Phase correction of skull aberration with 1.75-D and 2-D arrays using speckle targets

Abstract

Wavefront distortion induced by the skull severely impacts transcranial ultrasound. In particular, the skull reduces the ability to monitor flow in the cerebral vasculature. We present B-scan and color flow images, aberratred with polymer casts of skull bone, that have been improved with near-field phase correction techniques. These algorithms rely on speckle targets for estimation of the phase profile. Real-time adaptive imaging experiements in tissue-mimicking phantoms with 4 mm spherical voids were performed using multi-lag cross-correlation and multi-lag speckle brightness phase correction algorithms. The real-time adaptive imaging was con- structed on a Siemens Antares TM scanner and used a custom 3.5 MHz, 1.75-D transducer (8×96 elements). Phase correction with the 1.75-D array was performed in a matter of a few seconds. The contrast-to-speckle (CSR) of the spherical voids improved from 1.16 ± 0.39 in the aberrated images to 1.52 ± 0.37 in the phase corrected images. Using a 2.5 MHz, 2-D array on a real time 3-D scanner similar experiments with spherical voids were performed. The CSR of the lesions on the 3-D scanner improved from 0.9 in the aberrated images to 1.6 in the phase corrected images. In addition, phase correction was shown to improve the estimation of 3-D color flow when the transducer was focused on a speckle target through the skull cast. I. INTRODUCTION Over the last decade, with the availability of color flow and power Doppler combined with safe, effective intravenous ultrasound contrast agents, a renaissance has occurred in the evaluation of cerebrovascular disease using transcranial ultrasound in spite of the image degrading properties of the skull. The standardized examination procedure for transcranial ultrasound uses a 2 MHz phased array applied to the acoustic windows of the skull combined with contrast agents and color flow Doppler and/or power Doppler. Extensive recent reviews have described the role of ultrasound in the evaluation of stroke and other pathologies of the intracranial vascular system (1, 2) as well as the brain parenchyma (3). Furthermore, there is continuing progress in the measurement of cerebral perfusion with new ultrasound contrast agents and harmonic imaging techniques (4). In 2004, we showed the feasibility of real-time 3-D tran- scranial ultrasound imaging (5). However, imaging the brain with ultrasound is a difficult process because the skull has a significantly higher speed of sound (approximately 3000 m/s) than soft tissues (nominally 1540 m/s). At such a large discrepancy in sound speed, the phase and amplitude of the ultrasound wave become severely distorted, resulting in significant degradation in beam focusing and image quality.