Repeatability of Simultaneous 1 $$ {}^1 $$ H/ 23 $$ {}^{23} $$ Na MR Fingerprinting in Knee Cartilage at 7 T

This study evaluates the repeatability of a 3D simultaneous ${}^1$ H/ ${}^{23}$ Na MR fingerprinting (MRF) sequence in knee cartilage of healthy volunteers. Eight healthy volunteers underwent four knee scans each with 3D simultaneous ${}^1$ H/ ${}^{23}$ Na MRF at 7 T. Proton density (PD), tissue sodium concentration (TSC), and ${}^1$ H and ${}^{23}$ Na relaxation time maps were acquired over two visits with two consecutive acquisitions at both visits. Mean values and standard deviations of all MRF metrics were measured in three knee cartilage regions: patellar, femorotibial medial, and femorotibial lateral. Image processing included ${}^1$ H and ${}^{23}$ Na MRF dictionary matching, B ${}_1^{-}$ correction, TSC and PD quantification, and image registration between the scans. Repeatability was assessed using the coefficient of variation (CV) and intraclass correlation coefficient (ICC) for all measurements and Bland-Altman plots to compare intraday and interday measurement differences. Mean TSC values over all subjects and cartilage regions were 162  $\pm$  29 mM, with a CV of 12%  $\pm$  1%. Mean ${}^{23}$ Na T ${}_1$ (30  $\pm$  2 ms) and T ${}_{2\text{l}}$ (13  $\pm$  3 ms) values were relatively consistent (CV = 6%-19%), while T ${}_{2\text{s}}$ showed greater variability (1.62  $\pm$  1.60 ms, CV = 54%  $\pm$  11%). For ${}^1$ H, mean PD was 0.87  $\pm$  0.24 (CV = 20%  $\pm$  8%), mean T ${}_1$ was 1114  $\pm$  168 ms (CV = 11%  $\pm$  5%), and mean T ${}_2$ was 36  $\pm$  15 (CV = 20%  $\pm$  7%). ICC values suggested low-to-moderate discrimination power, with highest values observed for ${}^1$ H T ${}_1$ and lowest values for ${}^{23}$ Na relaxation times. The Bland-Altman plots suggest similar intraday and interday differences. This study shows that we can acquire quantitative ${}^1$ H and ${}^{23}$ Na MRF maps simultaneously in knee cartilage at 7 T. Repeatability was the highest for TSC and ${}^{23}$ Na T ${}_1$ . ICC was the highest for PD and ${}^1$ H T ${}_1$ .