
the International Labour Organisation,
and the International Radiation Protection Association
World Health Orgnization
Geneva, 1982
ISBN 92 4 154082 6
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letters.
CONTENTS
ENVIRONMENTAL HEALTH CRITERIA FOR ULTRASOUND
1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES
1.1. Summary
1.1.1. Scope and purpose
1.1.2. Introduction
1.1.3. Mechanisms of action
1.1.4. Biological effects
1.1.4.1 Airborne ultrasound
1.1.4.2 Molecules in living systems
1.1.4.3 Cells in suspension
1.1.4.4 Organs and tissues
1.1.4.5 Animal studies
1.1.4.6 Epidemiology and health risk evaluation
1.1.5. Exposure limits and emission standards
1.1.5.1 Occupational exposure to airborne ultrasound
1.1.5.2 Therapeutic use
1.1.5.3 Diagnostic use
1.1.5.4 General population exposure
1.2. Recommendations for further studies
1.2.1. Measurement of ultrasonic fields
1.2.2. Exposure of patients to diagnostic ultrasound
1.2.3. Biological studies
1.2.4. Training and education
1.2.5. Regulations and safety guidelines for equipment
2. PHYSICAL CHARACTERISTICS OF ULTRASOUND
2.1. Continuous, gated, and pulsed waves
2.2. Intensity distribution in ultrasound fields
2.2.1. Progressive wave fields
2.2.2. Standing waves
2.3. Speed of sound
2.4. Refraction and reflection
2.5. Characteristic acoustic impedance
2.6. Attenuation and absorption
2.7. Finite amplitude effects
3. MECHANISMS OF INTERACTION
3.1. Thermal mechanism
3.2. Cavitation
3.2.1. Introduction
3.2.2. Stable cavitation
3.2.3. Transient cavitation and studies concerned
with both stable and transient cavitation
3.2.4. Cavitation in tissues
3.3. Stress mechanisms
3.3.1. Radiation pressure, radiation force, and radiation torque
3.3.2. Acoustic streaming
4. MEASUREMENT OF ULTRASOUND FIELDS
4.1. Measurement of liquid-borne ultrasound fields
4.1.1. Measurement of the total power of an ultrasound beam
4.1.2. Spatial and temporal measurements
4.2. Measurement of airborne ultrasound fields
5. SOURCES AND APPLICATIONS OF ULTRASOUND
5.1. Domestic sources
5.2. Industrial and commercial sources
5.2.1. Airborne ultrasound exposure levels
5.3. Medical applications
5.3.1. Diagnosis
5.3.1.1 Exposure levels from diagnostic
ultrasound equipment
5.3.2. Therapy
5.3.2.1 Exposure levels from therapeutic
ultrasound equipment
5.3.3. Surgical applications
5.3.4. Other medical applications
5.3.5. Dentistry
6. EFFECTS OF ULTRASOUND ON BIOLOGICAL SYSTEMS
6.1. Introduction
6.2. Molecules in living systems
6.3. Cells
6.3.1. Effects on macromolecular synthesis and ultrastructure
6.3.1.1 Protein synthesis
6.3.1.2 DNA
6.3.1.3 Cell membrane
6.3.1.4 Intracellular ultrastructural changes
6.3.1.5 Summary
6.3.2. Effects of ultrasound on mammalian cell
survival and proliferation
6.3.3. Synergistic effects
6.3.4. Summary
6.4. Effects on multicellular organisms
6.4.1. Effects on development
6.4.1.1 Drosophila melanogaster
6.4.1.2 Mouse
6.4.1.3 Rat
6.4.1.4 Frog
6.4.1.5 Summary
6.4.2. Immunological effects
6.4.2.1 Summary
6.4.3. Haematological and vascular effects
6.4.3.1 Platelets
6.4.3.2 Erythrocytes
6.4.3.3 Blood flow effect
6.4.3.4 Biochemical effects
6.4.3.5 Effects on the haemopoietic system
6.4.3.6 Summary
6.4.4. Genetic effects
6.4.4.1 Chromosome aberrations
6.4.4.2 Mutagenesis
6.4.4.3 Summary
6.4.5. Effects on the central nervous system
and sensory organs
6.4.5.1 Morphological effects
6.4.5.2 Functional effects
6.4.5.3 Auditory sensations
6.4.5.4 Mammalian behaviour
6.4.5.5 The eye
6.4.5.6 Summary
6.4.6. Skeletal and soft tissue effects
6.4.6.1 Bone and skeletal tissue
6.4.6.2 Tissue regeneration - therapeutic effects
6.4.6.3 Muscle
6.4.6.4 Thyroid
6.4.6.5 Treatment of neoplasia
6.4.6.6 Summary
6.5. Human fetal studies
6.5.1. Fetal abnormalities
6.5.2. Fetal movement
6.5.3. Chromosome abnormalities
6.5.4. Summary
7. EFFECTS OF AIRBORNE ULTRASOUND ON BIOLOGICAL SYSTEMS
7.1. Auditory effects
7.2. Physiological changes
7.3. Heating of skin
7.4. Symptomatic effects
7.5. Summary
8. HEALTH RISK EVALUATION
8.1. General
8.1.1. Criteria
8.1.2. Mechanisms
8.1.3. In vitro experimentation
8.2. Diagnostic ultrasound
8.3. Therapeutic ultrasound
8.4. Hyperthermia
8.5. Dental devices
8.6. Airborne ultrasound
8.7. Concluding remarks
9. PROTECTIVE MEASURES
9.1. Regulations and guidelines
9.2. Types of standards for ultrasound
9.2.1. Standards for devices
9.2.1.1 Diagnostic ultrasound
9.2.1.2 Therapeutic ultrasound
9.2.1.3 Other equipment performance standards
9.2.2. Exposure standards
9.2.2.1 Airborne ultrasound
9.3. Specific protective measures
9.3.1. Diagnostic ultrasound
9.3.2. Therapeutic ultrasound
9.3.3. Industrial, liquid-borne and airborne ultrasound
9.3.4. General population exposure
9.4. Education and training
REFERENCES
APPENDIX I
APPENDIX II
APPENDIX III
WHO/IRPA TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR ULTRASOUND
Members
Dr V. B. Bindal, National Physical Laboratory, New Delhi, India
Dr P. D. Edmonds, Ultrasonic Program, Stanford Research
Institute, Menlo Park, California, USA
Dr D. Harder, Institute for Medical Physics and Biophysics,
University of Gottingen, Federal Republic of Germanya
Dr K. Lindström, Department of Biomedical Engineering,
University Hospital, Malmö, Sweden
Dr K. Maeda, Department of Obstetrics and Gynaecology, Tottori
University School of Medicine, Yonago, Japan
Dr V. Mazzeo, Department of Ophthalmology, University of
Ferrara, Ferrara, Italy (Vice-Chairman)
Dr W. Nyborg, Department of Physics, University of Vermont,
Burlington, Vermont, USA
Dr M. H. Repacholi, Radiation Protection Bureau, Department of
National Health and Welfare, Ottawa, Canada (Chairman)a
Dr H. F. Stewart, Bureau of Radiological Health, Department of
Health and Human Services, Food and Drug Administration,
Rockville, Maryland, USA (Rapporteur)
Dr M. Stratmeyer, Bureau of Radiological Health, Department of
Health and Human Services, Food and Drug Administration,
Rockville, Maryland, USA (Rapporteur)
Dr A. R. Williams, Department of Medical Biophysics,
University of Manchester, Manchester, United Kingdom,
(Rapporteur)
Representatives of other organizations
Dr W. D. O'Brien, American Institute of Ultrasound in Medicine,
Department of Electrical Engineering, University of Illinois
Urbana, Champaign, Illinois, USA
Dr C. Pinnagoda, International Labour Office, Geneva,
Switzerland
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a Members of the International Non-Ionizing Radiation Committee
of IRPA
Secretariat
Mrs A. Duchźne, Commissariat ą l'Energie Atomique, Departement
de Protection, Fontenay-aux-Roses, Francea
Dr E. Komarov, Scientist, Environmental Hazards and Food
Protection, Division of Environmental Health, World Health
Organization, Geneva, Switzerland (Secretary)
-------------------------------------------------------------------
a Members of the International Non-Ionizing Radiation Committee
of IRPA
NOTE TO READERS OF THE CRITERIA DOCUMENTS
While every effort has been made to present information in the
criteria documents as accurately as possible without unduly
delaying their publication, mistakes might have occurred and are
likely to occur in the future. In the interest of all users of the
environmental health criteria documents, readers are kindly
requested to communicate any errors found to the Division of
Environmental Health, World Health Organization, Geneva,
Switzerland, in order that they may be included in corrigenda which
will appear in subsequent volumes.
In addition, experts in any particular field dealt with in the
criteria documents are kindly requested to make available to the
WHO Secretariat any important published information that may have
inadvertently been omitted and which may change the evaluation of
health risks from exposure to the environmental agent under
examination, so that the information may be considered in the event
of updating and re-evaluation of the conclusions in the criteria
documents.
ENVIRONMENTAL HEALTH CRITERIA FOR ULTRASOUND
Further to the recommendations of the Stockholm United Nations
Conference on the Human Environment in 1972, and in response to a
number of World Health Assembly resolutions (WHA23.60, WHA24.47,
WHA25.58, WHA26.68) and the recommendation of the Governing Council
of the United Nations Environment Programme, (UNEP/GC/10, 3 July
1973), a programme on the integrated assessment of the health
effects of environmental pollution was initiated in 1973. The
programme, known as the WHO Environmental Health Criteria
Programme, has been implemented with the support of the Environment
Fund of the United Nations Environment Programme. In 1980, the
Environmental Health Criteria Programme was incorporated into the
International Programme on Chemical Safety (IPCS). The result of
the Environmental Health Criteria Programme is a series of criteria
documents.
A joint WHO/IRPA Task Group on Environmental Health Criteria
for Ultrasound met in Geneva from 7 to 11 June 1982. Mr G. Ozolins,
Manager, Environmental Hazards and Food Protection, opened the
meeting on behalf of the Director-General. The Task Group reviewed
and revised the draft criteria document, made an evaluation of the
health risks of exposure to ultrasound, and considered rationales
for the development of equipment performance standards and human
exposure limits.
The International Radiation Protection Association (IRPA)
became responsible for activities concerned with non-ionizing
radiation by forming a Working Group on Non-Ionizing Radiation in
1974. This Working Group later became the International Non-
Ionizing Radiation Committee (IRPA/INIRC) at the IRPA meeting in
Paris in 1977. The IRPA/INIRC reviews the scientific literature on
non-ionizing radiation and makes assessments of the health risks of
human exposure to such radiation. Based on the Health Criteria
Documents developed in conjunction with WHO, the IRPA/INIRC
recommends guidelines on exposure limits, drafts codes of safe
practice, and works in conjunction with other international
organizations to promote safety and standardization in the non-
ionizing radiation field.
Two WHO Collaborating Centres, the Radiation Protection Bureau,
Health and Welfare Canada, and the Bureau of Radiological Health,
Rockville, USA, cooperated with the IRPA/INIRC in initiating the
preparation of the criteria document. The final draft was prepared
as a result of several working group meetings, taking into account
comments received from independent experts and the national focal
points for the WHO Environmental Health Criteria Programme in
Australia, Canada, Finland, Federal Republic of Germany, Italy,
Japan, New Zealand, Sweden, the United Kingdom, the USA, and the
USSR as well as from the United Nations Environment Programme, the
Food and Agriculture Organization of the United Nations, and the
International Labour Organisation. The collaboration of these
experts, national institutions, and international organizations is
gratefully acknowledged. Without their assistance this document
could not have been completed. In particular, the Secretariat
wishes to express its thanks to Dr D. Harder, Dr C. R. Hill,
Dr M. H. Repacholi, Dr C. Roussell, Dr H. F. Stewart,
Dr M. E. Stratmeyer, and Dr A. R. Williams for their assistance
in the preparation of the draft document and to Dr Repacholi and
Dr Williams for their help in the final scientific editing of the
text.
