Journal of Ultrasound in Medicine, Vol 18, Issue 1 63-68, Copyright © 1999 by American Institute of Ultrasound in Medicine
Computing the mechanical index
T. Christopher
Department of Electrical and Computer Engineering and the Rochester Center for Biomedical Ultrasound, University of Rochester, New York 14627, USA.
A computational nonlinear beam propagation model was used to compute the
water path and in situ fields of a phased array transducer operating at 2
MHz. The computational source was matched to the transducer's z = 10 cm
focal plane field. Subsequent computed propagations considered this source
operating at source amplitudes up to 1.49 MPa in a water medium and in a
tissue medium with an attenuation of 0.3 dB cm(-1) MHz(-1). The mechanical
index was calculated in three ways based on these computations:
extrapolated from one low amplitude water path propagation, extrapolated
from a series of water path propagations using the existing Output Display
Standard protocol, and directly from a series of tissue path propagations.
These computed results suggest that extrapolation from derated measurements
of a low level water path field can provide mechanical index estimates
which progressively overestimate the in situ values. At the highest source
amplitude considered, the linearly extrapolated mechanical index was 29%
higher than the mechanical index computed by the tissue path propagations.
The Output Display Standard protocol offered improved accuracy but
consistently underestimated the in situ values. The maximum error for the
Output Display Standard protocol was 8%. A variation of the Output Display
Standard protocol in which mechanical index estimates were obtained from
the on-axis spatial peak in the derated temporal peak rarefactional curve
was also considered. The maximum error for this method was 3%. The results
considered here also demonstrated the feasibility of computational
investigations of high intensity clinical propagations.
