Journal of Ultrasound in Medicine, Vol 18, Issue 1 43-53, Copyright © 1999 by American Institute of Ultrasound in Medicine
Estimating in situ exposure in the presence of acoustic nonlinearity
F. A. Duck
Medical Physics Department, Royal United Hospital, Bath, United Kingdom.
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.