The document is based primarily on original publications listed
in the reference section. Additional information was obtained from
a number of general reviews including: Nyborg, (1977); Repacholi,
(1981); and Stewart & Stratmeyer (1982).
Modern advances in science and technology change man's
environment, introducing new factors which, besides their intended
beneficial uses, may also have untoward side-effects. Both the
general public and health authorities are aware of the dangers of
pollution by chemicals, ionizing radiation, and noise, and of the
need to take appropriate steps for effective control. The more
frequent use of ultrasound in industry, commerce, the home, and
particularly in medicine, has magnified the possibiity of human
exposure, increasing concern about possible human health effects,
especially in relation to the human fetus.
This document comprises a review of data, which are concerned
with the effects of ultrasound exposure on biological systems, and
are pertinent to the evaluation of health risks for man. The
purpose of this criteria document is to provide information for
health authorities and regulatory agencies on the possible effects
of ultrasound exposure on human health and to give guidance on the
assessment of risks from medical, occupational, and general
population exposure to ultrasound.
Subjects briefly reviewed include: the physical
characteristics of ultrasound fields; measurement techniques;
sources and applications of ultrasound; levels of exposure from
devices in common use; mechanisms of interaction; biological
effects; and guidance on the development of protective measures
such as regulations or safe-use guidelines.
In a few countries, concern about occupational and public
health aspects has led to the development of radiation protection
guidelines and the establishment of equipment emission or
performance standards, and limits for human exposure (mainly to
airborne ultrasound). Health agencies and regulatory authorities
are encouraged to set up and develop programmes which ensure that
the lowest exposure occurs with the maximum benefit. It is hoped
that this criteria document may provide useful information for the
development of national protection measures against non-ionizing
acoustic radiation.
Details of the WHO Environmental Health Criteria Programme,
including definitions of some of the terms used in the documents,
may be found in the general introduction to the Environmental
Health Criteria Programme, published together with the
environmental health criteria document on mercury ( Environmental
Health Criteria 1 - Mercury, Geneva, World Health Organization,
l976), now available as a reprint.
1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES
1.1. Summary
1.1.1. Scope and purpose
This document comprises a review of data which are concerned
with the effects of ultrasound exposure on biological systems and
are pertinent to the evaluation of health risks for man. The
purpose of this evaluation is to provide information for health
authorities and regulatory agencies on the possible effects of
ultrasound exposure on human health and to give guidance on the
assessment of risks from medical, occupational, and general
population exposure to ultrasound.
Subjects briefly reviewed include: the physical
characteristics of ultrasound fields; measurement techniques;
sources and applications of ultrasound; levels of exposure in
common use; mechanisms of interaction; and guidance on the
development of protective measures such as regulations or safe-use
guidelines.
1.1.2. Introduction
Ultrasound is sound (a mechanical vibration phenomenon) having
a frequency above the range of human hearing (typically above 16
kHz) which, unlike electromagnetic radiation, requires a medium
through which to propagate.
Exposure to ultrasound can be divided into two distinct
categories: airborne and liquid-borne. Exposure to airborne
ultrasound occurs in many industrial applications such as cleaning,
emulsifying, welding, and flaw detection and through the use of
consumer devices such as dog whistles, bird and rodent controllers,
and camera rangefinders, and commercial devices such as intrusion
alarms. Liquid-borne exposure occurs predominantly through medical
exposure in diagnosis, therapy, and surgery.
As with any other physical agent, ultrasound has the potential
to produce adverse effects at sufficiently high doses. In addition,
biological effects of unknown significance have been reported under
laboratory conditions at low exposure levels. However, the health
risks that may be associated with biological effects at the levels
of ultrasound currently encountered in occupational, environmental,
or medical exposure have not yet been defined.
Though, at present, there is no evidence of adverse health
effects in human beings exposed to diagnostic ultrasound, its
rapidly increasing use during pregnancy is still of special concern
in view of the known susceptibility of the fetus to other physical
and chemical agents.
1.1.3. Mechanisms of action
Acoustic energy may be transformed into several other forms of
energy, which may exist at the same time within any given medium.
The mechanisms of transformation into these other forms of energy
are conventionally subdivided into three major categories
comprising a thermal mechanism, a cavitational mechanism, and other
mechanisms including streaming motions.
When ultrasound is absorbed by matter, it is converted into
heat producing a temperature rise in the exposed subject. An
ultrasound wave produces alternate areas of compression and
rarefaction in the medium and the pressure changes produced can
result in cavitation. This phenomenon occurs when expansion and
contraction of nuclei or gas bubbles (in liquids and body tissues)
cause either simple oscillations or pulsations (stable cavitation),
or violent events (transient or collapse cavitation), where the
collapse of the bubbles produces very high instantaneous
temperatures and pressures. Theoretical analyses have predicted
that a single cycle of ultrasound, at a sufficient amplitude level,
can produce a transient cavitation event in an aqueous medium in
which appropriate nucleation sites are present. This prediction has
not yet been verified experimentally.
Streaming motions and shearing stresses can occur within the
exposed system through stable cavitation; twisting motions
(radiation torque) have also been observed in biological systems
exposed to ultrasound.
Unlike ionizing radiation, where the basic physical mechanism
of interaction stays the same with increasing exposure rate, the
dominant mechanism of ultrasound action on biological systems can
change as the acoustic intensity, frequency, and exposure
conditions change.
It is generally agreed that diagnostic devices emitting space-
and time-averaged intensities of the order of a few milliwatts/cm2
are unlikely to cause temperature elevations in human beings that
would be regarded as potentially damaging. It is not known whether
some form of cavitational activity could occur in vivo at these
time-averaged intensities when pulse-echo devices are used. It has
been suggested that the elevated temperatures associated with the
use of higher spatial average temporal average (SATA) intensities
(0.1-3 W/cm2) contribute to the beneficial therapeutic effects of
ultrasound. In addition, gas bubbles have been detected in vivo
following therapeutic exposures, indicating that a form of
cavitational activity has occurred.
1.1.4. Biological effects
Very few systematic studies have been undertaken to determine
threshold levels for observed effects of ultrasound. Nearly all of
the reports in the literature have tended to be phenomenological in
nature, without evidence from further investigations to determine
the underlying mechanisms of action. Furthermore, most reports have
not yet been confirmed by more than one laboratory. Some studies
have been performed using exposure times longer than would normally
be encountered in the clinical situation and this has made the
evaluation of health risks from exposure to ultrasound extremely
difficult. Thus, there is an urgent need for more carefully
coordinated, systematic research in critical areas.
The health implications from a number of effects already
reported indicate the need for a prudent approach to the ultrasound
exposure of human subjects, even though the benefits of this
imaging modality far outweigh any presumed risks.
1.1.4.1. Airborne ultrasound
Exposure of human beings to low frequency ultrasound (16 -100
kHz) can be divided into two distinct categories; one is via
direct contact with a vibrating solid or through a liquid coupling
medium, and the other is through airborne conduction.
For airborne ultrasound exposure, at least one of the critical
organs is the ear. Effects reported in human subjects exposed to
airborne ultrasound include; temporary threshold shifts in sound
perception, altered blood sugar levels, electrolyte imbalance,
fatigue, headaches, nausea, tinnitus, and irritability. However,
in many instances, it has been difficult to state that the observed
effects were caused by airborne ultrasound because they were
subjective and there was often simultaneous exposure to high levels
of audible sound.
The use of experimental animals to study the effects of
airborne ultrasound has serious drawbacks because they have a
greater hearing acuity, wider audible frequency range, and a
greater surface-area-to-mass than man and most have fur-covered
bodies.
1.1.4.2. Biological Molecules
Studies of the exposure of biological molecules in solution to
liquid-borne ultrasound have, in general, served to indicate the
importance of cavitation as a mechanism of ultrasound action and to
identify which biological molecules preferentially absorb the
energy. It is not possible to extrapolate the results of such
studies to the in vivo situation.
1.1.4.3. Cells in suspension
There is evidence that ultrasound can change the rate of
macromolecular synthesis and cause ultrastructural changes within
cells. Alterations in cell membrane structure and function have
been reported from exposure to pulsed and continous wave (cw)
ultrasound using commercial diagnostic devices.
Conflicting results have been reported on the effects of
ultrasound on DNA. Unscheduled DNA synthesis (indicating possible
damage and subsequent repair to the DNA) has been reported
following exposure to pulsed diagnostic ultrasound and cw
therapeutic ultrasound.
Some evidence has been produced that alterations in cell
surface activity may persist for many generations.
1.1.4.4. Organs and tissues
Studies on skeletal tissue indicate that bone growth may be
retarded following exposure to ultrasound at high therapeutic
intensities, even if the transducer is kept in motion during
treatment. If the transducer is held stationary, bone and other
tissue damage occurs at lower intensities.
Both in vitro and in vivo exposures of muscle tissue have
been reported to trigger contractions. Therapeutic intensities of
ultrasound have also been reported to alter thyroid function in
man.
1.1.4.5. Animal studies
Fetal weight reduction has been observed following exposure of
rodent fetuses in utero. The lowest cw average intensity levels
that have been observed to induce fetal weight reduction in mice
are in the low therapeutic range. Some studies indicate that fetal
abnormalities and maternal weight loss also occur.
1.1.4.6. Epidemiology and health risk evaluation
To date, adverse effects have not been detected from exposure
to diagnostic ultrasound. However, it is of particular concern
that adequate epidemiological studies have not yet been performed,
and that soon most human fetuses in technologically developed
countries could be subjected to at least one ultrasound
examination. If such epidemiological studies are not carried out
very soon, there will not be any "control" populations to
compare with populations exposed to ultrasound.
Most of the human studies that have been performed have
suffered from inadequate control matching, too few cases, or a
variety of other problems and though, in general, adverse effects
have not been reported, these studies are inconclusive and of very
little value. The possibility of reduced weight resulting from in
utero exposure, which was reported recently, still needs further
investigation, especially in light of previous reports of reduced
body weight in animal fetuses exposed in utero.
1.1.5. Exposure limits and emission standards
1.1.5.1. Occupational exposure to airborne ultrasound
Occupational exposure limits for airborne ultrasound have
already been established or have been proposed in Canada, Japan,
Sweden, the United Kingdom, the USA, and the USSR. All standards
or proposed standards or regulations are similar, in that each has
a "step" allowing exposure to sound pressure levels above 20 kHz.a
1.1.5.2. Therapeutic use
Regulations which incorporate maximum output levels for
therapeutic ultrasound equipment exist in some countries (e.g.,
Canada) and have been proposed as a requirement by one technical
sub-committe of the International Electrotechnical Commission.
Other countries, such as the USA, have not incorporated a limit on
output levels in their ultrasound therapy products standard.
1.1.5.3. Diagnostic use
Given the current biological and biophysical data base, there
does not appear to be sufficient information to establish
quantitative limits on output levels for diagnostic ultrasound
equipment.
1.1.5.4. General population exposure
Ultrasound is used in many consumer products (e.g., camera
range-finders and TV controls, burglar alarms etc.) but little is
known about their potential health effects in the general
population, although they are thought to be negligible.
