Purpose: Anisotropic transverse R2 (1/T2 ) relaxation of water proton is sensitive to cartilage degenerative changes. The purpose is to develop an efficient method to extract this relaxation metric in clinical studies.
Methods: Anisotropic R2 can be measured inefficiently by standard R2 mapping after removing an isotropic contribution obtained from R1ρ mapping. In the proposed method, named as a unique anisotropic R2 of collagen degeneration (ARCADE) mapping, an assumed uniform isotropic R2 was estimated at magic angle locations in the deep cartilage, and an anisotropic R2 was thus isolated in a single T2W sagittal image. Five human knees from 4 volunteers were studied with standard R2 and R1ρ mappings at 3T, and anisotropic R2 derived from ARCADE on the T2W (TE = 48.8 ms) image from R2 mapping was compared with the composite relaxation (R2 - R1ρ ) using statistical analysis including Student's t-test and Pearson's correlation coefficient.
Results: Anisotropic R2 (1/s) from ARCADE was highly positively correlated with but not significantly different from standard R2 - R1ρ (1/s) in the segmented deep (r = 0.83 ± 0.06; 8.3 ± 2.9 vs. 7.3 ± 1.9, P = .50) and the superficial (r = 0.82 ± 0.05; 3.5 ± 2.4 vs. 4.5 ± 1.6, P = .39) zones. However, after eliminating systematic errors by the normalization in terms of zonal contrast, anisotropic R2 was significantly higher (60.2 ± 18.5% vs. 38.4 ± 16.6%, P < .01) than R2 - R1ρ as predicted.
Conclusion: The proposed anisotropic R2 mapping could be an efficient alternative to the conventional approach, holding great promise in providing both high-resolution morphological and more sensitive transverse relaxation imaging from a single T2W scan in a clinical setting.
Keywords: anisotropic R2; chemical exchange effect; composite relaxation metric R2 − R1ρ; human knee cartilage; magic angle effect.
© 2019 International Society for Magnetic Resonance in Medicine.