Diagnostic ultrasound ( PDFDrive )

CHAPTER 2 Biologic Effects and Safety 45
10
MI=2
5
MI=1
Threshold pressure (MPa)
2
1
5
190 W cm –2
10 W cm –2
MI=0.3
2
0.1
5 1 2
5 10
Frequency (MHz)
Adult mouse lung (10 µs)
Adult mouse lung (1 µs)
Neonatal mouse lung (10 µs)
Fruit fly larvae (10 µs)
Elodea leaves (5 µs)
FIG. 2.9 Threshold for Bioeffects From Pulsed Ultrasound Scan Using Low Temporal Average Intensity. Data shown are the threshold
for effects measured in peak rarefactional pressures (p − in Fig. 2.1) as a function of ultrasound frequency used in the exposure. Pulse durations
are shown in parentheses in the key below the graph. Also shown for reference purposes are the values for the mechanical index (MI) and the
local spatial peak, pulse average intensity (I SPPA ). (With permission from American Institute of Ultrasound in Medicine. Consensus Report on Potential
Bioeffects of Diagnostic Ultrasound. J Ultrasound Med. 2008;27:503-515. 22 )
predicted trade-of assumes short-pulse (a few acoustic cycles)
and low–duty cycle ultrasound (<1%). his relatively simple result
can be used to gauge the potential for the onset of cavitation
from diagnostic ultrasound. he MI was adopted by the FDA,
AIUM, and NEMA as a real-time output display to estimate the
potential for bubble formation in vivo, in analogy to the TI. As
previously stated, the collapse temperature for inertial cavitation
is very high. For MI, a collapse temperature of 5000 kelvins (K)
was chosen based on the potential for free radical generation,
and the frequency dependence of the pressure required to generate
this thermal threshold takes a relatively simple form. he MI is
a type of “mechanical energy index” because the square of the
MI is about proportional to mechanical work that can be performed
on a bubble in the acoustic rarefaction phase.
Results from several investigators have speciied the MI value
above which bioefects associated with cavitation are observed
in animals and insects. 22 In Fig. 2.9 the dotted lines are calculations
for several MI values; all the efects appear to occur at an MI
value of 0.3 or greater. In many of these cases, however, stable
pockets of gas (gas bodies) are known to exist in the exposed
tissues. Also, other body areas containing gas bodies might be
particularly susceptible to ultrasound damage, including the
intestinal lining. 85
In response to this potential, the AIUM issued a safety
statement related to the potential for bioefects related to the
interaction of ultrasound with naturally occurring nuclei. 86
Experimentation continues, and it remains to be seen if such
damage occurs in human tissue.
Inherent in the formulation of the MI are the conditions only
for the onset of inertial cavitation. he degree to which the
threshold is exceeded, however, relates to the degree of potential
bubble activity, which may correlate with the probability of a
bioefect. Note that, given present knowledge, exceeding the
cavitation threshold does not mean there will be a bioefect.
Below an MI of about 0.4, the physical conditions do not favor
bubble growth, even in the presence of a broad bubble nuclei
distribution in the body, which is in reasonable agreement with
the results of Fig. 2.9. Moreover, whereas the TI is a time-averaged
measure of the interaction of ultrasound with tissue, the MI is
a peak measure of this interaction. hus there is a desirable
parallel between these two measures, one thermal and one
mechanical, for informing the user of the extent to which
the diagnostic tool can produce undesirable changes in
the body.
Summary Statement on Gas
Body Bioeffects
he AIUM statements concerning bioefects in body areas with
gas bodies include several conclusions, summarized as follows 86 :