1.2. Recommendations for Further Studies
1.2.1. Measurement of ultrasonic fields
One of the difficulties of establishing a comprehensive body of
information with respect to the biological and health effects of
ultrasound has been the lack of adequate instrumentation to measure
the various exposure parameters. However, reliable methods for the
measurement of ultrasound field parameters, such as total radiated
power, and the various intensities in the ultrasound fields, are
now available in a few national or research institutions.
Most devices used to measure ultrasound power and the various
temporal and spatial intensity parameters for liquid-borne
ultrasound are not suitable for routine surveys in the work place.
There is an urgent need for the development of portable, rugged
instrumentation that will measure accurately both total power and
the relevant intensity parameters.
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a The International Radiation Protection Association is proposing
guidelines on limits of exposure to airborne acoustic energy for
both workers and the general population.
Furthermore, a substantial research effort is still needed to
develop a system of dosimetric variables relevant to the production
of and protection against adverse health effects of ultrasound in
medical and industrial applications.
1.2.2. Exposure of patients to diagnostic ultrasound
Information concerning the ultrasound exposure of patients
during diagnostic examinations has often not been available in the
past. Manufacturers are now increasingly supplying diagnostic
ultrasound equipment together with appropriate data to enable users
to evaluate the level to which the patient is exposed, and to
decide which devices would give the lowest exposure commensurate
with good diagnostic quality. This trend is commendable and should
be strongly encouraged.
Until the potential health effects of exposure to ultrasound
have been properly evaluated, it is recommended that manufacturers
should aim at keeping the output levels necessary for examinations
as low as readily achievable. This priority should apply to all
diagnostic techniques where the exposure time required to conduct
the examination can be minimized.
It is strongly recommended that patients should only be exposed
to ultrasound for valid clinical reasons.
1.2.3. Biological studies
Most bioeffect studies have been conducted on cell suspensions,
plants, insects, and other animal systems. However, it should be
noted that some of these biological systems accentuate certain
mechanisms of interaction to the extent that effects are observed
under exposure conditions that would not apply to, or would not
induce effects in human beings. Controversy continues as to the
exact mechanisms by which the effects of ultrasound are induced.
It is often possible to distinguish between dominant thermal and
non-thermal mechanisms, but the type of non-thermal effect remains
open to discussion. Cavitation is a well established mechanism of
action, but other non-thermal mechanisms may be involved in the
production of some ultrasound effects. With more complete
information on biological and physical mechanisms, studies can be
undertaken to determine possible thresholds (if they exist) for
bioeffects and the biophysical knowledge could be used to predict
potential bioeffects.
(a) Molecules and cells
It is recommended that studies be conducted at both the
molecular and cellular levels on interactions between ultrasound
and biological systems. Such information is needed to evaluate the
importance of the interaction mechanisms involved and to clarify
areas and end-points that need further study at higher levels of
biological organization.
(b) Immunological studies
Recent studies suggest that ultrasound may induce immunological
responses in laboratory animals. Because of the fundamental
importance of the immune system, any effects that might be induced
by ultrasound should be systematically investigated.
(c) Haematological studies
Ultrasound at therapeutic intensities has been shown to cause
platelet aggregation and other haematological alterations in vitro.
Results of some studies suggest that similar effects may occur in
vivo. This suggestion needs to be investigated further to assess
possible adverse consequences in vivo.
(d) Effects on DNA
Recent studies reporting repair to DNA, observed as unscheduled
DNA synthesis, need to be substantiated. Of particular importance
is the investigation of damage to DNA from pulsed ultrasound with
intensities in the diagnostic range.
(e) Genetic effects
Reports of sister chromatid exchanges, increased transformation
frequency, and changes in the cell membrane and cell motility, seen
many generations after a single exposure to ultrasound, suggest a
"genetic" effect. Because these results have not been adequately
confirmed, they cannot, at present, be extrapolated to the in vivo
situation; and need further investigation.
(f) Fetal studies
A number of reports indicate that lower fetal weight and
increased fetal abnormalities occur following exposure to
ultrasound in the low therapeutic intensity range. Studies should
be undertaken to establish exposure thresholds (if any) for effects
on the fetus exposed on various days during gestation. The
importance of the ratio of temporal average to temporal peak
intensities in relation to the production of fetal effects also
needs considerable investigation.
Since gross effects appear to occur only at high ultrasound
intensities, research workers should concentrate their efforts on
subtle effects, particularly in the fetus, which in many instances
receives a whole-body exposure to ultrasound. Wherever possible,
studies should be related to clinical situations.
Only one study on human beings suggests that lower birthweights
may result from exposure to diagnostic ultrasound in utero.
As the practice of ultrasound diagnosis becomes more
widespread, it will be difficult to find adequate control
populations and opportunities for satisfactory epidemiological
studies may become increasingly rare. It is strongly recommended
that cost-effective, well-designed studies be conducted soon and
coordinated at both the national and international levels.
Short-term studies where specific end-points, such as
haematological effects, can be identified, also need to be
conducted. Investigations should be made on patients undergoing
ultrasound therapy, since the average intensities used are
significantly higher than those used in diagnosis. To date, such
studies do not seem to have been undertaken.
(g) Behavioural studies
Studies on rodents suggest that behavioural effects may be seen
in newborn that have been exposed in utero. If these studies are
confirmed, systematic studies on human newborn will be necessary,
to determine whether such effects occur in man.
(h) Synergism
It is common for patients to undergo diagnostic examinations,
on the same day, in both the ultrasound and X-ray departments of
hospitals. Some evidence has been produced indicating that X-rays
may enhance ultrasound effects. Increased chromosome aberration
rates in somatic cells have been observed following combined
exposure to ultrasound and X-rays. Preliminary reports also
suggest that ultrasound may have a synergistic action with such
agents as heat, viruses, and drugs. Such synergistic effects need
to be investigated further.
(i) Airborne ultrasound
Few studies have been reported on the effects of airborne
ultrasound on man. Earlier reports of headaches and nausea seem to
have been largely attributed to subharmonics of the ultrasound beam
in the audible range. However, there has been a number of reports
of similar symptoms from people exposed to devices such as
ultrasound intrusion alarms. This indicates that further
investigation in this area is necessary.
1.2.4. Training and education
Since the ultrasound exposure levels currently employed in
physiotherapy are well within the range in which adverse health
effects have been confirmed, it is recommended that all operators
of such equipment receive formal training (up to l year) before
treating patients. These operators should also ensure that their
equipment is properly maintained and calibrated to make sure that
patients receive only the prescribed "dose".
Operators of diagnostic ultrasound equipment should also
receive appropriate formal training on the use and safety of this
clinical modality. They should be properly instructed on
maintaining and calibrating the equipment to ensure that the
ultrasound exposure of the patient is minimized while maximizing
the quality of the image.
In commercial, industrial, and research establishments where
devices emitting airborne and/or liquid-borne ultrasound operate,
all potentially exposed employees should be properly instructed
with regard to safety precautions appropriate for the equipment
being used.
Consumers using devices that emit airborne ultrasound should
familiarize themselves with the safety precautions provided by the
manufacturer.
1.2.5. Regulations and safety guidelines for equipment
Protective measures include the use of either mandatory
standards (regulations) or guidelines on equipment emission and
performance.
Where appropriate, safety guidelines should be provided for
operators of equipment that emits airborne ultrasound. In many
cases, such guidelines should recommend the use of hearing-
protectors and appropriate warning signs.
As surveys indicate, many ultrasound therapy devices do not
give the output levels indicated on the control console, so
mandatory standards or regulations are recommended for this type of
equipment. Such standards should include accuracy specifications
for the output power, output intensity, and timer setting.
The establishment of guidelines on the performance of
diagnostic ultrasound equipment is recommended and these should
include requirements concerning the image quality and stability,
and quality assurance measures. At present, there does not appear
to be a need to limit the output exposure levels of diagnostic
ultrasound equipment, other than to recommend strongly that the
lowest output levels be used commensurate with image quality,
adequate to obtain the necessary diagnostic information.
2. PHYSICAL CHARACTERISTICS OF ULTRASOUND
Ultrasonic energy consists of mechanical vibrations occurring
above the upper frequency limit of human audibility (generally
accepted as about 16 kHz). Ultrasound consists of a propagating
disturbance in a medium, which causes subunits (particles) of the
medium to vibrate. The vibratory motion of the particles
characterizes ultrasonic (acoustic) energy propagation. Unlike
electromagnetic radiation, acoustic energy cannot be transmitted
through a vacuum. The transmission through the medium depends to a
great extent on the ultrasound frequency and the state of the
medium, i.e., gas, liquid, or solid.
Ultrasound may propagate in different modes. In solids, two
important modes include compressional (longitudinal) waves and
shear (transverse) waves (Fig. 1). The propagation velocities of
these two modes are generally different.
Ultrasound propagates in gaseous, liquid, or solid media,
mainly in the form of longitudinal or compressional waves formed by
alternate regions of compression and rarefaction of the particles
of the medium, which vibrate in the direction of energy
propagation. The distance between two consecutive points of
maximum compression or rarefaction is called the wavelength.
Transverse (shear) waves mainly propagate in solids, and are
characterized by particle displacement at 90° to the direction of
propagation. At a bone/soft tissue interface, one type of wave can
give rise to another (mode conversion). If a longitudinal wave
propagating in soft tissue strikes bone at an angle, both
longitudinal and transverse waves may be excited in the solid
medium. This phenomenon can result in heating at the bone surface.
Results of heating in bone have been reported by Lehmann & Guy
(1972) and Chan et al. (1974).
The passage of a sound wave through a medium can be
characterized by several variables, associated with the movement of
particles in the medium. These include: acoustic pressure ( p),
particle displacement (xi), particle velocity ( v), and particle
acceleration ( a). Under idealized conditions each of these
quantities varies sinusoidally with space and time (Appendix I).
The acoustic pressure ( p) is the change in total pressure at
a given point in the medium at a given time, resulting in compression
where p is positive, and expansion where p is negative, as a
result of the action of the ultrasound waves. The displacement (xi)
is the difference between the mean position of a particle in the
medium and its position at any given instant in the time ( t). The
particle velocity ( v) is the instantaneous velocity of a vibrating
particle at a given point in the medium. This should not be confused
with the speed of sound ( c). The latter is the speed with which
the wave propagates through the medium, even though the individual
particles of the medium vibrate only about their mean positions with
no bulk movement of matter. The speed of sound ( c) is a constant
that depends on the physical properties of the medium; it is
discussed in section 2.3. As a result of the sinusoidal variation
in particle velocity ( v), each particle experiences an acceleration
( a) which also varies with time and position; it has positive
values when v increases, and negative values when v decreases.
The relationship between the intensity and various particle
parameters such as acoustic pressure, displacement, velocity, and
acceleration (Appendix I, Table 1) may be of importance when
analysing some biological effects reported in the literature.
For comparative purposes, it is worth noting an important
difference between ionizing radiation and ultrasound. To increase
the intensity of a beam of X-rays of a given spectral distribution,
the photon flux is increased. The energy of each individual photon
remains unchanged. Therefore, the interaction mechanism for each
photon remains the same, but the number of interactions per unit
time increases because of the increased number of photons. To
increase the intensity of a beam of ultrasound of fixed frequency,
the amplitude of the particle parameters (pressure, displacement,
velocity, acceleration) is increased, to obtain a higher energy
flux per unit area. Change in the magnitude of the particle
parameters may affect the relative importance of different
mechanisms of interaction with matter at different intensities.
