Assessment of the patellofemoral cartilage: Correlation of knee pain score with magnetic resonance cartilage grading and magnetization transfer ratio asymmetry of glycosaminoglycan chemical exchange saturation transfer

Purpose Biochemical imaging of glycosaminoglycan chemical exchange saturation transfer (gagCEST) could predict the depletion of glycosaminoglycans (GAG) in early osteoarthritis. The purpose of this study was to evaluate the relationship between the magnetization transfer ratio asymmetry (MTR_asym) of gagCEST images and visual analog scale (VAS) pain scores in the knee joint. Materials and methods This retrospective study was approved by the institutional review board. A phantom study was performed using hyaluronic acid to validate the MTR_asym values of gagCEST images. Knee magnetic resonance (MR) images of 22 patients (male, 9; female, 13; mean age, 50.3 years; age range; 25–79 years) with knee pain were included in this study. The MR imaging (MRI) protocol involved standard knee MRI as well as gagCEST imaging, which allowed region-of-interest analyses of the patellar facet and femoral trochlea. The MTR_asym at 1.0 ppm was calculated at each region. The cartilages of the patellar facets and femoral trochlea were graded according to the Outerbridge classification system. Data regarding the VAS scores of knee pain were collected from the electronic medical records of the patients. Statistical analysis was performed using Spearman's correlation. Results The results of the phantom study revealed excellent correlation between the MTR_asym values and the concentration of GAGs (r = 0.961; p = 0.003). The cartilage grades on the MR images showed significant negative correlation with the MTR_asym values in the patellar facet and femoral trochlea (r = −0.460; p = 0.031 and r = −0.543; p = 0.009, respectively). The VAS pain scores showed significant negative correlation with the MTR_asym values in the patellar facet and femoral trochlea (r = −0.435; p = 0.043 and r = −0.671; p = 0.001, respectively). Conclusion The pain scores were associated with the morphological and biochemical changes in articular cartilages visualized on knee MR images. The biochemical changes, visualized in terms of the MTR_asym values of the gagCEST images, exhibited greater correlation with the pain scores than the morphological changes visualized on conventional MR images; these results provide evidence supporting the theory regarding the association of patellofemoral osteoarthritis with knee pain scores.