Radiofrequency excitation–related ²³Na MRI signal loss in skeletal muscle, cartilage, and skin

Purpose To assess the sodium MRI signal loss resulting from typically used RF excitation pulses in human skeletal muscle, patellar cartilage, and skin. Methods A double flip‐angle experiment was performed 3 times on the knees of 5 healthy volunteers with prescribed ω 1 = γ B 1 of 1.67 kHz, 0.333 kHz, and 0.167 kHz. This was done to search for ω 1 ‐dependent increased rates of sodium‐23 central resonance flipping known to result from residual quadrupole splitting ( ω Q ), as this flip‐angle effect is associated with signal loss. This study facilitated in vivo regression of Gaussian‐distributed residual quadrupole splitting SD ( ω Q (SD) ) as well as T 2fast and T 2slow . 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. Results Sodium‐23 central resonance flipping was significantly greater than prescribed (44% cartilage, 23% skin, 9% muscle) using ω 1 = 0.167 kHz, but only 4% cartilage, 5% skin, and 2% muscle using ω 1 = 1.67 kHz. Regression yielded ω Q (SD) = 420 ± 50 Hz for cartilage but no significant ω Q (SD) for skin or muscle. This points to rapid biexponential relaxation as the cause of the flip‐angle effect for skin/muscle. The T 2fast(60%) /T 2slow(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. Conclusion High‐power (short) RF pulses are necessary to reduce excitation‐related signal loss, particularly for sodium‐23 imaging of cartilage and skin.