Minimum variance beamforming for high frame-rate ultrasound imaging

This paper investigates the application of adaptive beamforming in medical ultrasound imaging. A minimum variance (MV) approach for near-field beamforming of broadband data is proposed. The approach is implemented in the frequency domain, and it provides a set of adapted, complex apodization weights for each frequency sub-band. As opposed to the conventional, Delay and Sum (DS) beamformer, this approach is dependent on the specific data. The performance of the proposed MV beamformer is tested on simulated synthetic aperture (SA) ultrasound data, obtained using Field II. For the simulations, a 7 MHz, 128-element, phased array transducer with lambda/2-spacing was used. Data is obtained using a single element as the transmitting aperture and all 128 elements as the receiving aperture. A full SA sequence consisting of 128 emissions was simulated by gliding the active transmitting element across the array. Data for 13 point targets and a circular cyst with a radius of 5 mm were simulated. The performance of the MV beamformer is compared to DS using boxcar weights and Hanning weights, and is quantified by the Full Width at Half Maximum (FWHM) and the peak-side-lobe level (PSL). Single emission {DS Boxcar, DS Hanning, MV} provide a PSL of {-16,-36,-49} dB and a FWHM of {0.79,1.33,0.08} mm = {3.59 lambda, 6.05 lambda, 0.36 lambda}. Using all 128 emissions, {DS Boxcar, DS Hanning, MV} provide a PSL of 1-32, -49,-65} dB, and a FWHM of {0.63, 0.97, 0.08} mm = {2.86 lambda, 4.41 lambda, 0.36 lambda}. The contrast of the beamformed single emission responses of the circular cyst were calculated to {-18, -37, -40} dB. The simulations have shown that the frequency sub-band MV beamformer provides a significant increase in lateral resolution compared to DS, even when using considerably fewer emissions. An increase in resolution is seen when using only one single emission. Furthermore, it is seen that an increase of the number of emissions does not alter the FWHM. Thus, the MV beamformer introduces the possibility for high frame-rate imaging with increased resolution.