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

Objective: To evaluate the feasibility of in vivo ²³Na imaging of the corticomedullary ²³Na gradient and to measure ²³Na transverse relaxation times (T2*) in human kidneys. Methods: 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 ²³Na-only spine-array coil. For morphological ²³Na-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. ²³Na 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 ²³Na signal decay monoexponentially on a pixel-by-pixel basis, using least squares fit. Results: Mean corticomedullary ²³Na-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 ²³Na- T2*relaxation times differed statistically significantly (P < 0.001) between the cortex (17.9 ± 0.8 ms) and medulla (20.6 ± 1.0 ms). Conclusion: The aim of this study was to evaluate the feasibility of in vivo ²³Na MRI of the corticomedullary ²³Na gradient and to measure the ²³Na T2*relaxation times of human kidneys at 7 T. Key Points: • High field MR offers new insights into renal anatomy and physiology. • ²³ Na MRI of healthy human kidneys is feasible at ultra-high field. • Renal ²³ Na concentration increases from the cortex in the medullary pyramid direction. • In vivo measurements of renal ²³ Na-T2*times are demonstrated at 7.0 T.