2.1. Continuous, Gated, and Pulsed Waves
The differences between continuous wave, gated (amplitude-
modulated), and acoustic-burst pulsed waves are shown in Fig. 2. A
continuous wave at a single frequency is a simple sinusoidal wave
having constant amplitude. Amplitude-modulated waveforms are used
in some equipment, for example, pulsed therapy equipment. An
acoustic burst is the type of pulse used in pulse echo diagnostic
equipment. It can represent the variation of pressure as a function
of distance at a fixed instant in time, or as a function of time at
a fixed point in space. For the pulsed wave, the pressure amplitude
is not constant and is zero for part of the time. No acoustic
energy is being emitted between pulses and the ultrasound
propagates through the medium as small packages of acoustic energy.
Pulsed waves can have any combination of on/off times. Thus, it is
important to specify exactly the time regimen of the pulsed beam.
Pulsed ultrasound with short and widely-spaced pulses
(typically microsecond (µs) pulses spaced at intervals of
milliseconds (ms) is used for diagnostic purposes, whereas
continuous waves (cw) are often used in therapeutic applications
of ultrasound and in most Doppler devices. Though the temporal
(time) average of the sound intensity produced by a diagnostic
pulse echo machine is usually about 1000 times less than the
intensity in a therapeutic ultrasound beam, the acoustic pressure
and the particle displacement, velocity, and acceleration during
the pulse may reach peak values an order of magnitude greater than
those in cw therapeutic ultrasound.
A particular complex time structure of the ultrasound field
may occur with real-time diagnostic devices that have an array of
transducers, where acoustic beams emitted by adjacent elements of
the array sequentially contribute to the acoustic intensity at a
point in space. The temporal characteristics of ultrasound fields
such as pulse duration, pulse repetition frequency, and temporal
peak intensity have been reported by several investigators
(Barnett, 1979; Child et al., 1980a; Lewin & Chivers, 1980;
Sarvazyan et al., 1980). A distinction must be made between the
spatial peak intensity and the spatial average intensity (Appendix
I, Table 1); great differences between particle parameters can
occur over space as well as time. Considerable spatial variations
in pressure occur in a standing wave field (section 2.2.2).
2.2. Intensity Distribution in Ultrasound Fields
Many of the ultrasound fields encountered during exposure of
human subjects, or in related biological studies, may be quite
complex, but most can be considered to be somewhere between two
extreme types: the progressive wave field and the standing wave
field. In the first case, it is possible to define and measure a
flux of energy along the direction of propagation in terms of any
of the four parameters ( p, xi, v, a) (Appendix I, Table 1).
2.2.1. Progressive wave fields
The ultrasonic field produced by a transducer obeys all the
physical laws of wave phenomena. It can be thought of as being
produced by many small point sources making up the transducer face
and thus producing a characteristic interference pattern at any
point in the field. As ultrasound is propagated from the transducer,
there is a zone where the overall beam size remains relatively
constant (the near field), though there are many variations of
intensity within the zone itself, both across and along the beam
axis. This zone is followed by a zone where the beam diverges and
becomes more uniform (the far field). Fig. 3 illustrates the near
field (or Fresnel region) with the transition into the far field
(or Fraunhofer region) for cw operation. For a circular piston
source of diameter D radiating sound of wavelength lambda, the
Fresnel zone extends from the transducer to a distance equal to
D2/4 lambda (when D is much greater than lambda); beyond this
distance is the Fraunhofer zone of the transducer. A numerical
analysis of the near field of a vibrating piston has been described
in the literature (Zemanek, 1971). For a given radius of the
transducer, the near field becomes more complex (exhibiting more
maxima and minima) as the wavelength of the ultrasound becomes
shorter. The acoustic field of a pulsed transducer can be thought
of as being composed of contributions from all the frequencies
within the bandwidth of a short pulse. It has been shown (Wien &
Harder, 1982) that the near field is less structured than that of a
cw transducer, and that the length of the near field corresponds to
that of a cw transducer oscillating at the centre frequency of the
pulsed field.
In the far field of any transducer, the acoustic intensity
is proportional to the square of the acoustic pressure. The
directivity of the beam in the far field is determined by
diffraction, in the same way that a light wave is affected by a
small aperture; the higher the frequency of ultrasound produced for
a given transducer size, the more directional is the beam. Further-
more, if the frequency is held constant but the diameter is reduced,
the beam divergence increases. Equation 2.1 is the formula for
conveniently determining the angle of divergence (theta) in the far
field (Kinsler & Frey, 1962) as shown in Fig. 3.
Sin theta = 1.22 lambda/D Equation 2.1
For the diagnostic transducers used for pulse echo imaging
purposes, the beam width determines the minimum lateral resolution
that can be expected. For this reason, many diagnostic transducers
are focused to decrease the beam width and enhance lateral
resolution.
The intensity distribution along the axis of such a transducer
is such that an axial intensity peak occurs at some distance from
the transducer. This peak is a common feature of both focused and
nonfocused fields, and its existence is an important factor in
characterizing ultrasound fields and in the interpretation of some
of the biological data. The ultrasonic intensity at this highest
main axial peak of the field is referred to as the spatial peak
intensity of the field. For exposure in experimental studies, the
spatial peak intensity may refer instead to the local maximum,
within the exposed region. It is also possible to define a spatial
average intensity as the ratio of the power to the beam cross-
sectional area, in the plane of interest. The definition of beam
cross section (Appendix II) allows a choice of the amplitude at the
lateral margin of the beam. Therefore, values of spatial average
intensity will depend on this choice and caution should be
exercised when comparing reports from different laboratories.
For a theoretical plane circular piston source in an infinite
non-reflecting medium, the spatial maximum intensity in the near
field is 4 times greater than the spatial average intensity at the
transducer surface (Zemanek, 1971; Nyborg, 1977). In actual
practice, this ratio typically has values ranging from about 2 to 6
for unfocused transducers, though higher values may be encountered,
depending on such factors as the nature of the piezoelectric
material used and how it is mounted in the applicator housing
(Stewart et al., 1980).
The intensity of the ultrasonic field produced by the
transducer also varies with time, if the ultrasound is pulsed.
Intensity averaging can be carried out in the time domain and it is
therefore necessary to distinguish between time (or "temporal")
average (such as the average over the total time or over the pulse
duration) and temporal peak intensities (Appendix II).
2.2.2. Standing waves
Standing waves can occur when cw ultrasound is propagating into
a confined space, so that the ultrasound waves are reflected back
from an interface and travel past each other in opposite directions.
This may be the case, for example, within a small room or in a
small container of water in the absence of absorbing materials.
The resultant waveform, at any instant, is obtained by adding the
wave pressures at each point. The acoustic energy distribution is
characterized by a stationary spatial pattern with minima and
maxima of pressure amplitude, called "nodes" and "antinodes",
respectively. Under the conditions applied during medical diagnosis
and therapy (generally in the range 1-10 MHz), a progressive wave
field usually predominates, though there may be an appreciable
standing wave component if, for example, there is a bone/tissue or
tissue/gas interface within the beam. The possibility of the
occurrence of standing waves is usually of less importance with
pulsed ultrasonic irradiation, because they can only exist during
the pulse overlap time at a given spatial location.
2.3. Speed of Sound
The speed ( c) at which ultrasonic vibrations are transmitted
through a medium is inversely proportional to the square root of
the product of the density (rho) and the adiabatic compressibility
( B) of the material, such that c = (rho B)-0.5. The speed
together with the frequency ( f) of the ultrasound determine the
wavelength lambda (lambda = c/f) of the waves that are propagated.
For example, the propagation velocity of ultrasound in most human
soft tissues ranges from approximately 1450 to 1660 m/s, so that
frequencies of 1 MHz correspond to a wavelength in the range of
1.4-1.7 mm respectively. Thus, ultrasonic diagnostic imaging
procedures carried out in this frequency range have the potential
for providing resolution of the order of 1 mm. Knowledge of the
speed at which ultrasound is transmitted through a medium is used
in diagnostic applications for the conversion of echo-return time
into the depth of tissue being imaged. Values of sound speed for
some other media of interest are given in Table 1 which shows that
the speed of sound is highest in solids, somewhat lower in liquids
and soft tissues, and very much lower in gases.
2.4. Refraction and Reflection
When an ultrasound wave encounters an interface between two
media, the dimensions of which are large compared with the wave-
length, part of the wave will be reflected back into the first
medium with the same speed. The rest of the wave will be
transmitted or refracted into the medium beyond the interface and
will travel with the velocity of propagation in that medium (Fig.
4). For reflection, the angles of incidence (thetai) and
reflection (thetar) are equal; for transmission the angles of
incidence and refraction are generally unequal. When the
ultrasonic wavelength is equal to or greater than the dimensions of
the reflecting object, the incident beam is scattered in all
directions.
The ratio of the characteristic impedances ( Zo) of any two
media on either side of an interface (see the following section)
determines the degree of reflection and refraction or transmission
of the incident wave.
2.5. Characteristic Acoustic Impedance
The characteristic acoustic impedance of a medium is the
product of the density (rho) and the speed ( c) of sound in that
medium. The extent to which ultrasonic energy is transmitted or
reflected at an interface separating two continuous isotropic media
is determined by the ratio of the characteristic acoustic
impedances of the media. The closer this impedance ratio is to 1,
the more energy is transmitted into the second medium and the less
is reflected from the interface. At an interface between human
tissue and air, only about 0.01% of the incident energy is
transmitted, the remainder being reflected. This illustrates the
importance of using a coupling medium between the transducer and
human tissue for both therapeutic and diagnostic ultrasound
applications. Strong reflections (close to 50%) also occur at
bone/tissue interfaces; thus bone/tissue and tissue/gas interfaces
constitute an important limitation on the accessibility of some
human anatomical regions to diagnostic ultrasonic investigation.
2.6. Attenuation and Absorption
As an ultrasound beam is transmitted through a heterogeneous
medium such as soft tissue, its intensity is reduced or attenuated
through a number of mechanisms, including beam divergence,
scattering, absorption, reflection, diffraction, and refraction.
Beam divergence refers to the spreading of the beam in the far
field through diffraction effects (section 2.2.1). For a given
transducer radius, this phenomenon is greater at lower frequencies.
As the beam area becomes larger, the intensity is reduced.
Scattering refers to the reflection of the incident ultrasound
from interfaces (i.e., surfaces separating media of different
characteristic acoustic impedances) with dimensions close to or
less than the ultrasound wavelength. In this case, the incident
beam is scattered in all directions. Ultrasound impinging on blood
cells, for example, would be scattered. When scattering occurs, it
is greater at higher ultrasonic frequencies.
Absorption of ultrasound occurs when the ordered vibrational
energy of the wave is dissipated into internal molecular motion,
i.e., into heat. There are many mechanisms by which ultrasound
absorption occurs in a medium, including viscous loss, hysteresis
loss, and relaxation processes.
The acoustic pressure amplitude px of the progressive
ultrasound wave of initial acoustic pressure amplitude po, at
a distance x for a nondiverging beam, in any uniform medium,
is described by the relationship:
px = poe-alphax Equation 2.2
where e is the base of natural logarithms and alpha is the
amplitude attenuation coefficient of the medium (as defined in
Appendix I) for a given frequency. Alpha is a measure of the rate
at which an ultrasonic wave decreases in amplitude as a function of
distance by other than geometric means as it propagates through a
medium. For any given medium, a increases with increasing
frequency. Because the acoustic intensity is proportional to the
square of the acoustic pressure, attenuation can be expressed also
in terms of intensity:
Ix = Ioe-2alphax Equation 2.3
Attenuation is important from several points of view. First,
it results in a decrease in intensity at various depths in the
medium and determines the amount of acoustic energy that can reach
structures of interest, either for imaging or therapeutic purposes.
