Quantitative 23Na‐MRI of the intervertebral disk at 3 T

Monitoring the tissue sodium content (TSC) in the intervertebral disk geometry noninvasively by MRI is a sensitive measure to estimate changes in the proteoglycan content of the intervertebral disk, which is a biomarker of degenerative disk disease (DDD) and of lumbar back pain (LBP). However, application of quantitative sodium concentration measurements in 23Na‐MRI is highly challenging due to the lower in vivo concentrations and smaller gyromagnetic ratio, ultimately yielding much smaller signal relative to 1H‐MRI. Moreover, imaging the intervertebral disk geometry imposes higher demands, mainly because the necessary RF volume coils produce highly inhomogeneous transmit field patterns. For an accurate absolute quantification of TSC in the intervertebral disks, the field variations have to be mitigated. In this study, we report for the first time quantitative sodium concentration in the intervertebral disks at clinical field strengths (3 T) by deploying 23Na‐MRI in healthy human subjects. The sodium maps were calculated by using the double‐angle method and a double‐tuned (1H/23Na) transceive chest coil, and the individual effects of the variation in the field patterns in tissue sodium quantification were calculated. Phantom measurements were conducted to evaluate the quality of the Na‐weighted images and mapping. Depending on the disk position, the sodium concentration was calculated as 161.6 mmol/L–347 mmol/L, and the mean sodium concentration of the intervertebral disks varies between 254.6 ± 54 mmol/L and 290.1 ± 39 mmol/L. A smoothing effect of the correction on the sodium concentration maps was observed, such that the standard deviation of the mean sodium concentration was significantly reduced with mitigation. The results of this work provide an improved integration of quantitative 23Na‐MRI into clinical studies in intervertebral disks such as degenerative disk disease and establish alternative scoring schemes to existing morphological scoring such as the Pfirrmann score.