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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 beams focus- TIC cranial bone is at or near the skins 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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This note was uploaded on 11/22/2010 for the course ULTRASOUND 729 taught by Professor Barnes during the Spring '10 term at El Centro College.
- Spring '10