Second, attenuation by scattering can result in ultrasonic energy
reaching unintended structures. Third, attenuation is important,
because it is due in part to an absorption process in which the
propagating energy is permanently modified (for example, converted
into heat energy which causes a temperature rise in tissue). In
therapeutic applications, energy absorption and heat generation in
tissue are usually the intended results.
Attenuation is greater in some soft tissues than in others.
This variation is exploited in therapy for differential absorption
and heating of ligaments and tendons in surrounding muscular tissue
(Lehmann et al., 1959; Stewart et al., 1982).
Because of the depth of penetration desired, the frequencies
used for therapy purposes range from about 0.5 to 3 MHz. For
diagnostic purposes, the upper limit of the range for imaging in
abdominal areas is about 10 MHz. Frequencies up to 20 MHz are used
for small structures such as the eye, which have a lower attenuation
coefficient and shorter imaging depth.
Absorption is considerably higher in bone than in soft tissues.
This is one reason why bone may constitute a critical organ for
some forms of ultrasonic exposure, especially ultrasound therapy,
even though there is a strong reflection from a bone/soft tissue
interface. Bone damage has been reported in experimental animals
(Barth & Wachsmann, 1949; Kolar et al., 1965) at levels just higher
than those normally employed in physiotherapy (i.e., 3-4 W/cm2)
(section 6.4.6). In addition, ultrasound exposure of a bone/tissue
interface can result in sudden and sometimes pronounced periosteal
pain arising from a buildup of heat at the interface. At the
bone/tissue interface, some of the longitudinal oscillations
(particles oscillating in the direction of propagation) are
transformed into transverse oscillations. The transverse
oscillations (shear waves) are more readily absorbed than
longitudinal waves. This can produce local heating at the
bone/tissue interface causing periosteal pain (Lehmann et al.,
1967).
2.7. Finite Amplitude Effects
Another effect that may be important when ultrasound is applied
in biomedical research, diagnosis, or surgery results from the
finite amplitude of the particle velocity of the ultrasonic wave-
front. In linear acoustics, two familiar assumptions are made:
(a) that the transmitted frequency is the only frequency produced;
and (b) that when the input amplitude is increased, the amplitude
at remote points in the field increases proportionally. These
linear assumptions are not valid when considering finite-amplitude
effects. For a more detailed explanation, the reader is referred
to Beyer & Letcher (1969).
It has been shown (Beyer & Letcher, l969; Muir & Carstensen,
1980; Carstensen et al., 1981) that the frequencies and intensities
used in pulsed diagnostic ultrasonics can potentially create
significant distortion of sound waves in water.
Table 1. Typical values of ultrasonic properties of various media at
1 MHz.
--------------------------------------------------------------------------
Characteristic Amplitude
Medium Ultrasonic acoustic Attenuation absorption
speeda impedanceb coefficientc coefficient
c Zo=rho x c alpha alpha a
(m/s) (103 kg/s m2) (Np/cm) (Np/cm)
--------------------------------------------------------------------------
air (dry) 343.6 0.45 0.18 0.18
(20°C)
water (37°C) 1480 1480 0.0002 0.0002
amniotic fluid 1530-1540 1540-1560 0.0008 ND
aqueous humour ) 1530-1540 1540-1560 0.005-0.08 ND
vitreous humour)
blood ) 1555-1525 1560-1580 0.001-0.002 ND
plasma)
testis 0.03-0.04 0.01-0.02
fat 1450-1490 1360-1400 0.07-0.24 ND
liver )
kidney) 1560-1600 1580-1620 0.07-0.3 0.02-0.05
brain )
heart )
spleen ) 1510-1600 1580-1620 0.07-0.3 ND
pancreas)
muscle 1560-1600 1620-1700 0.06-0.16 ND
uterus 1600-1660 0.02-0.20 ND
lens 1600-1660 0.02-0.20 ND
skin ) 1720-2000 0.04-0.50 ND
tendon)
bone 3000-3300 4000-7000 1.3-3 ND
lung 500-1000 2-3 ND
--------------------------------------------------------------------------
Note: These values are for animal tissue and are for illustrative
purposes only; published data are not always consistent. Actual
measured values may show quite strong variability with factors
such as tissue preparation temperature and intensity.
a Velocity of longitudinal waves. ND = not determined
b Estimated from published data.
c Attenuation is approximately proportional to frequency: alpha=alpha1 fm,
where alpha1 is the attenuation coefficient at 1 MHz, f is the frequency
in MHz, and known values of m lie between 0.76 (tendon) and 1.14 (brain).
3. MECHANISMS OF INTERACTION
When acoustic energy is absorbed by matter, it is converted
into heat, the consequent temperature elevation depending on the
amount of energy absorbed, the specific heat of the medium, and the
dynamic balance between heat deposition and removal. In contrast
to X-rays, for example, commonly used ultrasound beams can carry
appreciable amounts of energy and thus one mechanism of action of
potential biological importance is thermal. A second phenomenon
that is well known to be associated with ultrasonic energy, and to
play a major role in many of the biological changes that have been
induced by ultrasound applied in vitro, is cavitation. However,
not all the evidence of biological and biochemical changes induced
by ultrasound can be explained on the basis of either heat or
cavitation. It is necessary to be aware of a further group of
established and/or physically predictable stress mechanisms, and of
the possible existence of other biophysical mechanisms, hitherto
undocumented. Finally, it should be noted that the different
mechanisms, as classified in this manner, are not necessarily
independent; for example, the biological expression of a physical
stress directly induced by the passage of ultrasound may well be
influenced by the temperature of the irradiated structure. Examples
of reviews of ultrasound mechanisms are those published by Nyborg
(1977, 1979, 1982) and Repacholi (1981).
3.1. Thermal Mechanism
Several reviews concerning the elevation of temperature
resulting from ultrasound exposure have been published (Lele, 1975;
Nyborg, 1977).
When ultrasound interacts with matter, part of the energy of
the beam will be absorbed and converted into heat. The rate (Q) at
which heat is generated per unit volume within a medium is given by
the equation Q=2 Iaalpha a; where alpha a is the amplitude absorption
coefficient of the medium and Ia is the intensity of a plane
travelling ultrasound wave (Appendix I). Without heat conduction
away from the exposed site, the rate of temperature rise will be
(Dunn, 1965):
d T/d t = 2alpha a Ia/rho Cm Equation 3.1
where d T/d t is the temperature rise per unit time, rho is the
ambient density of the medium, and Cm is the specific heat per unit
mass.
Consider an example of soft tissue exposed to an ultrasound
beam of intensity 1 W/cm2. If rho = 1 g/cm3, Cm = 1 cal/g/°C and
alpha a is 0.1 Np/cm, the temperature rise d T/d t is then 0.048°C/s,
when heat conduction is neglected.
If the effect of heat conduction away from exposed matter is
considered, it will be appreciated that, following an initial rise,
the temperature will tend towards an equilibrium value. Calculations
covering this behaviour for a spherical model have been given by
Nyborg (1977); some results are shown in Fig. 5. For this model
(a spherically symmetrical object exposed in an isotropically
conducting medium), the increase in equilibrium temperature is
proportional to the square of the radius, as is the time required
to attain that temperature. Thus, a small body uniformly exposed
to ultrasound will experience a small but rapid temperature rise,
whereas a large body, uniformly exposed to the same ultrasound
intensity, will reach a higher final temperature, but over a longer
period of time. It follows that temperature elevations resulting
from local heating on a scale comparable to cellular dimensions
(10-50 µm), which presumably occurs as a result of local absorption
mechanisms, will be insignificant in practice. This conclusion was
reached independently by Love & Kremkau (1980).
In practice, the biological expression of heat-induced damage
is found to depend both on the maximum temperature achieved and on
the time period for which that temperature is maintained. According
to Lele (1975), exposure of mice to a temperature elevation of
2.5 - 5.0°C for an hour or more during pregnancy caused a
significant increase in the number of fetal abnormalities.
3.2. Cavitation
3.2.1. Introduction
Under certain conditions, the application of ultrasound to a
liquid (or quasi-liquid) medium gives rise to activity involving
gaseous or vaporous cavities or bubbles in the medium. This
phenomenon, termed cavitation, may require pre-existing nuclei,
i.e., bodies of gas with dimensions of the order of micrometres
or smaller which are stabilized in crevices or pores, or by other
means, in the medium. Reviews of the subject have been given by
Flynn (1964), Coakley & Nyborg (1978), Neppiras (1980), and Apfel
(1981).
It has proved useful (Flynn, 1964) to distinguish between
stable and transient cavitation. Both of these are important
mechanisms for biological effects of ultrasound, the former being
especially relevant at lower intensity levels (e.g., 300 mW/cm2
or less in water) and the latter at higher levels. In many
experiments, both types of cavitation occur simultaneously, but in
certain situations only stable cavitation occurs.
3.2.2. Stable cavitation
In some media, gas bubbles exist which are of such a size
that they are resonant in the sound field and oscillate with large
amplitude. When a bubble expands and contracts during the
ultrasound pressure cycle, the surrounding medium flows inwards and
outwards with a higher velocity than if the gas bubble were absent.
As a rough guide, the resonant diameter of a cavitation bubble in
water at 1 MHz is about 3.5 µm. Alternatively, gaseous nuclei may
exist in the medium which are initially smaller than resonance size
but which grow to that size in an applied sound field through the
process of rectified diffusion.
When a gas bubble pulsates, its motion is not usually
spherical, either because of distortion by an adjoining boundary
or because of surface waves set up by the ultrasound field.
Asymmetric or non-uniform oscillation of the air-liquid interface,
at the surface of an air pocket or bubble, causes a steady eddying
motion to be generated in the immediately adjoining liquid, often
called microstreaming, in which the velocity gradients may be high.
If biopolymer molecules or small biological cells are suspended in
liquid near a pulsating bubble, they may be swept into a region of
high velocity gradient. Such a situation can also occur if a small
bubble pulsates near a cell membrane causing the membrane to
vibrate, producing streaming motions within the cell. The
biological system will then be subjected to shearing action and
damage may occur, such as fragmentation of macromolecules and
membranes (Nyborg, 1977).
Significant biological effects occur in suspensions near
resonant bubbles, even at low spatial peak temporal average (SPTA)
intensity levels. For example, Barnett (1979), and Miller et al.
(1979) found that blood platelets tended to aggregate around
artificial holes (forming gas bubbles) in a membrane, and Williams
& Miller (1980), using similar membrane material (containing gas-
filled pores) observed ATP release from red blood cells. All of
these effects were observed at SPTA levels considerably lower than
0.1 W/cm2.
These findings are consistent with the theory of microstreaming
and with experimental information on the response of biological
cells to hydrodynamically generated viscous stress (Glover et al.,
1974; Brown et al., 1975; Anderson et al., 1978; Dewitz et al.,
1978, 1979). For example Nyborg (1977) estimated that a bubble of
3 µm radius in blood plasma, caused to pulsate by ultrasound at an
intensity of 1 mW/cm2 with a frequency of about 1 MHz (to which the
bubble is resonant), would generate a microstreaming field in which
the maximum viscous stress would greatly exceed 100 N/m2. The
latter is an intermediate value for hydrodynamically generated
viscous stress which causes cell lysis.
Pulsating bubbles also produce microstreaming in organized
tissues. Martin et al. (1978) reported acoustic streaming motions
in plant and mammalian systems, using Doppler fetal heart monitors
under experimental conditions that ensured the existence of gas
bubbles. According to Akopyan & Sarvazyan (1979), streaming can
produce changes in the relative positions of intracellular
organelles and breaks in cytoplasmic structures.
