Abstract
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~1ρ~‐weighted images of the same plugs. At the “magic angle” orientation (when the normal to the surface of cartilage was 54.7° with respect to B~0~), neither the T~2~ nor the T~1ρ~ images demonstrated laminae. At the same time, T~1ρ~ values were greater than T~2~ at both orientations throughout the cartilage. T~1ρ~ dispersion (i.e., the dependence of the relaxation rate on the spin‐lock frequency ω~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~1ρ~ 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. © 2004 Wiley‐Liss, Inc.