By National Council on Radiation Protection and Measurements
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In soft tissues containing water, the speed, c, tends to increase with the solidlwater ratio. Thus, for the lens of the eye, c is about 7% greater than for the aqueous and vitreous humors. Also, c exceeds 1700 m/s for skin and tendon, in which collagen and other proteins are relatively abundant. 5 ACOUSTIC PROPERTIES OF TISSUES / 35 quency but this variation is small (usually less than one percent) over the range of frequencies of interest here. 8, the reflection of a beam a t normal incidence to a planar interface between two media depends on the characteristic impedance pc of the two materifils.
In the later sections, evidence is given that these stresses cause alterations of biological particles which pass through or near the boundary layer. 6 / 2. BASIC PHYSICS OF ULTRASOUND Generation of Harmonics By a "harmonic" is meant a component of the frequency spectrum whose frequency is nf, where f is the fundamental frequency and n is an integer called the "order" of the harmonic. We have already noted that in the theory of nonlinear acoustics, the second-order approximation predicts the existence of second-harmonic contributions to the pressure, velocity and other acoustic variables.
4 which would be most representative of, or would underestimate, the phase-independent attenuation. (The S U B C U T A N E O U S FAT ABDOMINAL MUSCLE B L A D D E R WALL- assumed art of abdominal muscle and uterus 4 URINE 5 UTERINE MUSCLE 6 GESTATIONAL S A C 1 2 3 Fig. 8. Modeled example of ultrasound path to gestational sac. 38 / 2. 21, equal to a (z' - z), where a is the attenuation coefficient and (2' - z) the thickness. 6 dB/cm. 6 dB. 8. 01 for this fraction. 04 dB. 3. 36 x 10' g/s cmZ. F/M: fat/muscle.
Biological effects of ultrasound : mechanisms and clinical implications by National Council on Radiation Protection and Measurements