3.2.3. Transient cavitation and studies concerned with both
stable and transient cavitation
In contrast to stable cavitation, transient (or collapse)
cavitation is more violent and occurs at higher ultrasound
intensity levels. When a gas bubble or nucleus within the
medium is acted on by an ultrasound field having a high pressure
amplitude, it may expand to a radius of twice the original value or
more, then collapse violently. In the final stages of collapse,
kinetic energy given to a relatively large volume of liquid has to
be dissipated in an extremely small volume, and high temperatures
and pressures result. Idealized thermodynamic calculations show
that for a compression in which no heat escapes from the cavity at
the end of the cavity's existence, the final temperature is around
8000 K and the pressures are greater than 109 Pa (104
atmospheres). Of course, the idealized assumption of a
thermodynamically closed system is not valid under such extreme
conditions. Sutherland & Verrall (1978) report that, under actual
conditions, not all the heat remains trapped in the cavity during
collapse; some is conducted away, resulting in estimated
temperatures of the order of 3500 K. It seems reasonable to
assume that effects on biological systems may be induced at least
by the mechanical shock waves and high temperatures generated
during the bubble collapse.
Chemical changes are commonly produced by cavitation. The
combination of high pressures and temperatures can generate aqueous
free radicals and hydrated electrons (highly reactive chemical
species) within the exposed medium by the dissociation of water
vapour in the bubble during its contraction. Chemical interactions
of biomacromolecules with these free radicals often result
(especially with hydrogen H- and hydroxyl 0H- radicals), and
significantly alter their properties. This can be accompanied by
the formation of such compounds as nitrous acid (HNO2), nitric acid
(HNO3), and hydrogen peroxide (H2O2) (Akopyan & Sarvazyan, 1979).
Studies show that transient cavitation does not occur unless
the intensity exceeds some threshold value which is very dependent
on experimental conditions. The cavitational threshold SPTA
intensity was determined by Esche (1952) and Hill (1972a) for
frequencies ranging from 0.25 to 4 MHz, in air-equilibrated water,
for cw ultrasound. The threshold intensity was in the range of a
few watts per square centimetre and was frequency dependent. The
higher the frequency, the higher the intensity required to produce
cavitation.
Pulsing conditions have a marked influence on cavitation.
Hill & Joshi (1970) found that, at shorter pulse durations, the
cavitation threshold increased. Alternatively, as the pulse
duration decreased, the duty factor had to be increased to
produce cavitation at a given intensity. A model for acoustic
cavitation, according to which cavitation activity is optimized for
an appropriate choice of pulsing parameters, has been postulated
and confirmed experimentally by Ciaravino et al. (1981).
Higher ambient pressure causes higher threshold intensities for
cavitation. For a cw 1 MHz ultrasound beam, Hill (1972a) found
that the threshold intensity varied from just under 1 W/cm2 at an
ambient pressure of 105Pa (1 bar) to much greater than 16 W/cm2 at
1.75 x 105Pa (1.75 bar). Increasing the ambient pressure often
provides an effective means of inhibiting cavitation and thereby
ascertaining whether a previously observed response was due to
cavitation.
It has also been found that the threshold for cavitation
decreases with increasing temperature (Connolly & Fox, 1954) and
with increasing volume of the irradiated liquid (Iernetti, 1971).
Particularly important for the occurrence of cavitation is the
number and size distribution of gas nuclei within the medium.
Unfortunately, these quantities are not easily measured. The
number of available nuclei within a fluid medium greatly increases
when the medium is stirred or mechanically disturbed (Williams,
l982a).
3.2.4. Cavitation in tissues
Intracellular gas channels are commonly present in plant
tissues and greatly influence the biological response of these
tissues to ultrasound (Nyborg et al., l975; Carstensen, 1982).
Similarly, the responses of insects and insect eggs to ultrasound
are greatly influenced by the presence of microscopic airpores
(Child et al., 1980a, 1981a, 1981b). A characteristic of the
response of both plants and insects to pulsed ultrasound is that
the critical exposure parameter appears to be the temporal peak
rather than the temporal average of the intensity.
Much less is known about cavitation in mammalian tissues. In a
series of studies, Fishman (1968) was unable to detect significant
levels of haemolysis in the blood of human volunteers whose hands
were immersed in an 80 kHz cleaning bath for up to 45 min. However,
the external ears of rabbits developed numerous petechial haemmorrhages
when they were immersed for more than 3 min in a 55 kHz cleaning
bath (Carson & Fishman, 1976).
Lehmann (1965a), using dogs, reported that tissue damage, which
was attributed to cavitation, occurred at intensity thresholds of
1-2 W/cm2 for 1 MHz ultrasound applied by means of a stationary
applicator. When a stroking technique was used, these effects were
not observed at intensities up to 4 W/cm2. A dependence on ambient
pressure, observed for this biological effect is a strong indication
that the gas content of the tissue was involved in the reaction.
Thresholds of about 1.5 W/cm2 have been reported for soft tissue
damage due to cavitation caused by exposure to cw ultrasound with
the transducer in a stationary position (Hug & Pape, 1954). On the
basis of morphological findings and physical measurements, they
concluded that cavitation could be expected in tissues at
intensities in the range used for therapeutic purposes. Similar
data have also been reported by Lehmann & Herrick (1953). Other
reports of effects on experimental animals also indicate that
cavitation may have been responsible (O'Brien et al., 1979; Martin
et al., 1981).
Evidence for the existence of gaseous nuclei in tissues has
been given by ter Haar & Daniels (1981). They observed that the
production of gas bubbles in the legs of guinea-pigs exposed to
cw 0.75 MHz ultrasound at SATA intensities of 80 and 680 mW/cm2,
was associated with tissue interfaces. At 680 mW/cm2, sites
occurred throughout the entire cross-section of the leg with many
bubbles located intramuscularly. The rate of appearance of sites
increased with both intensity and duration of exposure. The
authors reported that an SATA intensity of 80 mW/cm2 appeared to be
close to an intensity threshold for stable bubble production in
tissues in vivo. In applying the theory for rectified diffusion to
these results, Crum & Hansen (1982) showed that they were
consistent with an assumption that gaseous nuclei with diameters in
the range of a few micrometres exist normally within tissues.
3.3. Stress Mechanisms
Stress mechanisms or non-thermal, non-cavitational mechanisms
of ultrasound action have been reviewed by Nyborg (1977) and Dunn &
Pond (1978). Ultrasound exposure produces various stresses within
biological systems, the magnitude and significance of which depend
on the detailed characteristics of the ultrasound field and the
biological system exposed. Lewin & Chivers (1980) proposed a
viscoelastic model of the cell membrane as a potential means of
investigation in connection with pulsed sources. Repacholi (1982)
found evidence that many biological effects on cell systems in
vitro may be due to forces both within and outside the cell, which
might be mediated by stress mechanisms.
Stresses or forces resulting from an ultrasound field acting on
heterogeneous regions within a medium can be categorized as follows
(Dunn & Pond, 1978):
(a) buoyancy forces that are oscillatory, have a time-
average equal to zero, and produce a radiation
pressure on bodies having a density different from
the surrounding medium;
(b) displacement or radiation forces that have a non-
zero time average and can cause an appreciable
relative velocity between the inhomogeneity and the
surrounding medium;
(c) viscosity-variation forces or viscous stresses that
result in acoustic streaming because of variations
in viscosity over the cycle of the applied ultrasound;
and
(d) the Oseen force, another time-averaged force, which
is due to the dependence of drag on the second power
of relative velocity.
3.3.1. Radiation pressure, radiation force, and radiation torque
There is evidence of radiation pressure (from ultrasound
pulses) being detected by the inner ear and giving rise to
disturbances that can be sensed by the brain as if they were
audible sound (Foster & Wiederhold, 1978). In addition, Gershoy &
Nyborg (1973) postulated that gradients of radiation pressure in
exposed plant tissue give rise to water flow in cytoplasmic
channels.
An example of the action of radiation force is the blood flow
stasis phenomenon reported by Dyson et al. (1971), where red blood
cells in the blood vessels of chick embryos exposed to an
ultrasonic standing-wave field, collected into parallel bands
spaced at half wavelength intervals. This has also been shown in
mammalian vessels (ter Haar et al., 1979).
Spinning of intracellular bodies exposed to highly non-uniform
ultrasound fields has been observed by various investigators (Dyer,
1965, 1972; Nyborg, 1977; Martin et al., 1978). When an ultrasound
field is propagated within a liquid, a twisting action may be
exerted on suspended objects, and on elements of the liquid itself.
For an asymmetrically shaped object such as a rod or disc, this
radiation torque varies with the orientation of the object relative
to the oscillation direction of the surrounding liquid, so that the
object tends to assume the position in which the torque on the
object is least. Such an effect may be important, when the effects
of ultrasound on asymmetrically shaped cells, organelles, or
macromolecules are considered. For a symmetrical object, steady
spinning will result. Theoretically, this spinning is expected in
non-uniform fields such as those existing at a boundary where a
progressive ultrasound wave impinges obliquely and is reflected
(Nyborg, 1977). In the latter situation, the object's velocity of
spinning ( v) is proportional to the ratio of the absorption
coefficient (alpha a) for the material in this spherical body and to
the coefficient of shear viscosity (eta) for the surrounding fluid.
Martin et al. (1978) observed the effects of radiation torque
in sonicated (2.1 MHz, 43 mW/cm2) leaves of Elodea and root tips of
Vicia faba. How radiation torque affects other macromolecular
structures or organelles within or outside cells is not known, at
present.
3.3.2. Acoustic streaming
When an ultrasound field is propagated within a liquid, the
particles of the liquid take part in an oscillatory flow. Consider
a particle oscillating in a direction parallel to a boundary. At
the boundary itself, the velocity of the liquid flow will be zero
provided the boundary is a fixed, rigid solid, and "non-slip"
conditions apply. Conditions may then exist for establishing
acoustic streaming, a time-independent circulatory motion of the
liquid. As part of this motion a thin boundary layer may exist
between the surface and the streaming liquid itself, within which
the velocity gradient is large. Such streaming has been observed
as circulatory flow in the vacuoles of plant cells (Nyborg, 1978).
However, there must be non-uniformity or some kind of asymmetry for
this streaming to be established. For an ultrasound field
propagating in a suspension of particles, relative motion occurs
between the particles and the fluid, where boundary layers are
established around each particle and give rise to an acoustic
streaming field. Such microstreaming was demonstrated near
vibrating gas bubbles by Elder (1959), who analysed four regimes of
streaming.
Early effects attributed to acoustic streaming were reported by
Nyborg & Dyer (1960), who demonstrated the migration of protoplasm
towards a needle vibrating at 25 kHz in intact cells of Elodea.
Selman & Jurand (1964) described the disorganization and subsequent
recovery of the arrangement of the endoplasmic reticulum following
irradiation for 5 min with 1 MHz ultrasound at intensities between
8 and 15 W/cm2. More recently, these stresses associated with
acoustic streaming have been suggested to be responsible for:
(a) altered cell surface charge (Repacholi, 1970;
Repacholi et al., 1971; Taylor & Newman, 1972);
(b) altered cell membrane permeability (Chapman, 1974;
Chapman et al., 1980; Al-Hashimi & Chapman, l981);
(c) separation of small fragments from cells (Dyson et
al., 1974; Nyborg, 1979; ter Haar et al., 1979);
(d) rupture and fragmentation of cell membranes (Williams,
1971; Brown et al., 1975; ter Haar et al., 1979); and
(e) reduced uptake of radioactive precursor in mammalian
cells in vitro (Repacholi, 1980).
