Computing the mechanical index

A computational nonlinear beam propagation model was used to compute the wa ter 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 operat ing at source amplitudes up to 1.49 MPa in a water medium and in a tissue m edium with an attenuation of 0.3 dB cm(-1) MHz(-1). The mechanical index wa s calculated in three ways based on these computations: extrapolated from o ne low amplitude water path propagation, extrapolated from a series of wate r path propagations using the existing Output Display Standard protocol, an d directly from a series of tissue path propagations. These computed result s suggest that extrapolation from derated measurements of a low level water path field can provide mechanical index estimates which progressively over estimate the in situ values. At the highest source amplitude considered, th e linearly extrapolated mechanical index was 29% higher than the mechanical index computed by the tissue path propagations. The Output Display Standar d protocol offered improved accuracy but consistently underestimated the in situ values. The maximum error for the Output Display Standard protocol wa s 8%. A variation of the Output Display Standard protocol in which mechanic al index estimates were obtained from the on-axis spatial peak in the derat ed 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 prop agations.