Measurements of the spatial coherence of the fundamental and second-harmonic beams for a clinical imaging system

Abstract

Spatial coherence of backscattered signals underlies correlation-based phase aberration corrections. The van Cittert–Zernike theorem relates frequency-independent spatial coherence to the autocorrelation of the transmit apodization. Previous studies suggest that the mainlobe of the nonlinearly generated harmonic beam is wider and exhibits lower sidelobes than a beam linearly generated at the harmonic frequency. The objective of this study was to measure the spatial coherence associated with fundamental and nonlinearly generated harmonic beams. Using data experimentally acquired from a clinical scanner (ATL HDI5000), two independent methods were employed to measure the spatial coherence. One approach measured the spatial coherence of backscatter from a tissue-mimicking phantom using rf signals from individual elements of a linear array. In the second approach, the effective apodization was determined by a linear angular spectrum backpropagation of hydrophone-sampled data from a transverse plane in the focal zone. The results show that the effective apodization of the nonlinearly generated harmonic beam is more aggressive than the actual transmit apodization. The spatial coherence associated with the second-harmonic beam differs from the spatial coherence of the fundamental beam, but is predicted by the effective apodization. [Supported in part by NIHHL40302 & ATL.]