4. MEASUREMENT OF ULTRASOUND FIELDS
The spatial distribution of ultrasound fields can be quite
complicated depending on such factors as focusing, the radius of
the transducer, the wavelength of the ultrasound, the distance from
the source, and even on the way in which the element of the
transducer is mounted (Zemanek, 1971). Any effect produced by
ultrasound will depend quantitatively on the temporal and spatial
characteristics of the ultrasonic field. It is therefore necessary
to consider the methods available for making physical measurements
to determine the relationships between the equipment output levels
used in human exposure and the results of biological studies.
These methods are divided into measurement techniques for
liquid-borne and airborne ultrasound. Several extensive reviews of
techniques for measuring liquid-borne ultrasound have been reported
in the literature (Stewart, 1975, 1982; Zieniuk & Chivers, 1976).
The phenomenon of solid-borne ultrasound, for example, in bone (Fry
& Barger, 1978) is also of interest, but will not be dealt with
here.
4.1. Measurement of Liquid-borne Ultrasound Fields
Measurements necessary to characterize ultrasound fields should
include all spatial and temporal characteristics. This will involve
measuring at least one (and possibly more) of the four field
parameters ( p, xi, v, a), discussed in section 2, over all relevant
conditions of space and time. Once these parameters are known, it
is possible to calculate the spatial and temporal behaviour of
power and intensity in the equivalent plane-wave field. In order to
characterize exposure, the total power should be specified as well
as the following intensities: spatial average temporal average
(SATA) intensity; spatial peak temporal peak (SPTP) intensity;
spatial peak temporal average (SPTA) intensity; and, if applicable,
spatial peak pulse average (SPPA) intensity and spatial average
pulse average (SAPA) intensity. These and other factors that are
important for the complete characterization of ultrasonic exposure
in the investigation of biological effects are summarized in Table 2.
Acoustic power and intensity have traditionally been used to
express exposure. They are the parameters specified in most
standards, e.g., the AIUM-NEMA (1981) standard, the Japanese
standards for diagnostic equipment (JIS 1979, 1980, 1981; JAS,
1976, 1978), and the standards of Canada (Canada, Department of
National Health and Welfare, 1981) and the USA (US Food and Drug
Administration, 1978) for the performance of ultrasound therapy
equipment.
Relatively little work has been carried out concerning
ultrasonic field measurements in tissue, though some measurements
and theoretical calculations to determine the ultrasonic field in
tissue have been reported (Chan et al., 1974). Instrumentation
used for internal field measurements include thermocouples for the
measurement of temperature rise at specific locations (Goss et al.,
1977) and miniature transducers inserted into bodies (Bang, 1972;
Lewin, l978).
Table 2. Biologically important exposure parameters
---------------------------------------------------------------------
(a) Continuous wave (cw) ultrasound
Frequency of ultrasound
SATA intensity
SPTA intensity (if focused)
(b) Pulsed ultrasound
Centre frequency
Pulse shape or frequency spectrum
Pulse duration
Pulse repetition frequency or duty factor
Frame repetition frequency (automatic scanners)
SPTP intensity
SPPA intensity
SPTA intensity
(c) General
Exposure time
Exposure fractionation (if not a single exposure)
Degree and periodicity of the modulation or interruption
Single transducer
Transducer diameter
Array dimensions (automatic scanners)
Type of field (focused or unfocused)
Focal area, focal length (if focused)
Other details of geometric conditions, such as:
Exposure under far-field or near-field conditions
Acoustic path length to organ or site of interest
Extent of standing wave component (if any)
Relation of the peak to the average intensity for
the beam cross section of interest, (i) if the source is
maintained in a fixed position and orientation during exposure;
(ii) if not fixed, the path and speed of motion
---------------------------------------------------------------------
Reported measurements of the attenuation between the abdominal
surface and the uterine cavity are shown in Table 3.
Instruments available for measuring liquid-borne ultrasound
include those that measure total power and those that can measure
point quantities over an area. With the latter, it is possible to
determine the distribution of the energy in the ultrasonic field.
Table 3. Reported attenuation between the abdominal surface and the
uterine cavitya
------------------------------------------------------------------------------
No. Average Attenuation Distance Frequency Species Reference
patients rate of (dB) (cm) (MHz)
attenuation
(dB/cm)
------------------------------------------------------------------------------
10 1.6 (mean) 2.25 mouse Bang &
Northeved
(1970)
8 0.5 - 1 2 - 4 2 - 4.5 2.25 man Bang (1972)
6 0.9 - 1.56 6 - 14 5 - 11 2.25 man Etienne et
al. (1976)
13 0.6 - 1.8 2 - 7.5 3 - 5.8 2.25 man Takeuchi et
al. (1977)
10 0.5 - 7.2 12 (mean) 6 2.0 man Morohashi
& Iizuka
(1977)
------------------------------------------------------------------------------
a From: Stewart & Stratmeyer (1982).
4.1.1. Measurement of the total power of an ultrasound beam
Measurement of total power is important for several reasons:
(a) the total power of an ultrasound field impinging on an extended
plane target can generally be measured more accurately than point
or spatial quantities; (b) it is commonly used to characterize
standard reference sources (such sources may be used in the
calibration of detectors that measure point quantities, e.g.,
hydrophones); and (c) on measuring the total power for a defined
field size, it is possible to calculate the mean intensity, usually
referred to as spatial average intensity.
Ultrasound measurement procedures are discussed by various
authors (O'Brien, 1978; Stewart, 1982). Several methods are
available for measuring total power, including radiation force,
calorimetry, and acoustico-optical techniques, but the one which is
usually favoured is radiation force. This method, which can be
used in the measurement of the total power output of ultrasound
equipment, is based on the fact that the surface of a reflecting or
absorbing target is performing a microscopic oscillation according
to the continuity of particle velocity ( v) and partitioning of the
momentum carried by the plane wave takes place at the surface.
Consequently, the time average of the acoustic pressure at this
non-stationary reference surface is non-zero. The resulting steady
pressure on the surface, multiplied by the exposure area, is called
the radiation force. The force produced is independant of frequency
and is proportional to the total ultrasonic power impinging on the
target. The radiation force ( F) in newtons is given by:
F = PD/ c Equation 4.1
where P is the incident acoustic power in watts, c is the
propagation velocity of the wave in m/s (in water c = 1.5 x 103
m/s at 30°C), and D is a dimensionless factor, determined by the
type of interface encountered by the ultrasonic field and the
direction in which the force produced by reflection or absorption
is measured.
Values for D in Equation 4.1 are shown in Table 4. The table
has been modified from that of Hueter & Bolt (1955) to a more
general situation (Stewart & Stratmeyer, 1982). By knowing the
type of interface a target presents to an ultrasonic field, and by
measuring the magnitude of the force the total power in the
acoustic field can be computed. Typically, a flat, totally
reflecting plate is used in radiation force devices. For this
situation, the only force produced by the reflected ultrasound is
in a direction normal to the plate. This force is given by 2 P/c
cos theta, where theta is the angle between the normal to the
reflecting surface and the ultrasound beam. If the direction of
measurement of force is not normal to the plate, only the component
in the direction of measurement will be determined. In this case,
the force measured is F = 2 P/c cos theta cos psi, where psi is the
angle between the normal to the reflecting surface and the
direction in which the force is to be measured.
If theta = psi, i.e., the ultrasound beam and the direction in
which the force is measured are the same, then F = 2 P/c cos2psi,
which is the equation usually associated with these devices
(Hueter & Bolt, 1955). For propagation in water, a collimated
beam of ultrasound exerts an apparent weight in the direction
of propagation equivalent to 0.136 cos2psi mg/mW or 0.067 mg/mW
for psi = 45°.
The relationship in equation 4.1 applies for both cw and pulsed
ultrasonic fields, provided that P is taken as a time-averaged value.
Because of inertia, the system cannot respond to the temporal
variation of the pulsed ultrasound, unless the pulse repetition
rate is extremely slow. Many practical radiation force systems for
measuring the output from both therapy and diagnostic sources have
been described in the literature (Rooney, 1973; Stewart, 1975;
Robinson, 1977; Brendel et al., 1978; Carson et al., 1978; Bindal &
Kumar, 1979, 1980; Bindal et al., 1980; Carson, 1980; Shotton,
1980).
Table 4. Values of the constant D for various physical situations
for a plane progressive ultrasound fielda
--------------------------------------------------------------------
Physical situation Dx Dy
--------------------------------------------------------------------
Perfect absorber,
normal incidenceb
r = 1 1 1 cos psi
Perfect reflector,
normal incidence
r = O or infinite 2 2 cos psi
Perfect reflector,
ultrasound incident
at angle theta to
reflectorb
r = O or infinite 2 cos2theta 2 cos theta cos psi
Nonreflecting interface,
normal incidenceb
r = 1, c1=/=c2 1- c1/ c2 (1- c1/ c2) cos psi
For c1 < c2, force in direction of
propagation
For c1 > c2, force directed opposite
to direction of propagation
Partially reflecting
interface, normal
incidence
Z2=/= Z1, c1=/= c2 2[(r-1)2/(r+1)2] 2[(r-1)2/(r+1)2] cos psi
--------------------------------------------------------------------
a From: Hueter & Bolt (1955) and Stewart & Stratmeyer (1982).
b r = Z2/ Z1, the impedance ratio at an interface, where Z = rho c.
x where the direction of ultrasound propagations is the same as the
direction in which the force is measured.
y where the direction of ultrasound is not the same direction in
which the force is measured.
c = the velocity of ultrasound in the medium.
rho = the density of the medium.
theta = the angle between the normal to the reflecting surface and the
incident ultrasound beam axis.
psi = the angle between the normal to the reflecting surface and the
direction in which the force is measured.
=/= - not equal to
Note:
(1) When the direction of the incident ultrasound beam is the same
as the direction in which the force is measured, then psi = theta
and the value of D for a reflecting surface becomes 2 cos2theta;
this is usually the case in practice.
(2) When the direction in which the force is measured is the same
as the direction of the normal to the reflecting surface, then psi
= 0 and the value of D for a reflecting surface becomes 2 costheta.
4.1.2. Spatial and temporal measurements
Ideally, to measure the spatial and temporal characteristics of
ultrasound, a detector is needed that is small compared with the
wavelength of the ultrasound field and has a response function
(i.e., the quotient of the electric output signal and the acoustic
imput signal) that is flat over the frequency of interest, combined
with high sensitivity, low noise, and a wide acceptance angle.
Miniature piezoelectric hydrophones, though not ideal, are used
extensively to determine the spatial distributions and temporal
pressure waveforms and, when properly calibrated against an
appropriate standard, can provide a satisfactory measurement
method. Wells (1977) describes various types of hydrophones that
have been used. Devices of this type respond to the instantaneous
local value of the acoustic pressure in the field. However, not
all commercially available hydrophones are frequency independent in
their sensitivity, and this presents a major problem. The
frequency responses of several hydrophones have been reported in
the literature (Harris et al., 1977; Lewin, 1978, 1981a, b; Harris,
1981).
The International Electrotechnical Commission (IEC, 1981) and
the American Institute for Ultrasound in Medicine/National
Electrical Manufacturers Association joint task group (AIUM-NEMA,
1981) have both recommended the use of hydrophones for the
measurement of spatial and temporal exposure parameters for
diagnostic ultrasound equipment. Comparison of the reciprocity
technique for the calibration of ultrasonic hydrophones with that
of planar scanning in a field of known acoustic power has shown
that both methods yield consistent results (Gloerson et al., 1982).
The choice of method depends on convenience and the interest and
background of the user.
