3-D motion tracking and vascular strain imaging using bistatic dual aperture ultrasound acquisitions

Objective. This study demonstrates high volume rate bistatic 3-D vascular strain imaging, to overcome well-known challenges caused by the anisotropic resolution and contrast inherent to ultrasound imaging. Approach. Using two synchronized 32 x 32 element matrix arrays (3.5 MHz), coherent 3-D ultrasound images of ex vivo porcine aortas were acquired at 90 Hz during pulsation in a mock circulation loop. The image data of interleaved transmissions were coherently compounded on one densely sampled Cartesian grid to estimate frame-to-frame displacements using 3-D block matching. The radial displacement components were projected onto mesh nodes of the aortic wall, after which local circumferential and radial strain estimates were calculated with a 3-D least squares strain estimator. Main results. The additional reflection content and high-resolution phase information along the axis of the second transducer added more distinctive features for block matching, resulting in an increased coverage of high correlation values and more accurate lateral displacements. Compared to single array results, the mean motion tracking error for one inflation cycle was reduced by a factor 5-8 and circumferential elastographic signal-to-noise ratio increased by 5-10 dB. Radial strain remains difficult to estimate at the transmit frequency used at these imaging depths, but may benefit from more research into strain regularization and sub-pixel interpolation techniques. Significance. These results suggest that multi-aperture ultrasound acquisition sequences can advance the field of vascular strain imaging and elastography by addressing challenges related to estimating local-scale deformation on an acquisition level. Future research into 3-D aberration correction and probe localization techniques is important to extend the method's applicability towards in vivo use and for a wider range of applications.