In vivo sodium (23Na) imaging of the human kidneys at 7 T: Preliminary results

To evaluate the feasibility of in vivo Na-23 imaging of the corticomedullary Na-23 gradient and to measure Na-23 transverse relaxation times (T2*) in human kidneys. In this prospective, IRB-approved study, eight healthy volunteers (4 female, 4 male; mean age 29.4 +/- 3.6 years) were examined on a 7-T whole-body MR system using a Na-23-only spine-array coil. For morphological Na-23-MRI, a 3D gradient echo (GRE) sequence with a variable echo time scheme (vTE) was used. T2* times were calculated using a multiecho 3D vTE-GRE approach. Na-23 signal-to-noise ratios (SNR) were given on a pixel-by-pixel basis for a 20-mm section from the cortex in the direction of the medulla. T2* maps were calculated by fitting the Na-23 signal decay monoexponentially on a pixel-by-pixel basis, using least squares fit. Mean corticomedullary Na-23-SNR increased from the cortex (32.2 +/- 5.6) towards the medulla (85.7 +/- 16.0). The SNR increase ranged interindividually from 57.2 % to 66.3 %. Mean Na-23-T2* relaxation times differed statistically significantly (P < 0.001) between the cortex (17.9 +/- 0.8 ms) and medulla (20.6 +/- 1.0 ms). The aim of this study was to evaluate the feasibility of in vivo Na-23 MRI of the corticomedullary Na-23 gradient and to measure the Na-23 T2* relaxation times of human kidneys at 7 T. Key Points High field MR offers new insights into renal anatomy and physiology. Na-23 MRI of healthy human kidneys is feasible at ultra-high field. Renal Na-23 concentration increases from the cortex in the medullary pyramid direction. In vivo measurements of renal Na-23-T2* times are demonstrated at 7.0 T.