Spatial variation in T1 of healthy human articular cartilage of the knee joint

The longitudinal relaxation time T(1) of native cartilage is frequently assumed to be constant. To redress this, the spatial variation of T(1) in unenhanced healthy human knee cartilage in different compartments and cartilage layers was investigated. Knees of 25 volunteers were examined on a 1.5 T MRI system. A three-dimensional gradient-echo sequence with a variable flip angle, in combination with parallel imaging, was used for rapid T(1) mapping of the whole knee. Regions of interest (ROIs) were defined in five different cartilage segments (medial and lateral femoral cartilage, medial and lateral tibial cartilage and patellar cartilage). Pooled histograms and averaged profiles across the cartilage thickness were generated. The mean values were compared for global variance using the Kruskal-Wallis test and pairwise using the Mann-Whitney U-test. Mean T(1) decreased from 900-1100 ms in superficial cartilage to 400-500 ms in deep cartilage. The averaged T(1) value of the medial femoral cartilage was 702+/-68 ms, of the lateral femoral cartilage 630+/-75 ms, of the medial tibial cartilage 700+/-87 ms, of the lateral tibial cartilage 594+/-74 ms and of the patellar cartilage 666+/-78 ms. There were significant differences between the medial and lateral compartment (p<0.01). In each cartilage segment, T(1) decreased considerably from superficial to deep cartilage. Only small variations of T(1) between different cartilage segments were found but with a significant difference between the medial and lateral compartments.