2C-6 Optimal Contrast Resolution Beamforming

The delay-and-sum (DAS) beamformer, utilized in current state of the art ultrasound imaging systems, applies time delays on each receive channel to focus the returning echoes and weights the focused RF data on each receive channel prior to beamsummation. Apodization functions like the Hamming and Nuttall windows reduce the sidelobe levels the DAS beamformer´s point spread function (PSF). As a result, apodization greatly impacts the contrast and resolution of the final output image. We propose a beamformer architecture for broadband imaging systems that uses unique finite impulse response (FIR) filters on each receive channel The filter weights are constructed to maximize the contrast resolution of the system´s spatial response. We present simulation results showing that FIR filters of modest tap lengths (3-7) can yield marked improvement in image contrast and point resolution. This increase in contrast resolution comes at the expense of a decrease in beamformer sensitivity. We investigate the effects of magnitude and phase aberration on the FIR beamformer by simulating near field thin phase screen aberrators. Specifically we examine the performance of the FIR beamformer in the presence of magnitude aberration characterized by an a priori root-mean- square (RMS) strength of 5 dB and a full-width at half-maximum (FWHM) correlation length of 2.1 mm as well as in the presence of phase aberration characterized by an a priori RMS strength of 28 ns and a FWHM correlation length of 3.6 mm. Contrast resolution results show that the aberrated FIR beamformer outperforms the unaberrated DAS beamformer by almost 10 dB. We believe that our array pattern synthesis technique and beamformer architecture have the potential to significantly improve the contrast resolution performance of broadband imaging systems.