Imaging Myocardial Tagging

It has been postulated that rotation of the left ventricular apex with respect to the base is a component of normal systolic function in humans, but it has been difficult to measure it noninvasively. Tagging is a new magnetic resonance imaging technique that labels specific areas of myocardium by selective radio-frequency excitation of narrow planes orthogonal to the imaging plane before acquiring an image. Tags appear as black lines and persist in myocardium for 400-500 msec and, if applied at end diastole, will move with the myocardium through systole. Tagging was used to noninvasively quantify left ventricular torsion and circumferentiallongitudinal shear (shearcL) in humans. Eight normal volunteers, aged 24-38 years, were imaged in a 0.38-T iron-core resistive magnet. Five short-axis left ventricular images, positioned to encompass the entire left ventricle (LV), were obtained separately at end systole. Four equiangular radial tags had been applied at end diastole, intersecting the myocardium at eight locations. We calculated the difference in angular displacement of each epicardial and endocardial tag point (a tag point being where the tag crossed the epicardium or endocardium) at end systole from the systolic position of the corresponding tag point on the basal plane. This value was called the torsion angle. From this, shearcL, the angle inscribed on the epicardial or endocardial surface between the systolic tag position, the corresponding basal tag position, and its projection onto the slice of interest could be calculated at 32 pointsin the left ventricular wall. The mean of the torsion angles of the eight locations on the apical slice, relative to the mean of the torsion angles of the base for endocardial points (endocardial torsion), was 19.1±2.0° (mean±SEM,p<0.001), counterclockwise when viewed from the apex. Epicardial torsion (counterclockwise, 11.2±1.30; p<0.001) was 8± 1.90 less than the endocardium (p<0.01). Torsion (mean of torsion angles between base and apex) in the posteroseptal regions was less than in anterolateral regions for both endocardium (12.4±2.90 vs. 23.1±4.4°, p<0.001) and epicardium (6.4±3° vs. 12.8±3.10, p<0.04). The torsion angle increased with distance from the base for both epicardium and endocardium. Different amounts of torsion, however, were found to result in similar amounts of shearCL for both epicardium (5.0±0.60) and endocardium (4.0+0.50), which did not increase with distance from the base. Therefore, torsion varies with distance from the base and from the center of the LV but constancy of shearcL at each level of the LV is achieved. This constancy of shearCL may represent an important principle by which stress through and along the left ventricular wall is equalized during normal ejection. (Circulation 1990;81:1236-1244)