T2*Of Calcified Cartilage And Osteochondral Junction At 3 Tesla And 7 Tesla Field Strength And Histological Correlation

Purpose: Understanding the pathophysiology of osteoarthritis (OA) requires a clear definition of the tissues underneath the articular cartilage, including the osteochondral junction (OJ) or tidemark and calcified cartilage (CC). Calcified cartilage begins to advance into the articular cartilage in late-stage disease, leaving a footprint of multiple tidemarks as the mineralization front advances. These findings can be detected histologically whereas MR assessment becomes challenging, due to size (width) and the tissues rapid relaxation time. The aim of this study was to compare a quantitative T2*, thickness of CC and OJ using vTE at 3T and 7T, as well as to correlate with histomorphometry. Methods: Six ex-vivo human knees were used in the study (mean age 76 +/- 7 years). The ex-vivo knees were examined with a 3T whole-body system (Tim-Trio, Siemens Healthcare, Erlangen, Germany) and 7T investigational whole body system (Magnetom 7Tesla, Siemens Healthcare, Erlangen, Germany) using similar 8-channel knee coils. T2* maps were calculated from an isotropic 3D multi-echo vTE-sequence using ten sequentially shifted echo times TE =[0.75, 3.51, 5.87, 8.23, 10.6, 12.96, 15.33, 17.69, 20.06, 22.42] ms using a mono-exponential fit least square analysis performed in IDL 6.3 (Interactive Data Language, Research Systems, Inc, Boulder, CO, USA). The fitting function was S=S0×e-TE/T2* + offset, where offset was estimated from the TE=0.75ms image noise. The thickness measurements were performed on subtracted vTE images with TE=0.75ms and TE=22.42ms with highlighted fast relaxing tissues (optimized for T2* in range of 1 to 10 ms) in JiveX software (JiveX 4.3, VISUS Technology Transfer GmbH, Bochum, Germany). Regions of Interest (ROI) on femoral condyle cartilage (medial and lateral) were drawn on the weight-bearing zone and transferred to T2* maps. Sagittal sections were stained with H&E and Safranin-O Fast Green, examined and graded by Mankin score. Outcome measurements included: Mean T2* relaxation times at 7T and 3T, CC and OJ thickness measurements from T2*, histomorphometry and Mankin scores. Results: Mean T2* relaxation times (ms) were lower at 7T (mean: 5.6 ± 3.99) compared with 3T (mean: 7.4 ± 3.4) (p<0.06). CC and OJ thickness measured from substracted vTE images were significantly higher at 3T compared with 7T (p<0.047). The correlation between T2* at 7T and OJ thickness was r=-0.629. The correlation between T2* at 7T and CC and OJ thickness measured by substracted vTE images was r=-0.576. Mean Mankin score was 5.5 ± 1.8. The correlation between CC and OJ thickness measured by substracted vTE images and Mankin was r= 0.593. Conclusions: A decrease of T2* was expected at the higher field strength because magnetic field inhomogeneities and susceptibility effects are much more pronounced at 7T. Seven Tesla vTE sequences showed higher correlation with histomorphometric measurements and Mankin scores than 3T. Three-dimensional vTE sequence on 3T and 7T field strength showed feasibility to precisely assess the osteochondral junction and calcified cartilage in an ex-vivo knee. Since these tissues have a very short T2 relaxation time (∼ 5-10 ms), vTE sequences allow accurate calculation of T2* in these challenging tissues.