Imaging myocardial fiber disarray and intramural strain hypokinesis in hypertrophic cardiomyopathy with MRI

Purpose: To examine the relationship between myofiber disarray and myocardial hypokinesis in human hypertrophic cardiomyopathy (HCM) using noninvasive cardiac diffusion and strain MRI. Materials and Methods: Five patients with a diagnosis of HCM and five age-matched healthy volunteers were studied and compared. Cardiac diffusion MRI was applied to map in vivo myocardial fiber architecture. Cardiac strain MRI was applied to map myocardial motion. By acquiring registered diffusion and strain MRI images in vivo on both normal and HCM hearts, we investigated in HCM the relationship between myofiber disarray and systolic strain components, including radial, fiber, and cross-fiber strains, and sought to determine the mechanism behind disarray-related myocardial dysfunction in HCM. Results: Regionally disordered fiber orientation and reduced diffusion fractional anisotropy (FA) were found in HCM, demonstrating myofiber disarray. Intramural myocardial strain hypokinesis (reduced radial, fiber, an cross-fiber strains) was also observed in HCM in the same region, and was found to be correlated with FA. The correlation between FA and hypokinesis was found to be stronger in the cross-fiber direction (part of the passive myocardial function) compared to the fiber direction (part of the active myocardial function). This is consistent with the hypothesis that the principal feature of HCM is the reduction of myocardium passive compliance. The angle between principal shortening and fiber orientation was markedly disordered in HCM, indicating an abnormal transmural coupling. Conclusion: Myofiber disarray in HCM is correlated with abnormalities of both passive and active myocardial function, and the normal patterns of fiber shortening and wall thickening are deranged in HCM.