Image amplitude estimation with the minimum variance beamformer

We have applied the minimum variance (MV) and Delay-and-sum (DAS) beamformers (BF) to Field II simulated ultrasound data. A 2.5 MHz 64-element linear array transducer is used. For the MV beamformer, subarray length of L = 28, temporal averaging over four wavelengths and a diagonal loading factor corresponding to 10% of the signal energy have been used. The chosen temporal averaging gives MV images with speckle statistics comparable to DAS images. Speckle images containing areas with 9 different intensities with a relative intensity difference ranging from -24 dB to +24 dB were simulated. The images also included bright point targets on speckle background. All images were normalized to have the same speckle intensity in the background. The MV beamformer gives point responses with the same amplitude level, but approximately 34% of the -3 dB beamwidth compared to the DAS beamformer. The edges between the different intensity areas and the shape of the areas were also more well defined in the MV case. None of the beamformers estimate amplitudes in speckle areas correctly, but the DAS beamformer is closer to the true value than the MV beamformer by 0.2-0.8 dB.