Diagnostic ultrasound ( PDFDrive )

46 PART I Physics
1. Current ultrasound systems can produce cavitation in vitro
and in vivo and can cause blood extravasation in animal tissues.
2. he MI can gauge the likelihood for cavitation and apparently
works better than other ield parameters in predicting
cavitation.
3. Several interesting results have been observed concerning
animal models for lung damage, which indicate a potential
very low threshold for damage, but the implications for human
exposure have not yet been determined.
4. In the absence of gas bodies, the threshold for damage is
much higher. (his is signiicant because ultrasound examinations
may be performed predominantly in tissues with no
identiiable gas bodies.)
The AIUM Statement on Mammalian
Biological Effects in Tissues With Naturally
Occurring Gas Bodies 86
APPROVED MARCH 25, 2015
Biologically signiicant adverse nonthermal effects have been
identiied in tissues containing stable bodies of gas for
diagnostically relevant exposure conditions. Gas bodies occur
naturally in postnatal lungs and in the folds of the intestinal
mucosa. This statement concerns naturally occurring gas
bodies encountered in pulmonary and abdominal ultrasound,
whereas a separate statement deals with the use of gas
body contrast agents.
1. The outputs of some currently available diagnostic
ultrasound devices can generate levels that produce
capillary hemorrhage in the lungs 60 and intestines 50 of
laboratory animals.
2. Thresholds for adverse nonthermal effects associated
with naturally occurring gas bodies depend on tissue
characteristics and the physiologic status, including
anesthesia, 87 and on physical ultrasound parameters,
including attenuation by intervening tissue, exposure
duration, ultrasonic output, frequency, pulse duration,
and pulse repetition frequency. 50
3. A mechanical index (MI) a has been formulated to assist
users in evaluating the likelihood of mechanical
(nonthermal) adverse biological effects for diagnostically
relevant exposures, and its value is displayed on screen
in accordance with output display speciications.
4. The minimum threshold value of the experimental MI
(the in situ value of the peak rarefactional pressure
amplitude divided by the square root of the frequency)
for pulmonary capillary hemorrhage in laboratory
mammals is approximately 0.4. The corresponding
threshold for the intestine is MI = 1.4.
5. The implications of these observations for human exposure
during thoracic or abdominal ultrasound examinations are
yet to be determined.
a The MI is equal to the derated peak rarefactional pressure (in
megapascals) at the point of the maximum derated pulse intensity
integral divided by the square root of the ultrasonic center frequency
(in megahertz). 12
AIUM, American Institute of Ultrasound in Medicine.
Reproduced with permission from American Institute of Ultrasound in
Medicine. AIUM Statement on Mammalian Biological Effects of Heat:
AIUM; 2015. Available from: http://www.aium.org/
oficialStatements/6. 86
OUTPUT DISPLAY STANDARD
Several groups, including the FDA, AIUM, and NEMA, have
developed the Standard for Real-Time Display of hermal and
Mechanical Acoustical Output Indices on Diagnostic Ultrasound
Equipment, which introduces a method to provide the user with
information concerning the thermal and mechanical indices.
Real-time display of the MI and TI will allow a more informed
decision on the potential for bioefects during ultrasound examinations
(Fig. 2.10). he standard requires dynamic updates of
the indices as instrument output is modiied and allows the
operator to learn how controls will afect these indices. Important
points to remember about this display standard include the
following:
• he MI should be clearly visible on the screen (or the operator
should be alerted by some other means) and should begin to
appear when the instrument exceeds a value of 0.4. An exception
is made for instruments incapable of exceeding index
values of 1; these are not required to display the bioefects
indices such as MI.
• When the indices are required to be displayed, both indices
are to be displayed at all times.
• he standard also requires that appropriate default output
settings be in efect at power-up, at new patient entry, or
when changing to a fetal examination. Ater that time the
operator can adjust the instrument output as necessary to
acquire clinically useful information while attempting to
minimize the index values.
• As indicated previously, the bioefects indices do not include
any factors associated with the time taken to perform the
scan. Eicient scanning is still an important component in
limiting potential bioefects.
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FIG. 2.10 Display of Bioeffects Indices. Typical appearance of an
ultrasound scanner display showing (right upper corner) the thermal
index in the Bone Thermal Index (TIB) and mechanical index (MI) for an
endocavitary transducer.