Abstract
Developmental dysplasia of the hip (DDH) is a common risk factor of early osteoarthritis (OA), with insufficient coverage of the femoral head by the acetabulum which leads to excessive cartilage stresses in the hip joint. Knowledge of the molecular health of cartilage using MRI may diagnose and stage chondral disease, but more importantly allows for treatment stratification and prognostication. Delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) is a validated MRI technique for detecting early loss of proteoglycan (PG). However, it requires an injection of contrast agent and exercise prior to the scan. MRI techniques such as T1ρ and T2 mapping have also been shown to be sensitive to early biochemical changes in cartilage but can be performed without any contrast injection. In this study we evaluate three quantitative MR techniques (dGEMRIC, T1ρ and T2 mapping) in patients with DDH. Our hypothesis is that both T1ρ and T2 correlate with dGEMRIC, and thus may be effective non-contrast based techniques for biochemical cartilage mapping in DDH hips.
Seven informed and consented patients (mean age: 31.1 years) with DDH were enrolled in this IRB approved MRI study before surgery. DDH was defined as a lateral center-edge angle under 25º and acetabular index >13º on the plain x-ray. All subjects underwent two successive MRI sessions at 3T: In the first cartilage T1ρ and T2 mapping were performed. After leaving the scanner the subjects were injected with 0.4ml/kg Dotarem (i.v.), walked for 15min and rested for 25min before returning into the MRI. dGEMRIC (T1post) mapping was initiated approximately 45min after the injection. Image post-processing, registration and cartilage segmentation was performed with Matlab. The joint was subdivided into anterior and posterior regions in the sagittal plane and into lateral, intermediate and medial zones in the transverse plane, resulting in six region of interest (ROIs): antero-lateral, antero-intermediate, antero-medial, postero-lateral, postero-intermediate and postero-medial. The correlation between the dGEMRIC and T1ρ and dGEMRIC and T2 were evaluated using Spearman's Rho and tested for significance.
The analysis of all six cartilage ROIs for all subjects resulted in a significant (p < 0 .001) negative correlation (Rho = −0.50) between the dGEMRIC index (T1post) and the T1ρ relaxation time. The dGEMRIC index and T2 correlated positive (Rho = 0.55) and significant (p < 0 .001).
Although this pilot study has a small sample size a negative correlation between dGEMRIC and T1ρ was found in patients with DDH. Both methods are known to probe the PG content of cartilage, where a decreased PG content leads to lower dGEMRIC index and an increased T1ρ value. The correlation coefficient was moderate, but significant, which shows that T1ρ mapping as an effective tool to probe the cartilage PG content similar to dGEMRIC. A comparable, but positive correlation was found between dGEMRIC and T2. T2 is sensitive to the cartilage collagen content with a decreased T2 value in degenerated cartilage. In symptomatic DDH, where an onset of OA is assumed, both PG depletion and collagen decay are in progress and can be evaluated using these mapping techniques.
