half-value distance

half-value distance

; H-VD the tendency for ultrasound (US) intensity to become absorbed by surrounding tissues, and attenuate to the point of therapeutic ineffectiveness; US penetration is ∼ 4 mm at 3 MHz, and 11 mm at 1 MHz in irregular soft tissues; the higher the US frequency, the greater its absorption by surrounding tissues, and the greater the rate of its conversion into heat energy at tissue interfaces