User's Manual

Clarius Ultrasound Scanner - HD3 Scanners Safety Topics
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Bioeffects
Thermal
Thermal bioeffects refers to heat generated whenever ultrasound energy is absorbed. The
amount of heat produced depends on the ultrasound's intensity, exposure time, and the
tissue's absorption characteristics.
Tissue absorbs ultrasound energy to varying degrees depending on the tissue's absorption
characteristics. Absorption characteristics are quantified by the absorption coefficient:
• Fluids: Their absorption coefficient is almost zero. Fluids such as amniotic fluid, blood, and
urine absorb very little ultrasonic energy. That means the ultrasound goes through the
fluid with very little decrease. And there's little temperature elevation in the fluid.
• Bone: Its absorption coefficient is very high. Dense bone absorbs the energy very quickly
and causes the temperature to rise rapidly. Adult bone absorbs nearly all acoustic energy
impinging on it. Fetal bone absorption coefficients vary greatly depending on the degree
of ossification.
• Soft tissue: Soft tissue varies in density depending on the organ, but the density does not
vary much within an organ. We call it soft tissue to distinguish it from hard tissue such as
bone. Also, the tissue density within a particular organ is not always the same. But for our
purposes, we assume that attenuation is uniform throughout the organ. We call this a
homogeneous soft tissue model.
Attenuation is caused by:
• Absorption: Energy converted to heat.
• Scattering: Redirection of ultrasound.
Mechanical (Non-Thermal)
Mechanical bioeffects are threshold phenomena, such as cavitation, that occur when the
output exceeds a certain level. This threshold varies by tissue type.
Cavitation is the interaction of ultrasound with gas bubbles, causing rapid and potentially large
changes in bubble size. These bubbles originate within materials at locations termed
nucleation sites, the exact nature and source of which are not well understood in a complex
medium such as tissue or blood. The change in bubble size may increase temperature and
pressure within the bubble, causing mechanical stress on surrounding tissues, precipitate fluid
microjet formation, and generate free radicals. Gas-containing structures, such as lungs, are
most susceptible to the effects of acoustic cavitation; however, such higher frequency
ultrasounds do not provide sufficient time for significant bubble growth; therefore, cavitation
is unlikely to occur under these circumstances. Factors that produce cavitation include
pressure (compressional, rarefactional), frequency, focused/unfocused beam, pulsed/
continuous waves, degree of standing waves, boundaries, and the nature and state of material.