The peak rarefactional pressure generated by medical ultrasound systems in water and tissue: a numerical study

Abstract

Current estimates of in-situ exposure are based on de-rating field measurements made in waterto allow for the attenuation of tissue, using a specific attenuation coefficient of 0.3 dB cm-1 MHz-1. This process assumes that the propagation process is linear. However for medical ultrasound systems nonlinear propagation effects can be significant. In order to explore improved methods of characterising finite amplitude fields an extensive programme of modelling has been performed with the aim of investigating the relationship between finite amplitude fields in tissue and water. This utilised a finite difference solution to the KZK equation to model 35 fields using starting conditions typical of medical ultrasound arrays. In each case the field was modelled in water and then in homogeneous tissue, assuming the specific attenuation coefficient of 0.3 dB cm-1 MHz-1. This enabled the de-rated peak rarefactional acoustic pressure (pr,α) at specific locations, derived from water predictions, to be compared with the corresponding predictions for the peak rarefactional pressure pr in tissue. The results show that a nonlinear propagation parameter and measurement range can be used to give a reasonably good indication of the extent to which pr,α underestimates pr in tissue. Corresponding results for the pulse intensity integral do not demonstrate such a simple relationship. (Work supportedby EPSRC under grant GR/R43747.