Relation of regional cross-fiber shortening to wall thickening in the intact heart. Three-dimensional strain analysis by NMR tagging.

BACKGROUNDThe mechanism by which small amounts of myofiber shortening lead to extensive wall thickening is unknown. When isolated fibers shorten, they thicken in the two orthogonal directions. In situ fibers, however, vary in their orientation through the wall, and each is tethered to near or distant neighbors, which allows shortening to occur both in the direction of the fibers and also perpendicular to them. This "cross-fiber" shortening may enable the wall to shorten in two directions and thereby thicken extensively in the third.METHODS AND RESULTSNuclear magnetic resonance tagging is a noninvasive method of labeling and tracking myocardium of the entire heart in three dimensions that does not interfere with myocardial motion. To investigate the presence and importance of cross-fiber shortening in the intact left ventricle, 10 closed-chest dogs were studied by nuclear magnetic resonance tagging. Five short-axis and four long-axis images were acquired to reconstruct 32 cubes of myocardium in each dog at...