Experimental Methodologies for the Visualization of Cardiac Myofiber Orientations within Perfusion‐fixed Human Hearts

Abstract

Cardiac myofiber structure and organizations play critical roles in the electrical and mechanical properties of the heart. Diffusion tensor magnetic resonance imaging (DTMRI) has been a useful imaging modality to visualize the myocardial fiber orientation with emerging clinical application. DTMRI takes advantage of the limited diffusion of water along the cardiac fiber’s longitudinal axis and provides the dominant direction of diffusion [1]. Until recently, DTMRI usage was relatively limited to clinical imaging of white matter within the brain. While there are limitations for cardiac use in vivo, due to tissue displacement and non‐rigid deformation, DTMRI is becoming a powerful diagnostic tool to assess structural heart damage. The data can also be the input into numerous computational simulations of the heart’s electrical activities. The Visible Heart®; Laboratories at the University of Minnesota, in collaboration with Lifesource, an organ procurement organization, maintains a large collection of human hearts that were donated for research. These organs are received fresh and subsequently preserved with 10% buffered formalin in the approximate end‐diastolic state for future anatomical studies. For the present studies, the hearts were rinsed of formalin and completely submerged in an agarose gel in attitudinally correct positions. The hearts are next scanned in a clinical 3T MRI scanner using a diffusion tensor sequence. The raw DTMRI DICOM files are then reconstructed using Diffusion Toolkit and visualized in TrackVis software [2]. This outputs a 3‐dimensional model that allows for visualization of the relative orientations of the myocardial fibers. Whole heart tissue models can also be generated using Materialise Mimics software from these scans. MRI scans of a heart from a 50 year old male with a history of myocardial infarction can be seen in Figure 1. The DTMRI axial images (Figure 1C) provide insights relative to the orientations of the myocardial fibers; which can then be transformed into a 3D diagram (Figure 1D) of directional eigenvectors. Future work will be done to improve resolutions by extending the number of averages acquired during such scanning. Increasing resolution will allow us to assess structural heart damage stemming from myocardial injuries: e.g. from ablations or due to myocardial infarction. Additionally, we hope to combine DTMRI with cardiac tissue models to create a 3D print that highlight regional myocardial fiber orientations. The fiber orientations from the DTMRI scans can also be incorporated into computer simulations, which may help elucidate the activation pathways of the heart. In some cases today, DTMRI can be utilized clinically to improve one’s understanding of the conduction system of the heart, which has implications in lead placement for pacing and ablation procedures. Researchers and clinicians can learn critical information relative to cardiac anatomy from DTMRI.An overview of the methods used to generate a 3D reconstruction of diffusion tensor MRI data. The resulting image is further refined to remove extraneous dataFigure 1