#3063 MRI assessment of skin and muscle sodium (23Na) and fluid volume in haemodialysis patients
Authors: Prestwich Ben , Noble Rebecca , White Kelly , Taal Maarten , Selby Nicholas , Francis Susan 
Published: May 2024

Abstract

Background and Aims
Haemodialysis (HD) is life sustaining for patients with end-stage kidney disease (ESKD). In healthy people, sodium balance is regulated by the kidneys; in ESKD this is achieved by sodium removal during HD. Recent evidence suggests non-osmotically stored sodium in the muscle and/or skin may be a critical factor impacting the development of hypertension and cardiovascular disease (CVD). Sodium (23Na) MRI allows skin and muscle sodium storage assessment and may provide a valuable tool in evaluating sodium storage in dialysis patients. Here, 23Na MRI is used to measure muscle and skin tissue sodium concentration (TSC) in HD participants. HD participants are scanned before and after a single haemodialysis session using 23Na MRI as well as 1H T2 relaxometry to study changes in fluid status.

Method
Data were collected on HD patients who underwent a pre-HD 23Na MRI calf scan, then had their usual dialysis session with a dialysate sodium of 137 mmol/L, followed by a repeat post-HD 23Na MRI scan. Patient dialysis vintage, residual renal function, and ultrafiltration volume, and blood measures pre- and post-HD were collected. 1H scans were acquired for localisation and muscle segmentation (mDIXON) along with T1 and T2 mapping. 23Na images were acquired for (TSC) quantification. Reference bottles (10, 20, 30 and 40 mmol/L NaCl) were placed above the leg to calibrate TSC muscle and skin maps. Regions-of-interest (ROIs) of each muscle group and the skin were manually segmented on mDIXON scans. In each muscle group the voxel-wise mode (to avoid influence of signals from vessels) of TSC and T2 was estimated. A paired-sample t-test was performed between metrics pre-HD and post-HD. Absolute measures of ΔTSC and ΔT2 were correlated with clinical measures.

Results
Ten people receiving HD were recruited (age 55-73 yrs, 5 M:5F). Figure 1 shows TSC maps of the HD patients pre-HD and post-HD, with a significant reduction (p &lt; 0.05) in TSC in the extensor, peroneus and gastrocnemius muscles post-HD but skin sodium showed no detectable change. Post-HD TSC significantly positively correlated with post-HD Systolic Blood Pressure (SBP) (P &lt; 0.004) but not with plasma sodium. 1H T2-maps (Fig. 2A) show a significant reduction (p &lt; 0.01) in muscle T2 post-HD compared to pre-HD all muscle groups (Fig. 2B). There were no significant correlations between T2 and 23Na measures, or T2 and clinical measures. There was no significant detectable change in 1H T1.

Conclusion
Post-HD TSC values are consistent with published data from patients dialysed against a dialysate sodium of 137 mmol/L, and shows a significant correlation of post-HD TSC with post-HD SBP. 1H muscle T2 reduced post-HD, consistent with published data, as patients change from a pre-HD hypervolemia state closer to a post-HD to euvolemia state after fluid removal. There was no correlation of absolute or ΔTSC and 1H T2 and ΔT2 values, suggesting different mechanisms for equilibrium conditions of water and sodium ion concentrations. Future studies will study the effect of dialysate sodium on muscle TSC and mechanistic links to CVD, and include the use of a dedicated skin coil to study skin sodium alongside bioimpedance fluid measures.