Estimating in situ exposure in the presence of acoustic nonlinearity.

Evidence is presented for excess attenuation of pulsed ultrasound due to finite amplitude effects in water. Measurements on a modern scanner are used to demonstrate that linear derating can underestimate many exposure quantities, including all safety indices apart from the cranial thermal index. More appropriate methods for estimating in situ exposure are reviewed. A preferred procedure that requires exposure measurements to be made in water under "small signal" conditions is selected. A spectral index is defined that is proposed as an indicator of finite amplitude effects, where spectral index = 0.1 defines the threshold between nonlinear and quasi-linear conditions.