3D/4D Ultrasound Imaging

REVERBERATIONS - ULTRASOUND BIOSAFETY - NELSON ET.AL. 2008
1
Ultrasound Biosafety Considerations for the
Practicing Sonographer/Sonologist
Thomas R. Nelson, J. Brian Fowlkes, Jacques S. Abramowicz
Abstract— The purpose of this article is to present the practicing sonologist or sonographer with an overview of the biohazards
of ultrasound and guidelines for safe usage.
Index Terms— Ultrasound, bioeffects, fetal, mechanical index,
thermal index, output display standard
I. OVERVIEW
Ultrasound is an imaging modality that has important diagnostic value. While useful in a variety of applications, diagnostic ultrasound is particularly useful in prenatal diagnosis with
over 250 million fetal ultrasound examinations performed per
year in USA. To date, there is no evidence that diagnostic
ultrasound produces harm in the developing fetus or other
tissues when used properly.
There are however an increasing range of US studies
being performed. Newer technologies can have higher acoustic
output levels than earlier equipment. Also, subtle or transient
effects of diagnostic ultrasound, such as changes in membrane
permeability, are completely understood.
Therefore, diagnostic ultrasound should be used prudently
with ultrasound examinations performed only by trained, competent personnel. To ensure continued safety it is essential
to maintain an awareness of the potential for bioeffects,
especially with newer equipment and more sophisticated procedures.
The purpose of this article is to review ultrasound biosafety
considerations for the practicing sonographer and sonologist
as well as provide references for more detailed discussion and
guidance with an emphasis on fetal imaging.
II. U LTRASOUND ACOUSTIC O UTPUT AND B IOEFFECTS
The embryo and fetus are particularly sensitive to energy
deposition from many imaging modalities, including ultrasound. Up to eight weeks after conception, organogenesis is
Address correspondence to:
Thomas R. Nelson, Ph.D., University of California, San Diego, La Jolla,
CA, [email protected];
J. Brian Fowlkes, Ph.D., University of Michigan, Ann Arbor, MI,
[email protected]
Jacques S. Abramowicz, M.D., Rush University, Chicago, IL,
jacques [email protected]
taking place in the embryo. This is a period when cell damage
might lead to anomalies or subtle developmental changes.
Furthermore, the brain and spinal cord continue to develop
through to the neonatal period. Little information is available
regarding possible subtle biological effects at diagnostic levels
in either the fetus or adult.
The presence of bone within the ultrasound beam greatly
increases the likelihood of a temperature rise due to direct
absorption in the bone itself and due to conduction of heat
from bone to adjacent tissues. Generally the temperature rise
is greatest at bone surfaces and adjacent soft tissues.
Fetal effects vary with the increasing mineralization of the
developing bone raising the potential for heating of sensitive
tissues such as brain and spinal cord although the increasing
fetal size in later gestation may provide some additional
resistance to thermal insult. It also is important that care be
taken to minimize fetal eye exposure.
In general, one should limit exposure time commensurate
with acceptable diagnostic evaluation. At present there is
no reason to withhold diagnostic scanning during pregnancy
provided it is medically indicated and is used prudently by
fully trained operators [2]
III. S TANDARD FOR R EAL -T IME D ISPLAY OF T HERMAL
AND M ECHANICAL ACOUSTIC O UTPUT I NDICES ON
D IAGNOSTIC U LTRASOUND E QUIPMENT
Before 1976, there were no limits to the permissible acoustic
output from diagnostic ultrasound equipment. In 1976, the US
Food and Drug Administration (USFDA) began regulating the
output levels of machines to be no more than 94 mW/cm2
spatial-peak temporal-average (SPTA) intensity for fetal use.
This regulatory output level was established based on the
predicate devices in use in the market at that time and the
apparent safety of ultrasound as understood at that time.
