Stroke is a significant cause of mortality and disability in America. Due to differences in the treatment of ischemic and hemorrhagic stroke, imaging must be performed before administration of therapeutic medication. Unfortunately, the current standard imaging methods, namely CT and MRI, require specialized locations and staff, which can induce delays in triage, and therefore, treatment time. Recent work suggests that ultrasound tomography (UST) is capable of imaging in vivo tissue properties and may have potential as a diagnostic tool during stroke treatment which could be performed at the point of injury rather than at a local hospital. In this work, we investigate the feasibility of using UST imaging to image the brain via in silico, in vitro, and ex vivo studies. The results of this work indicate some of the challenges which must be overcome to effectively image in vivo stroke patients.
Phase aberration is one the key sources of image degradation in handheld B-mode ultrasound imaging. Sound speed heterogeneities create phase aberrations in the image by inducing additional tissue-dependent delays and diffractive effects that conventional beamforming does not incorporate. For this reason, the Fourier split-step angular spectrum method is used to simulate pressure fields in a heterogeneous sound speed medium and create B-mode images based on the cross-correlation of transmitted and received wavefields. Because the strongest aberrations are caused by a laterally varying sound speed profile, this work presents a new sound speed estimator that can be used to correct for aberrations in laterally varying media. Phantom experiments show a 58-76% improvement in point target resolution and a 2.5x improvement in contrast-to-noise ratio because of the proposed sound speed estimation and phase aberration correction scheme.
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