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 plane waves 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 giving access to estimates of individual transmit element signals, but tradeoffs in image quality between these encodings have not been previously studied. We evaluate in simulation several commonly used time delay and amplitude encodings and investigate the optimization of the parameter space for each. Using the signal-to-noise ratio (SNR), point resolution, and lesion detectability, we found tradeoffs between focused beams, plane waves, and Hadamard weight encodings. Beams with broader geometries maintained a wider field of view after decoding at the cost of the SNR and lesion detectability. Focused beams and plane waves 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 improved image quality using the spatial encoding approach for imaging.