In 1992, at the request of manufacturers and end users
interested in obtaining specific improvements in the diagnostic
capabilities of ultrasound, the USFDA changed this limit to
720 mW/cm2 , except for eye scanning. Along with the change
in output limits, the USFDA also mandated that machines
capable of producing higher outputs be able to display to the
REVERBERATIONS - ULTRASOUND BIOSAFETY - NELSON ET.AL. 2008
diagnostician some indication of the likelihood of ultrasoundinduced bioeffects that is known as the Standard for RealTime Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment, more
commonly known as the Output Display Standard (ODS) [3].
Historically, the initial approach was that implementation of
the ODS would remove limits on machine output placing full
responsibility on the operator. Subsequently, it was decided to
keep the limits and display the output.
The Output Display Standard consists of the mechanical
index (MI) and the thermal index (TI). The MI is an onscreen indicator of the potential for ultrasound to induce
inertial cavitation in tissues. The TI is an on-screen indicator
of the magnitude of temperature rise. The TI is a model-based
approach to determine where the maximum temperature and
location in the acoustic field would be expected based on
the imaging parameters and the acoustic propagation model
selected.
While not perfect, TI and MI should be accepted as the most
sensible methods of thermal and non-thermal risk estimation
currently available. Nevertheless, for the acoustic indices to be
meaningful, the diagnostician must be familiar with ultrasound
safety issues and their implications for fetal and patient
imaging studies.
Implementation of the ODS puts much greater responsibility
for patient safety on the ultrasound end user. Adherence to
the ALARA (As Low As Reasonably Achievable) principle
is recommended. An additional major requirement for the
acceptance by the USFDA of the ODS pertains to adequate education of end users. This requires information about the ODS
be provided by the manufacturer, which is most commonly in
the form of the Medical Ultrasound Safety publication from
the AIUM [4]. However, the goal should be understanding
of the potential for bioeffects through initial applications
training by the vendor, ongoing CME courses and vigilant
daily awareness of acoustic output.
IV. ACOUSTIC P HYSICS
A. Physical Properties of Ultrasound
Ultrasound is mechanical energy that propagates longitudinally through elastic media creating alternating zones of
compression and rarefaction. Ultrasound imaging typically
uses short pulses (Fig. 1) with acoustic energy reflected back
toward the transducer from interfaces having different acoustic
properties. Typical biomedical ultrasound imaging parameters
are shown in Table I [5].
The majority of prenatal bioeffects epidemiological studies
have been based on ultrasound exposures occurring before
1992 using equipment regulated per the pre-1992 output limits
of 94 mW/cm2 . Since 1992 equipment is capable of operating
with much higher limits of up to 720 mW/cm2 with the
specific acoustic output under the direct control of the operator
and with the expectation that ALARA techniques will be
utilized.
The acoustic power is the rate of energy production, absorption or flow. The SI (Le Système International d’Unitès)
unit of power is the Watt (1 Joule/sec). The acoustic intensity
2
Fig. 1. Acoustic pulses showing where and how acoustic output is measured
and reported for diagnostic medical ultrasound. SA = Spatial Average SP =
Spatial Peak PA = Pulse Average TA = Temporal Average TP = Temporal
Peak
TABLE I
D IAGNOSTIC M EDICAL U LTRASOUND P ROPERTIES
Parameter
Values
Speed of Sound
1.54 mm/ µs
Frequency
1 - 15 MHz
Wavelength
0.1 - 1.5 mm
Pulse Length
3-5 cycles
Attenuation
1 dB/(cm-MHz)
Frame Rate
up to 150 fps (30 fps typical)
is the rate of energy flow through a unit area (Watts/cm2 ).
Acoustic output is measured for a variety of pulse conditions
with the intensity relationships between the various measured
parameters shown in Table II.
The current USFDA output limits for diagnostic ultrasound
are ISP T A <720 mW/cm2 although Doppler measurements can
exceed an ISP T A of 1000 mW/cm2 under some conditions.
The acoustic output depends on the output power, the pulse
repetition frequency (PRF), the scanner operating mode (i.e.
B-mode, M-mode, pulsed, color or power Doppler imaging,
etc.) [6] [7].
