By definition, Grade 0 cartilage has both normal morphology and signal intensity; however, signal intensity of normal cartilage varies with depth from the articular surface , location in the joint , age of the patient , and with the particular pulse sequence used to acquire the image. Recognizing the normal intensity variation in cartilage is necessary to identify focal areas of cartilage injury associated with trauma, and to avoid erroneously interpreting nonuniform signal as disease .
As illustrated in Fig. 2, the signal intensity of articular cartilage on fat-suppressed, PD-weighted FSE images increases toward the articular surface. This is primarily a function of regional and zonal differences in the Type II collagen matrix, which has a strong influence on the T2 relaxation of cartilage [60,61]. In cartilage subjected to repetitive compressive strain, the Type II collagen demonstrates a highly organized zonal architecture , illustrated in Fig 2B. With scanning electron microscopy, the collagen matrix is organized in a leaflike architecture, with the preferential orientation perpendicular to bone in the deep and middle layers, and curving to become parallel to bone at the articular surface . With very high-resolution images, the darker deep and middle zones of cartilage have a striated appearance, with alternating fine bands of high and low signal intensity radiating from the bone cartilage interface [57,63-65]. Closer to the articular surface, the higher water content and lower anisotropy and oblique orientation of the collagen fibers increases the T2 relaxation time, leading to a gradual increase in signal intensity of the superficial zone. At the articular surface, collagen fibers are oriented parallel to the articular surface in a thin layer termed the lamina splendens . This layer is too thin to resolve on standard clinical imaging. In addition to differences in cartilage T2 with respect to depth from the articular surface, there are differences in cartilage T2 with respect to location in the joint and orientation of the collagen fibers to the applied magnetic field [57,66,67]. The thickness of the hyperin-tense superficial zone varies within the joint. For femoral and tibial cartilage, it is thinner in high-load-bearing regions of the joint not covered by the meniscus, and is thicker in the low-load-bearing regions beneath the meniscus. Oblique orientation of the collagen fibers to the magnetic field results in a more uniform elevated signal in cartilage of the anterior and posterior femoral con-dyles . The layers of signal intensity described above are most conspicuous in the patella and tibial plateau . In thin cartilage such as the femoral condyle, ankle, or hip there is generally insufficient spatial resolution to resolve zonal differences in cartilage T2.
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