Improved sector-scan motion tracking using joint azimuthal-elevational estimation

Ultrasound-based motion tracking is widely used to estimate both tissue motion, for diagnostic purposes, and to estimate bulk transducer motion relative to tissue in freehand 3D imaging, where successive 2D ultrasound scan planes are registered in a 3D volume. Speckle-tracking and decorrelation-based methods are used to estimate motion in the azimuthal and elevational planes. However, the performance of speckle-tracking is significantly degraded in sector-scan systems due to point spread function (PSF) rotation with lateral motion. In these systems, when there is both azimuthal and elevational motion, the accuracy of estimates in both planes are reduced. In this paper, we develop a new method for joint azimuthal-elevational motion estimation based on the complex correlation of individual IQ-demodulated sector-scan A-lines arising from tissue motion in 3D space. Using a new statistical model, we show that the phase of per-line decorrelation is linearly related to the dot product of the tissue motion vector and each A-line´s direction unit vector. Motion-tracking efficacy is demonstrated by improved freehand imaging of a known target (anatomically accurate 3D-printed lumbar spine model) in a tissue-mimicking phantom, with a root mean squared (RMS) surface distance error of 1.2 mm, compared to 2.43 mm using a conventional approach. These data indicate that the new algorithm is capable of improved tracking performance for sector-scan systems, enabling effective freehand 3D scanning.