B. Mechanical Index
The Mechanical Index (MI) is intended to offer a rough
guide to the likelihood of the occurrence of cavitation and is
related to the intensity of the pulse. The MI is defined as the
maximum value of the peak negative pressure divided by the
square root of the acoustic center frequency. The MI is based
REVERBERATIONS - ULTRASOUND BIOSAFETY - NELSON ET.AL. 2008
TABLE II
ACOUSTIC I NTENSITY O UTPUT M EASUREMENT PARAMETERS
Spatial
Peak
Temporal Peak
ISP T P
the highest intensity measured at any
point in the ultrasound beam and at any
time. It is highest value of the measured
intensities. (Indicator of potential mechanical bioeffects and cavitation.)
Spatial
Peak ISP P A the highest intensity measured at any
Pulse Average
point in the ultrasound beam averaged
over the temporal (time) duration of the
pulse.
Spatial
Peak ISP T A the highest intensity measured at any
Temporal
point in the ultrasound beam averaged
Average
over the pulse repetition period. (Indicator of the magnitude of thermal
bioeffects)
Spatial Average ISAT P the average intensity over a selected
Temporal Peak
area, such as the transducer face but at
the peak in time.
Spatial Average ISAP A the average intensity over a selected
Pulse Average
area, such as the transducer face, averaged over the temporal duration of
pulse.
Spatial Average ISAT A the average intensity over a selected
Temporal Averarea, such as the transducer face, avage
eraged over the pulse repetition period.This measurement of intensity is
frequently quoted and is the lowest
value of the measures of intensity.
NB: definitions are given in order of decreasing intensity value for
the same pulse conditions.
on the derated peak rarefactional pressure. 1
Cavitation is the phenomena wherein bubbles form in a
liquid when the local pressure (such as might be produced by
the rarefaction part of a passing ultrasound wave) falls below
the vapor pressure of the liquid.
Cavitation generally falls into two types: (1) inertial, or
transient, cavitation in which the formed bubble rapidly collapses forming a shock wave that can be capable of biological
damage and (2) non-inertial cavitation wherein the formed
bubble oscillates in the acoustic field.
Ultrasound contrast agents also can play a role in cavitation. Ultrasound contrast agents typically are stable gas-filled
microbubbles. While cavitation can occur without ultrasound
contrast agents, their presence can increase the probability of
cavitation and other non-thermal effects [1]. Animal studies
have shown that ultrasound contrast agents potentially can
enhance cavitation and micro-streaming effects resulting in
microvascular damage or capillary rupture [8].
Ultrasound contrast agents also have been associated with
induction of premature ventricular contractions in echocardiography using high MI pulses. Appropriately triggered excitation can be used to reduce the occurrence of such effects. In
general, the risk of cavitation increases with increasing MI [2].
The potential for ultrasound non-thermal biohazards exists
if equipment is used imprudently. Damage has been demonstrated in animal models for tissues having gas pockets and at
MI values greater than 0.3. Thus one should avoid unnecessary
1 NB: the maximum value of peak negative pressure anywhere in the
ultrasound field measured in water is reduced by an attenuation factor equal to
that which would be produced by a medium having an attenuation coefficient
of 0.3 dB /(cm-MHz) typically referred to as a process called derating
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exposure to tissues such as the neonatal lung. As a general
strategy the MI should be kept as low as possible while still
obtaining the necessary diagnostic information.
C. Thermal Index
The Thermal Index (TI) is defined as the ratio of the emitted
acoustic power to the power required to raise the temperature
of tissue by 1◦ C. The TI is intended to give a rough guide to
the likely temperature rise that might be produced after long
exposure, although technically the TI is not a measure of the
precise temperature rise. A larger TI value represents a higher
temperature and a higher risk (e.g. a TI of 2 means a higher
risk than a TI of 1.5 but not as high risk as a TI of 3).
There are three forms of TI that may be displayed, according
to the application:
1) The TI for soft tissues (TIS). TIS assumes that the ultrasound beam does not impinge on bone only insonating
soft tissue, such as for the first trimester.
