Figure 2.

The Proposed Effect of Transmural Helical Myocyte Orientation on FA

Streeter’s classic micrograph of transmural variation of helix angles from endocardium (0% wall thickness) to epicardium in a canine left ventricle (15)(A). Approximately 3 DT-CMR imaging voxels (2.8 × 2.8 × 8 mm³) span the myocardium transmurally in diastole (B). In the endo- and epicardial voxels, myocytes are progressively changing from a longitudinal to circumferential orientation and vice versa, respectively. There are no marked transmural differences in myocyte diameter or fibrous tissue in healthy myocardium (40). Therefore, the wide distribution of myocyte orientations in the endo- and epicardium will reduce FA. Conversely, the narrow distribution of orientations in the mid-wall where myocytes are consistently circumferentially orientated will elevate FA (C). Myocyte disarray and fibrosis in the mid-wall will reduce FA, due to the wide distribution of disorganized myocyte orientations and expanded extracellular space (39), but the overall mean voxel helix angle remains in the circumferential orientation (D). Thus, helix angle is expected to be normal despite abnormal FA. LV = left ventricular; other abbreviations as in Figure 1.