Feasibility of 7 T 39K/23Na Magnetic Resonance Imaging for assessing muscular ion balance in hypokalemic periodic paralysis

Objectives Recently introduced potassium-39-( 39 K)-MRI provides a noninvasive approach to assess typically high intracellular K + -levels and can be combined with sodium-23-( 23 Na)-MRI. The aim of this study was to evaluate 39 K/ 23 Na muscle ion homeostasis in hypokalemic periodic paralysis (HypoPP), a rare muscular ion channelopathy, using 7 T MRI. Materials and Methods Lower legs of patients with HypoPP and healthy controls were prospectively examined between August 2022 and July 2023 (case-control study). Scanning protocol at 3 T included T 1 -weighted, T 2 -weighted STIR sequences, a 6-point-Dixon-type gradient echo and a T2-mapping sequence. 39 K/ 23 Na data were acquired at 7 T using acquisition-weighted Stack-of-Stars sequences. Apparent tissue 39 K/ 23 Na concentrations (aTPC/aTSC) were calculated after correcting for partial-volume and relaxation effects and corrected for proton-density fat-fractions to account for fatty replacement. A 23 Na-inversion-recovery ( 23 Na-IR) sequence served to introduce a stronger intracellular weighting. Differences in central tendency between the HypoPP and control groups and correlations were analyzed. Results Thirteen HypoPP-participants and 13 controls were included. Extent of fatty replacement/edema-like changes varied highly with the gastrocnemius medialis muscle most affected. The HypoPP group showed significantly increased aTSC in all 7 analyzed muscles and decreased aTPC in 3 specific muscles. Across all muscles, the mean aTSC was higher in the HypoPP group (median: 33.4 vs 22.5 mM, mean ± SD: 34.3 ± 6.8 vs 21.0 ± 4.8 mM, P < 0.001), whereas the mean aTPC was lower (98.7 vs 109.0 mM, 97.9 ± 12.0 vs 108.7 ± 10.4 mM, P = 0.02). The 23 Na-IR signal was strongly correlated with aTSC (r = 0.77, P < 0.001). Conclusions Combined 39 K/ 23 Na MRI at 7 T demonstrated alterations of sodium and potassium ion homeostasis in HypoPP. These findings could be helpful for a better pathophysiological understanding of HypoPP and may aid in future studies to assess disease extent or monitor treatment efficacy.