Whole knee joint mapping using a phase modulated UTE adiabatic T_1ρ (PM-UTE-AdiabT_1ρ ) sequence

Purpose: To develop a 3D phase modulated UTE adiabatic T_1ρ (PM-UTE-AdiabT_1ρ ) sequence for whole knee joint mapping on a clinical 3 T scanner.

Methods: This new sequence includes six major features: (1) a magnetization reset module, (2) a train of adiabatic full passage pulses for spin locking, (3) a phase modulation scheme (i.e., RF cycling pair), (4) a fat saturation module, (5) a variable flip angle scheme, and (6) a 3D UTE Cones sequence for data acquisition. A simple exponential fitting was used for T_1ρ quantification. Phantom studies were performed to investigate PM-UTE-AdiabT_1ρ 's sensitivity to compositional changes and reproducibility as well as its correlation with continuous wave-T_1ρ measurement. The PM-UTE-AdiabT_1ρ technique was then applied to five ex vivo and five in vivo normal knees to measure T_1ρ values of femoral cartilage, meniscus, posterior cruciate ligament, anterior cruciate ligament, patellar tendon, and muscle.

Results: The phantom study demonstrated PM-UTE-AdiabT_1ρ 's high sensitivity to compositional changes, its high reproducibility, and its strong linear correlation with continuous wave-T_1ρ measurement. The ex vivo and in vivo knee studies demonstrated average T_1ρ values of 105.6 ± 8.4 and 77.9 ± 3.9 ms for the femoral cartilage, 39.2 ± 5.1 and 30.1 ± 2.2 ms for the meniscus, 51.6 ± 5.3 and 29.2 ± 2.4 ms for the posterior cruciate ligament, 79.0 ± 9.3 and 52.0 ± 3.1 ms for the anterior cruciate ligament, 19.8 ± 4.5 and 17.0 ± 1.8 ms for the patellar tendon, and 91.1 ± 8.8 and 57.6 ± 2.8 ms for the muscle, respectively.

Conclusion: The 3D PM-UTE-AdiabT_1ρ sequence allows volumetric T_1ρ assessment for both short and long T₂ tissues in the knee joint on a clinical 3 T scanner.