4D Strain from Tagged Magnetic Resonance Images by Unwrapping the Harmonic Phase Images

Abstract

Accurate assessment of ventricular function is clinically important. Tagged magnetic resonance imaging (MRI) is the modality of choice for calculating parameters such as torsion, rotation and strain that characterize regional myocardial function. In this dissertation, a method for reconstructing three-dimensional strain over time from unwrapped harmonic phase measurements obtained from tagged MRI is presented. The procedure involved the placement of branch cuts before unwrapping the phase images to remove inconsistencies and obtaining displacement measurements. An affine prolate spheroidal B-spline (APSB) model was then used to reconstruct strain from the estimated displacement measurements. Initially, this method was implemented to obtain strain and torsion in the left ventricle (LV) by the manual placement of branch cuts through a graphical user interface (GUI) before unwrapping. This procedure is known as manual Strain from Unwrapped Phase (mSUP). The strain obtained was compared to 3D strain obtained from a feature-based method and 2D strain obtained from harmonic phase strain measurements. A method called the biventricular strain from unwrapped phase (BiSUP) for reconstructing three-dimensional plus time (3D+t) biventricular strain maps from unwrapped harmonic phase (HARP) images was then introduced. This is an extension of mSUP to obtain 3D+t strain in the right ventricle (RV). Accurate assessment of RV function is clinically important. Compared to LV, however, analysis of RV function is relatively difficult because of a lack of geometric symmetry and the comparatively thinner myocardium. Displacement estimates were obtained from unwrapped harmonic phase measurements, in a procedure similar to the mSUP, and were then used to reconstruct 3D strain using a discrete model free (DMF) approach. The DMF method of reconstruction does not require the model to have geometric symmetry and so performed efficiently to produce accurate strains. The BiSUP strain and displacements were compared to those estimated by a 3D feature-based (FB) technique and a 2D+t HARP technique. A computer-assisted Strain from Unwrapped Phase (caSUP) procedure to automatically place branch cuts was then devised. A combination of simulated annealing and exhaustive search methods were used to obtain the optimal branch cut configuration. The energy function that was minimized in the search methods aimed to maintain spatial and temporal continuity. This process reduced the manual intervention required to obtain 3D+t strain to nearly one-third of that of the GUI-based approach. The strain obtained was compared to 3D strain obtained from mSUP. To summarize, these works included validation and detailed comparisons of strains, torsion and rotation parameters of both the LV and the RV in human subjects of different pathologies and morphologies.