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the machine output intensities. The visual display of thermal and mechanical indices
during ultrasound imaging provides an aid to limit the output of the machine.
Sonographic evaluation of the human body, including potentially sensitive tissues,
such as developing fetus and the eye, have been performed on millions of patients
without documentation of serious adverse events. However, ultrasound waves have
the potential to cause significant biological effects, depending on ultrasound wave
characteristics and scanned tissues sensitivity. Physicians and sonographers must be
aware of these potential biological effects in assessing the overall safety of the
procedure. The biological effects of ultrasound depend on the total energy applied
to a given region. Thus, varying duration of exposure to wave emission, intensity and
frequency of the ultrasound beam, pulsed or continuous emission modality and
acoustic power, may lead to significant biological effects, that are commonly divided
in thermal and
Mechanical(
non-thermal) effects.
Thermal
The biological effects of ultrasound energy are related primarily to the production of
heat. Heat is generated whenever ultrasound energy is absorbed, and the amount of heat
produced depends on the intensity of the ultrasound, the time of exposure, and the specific
absorption characteristics of the tissue. As much as 70% of the total temperature increase
associated with ultrasound occurs within the first minute of exposure
, but temperature
continues to rise as exposure time is prolonged. Minimizing the exposure time is probably
the single most important factor for ensuring patient safety from thermal injury
. Other
important parameters to be considered are:
•
The relative protein content of each tissue, since absorption coefficients of tissues
are directly related to protein content; absorption coefficients vary between 1 (skin,
tendon, spinal cord) and 10 (bone) dB/cm MHz
•
The perfusion of the tissue, which has a dampening effect on heat generation and
physically allows heat to be carried away from the point of energy transfer.
•
Emission modality, since pulsed-wave ultrasound is extremely unlikely to
significantly heat tissues.
•
Beam width, since a wider beam width reduces the rate and extent of temperature
rise by permitting the energy to be distributed over a larger perfusion territory
Mechanical (Non-Thermal)
Ultrasound energy creates also mechanical forces independent of thermal effects,
thereby causing biologic effects that are not related to temperature rise alone, such as
cavitation, torque forces, oscillatory shear, radiation, pressure and microstreaming.
Cavitation
The interaction of ultrasound with gas bubbles or contrast agents causes rapid and
potentially large changes in bubble size. This process, termed cavitation, may increase
temperature and pressure within the bubble and thereby cause mechanical stress on
surrounding tissues, precipitate fluid microbe formation, and generate free radicals
. Gas-
