Acceleration of Phase-Modulated Ultrashort Echo Time Sequence for Fast T₂ and Adiabatic T_1ρ Mapping of the Knee...

To develop and validate an accelerated phase-modulated ultrashort echo time (PM-UTE) sequence for volumetric T₂ and adiabatic T_1ρ (AdiabT_1ρ) mapping of both short- and long-T₂ tissues in the knee joint.

The proposed method estimates and removes the T₁-dependent component from a single reference acquisition, eliminating repeated phase-cycled scans. PM-UTE-T₂ and PM-UTE-AdiabT_1ρ sequences were implemented on a 3T scanner. Four agarose phantoms, five ex vivo human knees, and five in vivo healthy knees were imaged. Single-exponential fitting was applied to compute T₂ and T_1ρ values in cartilage, meniscus, muscle, patellar tendon, anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL). Agreement between accelerated and conventional PM-UTE methods was evaluated using Pearson correlation and voxel-wise difference maps.

In phantoms, accelerated and conventional mapping showed excellent agreement (T₂: R = 0.99; T_1ρ: R = 0.99). Ex vivo studies demonstrated strong correlations (T₂: R = 0.97; T_1ρ: R = 0.97) across all tissues with minimal voxel-wise differences. In vivo measurements correlated strongly (T₂: R = 0.96; T_1ρ: R = 0.96), with reliable visualization of short- and long-T₂ structures. Across all tissues, T₂ and T_1ρ relaxation times from accelerated method closely matched conventional values, with differences smaller than their respective standard deviations. The acceleration reduced total scan time by ∼1.6-fold across phantom, ex vivo and in vivo experiments.

The accelerated PM-UTE technique enables accurate, efficient whole-knee T₂ and T_1ρ mapping, providing comprehensive multi-tissue characterization within a clinically feasible timeframe. This technique shows promise for early osteoarthritis detection and longitudinal monitoring.