Changes in tissue sodium concentration and sodium relaxation times during the maturation of human knee cartilage: Ex vivo ²³Na MRI study at 10.5 T

Purpose: To evaluate the influence of skeletal maturation on sodium (Na-23) MRI relaxation parameters and the accuracy of tissue sodium concentration (TSC) quantification in human knee cartilage.Methods: Twelve pediatric knee specimens were imaged with whole-body 10.5 T MRI using a density-adapted 3D radial projection sequence to evaluate Na-23 parameters: B-1(+), T-1, biexponential T-2*, and TSC. Water, collagen, and sulfated glycosaminoglycan (sGAG) content were calculated from osteochondral biopsies. The TSC was corrected for B-1(+), relaxation, and water content. The literature-based TSC (TSCLB) used previously published values for corrections, whereas the specimen-specific TSC (TSCSP) used measurements from individual specimens. Na-23 parameters were evaluated in eight cartilage compartments segmented on proton images. Associations between Na-23 parameters, TSCLB - TSCSP difference, biochemical content, and age were determined.Results: From birth to 12 years, cartilage water content decreased by 18%; collagen increased by 59%; and sGAG decreased by 36% (all R-2 >= 0.557). The short T-2* (T-2S*) decreased by 72%, and the signal fraction relaxing with T-2S* (fT(2S)*) increased by 55% during the first 5 years but remained relatively stable after that. TSCSP was significantly correlated with sGAG content from biopsies (R-2 = 0.739). Depending on age, TSCLB showed higher or lower values than TSCSP. The TSCLB - TSCSP difference was significantly correlated with T2*S (R-2 = 0.850), fT(2S)* (R-2 = 0.651), and water content (R-2 = 0.738).Conclusion: TSC and relaxation parameters measured with Na-23 MRI provide noninvasive information about changes in sGAG content and collagen matrix during cartilage maturation. Cartilage TSC quantification assuming fixed relaxation may be feasible in children older than 5 years.