2) The TI for bones (TIB). TIB assumes that the beam
impinges on bone at or near its focus that should be
displayed in second and third trimesters.
3) The TI for cranial bone (TIC). TIC assumes that the
transducer front face is very close to the bone, such as
when scanning in the adult cranium.
Generally, for TIS and TIB the maximum temperature and
its location in the acoustic field would be at the focal point of
the ultrasound beam but such is not necessarily the case for
TIC. However, note that errors in calculating TI values, and
the limitations of the simple models on which they are based,
means that TI values may underestimate the temperature
elevation by a factor of up to two or more although more
typically the TI overestimates the temperature [1].
A temperature elevation of less than 1.5◦ C does not present
a bioeffects risk to the embryo. A temperature elevation greater
than 4◦ C for 5 minutes can present a bioeffects risk to the
embryo. Spectral pulsed, color and power Doppler all have
the potential to reach these levels. The TI provides a rough
guide for sonographer/sonologist regarding the magnitude of
temperature increase. However, it is important to keep in mind
that the potential for bioeffects also exists with equipment that
is not adjusted properly or used prudently.
D. Acoustic Reporting Requirements
The acoustic reporting requirements are implemented by
USFDA using a two-track approach to marketing clearance,
Track 1 and Track 3. Track 1 is for devices that do not follow
the Output Display Standard and therefore have applicationspecific limits; Track 3 is for devices that conform to the
Output Display Standard. There is no longer a Track 2.
Systems that include fetal Doppler applications, except for
fetal heart rate monitors, should follow Track 3. Track 3 does
not apply to systems for which a display would be required
but which have fixed acoustic output.
Under Track 3, acoustic output will not be evaluated on
an application-specific basis, but the global maximum derated
ISP T A must be ≤720 mW/cm2 and either the global maximum
REVERBERATIONS - ULTRASOUND BIOSAFETY - NELSON ET.AL. 2008
MI must be ≤1.9 or the global maximum derated ISP P A
must be ≤190 W/cm2 . An exception is for ophthalmic use,
in which case the TI = max(TIS, TIC), and is not to exceed
1.0; ISP T A.3 ≤50 mW/cm2 , and MI ≤0.23.
The displayed TI and MI values are updated by the machine
as control settings are changed by the operator Fig. 2.
V. G UIDELINES FOR S AFE U SE
A. Medical endorsement
Diagnostic ultrasound equipment should only be used for
medical diagnosis. Ultrasound equipment should only be used
by persons who are fully trained in the safe and proper
operation of the equipment (see Appendix I for a description
of educational program content). The operator should have a
full awareness of machine settings and understand the effect of
those machine settings on thermal and mechanical bioeffects.
The initial power setting and scanner default protocols should
be set for lower power. Exposure times and power levels
should follow the ALARA principle during scanning.
B. Thermal and Mechanical Indices
As a general strategy, the on-screen TI and MI values should
be monitored after scanner adjustments and kept ALARA. Table III provides rule-of-thumb guidance for monitoring output
during scanning. Typically these are acoustic output targets
although it is appreciated that depending on the scanning
conditions and diagnostic requirements of the study these
levels may be exceeded for limited periods to ensure optimal
patient care.
For fetal scanning, the TIS should be used during the first
eight weeks after conception with the TIB after eight weeks. In
the fetus and adult, the TIS should be used for eye scanning. In
the neonate, pediatric and adult patient, the TIC should be used
when the ultrasound field is close to bone in the skull, such
as with transcranial Doppler that uses higher power levels,
otherwise the TIB should be used for everything else.
Ultrasound output levels during routine obstetric ultrasound
examinations as reflected by the displayed MI and TI vales are
generally low [9]. However, higher output levels, particularly
TI levels of greater than 1.5, can be achieved, especially
with Doppler, requiring extra diligence even though they may
account for only a very small part of examination time [10].
Special care, and reduced scanning times, should be used for
sensitive tissues such as those found in the embryo (<8 wks),
eye, head, brain, spine. In general, prolonged pulsed Doppler
is not recommended for sensitive tissues [1]. The presence
of pre-existing temperature elevation, such as with elevated
maternal temperatures, should be considered with regard to
minimizing scan times.
