Magic-angle effect in magnetic resonance imaging of articular cartilage: a review.

RATIONALE AND
OBJECTIVES: The laminar appearance of articular cartilage in magnetic resonance (MR) images has been a source of confusion, especially concerning the number, intensity, thickness, and origin of the layers. The laminar appearance is associated with the magic-angle effect in the MR imaging (MRI) of articular cartilage.
METHODS: This article introduces the topic with background information about cartilage and the magic-angle effect and then reviews the literature about the magic-angle effect. The review concludes with a brief discussion of the future directions of study and the potential clinical relevance of the laminae in MR images of articular cartilage.
CONCLUSIONS: The magic-angle effect is commonly seen in MR images of several tissues. The direct cause of the laminar appearance of articular cartilage is the T2 relaxation anisotropy in the tissue, which is closely linked to the structure of the collagen fibers, their orientation in the magnetic field, and the water-proteoglycan interaction that amplifies the prevailing orientation of the collagen fiber network. The laminar appearance of cartilage has an intrinsic spatial heterogeneity over the two-dimensional joint surface, which leads to inconsistencies in the reported total number of cartilage laminae and the laminar patterns observable in MRI, depending on where the sample was taken. Two additional thin, low-intensity laminae may also be visible at the boundaries of the cartilage with fluid and with bone; whether these boundary laminae are identified and counted with the others may introduce inconsistency in the results reported by various researchers.