Improve myocardial strain estimation based on deformable groupwise registration with a locally low-rank dissimilarity metric.

Background: Current mainstream cardiovascular magnetic resonance-feature tracking (CMR-FT) methods, including optical flow and pairwise registration, often suffer from the drift effect caused by accumulative tracking errors. Here, we developed a CMR-FT method based on deformable groupwise registration with a locally low-rank (LLR) dissimilarity metric to improve myocardial tracking and strain estimation accuracy.

Methods: The proposed method, Groupwise-LLR, performs feature tracking by iteratively updating the entire displacement field across all cardiac phases to minimize the sum of the patchwise signal ranks of the deformed movie. The method was compared with alternative CMR-FT methods including the Farneback optical flow, a sequentially pairwise registration method, and a global low rankness-based groupwise registration method via a simulated dataset (n = 20), a public cine data set (n = 100), and an in-house tagging-MRI patient dataset (n = 16). The proposed method was also compared with two general groupwise registration methods, nD + t B-Splines and pTVreg, in simulations and in vivo tracking.

Results: On the simulated dataset, Groupwise-LLR achieved the lowest point tracking errors (p = 0.13 against pTVreg for the temporally averaged point tracking errors in the long-axis view, and p < 0.05 for all other cases), voxelwise strain errors (all p < 0.05), and global strain errors (p = 0.05 against pTVreg for the longitudinal global strain errors, and p < 0.05 for all other cases). On the public dataset, Groupwise-LLR achieved the lowest contour tracking errors (all p < 0.05), reduced the drift effect in late-diastole, and preserved similar inter-observer reproducibility as the alternative methods. On the patient dataset, Groupwise-LLR correlated better with tagging-MRI for radial strains than the other CMR-FT methods in multiple myocardial segments and levels.

Conclusions: The proposed Groupwise-LLR reduces the drift effect and provides more accurate myocardial tracking and strain estimation than the alternative methods. The method may thus facilitate a more accurate estimation of myocardial strains for clinical assessments of cardiac function.