Finally, when the probe is held in a stationary position,
the freeze-frame (i.e.non-imaging) mode should be used. The
potential for probe self-heating also should be considered.
C. Non-diagnostic Applications
Non-diagnostic uses of ultrasound equipment such as repeated scans for equipment demonstration using normal subjects generally should not be performed. First trimester scans
4
TABLE III
S UGGESTED S CANNING G UIDELINES BASED ON ACOUSTIC O UTPUT
Target
Fetus (1st trimester)
Fetus (1st trimester)
Fetus (2nd / 3rd trimester)
Fetus (2nd / 3rd trimester)
Neonate, pediatric, adult
Neonate, pediatric, adult
Fetus (1st trimester)
Fetus (1st trimester)
Fetus (2nd / 3rd trimester)
Fetus (2nd / 3rd trimester)
Neonate, pediatric, adult
Neonate, pediatric, adult
Eye
Index
MI
TI
MI
TI
MI
TI
MI
TI
MI
TI
MI
TI
MI
Value
<0.5
<0.5
<1.0
<1.0
<1.0
<1.0
>0.5
>0.5
>1.0
>1.0
>2.5
>2.5
<0.32
Duration
unlimited
unlimited
unlimited
unlimited
unlimited
unlimited
<5 minutes
<5 minutes
<5 minutes
<5 minutes
<1 minute
<1 minute
<unlimited
should avoid color and power Doppler modes and should not
be carried out for the sole purpose of producing souvenir
videos or photographs.
Production of fetal souvenir pictures or videos during diagnostic clinical studies should not increase exposure levels
or extend the scan times beyond those needed for clinical
purposes. In these situations, instrument power levels for fetal
scanning should set TI and MI values less than 1.0. First
trimester scanning should use TI and MI values less than
0.5 and avoid Doppler modes. Frequent exposure of the same
subject is to be avoided. Operators should follow safe scanning
guidelines and ALARA principles.
For training purposes the subject should be informed as to
how the ultrasound scan relates to normal diagnostic studies.
The acoustic output should be kept as low as possible, in
concordance with the time/exposure equation as expressed in
[1] and Table III.
VI. C ONCLUSIONS
Technological advances in ultrasound imaging equipment
provide improved visualization, diagnosis and management.
Such improvements have significantly improved fetal imaging
and improved prenatal management. Further, more recent
technology, such as 3DUS, provides clear images of the
developing fetus that are recognizable to family and physician.
4DUS equipment further facilitates observing fetal movements
similar to real-time 2DUS imaging, potentially inviting longer
fetal viewing [11].
At present, 3D ultrasound does not introduce additional
safety considerations although 4D ultrasound with continuous
exposure offers the potential to prolong examination times and
thus increase the potential for bioeffects [2].
Some non-imaging procedures such as using Doppler for
fetal heart monitoring or peripheral pulse monitoring that use
low power levels are not contra-indicated even for extended
periods.
Patient scanning with ultrasound should utilize the lowest
possible acoustic output setting to obtain the necessary diagnostic information under the as low as reasonably achievable
(ALARA) principle. The reader is referred to recent articles in
the Journal of Ultrasound in Medicine providing an in depth
review of ultrasound bioeffects and exposimetry [8], [12]–[17].
REVERBERATIONS - ULTRASOUND BIOSAFETY - NELSON ET.AL. 2008
Fig. 2.
5
Examples of MI and TI display on ultrasound images from various manufacturers. (Images courtesy of Dr. D. Pretorius).
The promotion, selling, or leasing of ultrasound equipment
for making keepsake fetal videos is considered by the USFDA
to be an unapproved use of a medical device. Thus use of
a diagnostic ultrasound system for these purposes, without
a physicians order, may be in violation of state laws or
regulations [18].
