P28
containing structures (e.g., lungs, intestines) are most susceptible to the effects of acoustic
cavitation. Ultrasound wavelength has an important role in bubble formation and growth:
short wavelength ultrasound (observed at higher frequencies) does not provide sufficient
time for significant bubble growth; therefore, cavitation is less likely under these
circumstances compared with long wavelengths. The short half-life of cavitation nuclei
prevents most cavitation-related biological effects, unless ultrasound contrast agents are
also present. Contrast agents markedly reduce the threshold intensity for cavitation.
However, because of the relatively high viscosity of blood and soft tissue, significant
cavitation is unlikely, and cavitation has not been shown to occur with the ultrasound
exposure commonly used during a diagnostic examination.
Note:
Cavitation depends on:
• Frequency
• Pressure
• Focused/unfocused beams
• Pulsed/continuous ultrasound
• Degree of standing waves
• Nature and state of material
• Boundaries
Other effects
A variety of other physical forces may also be produced by ultrasound energy. Although
each of these effects can be demonstrated in vitro, there is no evidence that any of these
physical phenomena has a significant biological effect on patients.
ALARA Principles
The guiding principle for the use of diagnostic ultrasound is defined by the ALARA
(which means that we keep total ultrasound exposure as low as reasonably achievable
while optimizing diagnostic information). The decision as to what is reasonable has
been left to the judgment and insight of qualified personnel. According to
AIUM Medical
Ultrasound Safety (Third Edition)
, there are the following description" With new
ultrasound equipment, the on-screen output display (thermal index [TI] and
mechanical index [MI]) lets us determine the exposure level in terms of the potential
for bio effects. For equipment that does not have an output display, we depend on
whatever output information, such as intensity, decibels, or the percentage of power,
which the system provides. Because the threshold, if one exists, for diagnostic
ultrasound bioeffects is undetermined, it becomes our responsibility to control the
total exposure to the patient. Controlling the total exposure depends on the output
level and exposure time. The output level required for an examination depends on the
patient and the clinical need. Not all diagnostic examinations can be performed at very
low levels. In fact, using too low a level may result in poor data and the need to repeat
