Determination of temporal bone Isoplanatic Patch sizes for transcranial phase aberration correction

Abstract

1286Abstract-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.

Pitch-catch phase aberration correction of multiple isoplanatic patches for 3-D transcranial ultrasound imaging

“…In actuality, the skull has a finite thickness and is separated from the transducer by a few millimeters of extracranial tissues and vessels, resulting in a finite IP. For a phased-array scan, the near-field phase screen assumption begins to fail as scan angle increases from broadside (0°, 0°) to approximately ±16° [43], [45], decreasing the potential benefit of phase aberration correction [46]. Applying the appropriate phase correction maps to the appropriate steering angles could improve brightness and contrast in transcranial images over the entire field of view.…”

“…There is not a single, standard definition for the IP; it has been variously defined as the positions in the field over which the point spread function (PSF) increases by 10% [47], aberrators are correlated by 70% to 90% [44], or speckle/target brightness increases [43], [45]. IP size varies depending on the definition used, the scanning system, the correction method used, and on the individual skull [45].…”

“…IP size varies depending on the definition used, the scanning system, the correction method used, and on the individual skull [45]. Correction of multiple IPs has been investigated by Liu and Waag in ex vivo abdominal tissues [47], and by Ferndandez and Trahey [48] and Dahl et al .…”

“…Vignon et al . measured 36 ± 18° in ex vivo skulls and 33° in vivo in a single subject, both in the transverse plane, using a time-reversal mirror correction method (FDORT) [45]. This study also suggested that aberration correction effectiveness in transcranial sector scans might be divided into 3 broad categories of “effective,” “lowly effective,” and “ineffective” as scan angle increases, where “effective” denotes the center 30° of a transcranial sector scan.…”

“…The IP in a 2-D sector scan is often reported as an azimuthal angle and an axial depth. Because both previous measurements found the axial depth of the temporal bone IP to extend to the entire scan depth (12 cm [49] and 15 cm [45]), an infinite patch size in the axial direction will be assumed.…”