Magnetic Resonance Parameters in Quantitative Evaluation of Articular Cartilage

Articular cartilage is highly specialized avascular tissue that covers the articulating ends of bones. Its main function is to enable smooth and effortless movement of joints while transmitting and distributing loads from bone to bone. The regenerative capability of articular cartilage is limited. Since the tissue is aneural, there will be no signs of advancing joint disease, such as osteoarthritis, until it has reached an incurable state. Thus, a method to provide an early diagnosis would be of help in designing effective interventions to stop or at least slow down disease progression. As no disease modifying osteoarthritis drugs are currently available, sensitive diagnostics would also support the development of novel treatment strategies. Specific magnetic resonance imaging (MRI) methods, such as T2 relaxation time mapping and delayed gadolinium enhanced MRI of cartilage (i.e. dGEMRIC) have been proposed as ways of obtaining quantitative evaluation of articular cartilage. This study aims at the further development of these methods; to study how the specific MRI parameters reflect the structural and functional properties of articular cartilage and to address the potential limitations, such as the significance of precontrast T1 and the assumption of constant relaxivity of the contrast agent. For this purpose, relaxation properties of intact and spontaneously degenerated human and animal cartilage were studied at 9.4 T with reference to the biomechanical, biochemical and histological properties which were measured using well-established quantitative methods. The cartilage structure was assessed using quantitative polarized light microscopy (PLM). The concentration of Gd-DTPA in articular cartilage was evaluated by localized cryo-cell laser ablation inductively coupled plasma mass spectrometry (CC-LA-ICP-MS). The T2 relaxation time was found to be sensitive to both maturation related changes of cartilage and the structural integrity of the tissue. The structural properties of the collagen fibril network in articular cartilage, as detected by PLM, were clearly detectable by T2 mapping. In spontaneous degeneration, the T2 relaxation time showed a significant increase, especially in the superficial part of the tissue (increase from 45 ± 15 ms to 128 ± 109 ms, p < 0.05) with a concomitant change in its biochemical and biomechanical properties. The T1 relaxation time in the presence of Gd-DTPA 2− contrast agent, i.e. the dGEMRIC index, changed with cartilage degeneration and was closely related to the cartilage proteoglycan content (R = 0.817, p < 0.01) and water content (R = −0.870, p < 0.01). The concentration of Gd-DTPA, as measured by CC-LA-ICP-MS exhibited variations between different joint surfaces and along the cartilage depth, with a significantly higher Gd-DTPA concentration in tibial cartilage. The present results indicate that the relaxation properties are related to the structure and composition of articular cartilage and can provide information on cartilage degeneration in a non-invasive manner. The results also suggest that certain factors, such as the relaxivity of Gd-DTPA should be further considered when conducting dGEMRIC experiments. Furthermore, T2 experiments may provide the possibility to estimate the maturation of articular cartilage, via assessment of the related changes in tissue structure. By making certain assumptions, the functional properties of articular cartilage may be assessed using relaxation properties. Carefully controlled clinical studies are needed to establish the feasibility of the present MRI parameters, to improve the diagnosis of early osteoarthritis and to support the development of disease modifying treatment strategies. National Library of Medicine Classification: WE 141, WE 300, WE 304, WE 348, WN 160, WN 185 Medical Subject Headings: Joint Diseases/diagnosis; Osteoarthritis/diagnosis; Cartilage, Articular; Magnetic Resonance Imaging; Contrast Media; Gadolinium DTPA; Biomechanics; Microscopy, Polarization; Collagen; Proteoglycans Stud Kuopio University Publications C. Natural and ge: