MR imaging of knee cartilage with FEMR

Objective: Fluctuating equilibrium magnetic resonance (FEMR) is a rapid three-dimensional (3D) imaging sequence with high signal-to-noise ratio (SNR). FEMR may be useful for detecting cartilage defects in the knee. At 1.5 T, FEMR uses a TR with odd multiples of 2.2 ms for fat/water separation. With a TR of 6.6 ms, high-resolution 3D imaging of cartilage is possible. Design and patients: The knees of 10 volunteers and two patients were imaged on a GE Signa 1.5 T scanner using an extremity coil. Scans were preceded by a shimming sequence optimizing linear terms. Subjects were imaged with FEMR, proton-density fast spin-echo (PD-FSE), T2-weighted fast spin-echo (T2-FSE), and 3D fat-suppressed spoiled-gradient-recalled echo (3D-SPGR). Results: SNR and contrast-to-noise efficiency measurements for cartilage using FEMR were superior to those using PD-FSE, T2-FSE, and 3D-FS-SPGR. FSE images showed bright synovial fluid with limited cartilage detail. 3D-SPGR had comparable resolution to FEMR but suboptimal cartilage/fluid contrast and longer scan times (9 min versus 2 min). Cartilage surface detail, outlined by bright synovial fluid, was best seen on the FEMR images. Discussion: FEMR obtains high-resolution 3D images of the entire knee in 2 min with excellent cartilage/fluid contrast. FEMR is sensitive to field inhomogeneity and requires shimming. Surface defects are outlined by bright synovial fluid, and cartilage has higher signal-to-noise efficiency compared with PD-FSE, T2-FSE, and 3D-SPGR techniques.