| Phase aberration is a leading cause of transcranial ultrasound image degradation. In order to realign aberrated wavefronts, a delay map corresponding to the aberration can be computed from signals backscattered from a region of interest (ROI) in the medium, and used to correct the beamforming delays. However, such a map is only effective for correcting the aberration in a limited area called the Isoplanatic Patch (IP) around the ROI. This fundamentally limits the effectiveness of transcranial aberration correction to restore image quality. In this paper, IP sizes are measured in vitro for aberration correction with an X7-2 2D array (Philips Healthcare, Andover, MA) through 12 ex vivo human temporal bone samples. The angular IP size is found to be 36° ± 18°. An in vivo experiment confirms that the IP is limited angularly (~30°) but large in depth (~15cm). Small IP sizes and high refocusing effectiveness within the IP are correlated with high gradients in the measured phase aberration maps. This study indicates that phase aberration correction with a single delay map is only effective for transcranial ultrasound applications with a small angular field of view. |