title: Quantitative in vivo 23Na MR imaging of the healthy human kidney: determination of physiological ranges at 3.0T with comparison to DWI and BOLD.
abstract: OBJECTIVE
  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).
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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].
authors.affiliation: Institute of Clinical Radiology and Nuclear Medicine, University Medical Center Mannheim, Medical Faculty Mannheim, Heidelberg University, Theodor- Kutzer-Ufer 1-3, 68167, Mannheim, Germany, stefan.haneder@umm.de.
authors.forename: Stefan
authors.initials: S
authors.lastname: Haneder
authors.forename: Paul
authors.initials: P
authors.lastname: Kettnaker
authors.forename: Simon
authors.initials: S
authors.lastname: Konstandin
authors.forename: John N
authors.initials: JN
authors.lastname: Morelli
authors.forename: Lothar R
authors.initials: LR
authors.lastname: Schad
authors.forename: Stefan O
authors.initials: SO
authors.lastname: Schoenberg
authors.forename: Henrik J
authors.initials: HJ
authors.lastname: Michaely
mesh_terms: D000328:Adult; D015992:Body Mass Index; D038524:Diffusion Magnetic Resonance Imaging; D005260:Female; D005919:Glomerular Filtration Rate; D064368:Healthy Volunteers; D006801:Humans; D007668:Kidney / Q000502:physiology*; D007672:Kidney Cortex / Q000473:pathology; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012975:Sodium Isotopes / Q000737:chemistry; D055815:Young Adult
chemical_list: D012975:Sodium Isotopes