T1ρ‐relaxation mapping of human femoral‐tibial cartilage in vivo

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Abstract
Purpose: To demonstrate the in vivo feasibility of measuring spin‐lattice relaxation time in the rotating frame (T1ρ); and T1ρ‐dispersion in human femoral cartilage. Furthermore, we aimed to compute the baseline T1ρ‐relaxation times and spin‐lock contrast (SLC) maps on healthy volunteers, and compare relaxation times and signal‐to‐noise ratio (SNR) with corresponding T2‐weighted images.Materials and Methods: All MR imaging experiments were performed on a 1.5 T GE Signa scanner (GEMS, Milwaukee, WI) using a custom built 15‐cm transmit‐receive quadrature birdcage radio‐frequency (RF) coil. The T1ρ‐prepared magnetization was imaged with a single‐slice two‐dimensional fast spin‐echo (FSE) pulse sequence preencoded with a three‐pulse cluster consisting of two hard 90° pulses and a low power spin‐lock pulse. T1ρ‐dispersion imaging was performed by varying the spin‐lock frequency from 100 to 500 Hz in five steps in addition to varying the length of the spin‐lock pulse.Results: The average T1ρ‐relaxation times in the weight‐bearing (WB) and nonweight‐bearing (NWB) regions of the femoral condyle were 42.2 ± 3.6 msec and 55.7 ± 2.3 msec (mean ± SD, N = 5, P < 0.0001), respectively. In the same regions, the corresponding T2‐relaxation times were 31.8 ± 1.5 msec and 37.6 ± 3.6 msec (mean ± SD, N = 5, P < 0.0099). T1ρ‐weighted images have ∼20%–30% higher SNR than the corresponding T2‐weighted images for similar echo time. The average SLC in the WB region of femoral cartilage was 30 ±4.0%. Furthermore, SLC maps provide better contrast between fluid and articular surface of femoral‐tibial joint than T1ρ‐maps. The T1ρ‐relaxation times varied from 32 msec to 42 msec (∼31%) in the WB and 37 msec to 56 msec (∼51%) in NWB regions of femoral condyle, respectively, in the frequency range 0–500 Hz (T1ρ‐dispersion).Conclusion: The feasibility of performing in vivo T1ρ relaxation mapping in femoral cartilage at 1.5T clinical scanner without exceeding Food and Drug Administration (FDA) limits on specific absorption rate (SAR) of RF energy was demonstrated. J. Magn. Reson. Imaging 2003;18:336–341.