Reduction of residual dipolar interaction in cartilage by spin-lock technique

The influence of radiofrequency (RF) spin-lock pulse on the laminar appearance of articular cartilage in MR images was investigated. Spin-lock MRI experiments were performed on bovine cartilage plugs on a 4.7 Tesla small-bore MRI scanner, and on human knee cartilage in vivo on a 1.5 Tesla clinical scanner. When the normal to the surface of cartilage was parallel to B-0, a typical laminar appearence was exhibited in T-2-weighted images of cartilage plugs, but was absent in T-1p-weighted images of the same plugs. At the "magic angle" orientation (when the normal to the surface of cartilage was 54.7degrees with respect to B0), neither the T-2 nor the T-1p images demonstrated laminae. At the same time, T-1p values were greater than T-2 at both orientations throughout the cartilage. T-1p dispersion (i.e., the dependence of the relaxation rate on the spin-lock frequency omega(1)) was observed, which reached a steady-state value of close to 2 kHz in both parallel and magic-angle orientations. These results suggest that residual dipolar interaction from motionally-restricted water and relaxation processes, such as chemical exchange, contribute to T-1p dispersion in cartilage. Further, one can reduce the laminar appearance in human articular cartilage by applying spin-lock RF pulses, which may lead to a more accurate diagnosis of degenerative changes in cartilage. Magn Reson Med 52:1103-1109, 2004. (C) 2004 Wiley-Liss, Inc.