Comparison of spatial encodings for ultrasound imaging

Ultrasound pulse sequencing and receive signal focusing work hand-in-hand to determine image quality. These are commonly linked by geometry, for example using focused beams or planewaves in transmission paired with appropriate time-of-flight calculations for focusing. Spatial encoding allows a broader class of array transmissions but requires decoding of the recorded echoes before geometric focusing can be applied. Recent work has expanded spatial encoding to include not only element apodizations but also element time delays. This powerful technique allows for a unified beamforming strategy across different pulse sequences and increased flexibility in array signal processing given access to estimates of individual transmit element signals, but trade-offs in image quality between these encodings has not been previously studied. We evaluate in simulation several commonly used time delay and amplitude encodings and investigate optimization of the parameter space for each. Using signal-to-noise ratio, point resolution, and lesion detectability we found trade-offs between focused beams, planewaves, and Hadamard weight encodings. Beams with broader geometries maintained a wider field-of-view after decoding at the cost of signal-to-noise ratio (SNR) and lesion detectability. Focused beams and planewaves showed slightly reduced resolution compared to Hadamard weights in some cases, especially close to the array. We also found overall degraded image quality using random weight or random delay encodings. We validate these findings with experimental phantom imaging for select cases. We believe that these findings provide a starting point for sequence optimization and for improved image quality using the spatial encoding approach for imaging.