Estimating Aggregate Cardiomyocyte Strain Using In~Vivo Diffusion and Displacement Encoded MRI

Changes in left ventricular (LV) aggregate cardiomyocyte orientation and deformation underlie cardiac function and dysfunction. As such, in vivo aggregate cardiomyocyte "myofiber" strain (E ff ) has mechanistic significance, but currently there exists no established technique to measure in vivo E ff . The objective of this work is to describe and validate a pipeline to compute in vivo Eff from magnetic resonance imaging (MRI) data. Our pipeline integrates LV motion from multi-slice Displacement EN-coding with Stimulated Echoes (DENSE) MRI with in vivo LV microstructure from cardiac Diffusion Tensor Imaging (cDTI) data. The proposed pipeline is validated using an analytical deforming heart-like phantom. The phantom is used to evaluate 3D cardiac strains computed from a widely available, open-source DENSE Image Analysis Tool. Phantom evaluation showed that a DENSE MRI signal-to-noise ratio (SNR) ≥20 is required to compute E ff with near-zero median strain bias and within a strain tolerance of 0.06. Circumferential and longitudinal strains are also accurately measured under the same SNR requirements, however, radial strain exhibits a median epicardial bias of -0.10 even in noise-free DENSE data. The validated framework is applied to experimental DENSE MRI and cDTI data acquired in eight (N = 8) healthy swine. The experimental study demonstrated that E ff has decreased transmural variability compared to radial and circumferential strains. The spatial uniformity and mechanistic significance of in vivo E ff make it a compelling candidate for characterization and early detection of cardiac dysfunction.