It is important to keep in mind that diagnostic ultrasound
transmits energy into the fetus or patient. Diagnostic ultrasound studies of the fetus are generally considered to be safe
during pregnancy. However, diagnostic procedures should be
performed only when there is a valid medical indication.
Education is key to maintaining a safe and productive ultrasound scanning situation for patients. Unfortunately, recent
reports suggest that most ultrasound users are poorly informed
regarding safety issues during pregnancy [19]. Further efforts
in improving the education and training of the ultrasound community are needed to improve end user knowledge about the
acoustic output of the machines and safety issues (Appendix
I).
Proper use of diagnostic ultrasound equipment enhances the
care and management of patients in a safe manner and thus
provides a valuable contribution to improving healthcare when
used in a safe and appropriate manner. Ultimate responsibility
for the safe use of diagnostic ultrasound equipment resides in
the operator.
R EFERENCES
[1] Guidelines for the safe use of diagnostic ultrasound equipment, Safety
Group of the British Medical Ultrasound Society, 2000
[2] Clinical Safety Statement for Diagnostic Ultrasound, European Committee of Medical Ultrasound Safety (2006)
[3] National Council on Radiation Protection and Measurements (NCRP),
NCRP report 140, Exposure Criteria for Medical Diagnostic Ultrasound,
II: Criteria Based on All Known Mechanisms. Bethesda, MD: NCRP;
(2002)
[4] Medical Ultrasound Safety, American Institute of Ultrasound in Medicine,
14750 Sweitzer Lane suite 100, Laurel MD 20707-5906; 1994 (under
revision).
[5] T. L. Szabo. Diagnostic Ultrasound Imaging Inside Out, Elsevier, 2004
[6] Standard for real-time display of thermal and mechanical acoustic output
indices on diagnostic ultrasound equipment. Revision 2. AIUM/NEMA
Standards Publication - UD3; American Institute of Ultrasound in
Medicine, 14750 Sweitzer Lane suite 100, Laurel MD 20707-5906 or
National Electrical Manufacturers Association, 1300 North 17th Street,
Suite 1847, Rosslyn VA 22209, 1998 (or latest revision)
[7] Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment. American Institute of Ultrasound in Medicine, 14750 Sweitzer Lane
suite 100, Laurel MD 20707-5906; 1997 (or latest revision)
[8] Douglas L. Miller, Michalakis A. Averkiou, Andrew A. Brayman, E.
Carr Everbach, Christy K. Holland, James H. Wible, Jr, and Junru Wu,
Bioeffects Considerations for Diagnostic Ultrasound Contrast Agents, J
Ultrasound Med 2008 27: 611-632
[9] Sheiner et. al. Acoustic output as measured by MI and TI during routine
OB exams, J Ultrasound Med 2005; 24:16651670
[10] Sheiner et.al. Increased TI can be achieved during Doppler OB Sonography, J Ultrasound Med 2007; 26:7176
[11] Ultrasound Bioeffects: Fetal Safety AIUM Practice Guideline for the
Performance of an Antepartum Obstetric Ultrasound Examination,2003
(http://www.aium.org/publications/clinical/obstetrical.pdf)
[12] Jacques S. Abramowicz, J. Brian Fowlkes, Andrea C. Skelly, Melvin E.
Stratmeyer, and Marvin C. Ziskin, Conclusions Regarding Epidemiology
for Obstetric Ultrasound, J Ultrasound Med 2008 27: 637-644
[13] Bioeffects Committee of the American Institute of Ultrasound in
Medicine, American Institute of Ultrasound in Medicine Consensus
Report on Potential Bioeffects of Diagnostic Ultrasound: Executive Summary, J Ultrasound Med 2008 27: 503-515
[14] William D. OBrien, Jr, Cheri X. Deng, Gerald R. Harris, Bruce A.
Herman, Christopher R. Merritt, Naren Sanghvi, and James F. Zachary,
The Risk of Exposure to Diagnostic Ultrasound in Postnatal Subjects:
Thermal Effects , J Ultrasound Med 2008 27: 517-535
[15] Jacques S. Abramowicz, Stanley B. Barnett, Francis A. Duck, Peter D.
