In a recent study, we proposed a technique to correct aberration caused by the skull and reconstruct a transcranial B-mode image with a refraction-corrected synthetic aperture imaging scheme. Given a sound speed map, the arrival times were calculated using a fast marching technique (FMT), which solves the Eikonal equation and therefore is computationally expensive for real-time imaging. In this paper, we introduce a two-point ray tracing method, based on Fermat’s principle, for fast calculation of the travel times in the presence of a layered aberrator in front of the ultrasound probe. The ray tracing method along with the reconstruction technique are implemented on a graphical processing unite (GPU). The point spread function (PSF) in a wire phantom image reconstructed with the FMT and the GPU-implementation was studied with numerical synthetic data and experiments with a bone-mimicking plate and a sagittally-cut human skull. The numerical analysis showed that the error on travel-times is less than 10% of the ultrasound temporal period at 2.5MHz. As a result, the lateral resolution was not significantly degraded compared with images reconstructed with FMT-calculated travel times. The results using the synthetic, bone-mimicking plate and skull dataset showed that the GPU-implementation causes a lateral/axial localization error of 0.10/0.20 mm, 0.15/0.13 mm and 0.26/0.32 mm compared to a reference measurement (no aberrator in front of the ultrasound probe), respectively. For an imaging depth of 70 mm, the proposed GPU-implementation allows reconstructing 19 frames per second with full synthetic aperture (96 transmission events) and 32 frames per second with multi-angle plane wave imaging schemes (with 11 steering angles) for a pixel size of 200 µm. Finally refraction-corrected power Doppler imaging is demonstrated with a string phantom and a bone-mimicking plate placed between the probe and the moving string. The proposed approach achieves a suitable frame rate for clinical scanning while maintaining the image quality.
|Journal||IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control|
|Publication status||Accepted/In press - 6 Jul 2022|