Diagnostic ultrasound ( PDFDrive )

40 PART I Physics
acoustic power and derive the Cranial Bone hermal Index
(TIC).
Estimate of Thermal Effects
Ultrasound users should keep in mind several points when referring
to the TI as a means of estimating the potential for thermal efects.
First, the TI is not synonymous with temperature rise. A TI equal
to 1 does not mean the temperature will rise 1°C. An increased
potential for thermal efects can be expected as TI increases.
Second, a high TI does not mean that bioefects are occurring,
but only that the potential exists. he thermal models employed
for TI calculation may not consider factors that may reduce the
actual temperature rise. However, TI should be monitored during
examinations and minimized when possible. Finally, there is no
consideration in the TI for the duration of the scan, so minimizing
the overall examination time will reduce the potential for efects.
here have been proposals suggesting the inclusion of such a
dwell time efect, 21 but these have not been adopted.
Summary Statement on Thermal Effects
he AIUM statement concerning thermal efects of ultrasound
includes several conclusions that can be summarized as follows 22 :
• Adult examinations resulting in a temperature rise of up to
2°C are not expected to cause bioefects. (Many ultrasound
examinations fall within these parameters.)
• A signiicant number of factors control heat production by
diagnostic ultrasound.
• Ossiied bone is a particularly important concern for ultrasound
exposure.
• A labeling standard now provides information concerning
potential heating in sot tissue and bone.
• Even though an FDA limit exists for fetal exposures, predicted
temperature rises can exceed 2°C.
• hermal indices are expected to track temperature increases
better than any single ultrasonic ield parameter.
EFFECTS OF ACOUSTIC CAVITATION
Potential Sources for Bioeffects
Knowledge concerning the interaction of ultrasound with gas
bodies (which many term “cavitation”) has signiicantly increased
over time, although it is not as extensive as that for ultrasound
thermal efects and other sources of hyperthermia. Acoustic
cavitation inception is demarcated by a speciic threshold value:
the minimum acoustic pressure necessary to initiate the growth
of a cavity in a luid during the rarefaction phase of the cycle.
Several parameters afect this threshold, including initial bubble
or cavitation nucleus size, acoustic pulse characteristics (e.g.,
center frequency, pulse repetition frequency [PRF], pulse duration),
ambient hydrostatic pressure, and host luid parameters
(e.g., density, viscosity, compressibility, heat conductivity, surface
tension). Inertial cavitation refers to bubbles that undergo large
variations from their equilibrium sizes in a few acoustic cycles.
Speciically during contraction, the surrounding luid inertia
controls the bubble motion. 23 Large acoustic pressures are necessary
to generate inertial cavitation, and the collapse of these
cavities is oten violent.
The AIUM Statement on Mammalian Biological Effects of Heat 24
APPROVED MARCH 25, 2015
1. An excessive temperature increase can result in toxic effects
in mammalian systems. The biological effects observed
depend on many factors, such as the exposure duration, the
type of tissue exposed, its cellular proliferation rate, and its
potential for regeneration. Age and stage of development are
important factors when considering fetal and neonatal safety.
Temperature increases of several degrees Celsius above the
normal core range can occur naturally. The probability of an
adverse biological effect increases with both the duration and
the magnitude of the temperature rise.
2. In general, adult tissues are more tolerant of temperature
increases than fetal and neonatal tissues. Therefore,
higher temperatures and/or longer exposure durations
would be required for thermal damage. The considerable
data available on the thermal sensitivity of adult tissues
support the following inferences 10 :
a. For exposure durations up to 50 hours, there have
been no signiicant adverse biological effects observed
due to temperature increases less than or equal to
1.5°C above normal. 25
b. For temperature increases between 1.5°C and 6°C
above normal, there have been no signiicant adverse
biological effects observed due to temperature
increases less than or equal to 6 − [log 10 (t/60)]/0.6
where t is the exposure duration in seconds. For
example, for temperature increases of 4°C and 6°C,
the corresponding limits for the exposure durations t
are 16 minutes and 1 minute, respectively.
c. For temperature increases greater than 6°C above
normal, there have been no signiicant adverse
biological effects observed due to temperature
increases less than or equal to 6 − [log 10 (t/60)]/0.3
where t is the exposure duration in seconds. For
example, for temperature increases of 9.6°C and
6.0°C, the corresponding limits for the exposure
durations t are 5 and 60 seconds, respectively.
d. For exposure durations less than 5 seconds, there
have been no signiicant, adverse biological effects
observed due to temperature increases less than or
equal to 9 − [log 10 (t/60)]/0.3 where t is the exposure
duration in seconds. For example, for temperature
increases of 18.3°C, 14.9°C, and 12.6°C, the
corresponding limits for the exposure durations t are
0.1, 1, and 5 seconds, respectively.
3. Acoustic output from diagnostic ultrasound devices is
suficient to cause temperature elevations in fetal tissue.
Although fewer data are available for fetal tissues, the
following conclusions are justiied 10,26 :
a. In general, temperature elevations become
progressively greater from B-mode to color Doppler to
spectral Doppler applications.