Stratified-Medium Sound Speed Profiling for CPWC Ultrasound Imaging

Abstract

Coherent plane-wave compounding (CPWC) ultrasound is an important modality enabling ultrafast biomedical imaging. To perform CWPC image reconstruction for a stratified (horizontally layered) medium, one needs to know how the speed of sound (SOS) varies with the propagation depth. Incorrect sound speed and layer thickness assumptions can cause focusing errors, degraded spatial resolution and significant geometrical distortions resulting in poor image reconstruction. We aim to determine the speed of sound and thickness values for each horizontal layer to accurately locate the recorded reflection events to their true locations within the medium. Our CPWC image reconstruction process is based on phase-shift migration (PSM) that requires the user to specify the speed of sound and thickness of each layer in advance. Prior to performing phase-shift migration (one layer at a time, starting from the surface), we first estimate the speed of sound values of a given layer using a cosine similarity metric, based on the data obtained by a multi-element transducer array for two different plane-wave emission angles. Then, we use our speed estimate to identify the layer thickness via end-of-layer boundary detection. A low-cost alternative that obtains reconstructed images with fewer phase shifts (i.e., fewer complex multiplications) using a spectral energy threshold is also proposed in this thesis. Our evaluation results, based on the CPWC imaging simulation of a three-layer medium, show that our sound speed and layer thickness estimates are within 4% of their true values (i.e., those used to generate simulated data). We have also confirmed the accuracy of our speed and layer thickness estimation separately, using two experimental datasets representing two special cases. For speed estimation, we used iv a CPWC imaging dataset for a constant-speed (i.e., single-layer) medium, yielding estimates within 1% of their true values. For layer thickness estimation, we used a monostatic (i.e., single-element) synthetic-aperture (SA) imaging dataset of the threelayer medium, also yielding estimates within 1% of their true values. Our evaluation results for the low-cost alternative showed a 93% reduction in complex multiplications for the three-layer CPWC imaging dataset and 76% for the three-layer monostatic SA imaging dataset, producing images nearly similar to those obtained using the original PSM methods.

Refraction-Based Speed of Sound Estimation in Layered Media: An Angular Approach

“…As a result, specific methods have appeared in the last five years, making profit of the layered geometry of the liver and overlying tissues to compensate for superficial tissues. A first class of approaches use prior knowledge of superficial layers [15] or rely on prior segmentation of such layers [33], but the need for such priors makes them too complex to be used in most cases. Others simplify the tomographic inversion approach mentioned earlier to a one dimensional scenario where the inversion is only performed in the axial direction [34]- [37].…”

Stratified-Medium Plane-Wave Ultrasound Image Reconstruction via Frequency-Domain Migration

“…1 Phase-shift migration requires the user to specify all cz values in advance. Sound speed estimation in the context of CPWC imaging of layered media is addressed in [7], [8]. As for the special case of cz = const = c 0 over all z values, some of the corresponding frequency-domain CPWC beamforming techniques are discussed in [9]- [14].…”