Orientation of tendons in the magnetic field and its effect on T2 relaxation times.

In vitro studies of spin-spin relaxation times of the protons or hydrogen nuclei which make up the tendon demonstrated significant dependence of T2 on orientation at physiological levels of hydration. T2 varied from approximately 250 mu sec. with the tendon aligned with the magnetic field to 22 msec. at an angle of 55 degrees or 125 degrees and 4 msec. at deviations of +/- 15 degrees from 55 degrees and 125 degrees. 55 degrees and 125 degrees are the so-called "magic angles" at which 3 cos2 theta - 1 = 0. As a result, the signal from human tendons in vivo will depend on their orientation; however, T2 is generally short enough that the signal intensity will approach 0 in MR images regardless of the orientation of the tendons, thereby preventing this phenomenon from being observed in most studies.