Statistically significant differences in the spatial coherence of backscatter for fundamental and harmonic portions of a clinical beam

Correlation-based approaches to phase aberration correction rely on the spatial coherence of backscattered signals. The spatial coherence of backscatter was measured using a clinical linear array with a modified clinical imaging system (ATL HDI 5000). The spatial coherence results were verified using a 14 mm×14 mm pseudo-array scan in a transverse plane of the transmitted beam with a 0.6 mm hydrophone. An effective apodization was determined by backpropagating these values using a linear angular spectrum approach. The effective apodizations were compared with the spatial coherence measurements using the Van Cittert-Zernike theorem. The spatial coherence for the fundamental beam exhibited good agreement with the autocorrelation of the transmit apodization. The spatial coherence for the harmonic differed systematically and statistically from the autocorrelation of the transmit apodization. Additionally, our experimental results verify that the effective apodization of the nonlinearly-generated harmonic beam is more aggressive than the transmit apodization