Assessing muscle-specific potassium concentrations in human lower leg using potassium magnetic resonance imaging

Noninvasively assessing tissue potassium concentrations (TPCs) using potassium magnetic resonance imaging (³⁹ K MRI) could give valuable information on physiological processes connected to various pathologies. However, because of inherently low ³⁹ K MR image resolution and strong signal blurring, a reliable measurement of the TPC is challenging. The aim of this work was to investigate the feasibility of a muscle-specific TPC determination with a focus on the influence of a varying residual quadrupolar interaction in human lower leg muscles. The quantification accuracy of a muscle-specific TPC determination was first assessed using simulated ³⁹ K MRI data. In vivo ³⁹ K and corresponding sodium (²³ Na) MRI data of healthy lower leg muscles (n = 14, seven females) were acquired on a 7-T MR system using a double-resonant ²³ Na/³⁹ K birdcage Tx/Rx RF coil. Additional ¹ H MR images were acquired on a 3-T MR system and used for tissue segmentation. Quantification of TPC was performed after a region-based partial volume correction (PVC) using five external reference phantoms. Simulations not only underlined the importance of PVC for correctly assessing muscle-specific TPC values, but also revealed the strong impact of a varying residual quadrupolar interaction between different muscle regions on the measured TPC. Using ³⁹ K T₂ ^* decay curves, we found significantly higher residual quadrupolar interaction in tibialis anterior muscle (TA; ω_q  = 194 ± 28 Hz) compared with gastrocnemius muscle (medial/lateral head, GM/GL; ω_q  = 151 ± 25 Hz) and soleus muscle (SOL; ω_q  = 102 ± 32 Hz). If considered in the PVC, TPC in individual muscles was similar (TPC = 98 ± 11/96 ± 14/99 ± 8/100 ± 12 mM in GM/GL/SOL/TA). Comparison with tissue sodium concentrations suggested that residual quadrupolar interactions might also influence the ²³ Na MRI signal of lower leg muscles. A TPC determination of individual lower leg muscles is feasible and can therefore be applied in future studies. Considering a varying residual quadrupolar interaction for PVC of ³⁹ K MRI data is essential to reliably assess potassium concentrations in individual muscles.