Quantitative in vivo ²³Na MR imaging of the healthy human kidney: determination of physiological ranges at 3.0T with comparison to DWI and BOLD

The purpose of this prospective study was to assess the normal physiologic ranges of the renal corticomedullary ²³Na-concentration ([²³Na]) gradient at 3.0T in healthy volunteers. The corticomedullary [²³Na] gradient was correlated with other functional MR imaging parameters—blood oxygenation level dependent (BOLD) and diffusion-weighted imaging (DWI)—and to individual and physiologic parameters—age, gender, estimated glomerular filtration rate (eGFR), body mass index (BMI), and blood serum sodium concentration ([²³Na]_serum).

50 healthy volunteers (30 m, 20 w; mean age: 29.2 years) were included in this IRB-approved study, without a specific a priori preparation in regard to water or food intake. For ²³Na-imaging a 3D density adapted, radial gradient echo (GRE)-sequence (spatial resolution = 5 × 5 × 5 mm³) was used in combination with a dedicated ²³Na-coil and ²³Na-reference phantoms. [²³Na] values of the corticomedullary [²³Na] gradient were measured by placement of a linear region of interest (20 × 1 mm²) from the renal cortex in the direction of the renal medulla. By using external standard reference phantoms, [²³Na] was calculated in mmol/L of wet tissue volume (mmol/l WTV). Axial diffusion-weighted images (spatial resolution = 1.7 × 1.7 × 5.0 mm³) and 2D GRE BOLD images (spatial resolution = 1.2 × 1.2 × 4.0 mm³) were acquired. Mean values ± standard deviations for [²³Na], apparent diffusion coefficient (ADC) values, and R2* values were computed for each volunteer. The corticomedullary ²³Na-concentration gradient (in mmol/l/mm) was calculated along the area of linear concentration increase from the cortex in the direction of the medulla. Correlations between the [²³Na] and DWI, BOLD, and the physiologic parameters were assessed with Pearson correlation coefficients.

The mean corticomedullary [²³Na] for all healthy volunteers increased from the renal cortex (58 ± 17 mmol/l WTV) in the direction of the medulla (99 ± 18 mmol/l WTV). The inter-individual differences ranged from respective cortical and medullary values of 27 and 63 mmol/L WTV to 126 and 187 mmol/L WTV. No statistically significant differences in renal [²³Na] were found based on differences in individual or physiologic parameters (age, gender, [²³Na]_serum, BMI, GFR). No ADC or R2* gradients were identified, and [²³Na] did not correlate with these parameters.

Renal corticomedullary [²³Na] values increase from the cortex in the direction of the medullary pyramid, demonstrating wide inter-individual ranges and no significant correlations with age, gender, [²³Na]_serum, BMI, GFR, ADC, or R2* values. For future clinical evaluations, an approach relying on renal stimulation (e.g. pharmacologically induced diuresis) may be applicable to account for wide inter-individual ranges of normal [²³Na].