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

Abstract

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 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. Results Sodium‐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. Conclusion High‐power (short) RF pulses are necessary to reduce excitation‐related signal loss, particularly for sodium‐23 imaging of cartilage and skin.

Combined imaging of potassium and sodium in human skeletal muscle tissue at 7 T

“…Thus, in case , it might be beneficial to use FAs lower than 90° to mitigate this effect 21 . Kordzadeh et al recently observed a mild flip‐angle effect in the head of the gastrocnemius muscle using 23 Na MRI measurements of the knee 49 . However, the examination of the flip‐angle effect is challenging because the additional influences on the signal intensity corrected in this work—for example, caused by T 1 weighting—also vary with the effective FA.…”

“…21 Kordzadeh et al recently observed a mild flip-angle effect in the head of the gastrocnemius muscle using 23 Na MRI measurements of the knee. 49 However, the examination of the flip-angle effect is challenging because the additional influences on the signal intensity corrected in this work-for example, caused by T 1 weighting-also vary with the effective FA. In addition, particularly for the 39 K channel, an inhomogeneous distribution of the effective FA was found (compare Supporting Information Figure S1).…”

Sodium relaxometry using 23Na MR fingerprinting: A proof of concept

“…However, in future research these investigations could be expanded to an even larger range of parameter values by loosening such constraints. A more accurate description of the signal evolution in brain tissue could be achieved by an ISTO simulation using the actual pulses shapes 44 and by modeling a partly anisotropic environment 45 …”

“…However, in future research these investigations could be expanded to an even larger range of parameter values by loosening such constraints. A more accurate description of the signal evolution in brain tissue could be achieved by an ISTO simulation using the actual pulses shapes 44 and by modeling a partly anisotropic environment. 45 As the minimum k-space coverage per time frame is unclear in MRF, a simulation study preceded measurements, in which the minimum number of spokes required for stable relaxation quantification was determined for two different resolutions of 4 × 4 mm 2 and 2 × 2 mm 2 using a numerical head model.…”

Quantitative 23Na‐MRI of the intervertebral disk at 3 T

“…14 Second, as a spin-3/2 nucleus, 23 Na signal poses rapid biexponential transverse relaxation 15 and may also exhibit energy eigenstate shifting (or residual quadrupole splitting) as a result of the local electric field gradients due to the long-lived spatial orientation of the nuclear electric quadrupole moment. 16,17 Third, blurring results from the point spread function and from physiological motion. Fourth, strong partial volume effects because of the large voxel sizes lead to further inaccuracies.…”