VARIATION IN MR SIGNAL INTENSITY ACROSS NORMAL HUMAN KNEE CARTILAGE

Signal intensity (SI) of individual pixels on sagittal magnetic resona nce (MR) images of normal human knee cartilage was quantified to inves tigate whether it was related to cartilage proteoglycan content. In fi ve subjects, images were acquired with spin-echo sequences with a TR m sec/TE msec of 1,000 or 700/20 and a three-dimensional gradient-echo ( GRE) sequence (60/15). In a sixth subject, the GRE sequence alone was used with 15-degrees, 30-degrees, and 50-degrees flip angles. In all s ubjects, SI was maximal in pixel layers of the medial zone and minimal at both cartilage edges, resulting in the presence of a bell-shaped c urve of interpixel (zonal) SI variation across the cartilage thickness . The magnitude of SI was dependent on the pulse sequence and flip ang le, but the bell shape of the SI variation curve was independent of th em. For example, in the medial tibial cartilage, the peak SI was highe st with the 1,000/20 spin-echo sequence, intermediate with the 700/20 sequence, and lowest with the GRE sequence. The differences were stati stically significant. The bell-shaped SI variation curve resembled the curve for zonal variation in cartilage proteoglycan content but not t he curves for collagen or free water content. The physiologic basis fo r this resemblance and the potential usefulness of the findings for ea rly diagnosis of diseases such as osteoarthritis are discussed.