Micro-imaging of articular cartilage: T2, proton density, and the magic angle effect.

RATIONALE AND OBJECTIVES This study was designed to determine the relative influences of proton density versus collagen fiber orientation (through its influence on T2) in defining the layers of articular cartilage as seen in long-repetition-time magnetic resonance (MR) images. The authors mapped the T2 and proton densities of articular cartilage at 0 degree and 55 degrees with respect to the main magnetic field (B0) to determine the influence of T2 and water content on the normal laminar appearance of hyaline cartilage. MATERIALS AND METHODS Six patellae of white-tailed deer were imaged at 7 T. T2 and proton densities were calculated from echo time versus signal intensity plots obtained with a multiecho, composite pulse sequence. Regions of interest in the radial and transitional zones were compared with the articular facets at 0 degree and 55 degrees relative to B0. Transmission electron microscopy was performed for correlation. RESULTS At 0 degree, T2 was longer in the transitional than in the radial zone (29 vs 11 msec). AT 55 degrees, T2 increased in both radial and transitional zones, although the difference between the zones decreased (37 vs 29 msec). There was no difference in proton density between the two layers. CONCLUSION Collagen fiber orientation, through T2 effects, is the dominant influence on the appearance of layers in hyaline cartilage in long-repetition-time MR images; proton density is not a major factor, and the collagen fiber orientation in the transitional zone is not totally random.