We present a refraction-corrected sound speed reconstruction technique for layered media based on the angular coherence of plane waves. Previous work has successfully shown that sound speed estimation and refraction- corrected image reconstruction can be achieved using the coherence of full-synthetic aperture channel data. However, methods for acquiring the full-synthetic aperture dataset require a large number of transmissions, which can confound sound speed estimation due to the scatterer motion between transmit events, especially for in-vivo application. Furthermore, sound speed estimation requires producing full-synthetic aperture coherence images for each trial sound speed, which can make the overall computational cost quite burdensome. The angular coherence beamformer, initially devised as a quicker alternative to the more conventional spatial coherence beamformer, measures coherence between fully-beamformed I/Q channel data for each plane wave as opposed to the receive channel data prior to receive beamforming. As a result, angular coherence beamforming can significantly reduce the computation time needed to reconstruct a coherence image by taking advantage of receive beamforming. Previous work has used the coherence maximization of full-synthetic aperture channel data to perform sound speed estimation. By replacing spatial coherence with angular coherence, we apply a similar methodology to channel data from plane-waves to significantly reduce the computational cost of sound speed estimation. This methodology has been confirmed by both simulated and experimental channel data from plane waves.
We have fabricated a cylindrical intravascular ultrasound (IVUS) transducer array prototype capable of generating an acoustic radiation force impulse (ARFI) for shear wave elasticity imaging (SWEI). The prototype array was a 4-mm long, 2.5-mm diameter, 4 MHz PZT-8 tube, axially segmented into 12 elements on a 334 µm pitch. This transducer array was used in custom vessel phantoms and in ex vivo porcine artery experiments to investigate the potential for IVUS SWEI to distinguish soft lipid cores from stiffer surrounding tissues. By using this array transducer to generate a radially-directed ARFI “push”, and a Verasonics linear array probe to track displacements in planes parallel to the “push”, SWEI images of a vessel phantom with hard vessel walls and a soft inclusion were obtained. In tissue-mimicking phantoms, focusing the transducer array to a range of 5 mm generated ARFI displacements up to 1.36 and 1.76 times greater than unfocused excitations in the soft and stiff regions, respectively. The measured shear wave speed in the soft inclusion and stiff vessel wall was 0.97±0.59 m/s and 1.66±0.91 m/s, respectively, and was close to the calibrated measurements of 1.21±0.05 m/s and 1.56±0.05 m/s, respectively. A SWEI image of an ex vivo porcine renal artery was obtained using the prototype transducer and external tracking array, and showed an average shear wave speed of 3.97±1.12 m/s. These results demonstrate the potential of this IVUS array to enable SWEI, to quantifiably assess vulnerable vascular plaques.
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