Biochemical (T2, T2* and magnetisation transfer ratio) MRI of knee cartilage: Feasibility at ultra-high field (7T) compared with high field (3T) strength

Objective: This study compares the performance and the reproducibility of quantitative T2, T2* and the magnetisation transfer ratio (MTR) of articular cartilage at 7T and 3T. Methods: Axial MRI of the patella was performed in 17 knees of healthy volunteers (25.8±5.7 years) at 3T and 7T using a comparable surface coil and whole-body MR systems from the same vendor, side-by-side. Thirteen knee joints were assessed once, and four knee joints were measured three times to assess reproducibility. T2 relaxation was prepared by a multi-echo, spin-echo sequence and T2* relaxation by a multi-echo, gradient-echo sequence. MTR was based on a magnetisation transfer-sensitized, steady-state free precession approach. Statistical analysis-of-variance and coefficient-of-variation (CV) were prepared. Results: For T2 and T2*, global values were significantly lower at 7T compared with 3T; the zonal evaluation revealed significantly less pronounced stratification at 7T (p<0.05). MTR provided higher values at 7T (p<0.05). CV, indicating reproducibility, showed slightly lower values at 7T, but only for T2 and T2*. Conclusion: Although lower T2 and T2* relaxation times were expected at 7T, the differences in stratification between the field strengths were reported for the first time. The assessment of MT is feasible at 7T, but requires further investigation.