Comprehensive imager simulation for improved acoustic power control

Commercial ultrasound imagers must comply with the FDA 510(k) regulations to be marketed in the USA. Controlling the acoustic output to meet these requirements is complex. There are hundreds of thousands of discrete operating conditions available to the sonographer. Accurate measurements require "peaking" of the hydrophone in azimuth and elevation, and acquiring data as a function of range. The acoustic field needs to be characterized in 3 dimensions. It is impossible to measure the imager's output under each condition, so algorithmic means are needed to reduce the size of the problem. Even when simple linear dependencies (such as pulse repetition frequency) are taken into account, the time to obtain Thermal and Mechanical Indices for a new probe is formidable. One also must repeat the experiment each time changes are made to the transmitter hardware, or its waveforms. The authors present further results on how to speed the acquisition of data used for estimation of the output labeling parameters by guiding the water-tank measurements with a beam simulator. The linear simulation uses the FIELD II code from the Technical University of Denmark. The advantages and limitations of such an approach are detailed with reference to bracketing the maximum power search range. The authors also give an assessment of the reliability of the simulated values in view of nonlinear ultrasound propagation. Nonlinear effects are especially difficult to simulate since water offers little attenuation to the harmonics generated during propagation. Quite frequently, this nonlinearity is sufficient to redistribute the peak positions of the various acoustic intensities.