Table 1.
The comparison between different (representative) SAFT schemes.
| Algorithm description | Dimensions of improved lateral resolution | Achievable resolution (a) |
Wave-shape concept | Processing speed | Suppress SNR at focus (b) | ||
|---|---|---|---|---|---|---|---|
| 1D-SAFT+CF [56], [57] | Signals synthesized in one dimension (x-z or y-z) | 1 (Anisotropic lateral resolution) |
High (Comparable with resolution at focus) |
Cylindrical wave | Fast | Yes (if without SIR) | |
| 2D-SAFT+CF [66] | Signals synthesized in two dimensions (x-z and y-z) | 2 (Isotropic lateral resolution) |
Medium (Less than resolution at focus) |
Spherical wave | Yes (if without SIR) | ||
| A-SAFT +CF [68] |
Tubular targets | Signals synthesized perpendicularly to each target’s orientation | 2 (Isotropic lateral resolution) |
High (Comparable with resolution at focus) |
Cylindrical wave | Medium | Yes (if without SIR) |
| Point-like (or circular) target |
1(c) (Anisotropic lateral resolution) |
High (Comparable with resolution at focus) |
Yes (if without SIR) | ||||
| SIR-based SAFT [70] | Transducer’s SIR used to weight contributions | Depends on its combination with 1D-, 2D-, or Adaptive-SAFT | No (Homogenous SNR achieved) | ||||
| DMAS-SAFT [71] | Delay-multiply-and-sum used for beamforming | Depends on its implementation (in 1D-, 2D-, or Adaptive-SAFT manner) | Slow | Yes (if without SIR) | |||
(a)The best performance that could be achieved with each SAFT scheme. For 1D- and A-SAFT, the best resolution after reconstruction in the out-of-focus region is theoretically comparable with the original resolution at focus, thus achieving the depth-independent lateral resolution. 2D-SAFT shows a poorer performance compared to the 1D solution. (b) The SNR at the focal region is suppressed relative to the outside focal region if the SIR weight is not considered during the SAFT reconstruction. The weighing of SIR helps achieve a homogeneous SNR both far from and at the focus of the ultrasonic detector. (c) For point-like (or circular) targets, A-SAFT may lead to anisotropic lateral resolution, given that only one dimension synthesis is performed for a whole target’s shape when the target’s aspect ratio ≈ 1, as opposed to circular structured targets like vasculatures, where the synthesizing direction changes for each pixel based on the orientation of each branch of their network.