A Delta-Sigma beamformer with integrated apodization

This paper presents a new design of a discrete time Delta-Sigma (∆Σ) oversampled ultrasound beamformer which integrates individual channel apodization by means of variable feedback voltage in the Delta-Sigma analog to digital (A/D) converters. The output bit-width of each oversampled A/D converter remains the same as in an unmodified one. The outputs of all receiving channels are delayed and summed, and the resulting multi-bit sample stream is filtered and decimated to become an image line. The simplicity of this beamformer allows the production of high-channel-count or very compact beamformers suitable for 2-D arrays or compact portable scanners. The new design is evaluated using measured data from the research scanner SARUS and a BK-8811 192 element linear array transducer (BK Medical, Herlev, Denmark), insonifying a water-filled wire phantom containing four wires orthogonal to the image plane. The data are acquired using 12-bit flash A/D converters at a sampling rate of 70 MHz, and are then upsampled off-line to 560 MHz for input to the simulated ∆Σ beamformer. The latter generates a B-mode image which is compared to that produced by a digital beamformer that uses 10-bit A/D converters. The performance is evaluated by comparing the width of the wire images at half amplitude and the noise level of the images. The ∆Σ beamformer resolution has been found to be identical to that of the multi-bit A/D beamforming architecture, while the noise floor is elevated by approximately 6 dB.