Edmonds, Kullervo H. Hynynen, and Marvin C. Ziskin, Fetal Thermal
Effects of Diagnostic Ultrasound, J Ultrasound Med 2008 27: 541-559
[16] Charles C. Church, Edwin L. Carstensen, Wesley L. Nyborg, Paul L.
Carson, Leon A. Frizzell, and Michael R. Bailey, The Risk of Exposure
to Diagnostic Ultrasound in Postnatal Subjects: Nonthermal Mechanisms,
J Ultrasound Med 2008 27: 565-592
[17] Melvin E. Stratmeyer, James F. Greenleaf, Diane Dalecki, and Kjell A.
REVERBERATIONS - ULTRASOUND BIOSAFETY - NELSON ET.AL. 2008
Salvesen, Fetal Ultrasound: Mechanical Effects, J Ultrasound Med 2008
27: 597-605
[18] Carol
Rados,
FDA
cautions
against
ultrasound
keepsake
images,
FDA
Consumer,
Jan.-Feb.,
2004.
(www.fda.gov/fdac/features/2004/104 images.html)
[19] Sheiner et.al., What Do Clinical Users Know Regarding Safety of
Ultrasound During Pregnancy? J Ultrasound Med 2007; 26:319325
[20] Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers, Computed Imaging Devices
Branch Division of Reproductive, Abdominal, Ear, Nose, Throat and
Radiological Devices, Office of Device Evaluation, U.S. Department of
Health and Human Services , Food and Drug Administration, Center for
Devices and Radiological Health, Washington, DC, 1997
A PPENDIX I
FDA - E DUCATION P ROGRAM A BOUT U LTRASOUND
S YSTEMS U SING THE ODS ((T RACK 3 ) [20]
6.3 TRACK 3 - EDUCATION PROGRAM
1) 6.3.1 provide an ALARA education program for the clinical
end-user that covers the subjects listed below. ALARA is
an acronym for the principle of prudent use of diagnostic
ultrasound by obtaining the diagnostic information at an output
that is as low as reasonably achievable. This education program
should include explanations of:
a) the basic interaction between ultrasound and matter,
b) the possible biological effects,
c) the derivation and meaning of the indices,
d) a recommendation to use and the need for following the
ALARA principle in all studies
e) clinical examples of specific applications of the ALARA
principle.
f) A document published by the AIUM, ”Medical Ultrasound Safety” (AIUM, 1994, NB: new document forthcoming in 2008), is acceptable to FDA as meeting the
generic content of the educational program. The manufacturer also should provide information specific to its
device regarding ALARA.
2) 6.3.2 Minimum Requirements for Educational Material for
Track 3 Devices
a) 6.3.2.1 Bioeffects and Biophysics of Ultrasound Interactions
i) Brief description of ultrasound, diagnostic frequencies, energy levels
ii) Brief description of the change in policy which
requires user education
iii) Short history of ultrasound use and safety record
iv) Potential hazards at high output levels
v) Biological effect mechanisms–Thermal, Mechanical
vi) Exposure-effect studies (range of outputs)
vii) Risk versus benefit
viii) Present state of output levels–higher than historical
levels
ix) Proposed indices as indicators of thermal and mechanical effects
b) 6.3.2.2 Thermal Mechanisms
i) Describe thermal bioeffects–temperature rise
ii) Tissue type (soft, bone, fluid) and relative absorption
iii) Transducer type (frequency, focusing) and relationship to exposure
iv) Attenuation, absorption, scattering mechanisms in
different tissue types
v) Spatial volume of insonified tissue (at focus, or
elsewhere)
A) Homogeneity of tissue in insonified volume (effects of layering)
B) soft tissue
C) bone tissue (fetal, skull, other)
D) fluids, gas
6
c) 6.3.2.3 Nonthermal Mechanisms
i) Describe mechanical effects–cavitation and role of
bubbles
ii) Factors which produce cavitation:
A) pressure (compressional, rarefactional)
B) frequency
C) beam focusing
D) pulsed/continuous
E) standing waves
F) boundaries
G) type of material and ambient conditions
