T2 relaxation time of cartilage at MR imaging: comparison with severity of knee osteoarthritis

Abstract

Purpose: To evaluate differences in T2 values in femoral and tibial cartilage at magnetic resonance (MR) imaging in patients with varying degrees of osteoarthritis (OA) compared with healthy subjects and to develop a mapping and display method based on calculation of T2 z scores for visual grading and assessment of cartilage heterogeneity in patients with OA.

Materials and methods: Knee cartilage was evaluated in 55 subjects who were categorized with radiography as healthy (n = 7) or as having mild OA (n = 20) or severe OA (n = 28). Cartilage regions were determined with manual segmentation of an MR image acquired with spoiled gradients and fat suppression. The segmentation was applied to a map of T2 relaxation time and was analyzed in four knee cartilage compartments (ie, the medial and lateral tibia and femur). Differences between cartilage compartment T2 values and subject groups were analyzed with analysis of covariance. Correlations of cartilage T2 values with clinically reported symptoms and cartilage thickness and volume were examined. Cartilage T2 values were converted to z scores per voxel on the basis of normal population values in the same cartilage compartment to better interpret cartilage heterogeneity and variation from normal.

Results: Healthy subjects had mean T2 values of 32.1-35.0 msec, while patients with mild and severe OA had mean T2 values of 34.4-41.0 msec. All cartilage compartments except the lateral tibia showed significant (P <.05) increases in T2 relaxation time between healthy and diseased knees; however, no significant difference was found between patients with mild and severe OA. Correlation of T2 values with clinical symptoms and cartilage morphology was found predominantly in medial compartments.

Conclusion: Femoral and medial tibial cartilage T2 values increase with the severity of OA.

Figure 1

Example of registration between the T2-weighted MR image and the cartilage mask in six subjects (ie, two from each group). Sagittal view of femoral and tibial compartments outlined over the first echo of the dual two-dimensional spin-echo MR image (1,500/10 and 45; voxel size, 0.468 × 0.468 × 4 mm; examination time, 5 minutes 24 seconds; field of view, 12 cm; matrix, 256 × 256). Note that these are only single sections of the entire knee volume and may not reflect specific regions of disease that account for OA grade.

Figure 2

Graph shows distribution of T2 values in the medial femur cartilage compartment in 76-year-old man with severe OA. pct percentile.

Figure 3

Representative z score conversion sagittal MR images obtained in six subjects (ie, two from each group) (1,500/10 and 45; voxel size, 0.468 × 0.468 × 4 mm; examination time, 5 minutes 24 seconds; field of view, 12 cm; matrix, 256 × 256). Note the increase in area of regions of high z scores (indicated by yellow areas) in patients with mild and severe OA. Note that these are only single sections of the entire knee volume and may not reflect specific regions of disease that account for OA grade.