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 ~ 250 μsec. with the tendon aligned with the magnetic field to 22 msec. at an angle of 55° or 125° and 4 msec. at deviations of ±15° from 55° and 125°. 55° and 125° are the so-called 'magic angles' at which 3 cos² θ - 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.