Cardiac strain rate imaging using two-dimensional speckle tracking.

Ischemic heart disease typically produces regional abnormalities of myocardial (mechanical) function. Ultrasound-based cardiac strain rate imaging (SRI) has been proposed to characterize regional myocardial deformation using spatial derivatives of Doppler-based tissue velocity. As such, current SRI approaches suffer from angle-dependence, low strain-to-noise ratio (SNR), and range and velocity ambiguity. In this work, an alternate method for SRI is proposed using 2-D cross-correlation based speckle tracking techniques. To test the proposed approach, strain rate images were both simulated and measured on a thick-walled, cylindrical, tissue equivalent phantom modeling cardiac deformations. Results demonstrate that this 2-D SRI can yield high spatial resolution, high SNR, virtually angle-independent strain rate images of the time derivative of the strain magnitude. To fully characterize local deformation over a cardiac cycle with Lagrangian strain estimates, in addition to spatial registration by accumulating displacement measurements with the geometry changes taken into account, temporal registration is required to compensate for different sampling times between beams. Algorithms to implement spatial and temporal registration were developed. Since every point on the heart wall must return to its initial position after one beat, residual accumulation error (RAE) in the total displacement after exactly one cardiac cycle can be used as a robustness measure of tracking algorithms. RAE (or Baseline drift artifact) was observed in an initial in vivo porcine heart model. To address this issue and further understand the performance of 2-D SRI methods in a controlled 3-D environment, a 3-D left-ventricle simulation model with physiological deformation was developed. Results from the simulated ultrasonic sequences using this 3-D model demonstrate that out-of-plane motion plays a significant role in RAE. To optimize SNR in displacement estimates, a retrospective processing strategy employing different frame intervals for different periods in a cardiac cycle is also proposed. Accurate displacement estimates are critical for successful SRI. As a result, partial effort in this work has been dedicated to improve current 2-D speckle tracking algorithms. In traditional speckle tracking, lateral displacement (perpendicular to the beam direction) estimates are much less accurate than axial ones (along the beam direction). Therefore, adaptive incompressibility processing and synthetic lateral phase techniques were developed to improve lateral displacement estimates.Ph.D.Applied SciencesBiomedical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124376/2/3138127.pd