Determination of Tissue Potassium and Sodium Concentrations in Dystrophic Skeletal Muscle Tissue Using Combined Potassium (39K) and Sodium (23Na) MRI at 7 T.

Combined ²³Na/³⁹K MRI at 7 T can highlight ion disturbances in skeletal muscle tissue. In this work, we investigated if the apparent tissue potassium concentration (aTPC) can be determined in fatty replaced muscles of patients with facio-scapulo-humeral muscular dystrophy (FSHD) and if it can provide additional information to the fat replacement and the apparent tissue sodium concentration (aTSC). The lower leg of 14 patients (six females, eight males; mean age 47.7 ± 14.0 years) with genetically confirmed FSHD and 11 healthy controls (four females, seven males; mean age 47.0 ± 14.0 years) was examined at a 7-T MR system using a dual-tuned ²³Na/³⁹K birdcage RF coil. In addition, qualitative and quantitative ¹H MR measurements were performed at 7 T to assess the fat replacement and water accumulation. The aTPC and aTSC were determined in seven different muscle regions based on five external references phantoms and corrected for partial volume effects, relaxation biases, and reduced ion concentrations in fat. Results are expressed as median (interquartile range). The measured aTPC was strongly reduced in fat-replaced muscles and was close to zero in totally fat replaced muscles (aTPC = 4.3 mM [2.7 mM] for FF > 80%). After correction of aTPC values for reduced potassium concentration in fat, aTPC_fc values of patients in muscles with low or moderate fat fraction (FF < 30%) were similar to values of healthy subjects (patients: aTPC_fc = 85.6 mM [21.7 mM]; controls: aTPC_fc = 83.2 mM [22.3 mM]). However, muscles with FF > 30% showed reduced aTPC_fc and increased aTSC_fc compared with healthy controls (aTPC_fc = 28.9 mM [46.2 mM], aTSC_fc = 42.3 mM [17.6 mM]; controls: aTSC_fc = 15.0 mM [4.6 mM], aTPC_fc = 83.2 mM [22.3 mM]). No correlations were observed between the aTPC_fc and aTSC_fc, or between aTPC_fc and water T₂. We showed that a determination of the aTPC in dystrophic skeletal muscles is feasible using ³⁹K MRI at 7 T. Measured changes in aTPC_fc were greater than sole fat replacement and might therefore be used as an additional quantitative measure for dystrophic muscle tissue.