Improved elevational and azimuthal motion tracking using sector scans

Ultrasound data motion tracking is widely used to estimate relative tissue/transducer motion, for example in freehand 3-D imaging, in which successive 2-D ultrasound scan planes are registered in a 3-D 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 because of point-spread function rotation with lateral motion. 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 3-D space. We show that our method has performance benefits over both speckle-tracking and decorrelation-based tracking for motion estimation in sector-scan systems, particularly when there is both elevational and azimuthal motion. Motion-tracking efficacy is further demonstrated by improved freehand imaging of a known target (anatomically accurate 3-D-printed lumbar spine model) in a tissue-mimicking phantom, with an rms surface distance error of 1.2 mm, compared with 2.43 mm for conventional methods. These data indicate that the new algorithm is capable of improved tracking performance for sector scan systems, enabling effective freehand 3-D scanning.