# Implementation of Constrained Swept Synthetic Aperture Using a Mechanical Fixture
## 4 Discussion  
The simulation and phantom results suggest that both speckle tracking and channel correlation are potential techniques for SSA imaging with a constrained sweep. However, there are distinct trade-offs in these algorithms. Channel tracking showed clear advantages in the accuracy of estimation in these tasks and, importantly, greatly reduced the computational complexity of estimation. For example, in the phantom case of Figure 7, using a Ryzen 7 2700X 8-core CPU, speckle tracking required 8.5 s to beamform the set of 501 image frames (246 \(\times\) 164 pixels) used in the correlations and 13 s to perform the lateral cross-correlation search across frame pairs up to 10 frames apart and final weighted least squares estimate. The channel correlation approach took only 9 s in total (for the cross-correlation search) using the same estimation parameters. In addition to avoiding the need for image formation, channel correlation uses a reduced spatial sampling because it is based on the aperture spacing rather than the sampling requirements in the imaging field. While one could reduce the lateral pixel spacing for speckle tracking closer to the Nyquist limit, the required sampling will depend on the depth of estimation. In both methods, more computation is required for larger kernel and search regions. The above times should be used for comparison only as GPU beamforming would improve image formation times and a more optimized implementation of normalized cross-correlation (e.g., re-using overlapping calculations and reducing for loops) would improve the correlation times for both methods. The largest challenge in this approach is any motion of the fixture that occurs outside of the imaging plane. These motions cannot be estimated using either a tracking approach or degraded imaging performance as signals from across the sweep cannot be coherently combined. It is demonstrated here that, on relatively firm tissues (pork and beef), the fixture position was able to be maintained with sufficient accuracy to produce SSA images that are comparable to the phantom alone. Future work will explore the use of these techniques in vivo, where different choices of target may present different scanning challenges. It may be possible, for example, to add an adhesive to the linear fixture if sliding is a problem; however, if the tissue under the fixture compresses, then the fixture may still tilt relative to the anatomy of interest. It may  
Figure 11: Results of manual sweeps through various tissue phantoms using linear slide fixture. All images show a 50 dB dynamic range. (**Left** and **middle**) Zoomed images of point and lesion targets from reference (no sweep) and manual sweep data. Spatial axes labels are approximate because position varies slightly between sweeps and targets. (**Right top**) Sample SSA image through pork tissue (12 mm thick, above the dotted yellow line). (**Right bottom**) Estimated positions for each case using channel estimation method. be beneficial to explore coupling the device to skin with a gel pad rather than gel, providing a firmer surface for the fixture and a more consistent coupling across the sweep. It will also be necessary to study the role that in vivo target movement (e.g., skin deformation, cardiac-induced motion and breathing) has on the SSA images even when the fixture is stationary relative to the skin. A sufficiently rapid sweep relative to the target motion could prevent degradation, or adaptive motion filters may be required to process the data. This study was also limited in its analysis of acoustic clutter, which should be more directly assessed in more challenging imaging cases in the future. SSA imaging introduces a new dimension of variability in image quality not present in conventional imaging. Even after normalizing for the spatial/temporal dynamics of the sweep, the sweep extent will result in varying effective resolution depending on the length. It may be desirable to limit the sweep length in post-processing to achieve more consistent results between acquisitions.