## 4 Discussion
#### 4.2.2 Limitations  
The GB method needs to reconstruct an image of the aberrator in order to estimate its position and geometry. This is a challenging task for a thin temporal bone (1 mm thick) at 2.5 MHz because the near and far interfaces of the bone layer cannot be clearly distinguished. Accurate estimation of travel-times also requires an estimate (with the head-wave or the autofocus techniques [32]) or a guess (based on literature) of the wave-speed in each layer while each layer is assumed homogeneous. The assumption of homogeneous tissue layers likely breaks down in individuals with a heterogeneous complex cutaneous tissue layer or a temporal bone containing a diploe. Limitations in the Implementation of the Near-Field Phase-Screen Technique in This Work, Possible Improvements  
The near-field phase-screen aberration correction technique was most often applied with focused transmit waves [40]. However, to capture the transient fast phenomena in the brain (the blood velocity in the main cerebral arteries can approach 2 m/s [37]) and enable real-time feedback during clinical scans, an ultrafast imaging sequence relying on transmission of unfocused beams is preferred. Thus, the backscattered waves should be synthetically focused in transmit (for generating the coherent wavefront backscattered from a small volume) [37] so that the wavefront from a scatterer in an isoplanatic patch becomes well-distinguished from the background speckle noise and aberration profile estimation with the PS method is feasible. The focusing procedure includes delaying the RF data acquired by transmitting with each unfocused beam with respect to the focal point and coherent summation. Five steered plane waves are not optimal for efficient synthetic transmit focusing and, therefore, for estimating the aberration profile in each patch accurately. A better approach for the PS method would be to first estimate the APs in each patch with a large number (e.g., 50) of steered plane waves (or diverging waves) combined with synthetic transmit focusing or using focused transmit waves, and then using the estimated APs to correct the phase aberration in each patch when imaging with a high frame rate imaging sequence. Additionally, the PS approach can perform better if applied iteratively by improving transmit focusing at each step [40]. In this work, we used five transmit plane waves and a single estimation of the aberration profile to enable real-time imaging and live feedback to the operator. Moreover, we implemented the PS method with the beamsum channel correlation method; other techniques to estimate the AP, such as the multilagleast-squares cross-correlation method ([21]), may perform better. The main limitation of this study is that the aberrator was invariant in the out-of-plane dimension, while the temporal bone has a complex three-dimensional shape. Our study considered two-dimensional imaging with a phased-array transducer. However, as demonstrated earlier [21; 22; 23; 40], a two-dimensional matrix array transducer is required for efficient phase aberration correction with the PS method. Limitations in the Implementation of the Geometry-Based Aberration Correction Technique in This Work, Possible Improvements  
In this paper, we used the accelerated refraction-corrected (ARC) image reconstruction technique as the representative of the methods that use the position and geometry of the bone layer for aberration correction [34]. This approach is not iterative, and the image grid points are used to describe the near and far surfaces of the bone layer and find the shortest travel-time connecting a pixel to an array element (Fermat's principle). While this allows faster calculation of the travel-times and real-time image reconstruction with a GPU, there are more accurate ray-tracing methods (such as fast marching technique) that could outperform ARC at the expense of a higher processing time [31; 32; 33; 44].