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phyrev ch.9 - Thermal Mechanism – temperature elevation...

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Unformatted text preview: Thermal Mechanism – temperature elevation via absorption-bone is an absorber. Temp elevate at tissue-bone interface- SPTA intensity is related to tissue heating- any exam that causes a temperature elevation to greater than 41o C is potentially harmful to fetus Thermal index- number proposed by AIUM that relates to tissue heating. Allows closer estimation of the heat produced in tissues- TIS – soft tissue- TIB – bone at the beam’s focus- TIC – cranial bone is at or near the skin’s surface Focused Beam – less likely to cause temperature elevation in tissues, because a narrow beam heats only a small region of tissue Unfocused Beam – more likely to cause temp elevation in tissues Cavitation Mechanism – Microbubbles may be excited by US. Potential of near total energy absorption where the nuclei exist may lead to thermal injury. Microbubbles exist naturally in the body Mechanical Index – AIUM proposed that relates to cavitation, evaluate the probability of tissue damage due to cavitational forces - MI is higher or more likely to produce cavitation with : higher pressure and lower frequency sound Stable Cavitation – bubbles that oscillate (vibrate) in diameter when exposed to sound waves and amplitude. - Bubbles Do Not Burst, they intercept, reradiate and absorb acoustic energy- cause shear stress and microstreaming in surrounding fluid Transient (unstable, normal, inertial) Cavitation – the bubbles expand and oscillate violently and bubbles BURST....
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phyrev ch.9 - Thermal Mechanism – temperature elevation...

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