iii) Types of cavitation:
A) stable and inertial cavitation
B) microstreaming
C) nucleation sites
iv) Threshold phenomena for different types of tissues
v) Bioeffects data on animals (lung hemorrhage, intestinal hemorrhage)
d) 6.3.2.4 Benefits of Ultrasound vs. Risk
i) Benefits of use
ii) Risk of use
iii) Risk from not using ultrasound
iv) Increase in risk as acoustic output increases
v) Increase in diagnostic information as acoustic output
increases
vi) Increase in responsibility for user at higher output
levels
vii) The ALARA principle
A) controlling energy
B) controlling exposure time
C) controlling scanning technique
D) controlling system setup
E) effects of system capabilities
F) effects of operating mode (learn to distinguish)
G) effects of transducer capabilities
e) 6.3.2.5 The Output Display Standard
i) Purpose: To display exposure indices
ii) Mechanical Index (MI)
iii) Thermal Index (TI)
A) Soft Tissue Thermal Index (TIS)
B) Bone Thermal Index (TIB)
C) Cranial Bone Thermal Index (TIC)
iv) Thresholds for display of indices
A) (e.g., if system can exceed TI or MI of 1.0)
v) System display levels
A) (e.g., minimum TI displayed, minimum MI displayed, display increments)
vi) Explanation of the meaning of the TI and MI
A) threshold bioeffect levels vary depending on tissue type
B) bioeffect levels vary depending on frequency,
pressure
f) 6.3.2.6 Practicing the ALARA Principle
i) How to implement ALARA by using the TI and MI
indices
ii) Knowledge of system controls versus acoustic output
A) Overall gain and TGC versus increasing output
B) Dynamic range and post-processing versus increasing output
iii) Knowledge of system applications versus output
A) selection of appropriate range for task
iv) Knowledge of transducer effects on output
A) frequency
B) focusing
C) pulse length
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D) dwell time (scanned versus unscanned)
v) Knowledge of system operating mode versus output
A) B mode
B) Doppler (spectral, color flow, amplitude Doppler)
C) M mode
D) Control exposure time
E) Use the minimum possible to obtain information
vi) Clinical application examples–which indices are most
important?
A) fetal, cranial
B) fetal, Doppler
C) Adult thyroid
D) Adult carotid Doppler
A PPENDIX II
CME Q UESTIONS
1) (Multiple Choice) List the following modes in order of
increasing ultrasound intensity?
A. M-mode, B-mode, Color Flow Doppler, Pulsed
Doppler
B. B-mode, M-mode, Pulsed Doppler, Color Flow
Doppler
C. Color Flow Doppler, M-mode, B-mode, Pulsed
Doppler
D. Pulsed Doppler, M-mode, Color Flow Doppler,
B-mode
E. B-mode, M-mode, Color Flow Doppler, Pulsed
Doppler
Answer - E
2) (True or False) The mechanical Index (MI) is an
indication of the relative potential for cavitation
Answer - T
3) (True or False) The TI and MI are never shown together
on the ultrasound scanner image display
Answer - F
4) (True or False) Production of keepsake images is a
USFDA approved use of diagnostic ultrasound
Answer - F
5) The TI and MI change with scanner settings
Answer - T
6) (True or False) The Thermal Index (TI) is a rough guide
for sonographer/sonologist regarding the magnitude of
temperature increase
Answer - T
7) (True or False) Ultrasound scanning of patients should
always follow the ALARA principle
Answer - T
8) (Multiple Choice) Tissues most sensitive to ultrasound
exposure are?
A. embryo
B. eye
C. brain
7
D. spine
E. All of the above
Answer - E
9) (True or False) Regulatory acoustic output levels
for fetal ultrasound are higher now than when most
epidemiology bioeffects studies were performed.
Answer - T
10) (Multiple Choice) Acoustic output depends on?
A. PRF
B. Frequency
C. Operating mode
D. All of the above
Answer - D