Most conventional probes have resonances in the frequency range
of interest but distort the ultrasonic pulses being observed. Only
if the frequency characteristics of the probe are known, can
appropriate corrections be made. Another limitation in the use of
hydrophones is their directional sensitivity, for which correction
must be made. The use of the piezoelectric polymer polyvinylidene
fluoride as an ultrasonic hydrophone has been described (DeReggi et
al., 1978, 1981; Wilson et al., 1979; Shotton et al., 1980; Harris,
1981; Lewin, 1981b). Compared with ceramic, this material has an
acoustic impedance much closer to that of water and, because it is
available in sheets that have thickness resonances greater than 20
MHz, it promises to be useful as a broad-band, acoustically
transparent receiver. Hydrophones made with piezoelectric polymer
are commercially available.
4.2. Measurement of Airborne Ultrasound Fields
Both audible and ultrasonic fields are usually quantified in
terms of sound pressure level (SPL), in decibels (dB):
SPL (dB) = 20 log10( p/pr)
where p is the acoustic pressure in free air. The reference
pressure pr is usually taken as pr=20 micropascals (µPa),
which is equivalent to an acoustic intensity of Ir=10-12W/m2.
This is approximately the lowest intensity of audible sound
perceived by human subjects at 1000 Hz.
Since acoustic intensity is proportional to the square of
acoustic pressure, the sound level can equally be expressed by:
SPL (dB) = 10 log ( I/Ir)
Therefore, doubling the intensity I increases the SPL by 3
dB, whereas doubling the pressure p increases the SPL by 6 dB.
The actual determination of decibel levels at various positions
in an airborne ultrasound field can be made with several
commercially available systems (Michael et al., 1974; Herman &
Powell, 1981). These normally include a capacitor microphone
sensing element having a flat frequency response within the range
of interest, and signal processing circuitry. Usually, this
circuitry includes a set of one-third octave filters, so that the
additive SPL within any particular one-third octave frequency range
is indicated on the meter. A spectrum of SPL as a function of
frequency (to one-third octave resolution) can be obtained by
"stepping through" the filter set. When making SPL measurements,
humidity and temperature conditions should be taken into account.
Rapid advances are being made in the development of ultrasound
transducers for use in air, which have greatly improved resonance
frequency and resolving capacity. Commercially available
transducers include electrostatic types, with linear frequency
ranges up to a few hundred kHz (Frederiksen, 1977) and ceramic
types, with quarter-wavelength matching to air and resonant
frequencies up to 400 kHz (Kleinschmidt & Magori, 1981). At these
frequencies, the ultrasound wavelength in air is of the order of
1 mm, which enables the construction of a whole line of new
instrument systems using very narrow ultrasound beams (mm to cm)
for remote measurements over distances ranging from millimetres to
metres.
Applications using measurement of airborne ultrasound include:
industrial remote measurements (size, location, speed etc.),
anthropometrical measurements, and imaging of human beings
(Lindström et al., 1982). Measurements are performed using the
ultrasound pulse-echo method, which means that many techniques used
in diagnostic ultrasound can be transferred to high-frequency
airborne ultrasound, i.e., different forms of real-time scanners
(Lindström & Svedman, 1981).
Systems developed for measurement, control and imaging, and
working with high-frequency (50-1000 kHz) airborne pulse-echo
ultrasound, make use of narrow sound beams of high pulse intensity
but low duty rate (Lindström et al., 1982). Because of the short
pulse duration, determination of the intensity level should be
performed in a similar way to the procedure for diagnostic
ultrasound; i.e., using spatial and temporal measurements to
characterize the airborne ultrasound field.
5. SOURCES AND APPLICATIONS OF ULTRASOUND
For many years, ultrasound was only used in the detection of
submarines (Mason, 1976). The device, first produced by Paul
Langevin in 1917, was composed of a quartz crystal vibrating at 50
kHz, propagating ultrasound into the water and detecting the
reflected beam. Ultrasound was first used therapeutically in the
mid 1930s and for flaw detection between 1939 and 1945 (Firestone,
1945; Desch et al., 1946).
Since the Second World War, considerable progress has been made
in the development of new piezoelectric crystals, ferroelectric
ceramics, and magnetrostrictive materials, and the applications of
ultrasound have increased and diversified, particularly in recent
years. Fig. 6 includes examples of ultrasound devices used in
medicine, industry, consumer products, and signal processing and
testing, in relation to ultrasound frequency. Besides the
potential for occupational exposure to ultrasound in industrial and
medical applications, members of the general population are now
exposed to various consumer-oriented devices. However, medical
applications are the most rapidly increasing source of exposure.
This section includes a brief review of domestic, industrial,
commercial, and medical sources and applications of ultrasound.
5.1. Domestic Sources
An ever increasing number of consumer-oriented devices emitting
ultrasound are being manufactured. Examples are garage door
openers, television channel selectors, remote controls, burglar
alarms, dog whistles, bird and rodent scarers, traffic control
devices, and range-finders on cameras. In general, low intensities
and frequencies at the lower end of the ultrasound range (20-100
kHz) are used in these applications and the ultrasound is usually
propagated in air, so that the beam is rapidly attenuated over
short distances.
5.2. Industrial and Commercial Sources
The industrial and commercial applications of ultrasound have
been reviewed in a number of reports (Lemons & Quate, 1975;
Lynnworth, 1975; Shoh, 1975; Jacke, 1979; Repacholi, 1981; Rooney,
1981). Generally, these applications can be divided into two
categories (high- and low-power), depending on the power or
intensity levels involved. High-power applications usually rely
on compound vibration-induced phenomena occurring in the object or
material being irradiated. These phenomena include cavitation and
microstreaming in liquids, heating, and droplet formation at
liquid/liquid and liquid/gas interfaces. Some of the more common
applications of high-power ultrasound are described in Table 5
together with the ultrasound frequency and power or intensity range
used, where these variables are known. The most practical frequency
range for these applications is 20-60 kHz. Most industrial ultra-
sound is produced using an electrostrictive or magnetostrictive
transducer (Lynnworth, 1975), in which the dimensions of the
elements change in response to an applied electric or magnetic field.
Probably the oldest industrial application is cleaning by means
of cavitation and microstreaming mechanisms. Most cleaning tanks
operate at intensities below 10 W/cm2, 2 W/cm2 being commonly used.
Plastic welding with ultrasound became popular in the mid 1960s
and ultrasound is now used to assemble toys, appliances, and
thermoplastic parts. At frequencies above 20 kHz and intensities
of more than 20 W/cm2, sufficient heat is produced to melt the
plastic at the required locations. The principal advantages of
this method are speed, cleanliness, easy automation, and welding in
normally inaccessible places. An interesting application is the
ultrasonic sewing machine. Here woven or nonwoven fibres can be
"sewn" together without thread.
Metal welding was introduced commercially in the late 1950s and
is used in the semiconductor industry for welding or microbonding
miniature conductors. The process involves relatively low
temperatures, usually below the melting point of the metal. The
welding depends on ultrasonic cleaning. Ultrasonic shear causes
mutual abrasion of the two surfaces so that exposed plasticized or
metal surfaces can be joined under pressure to form a "solid-state"
bond. For this process, very high intensities are needed at the
welding tip (of the order of 2000 W/cm2 at frequencies ranging from
40 to 60 kHz).
Table 5. Industrial applications of high-power ultrasounda
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Application Description Frequency Power or intensity
(kHz) range
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cleaning and cavitating cleaning 18 - 100 usually below
degreasing solution scrubs parts 10 W/cm2 but up
immersed in solution to 100 W power
soldering and displacement of oxide approx. 2 - 200 W/cm2
brazing film to accomplish 30
bonding without flux
plastic welding welding soft and 20 - 60 usually 20 - 30
rigid plastic W/cm2 but power
below 1000 W output
metal welding welding similar and 10 - 60 up to 10 000
dissimilar metals W/cm2
machining rotary machining, usually
impact grinding using 20
abrasive slurry,
vibration-assisted
drilling
extraction extracting perfume, approx. about 500 W/cm2
juices, chemicals from 20
flowers, fruits, plants
atomization fuel atomization to 20 - up to 800 W
improve combustion 30 000
efficiency and reduce
pollution; also
dispersion of molten
metals
emulsification, mixing and homogenizing - -
dispersion, and liquids, slurries, and
homogenization creams
defoaming and separation of foam and - -
degassing gas from liquid,
reducing gas and foam
content
foaming of displacing air by foam - -
beverages in bottles or containers
prior to capping
electroplating increases plating rates approx. 30 W
and produces denser, 27
more uniform deposit
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Table 5. (contd.)
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Application Description Frequency Power or intensity
(kHz) range
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erosion cavitation erosion - -
testing, deburring,
stripping
drying drying heat-sensitive - -
powders, foodstuffs,
pharmaceuticals
cutting cutting small holes in approx. about 150 W
ceramics, glass, and 20
semi-conductors
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a From: Repacholi (1981).
Ultrasound soldering, without fluxes, has also been carried out
since the early 1950s. Cavitation in the molten solder erodes the
surface of metal oxides and exposes the clean metal to the solder.
Simultaneous cleaning and tinning of the metal can be effected
using ultrasonic intensities up to 100 W/cm2, at frequencies between
20 and 50 kHz.
The machining of metals and ceramics can be carried out using
an abrasive slurry between the vibrating tool and the work-piece.
With a rotary machine and axial ultrasonic vibration, metals and
other hard materials can be machined using diamond-impregnated core
bits. Ultrasonic cavitation accelerates the cutting action of the
water-cooled core bits. Usually, these devices operate at about 20
kHz.
In high-power applications, the materials being worked are
physically changed, whereas, in low-power applications, the
ultrasound is used to examine rather than alter the materials.
In many cases, low-power applications involve frequencies in the
megahertz range (Table 6). Applications include: the
determination of viscosity, transport properties, position, phase,
composition, anisotropy and texture, grain size, stress and strain,
elastic properties; the detection of bubbles, particles, and leaks;
non-destructive testing; acoustic emission; imaging and holography;
and counting by means of beam disruptions. Many of the devices
used in these applications have intrusive ultrasonic probes, but
non-invasive pulsed and resonance techniques are also used.
Table 6. Low-power applications of ultrasound in industrya
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Application Principle Frequency
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Measurement
of:
flow determining flow rates for gases, liquids, 1 - 10 MHz
and solids - Doppler technique
elastic relating speed of sound to resonance 25 kHz - 300 MHz
properties modes of polarization
temperature response to temperature dependence of up to 30 MHz
sound, speed, or attenuation
thickness timing round trip interval of pulse 2 - 10 MHz
density, resonant and non-resonant probe up to 50 kHz
porosity transmission
grain size ultrasound attenuation few MHz
of metals
pressure frequency of quartz crystal resonator 0.5 - 1 MHz
changes with applied pressure
level attenuation of ultrasound beam or measure around 100 kHz
travel time (pulse echo technique)
Counting beam interruptions counted 40 kHz
Gas leaks detection of ultrasonic "noise" 36 - 44 kHz
Flaw observe discontinuities in reflected 25 kHz to
detection beam 25 MHz (mW power)
Delay lines transform electric signal into ultrasound few MHz
and back again after ultrasound has
travelled a well-defined path
Burglar ultrasound beamed into room and a certain 18 - 50 kHz
alarms level of reflected beam is monitored; if (mW powers)
this level changes (with intruder) alarm
sounds
Pest frequency and intensity of ultrasound 18 - 50 kHz
control bothersome to pests - inaudible to human (mW powers)
beings
Sonar Doppler method determines presence and 5 - 50 kHz
velocity of object
Acoustic observe phase shift and attenuation of 100 - 3000 MHz
microscope ultrasound beam by the specimen
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a Adapted from: Lynnworth (