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

Purpose: To demonstrate the in vivo feasibility of measuring spin-lattice relaxation time in the rotating frame (T-1rho); and T-1rho-dispersion in human femoral cartilage. Furthermore, we aimed to compute the baseline T-1rho-relaxation times and spin-lock contrast (SLC) maps on healthy volunteers, and compare relaxation times and signal-to-noise ratio (SNR) with corresponding T-2-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 T-1rho-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 90degrees pulses and a low power spin-lock pulse. T-1rho-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 T-1rho-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 T-2-relaxation times were 31.8 +/- 1.5 msec and 37.6 +/- 3.6 msec (mean SD, N = 5, P < 0.0099). T-1rho-weighted images have similar to20%-30% higher SNR than the corresponding T-2-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 T-1rho-maps. The T-1rho-relaxation times varied from 32 msec to 42 msec (similar to31%) in the WB and 37 msec to 56 msec (similar to51%) in NWB regions of femoral condyle, respectively, in the frequency range 0-500 Hz (T-1rho-dispersion). Conclusion: The feasibility of performing in vivo T-1rho 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.