Simultaneous acquisition of T1ρ and T2 quantification in knee cartilage: repeatability and diurnal variation.

PURPOSE: To develop a robust sequence that combines T1ρ and T2 quantifications and to examine the in vivo repeatability and diurnal variation of T1ρ and T2 quantifications in knee cartilage. MATERIALS AND METHODS: Six healthy volunteers were scanned in the morning and afternoon on 2 days using a combined T1ρ and T2 quantification sequence developed in this study. Repeatability of T1ρ and T2 quantification was estimated using root-mean-square coefficients-of-variation (RMS-CV). T1ρ and T2 values from morning scans were compared to those from afternoon scans using paired t-tests. RESULTS: The overall RMS-CV of in vivo T1ρ and T2 quantification was 5.3% and 5.2%, respectively. The RMS-CV of am scans was 4.2% and 5.0% while the RMS-CV of pm scans was 6.0% and 6.3% for T1ρ and T2 , respectively. No significant difference was found between T1ρ or T2 values in the morning and in the afternoon. CONCLUSION: A sequence that combines T1ρ and T2 quantification with scan time less than 10 minutes and is robust to B0 and B1 inhomogeneity was developed with excellent repeatability. For a cohort with low-level daily activity, although no significant diurnal variation of cartilage MR relaxation times was observed, the afternoon scans had inferior repeatability compared to morning scans.

Zonal differences in meniscus MR relaxation times in response to in vivo static loading in knee osteoarthritis.

This study assessed the effects of static loading on MRI relaxation times of menisci in individuals with and without radiographic knee OA. High-resolution fast spin-echo (FSE) and T(1ρ)/T(2) relaxation time MR sequences were obtained with and without loading at 50% body weight in 124 subjects. T(1ρ)/T(2) relaxation times were calculated in menisci, and meniscus lesions were assessed through clinical grading. Student's t-test compared OA and control unloaded relaxation times as well as within-group changes with loading, Generalized Linear Models evaluated zonal variation, and ANCOVA compared loading response between groups. Unloaded T(1ρ) and T(2) in the middle and inner zones of the lateral anterior horn and outer zone of the medial posterior horn were significantly higher in OA and suggest that meniscal OA change occurs unevenly. Zonal T(1ρ) and T(2) showed differing patterns between anterior and posterior horns, suggesting differences in macromolecular organization. Significant increases with loading were seen largely in the T(2) of controls and less frequently in subjects with OA. In the medial posterior horn, T(1ρ) and T(2) decreased with loading in OA but changed negligibly in controls; these significantly different loading responses between groups may indicate load transmission failure in OA menisci.

Characterization of human osteoarthritic cartilage using optical and magnetic resonance imaging.

PURPOSE: Osteoarthritis (OA) is a degenerative disease starting with key molecular events that ultimately lead to the breakdown of the cartilage. The purpose of this study is to use two imaging methods that are sensitive to molecular and macromolecular changes in OA to better characterize the disease process in human osteoarthritic cartilage. PROCEDURES: Human femoral condyles were collected from patients diagnosed with severe OA during total knee replacement surgeries. T(1ρ) and T2 magnetic resonance measurements were obtained using a 3-Tesla whole body scanner to assess macromolecular changes in the damaged cartilage matrix. Optical imaging was performed on specimens treated with MMPSense 680 to assess the matrix metalloproteinase (MMP) activity. A linear regression model was used to assess the correlation of MMP optical data with T(1ρ) magnetic resonance (MR) measurements. Slices from a representative specimen were removed from regions with high and low optical signals for subsequent histological analysis. RESULTS: All specimens exhibit high T(1ρ) and T2 measurements in the range of 48-75 ms and 36-69 ms, respectively. They also show intense photon signals (0.376 to 7.89 × 10⁻4 cm²) from the activated MMPSense 680 probe, indicative of high MMP activity. The analysis of variance test of the regression model indicates a positive correlation between the MMP optical signal and T(1ρ) measurements (R² = 0.8936, P = 0.0044). Histological data also confirmed that regions with high MMP optical signal and intense T(1ρ) relaxation exhibit severe clefting, abnormal tidemarks, and irregular cellularity. CONCLUSIONS: The high T(1ρ) and T2 measurements suggest that there is a severe loss of proteoglycans with high water mobility in the damaged cartilage. The intense optical signals found in these specimens indicate the presence of active MMPs, and the positive correlation with T(1ρ) measurements implicates MMP's involvement in OA progression, characterized by a severe loss of proteoglycans in the cartilage matrix. The bimodal approach using optical and MR imaging may provide key molecular and macromolecular information of the disease pathway, offering insights toward the development of new tools for the early detection, treatment, and/or prevention of OA.

Prospective image registration for automated scan prescription of follow-up knee images in quantitative studies.

Consistent scan prescription for MRI of the knee is very important for accurate comparison of images in a longitudinal study. However, consistent scan region selection is difficult due to the complexity of the knee joint. We propose a novel method for registering knee images using a mutual information registration algorithm to align images in a baseline and follow-up exam. The output of the registration algorithm, three translations and three Euler angles, is then used to redefine the region to be imaged and acquire an identical oblique imaging volume in the follow-up exam as in the baseline. This algorithm is robust to articulation of the knee and anatomical abnormalities due to disease (e.g., osteophytes). The registration method is performed only on the distal femur and is not affected by the proximal tibia or soft tissues. We have incorporated this approach in a clinical MR system and have demonstrated its utility in automatically obtaining consistent scan regions between baseline and follow-up examinations, thus improving the precision of quantitative evaluation of cartilage. Results show an improvement with prospective registration in the coefficient of variation for cartilage thickness, cartilage volume and T2 relaxation measurements.