Radiofrequency Excitation-Related 23 Na MRI Signal Loss in Skeletal Muscle, Cartilage, and Skin.

PurposeTo assess the sodium MRI signal loss resulting from typically used RF excitation pulses in human skeletal muscle, patellar cartilage, and skin.MethodsA double flip‐angle experiment was performed 3 times on the knees of 5 healthy volunteers with prescribed ω1 = γB1 of 1.67 kHz, 0.333 kHz, and 0.167 kHz. This was done to search for ω1‐dependent increased rates of sodium‐23 central resonance flipping known to result from residual quadrupole splitting (ωQ), as this flip‐angle effect is associated with signal loss. This study facilitated in vivo regression of Gaussian‐distributed residual quadrupole splitting SD (ωQ(SD)) as well as T2fast and T2slow. Signal loss predicted from simulation was then compared with images acquired using 90° RF pulse lengths of 0.5 ms, 0.25 ms, and 0.15 ms.ResultsSodium‐23 central resonance flipping was significantly greater than prescribed (44% cartilage, 23% skin, 9% muscle) using ω1 = 0.167 kHz, but only 4% cartilage, 5% skin, and 2% muscle using ω1 = 1.67 kHz. Regression yielded ωQ(SD) = 420 ± 50 Hz for cartilage but no significant ωQ(SD) for skin or muscle. This points to rapid biexponential relaxation as the cause of the flip‐angle effect for skin/muscle. The T2fast(60%)/T2slow(40%) values were 1.6 ± 0.8 ms/16.1 ± 2.5 ms for muscle, 2.7 ± 0.9 ms/18.4 ± 2.5 ms for cartilage, and 0.4 ± 0.1 ms/9.3 ± 1.7 ms for skin. Simulation predicted signal loss of 6% ± 3%, 3% ± 1%, and 2% ± 1% for muscle, 16% ± 3%, 6% ± 1%, and 3% ± 1% for cartilage, and 26% ± 7%, 15% ± 4%, and 10% ± 3% for skin when using 90° RF pulse lengths of 0.5 ms, 0.25 ms, and 0.15 ms, matching experiment.ConclusionHigh‐power (short) RF pulses are necessary to reduce excitation‐related signal loss, particularly for sodium‐23 imaging of cartilage and skin.