Fat-suppressed quantitative ultrashort echo time magnetization transfer (UTE-MT) imaging of the knee joint

Objective To develop a simple fat-suppression strategy for more accurate quantitative ultrashort echo time magnetization transfer (UTE-MT) imaging of knee joint tissues. Materials and methods A narrow-bandwidth RF pulse centered on the fat peak was utilized for fat-selective imaging. These fat-selective images were subsequently subtracted from the MT-weighted 3D-UTE images to mitigate fat contamination in knee joint tissue imaging. This method was evaluated in an ex vivo human knee specimen and in three healthy volunteers at 3 T. Voxel-wise and ROI-based quantitative MT modeling were both performed to estimate macromolecular fraction (MMF), and results with and without fat suppression were compared. Results Fat-selective images revealed off-resonance artifacts from surrounding fatty tissues that extended into adjacent joint structures. Subtraction of the fat-selective images effectively suppressed these artifacts and improved visualization of cartilage, meniscus, and tendons. In ex vivo data, fat suppression stabilized MMF estimates by correcting voxels with abnormally elevated values (> 50%). MMF values were increased after fat suppression in articular cartilage (11.9 ± 1.3% vs. 17.3 ± 1.5%), meniscus (15.9 ± 0.8% vs. 18.2 ± 1.9%), patellar cartilage (13.5 ± 1.0% vs. 18.3 ± 0.4%), and patellar tendon (17.4 ± 2.8% vs. 24.6 ± 0.4%). Tissue contrast-to-noise ratios were also markedly improved. Discussion Subtraction of fat-selective images provides a simple and effective approach for suppressing fat interference in UTE-MT imaging, improving both image contrast and MT measurements.