Assessing the variability of 23 Na MRI in skeletal muscle tissue: Reproducibility and repeatability of tissue sodium concentration measurements in the lower leg at 3 T.

The goal of this study was to evaluate the reproducibility and repeatability of tissue sodium concentration (TSC) measurements using Na-23 MRI in skeletal muscle tissue. Na-23 MRI was performed at 3 T on the right lower leg of eight healthy volunteers (aged 28 +/- 4 years). The examinations were repeated at the same site after similar to 22 weeks to assess the variability over a medium-term period. Additionally, they were scanned at a second site shortly before or shortly after the first visit (within 3 weeks) to evaluate the inter-site reproducibility. Moreover, we analysed the effect of B-0 correction on the variability. Coefficients of variations (CVs) from mean TSC values as well as Bland-Altman plots were used to assess intra-site repeatability and inter-site reproducibility. In phantom measurements, the B-0 correction improved the quantitative accuracy. We observed differences of up to 4.9 mmol/L between the first and second visit and a difference of up to 3.7 mmol/L between the two different sites. The CV for the medium-term repeatability was 15% and the reproducibility CV was 9%. The Bland-Altman plots indicated high agreement between the visits in all muscle regions. The systematic bias of -0.68 mmol/L between site X and Y (P = 0.03) was slightly reduced to -0.64 mmol/L after B-0 correction (P = 0.04). This work shows that TSC measurements in healthy skeletal muscle tissue can be performed with good repeatability and reproducibility, which is of importance for future longitudinal or multicentre studies.