A novel method for characterization of nonlinear propagation and spatial averaging effects for ultrasound imaging systems

Harmonic imaging at frequencies up to 15 MHz is now routinely used in clinical practice and frequencies well beyond 20 MHz are considered for diagnostic ultrasound imaging applications. However, currently available measurement tools are not fully adequate to characterize such high frequency systems, primarily due to the combined effects of limited frequency responses and spatial averaging effects. To alleviate this problems, a comprehensive wave propagation model has been developed and tested. The model can predict the linear and nonlinear acoustic wave propagation generated by differently shaped acoustic radiators at virtually any point in the field and takes into account spatial averaging effects introduced by hydrophone probes and their associated frequency responses. The applicability of the model in hydrophone probe calibration up to 100 MHz is demonstrated. Also, a novel calibration technique termed Time-Gating Frequency Analysis (TGFA) is briefly described and calibration results in the frequency range up to 60 MHz for hydrophones having effective diameters between 150 and 500 μm are presented. Also presented are the results of the investigation that determined the effect of using hydrophone probes of different diameters and bandwidth on Spatial-Peak Pulse-Average Intensity (I_SPPA). It was found that the values of I_SPPA increased with decreasing effective aperture of the hydrophone probe and its bandwidth.