7-T Potassium (³⁹K) MRI to Assess Muscle K⁺ Depletion in Primary Aldosteronism

Background Although 98% of potassium (K⁺) resides intracellularly, current clinical diagnostics assess only extracellular K⁺ concentrations. Noninvasive imaging of tissue K⁺ distribution could provide novel insights into pathophysiologic processes in diseases such as primary aldosteronism (PA). PA is characterized by excessive aldosterone production, which leads to electrolyte imbalances, hypertension, and increased cardiovascular risk. Purpose To determine whether potassium 39 (³⁹K) MRI combined with sodium 23 (²³Na) MRI can help detect alterations in skeletal muscle K⁺ and Na⁺ distribution in participants with PA. Materials and Methods A prospective pre-post study in participants with PA and a cross-sectional case-control study in participants with PA and age- and sex-matched control participants were conducted between January 2019 and April 2024. Participants underwent 7-T ³⁹K and ²³Na MRI of the calf muscle before treatment and approximately 4 months after standard therapy (mineralocorticoid receptor antagonists or adrenalectomy). Control participants underwent a single scan. Apparent tissue K⁺ concentrations (aTPCs) and apparent tissue Na⁺ concentrations (aTSCs) were quantified; serum aldosterone and K⁺ were measured and correlated with imaging. Fifty-five male mice underwent sham surgery or deoxycorticosterone acetate-pellet implantation for translational chemical muscle K⁺ validation. Statistical tests included the Student t test or Mann-Whitney U test (between-group), Wilcoxon signed-rank test (within-participant), and Spearman correlation (two-sided α = .05). Results Forty-two participants were evaluated, including 21 participants with PA (mean age, 52 years ± 9 [SD]; 11 female participants) and 21 control participants (mean age, 53 years ± 9; 11 female participants). Participants with PA showed lower aTPC and higher aTSC than control participants (mean aTPC: 72.7 mmol/L ± 6.8 vs 79.1 mmol/L ± 10.0, P = .02; mean aTSC: 23.9 mmol/L ± 5.3 vs 19.0 mmol/L ± 3.0, P < .001). In within-participant comparisons, PA therapy increased aTPC (mean, 72.9 mmol/L ± 7.5 to 80.9 mmol/L ± 9.8; P = .001) but decreased aTSC (25.2 mmol/L ± 4.8 to 18.9 mmol/L ± 3.4; P < .001), with values approaching control participant values. Baseline aTPC did not correlate with serum K⁺ (r² = .02, P = .54). Mouse muscle chemistry mirrored MRI-based K⁺ depletion. Conclusion Combined ³⁹K and ²³Na MRI enabled noninvasive detection of aldosterone-mediated tissue electrolyte shifts. ClinicalTrials.gov Identifier: NCT04251780 © RSNA, 2026 Supplemental material is available for this article. See also the editorial by Ellermann in this issue.