Functional Sodium MRI in the Measurement of Corticomedullary Sodium Content and Its Role in Differentiating Between Transplanted Kidneys With Superior and Inferior Graft Function

ABSTRACT Background Sodium MRI ( 23 Na‐MRI) measuring tissue sodium content may directly assess the corticomedullary gradient (CMG) in the kidney. However, it is understudied in transplanted kidneys. Purpose To investigate associations between CMG in renal graft and urine concentrating ability, estimated glomerular filtration rate (eGFR), and biopsy‐determined fibrosis scores, and to determine if CMG could differentiate between renal grafts with superior and inferior graft function. Study Type Prospective. Subjects 57 participants (39 males) with renal transplantation. Field Strength/Sequence 3D T2‐weighted turbo spin echo sequence and 23 Na‐MR imaging at 3 T. Assessment Urine specific gravity and eGFR were assessed as measures of kidney function. Thirty‐eight participants underwent biopsy within 2 days of MRI, and the Banff fibrosis score was assessed. The average medulla to cortex ratio (MCR) was determined from 23 Na‐MRI analysis. Transplanted kidneys were divided into those with superior graft function (SGF, eGFR ≥ 45 mL/min/1.73 m 2 ) and inferior graft function (IGF, eGFR < 45 mL/min/1.73 m 2 ) groups. Statistical Tests Correlation analysis (Pearson or Spearman coefficient, r ), intraclass correlation coefficient, and area under the receiver‐operating characteristic curve (AUC). Results MCR was 1.27 ± 0.11 and mean urine specific gravity was 1.013 ± 0.005. MCR was significantly correlated with mean urine specific gravity ( r = 0.32) and with eGFR ( r = 0.795). MCR distinguished IGF ( n = 42) from SGF ( n = 15) with an AUC of 0.851 (95% CI: 0.732, 0.931) with a cutoff of ≥ 1.295. MCR was significantly correlated with the Banff fibrosis score of tubulitis in areas of interstitial fibrosis and tubular atrophy (t‐IFTA) ( r = 0.347). Data Conclusion 23 Na‐MRI has the potential to show the CMG in transplanted kidneys, with MCR being correlated with urine concentrating ability. In addition, the transplanted kidney CMG was related to eGFR and the Banff fibrosis score t‐IFTA. Evidence Level Level 2. Technical Efficacy Stage 2.