abstract :Cardiac function arises from the complex deformation of the heart during the cardiac cycle. This deformation results from the coordinated contraction of the cardiac muscle cells (cardiomyocytes). The cardiac microarchitecture is composed of a continuously branching and merging syncytium of cardiomyocytes. The aggregated cardiomyocytes form so-called “myofibers” that change their orientation from epicardium to endocardium in a helical fashion. Oriented in this unique architecture, the cardiomyocytes transform uniaxial “myofiber” contraction into the circumferential, longitudinal, and radial deformation of the whole heart. Measuring “myofiber” orientation and strain in vivo is crucial as it is suspected that transmural differences in function, cardiomyocyte disarray, and interstitial fibrosis contribute to Heart Failure.
Cardiac Diffusion Tensor imaging (cDTI) is the principal motion encoding MRI technique to measure cardiac microstructure in vivo. cDTI probes the mobility of water molecules within the tissue. In principle, diffusion encoding encode and decode the motion of diffusive water molecules using a pair of gradient. Because water molecules, hindered by the surrounding tissue microstructure, diffuse more in the direction of the cardiomyocyte long-axis, cDTI provides a direct measure of the aggregate cardiomyocyte (i.e. “myofiber”) orientation. New developments in cDTI have enabled in vivo imaging despite the inherent cardiac bulk motion. In Spin Echo (SE) approaches, a second-order motion compensated gradient design (M1=M2=0), decorrelate the diffusion effect from bulk motion.
Strain can be measured by motion tracking methods like Displacement ENcoding with Stimulated Echoes (DENSE) which provides spatially resolved maps of tissue movement. Combining strain and microstructural MRI provides access to a more mechanistically accurate description of cardiac function which allows the evaluation of the performance and wellness of the local cardiomyocyte function. This new approach could help understand the elevated cardiac torsion on patient with diabetic cardiomyopathies.