Non-ionising imaging

Ultrasound

Ultrasound (above $20$ kHz; medical uses $1$–$15$ MHz) is generated and detected by a piezoelectric crystal, which vibrates when an alternating p.d. is applied and produces an e.m.f. when squeezed by returning echoes. Pulses reflect at boundaries between tissues. The fraction reflected depends on the acoustic impedances $Z=\rho c$: $\dfrac{I_R}{I_0}=\dfrac{(Z_1-Z_2)^2}{(Z_1+Z_2)^2}$. A big mismatch reflects almost everything, so gel is used to couple the transducer to the skin. Ultrasound is attenuated exponentially: $I=I_0e^{-\mu x}$.