Changes and reproducibility of uptake in renal medulla on ¹⁸F-FDG PET/CT between before and after diuretics

Objectives Administration of diuretics after FDG-injection significantly reduces bladder activity; however, high uptakes in the renal medulla are still observed despite the significant decrease in tracer activity of adjacent renal pelvis. The purpose of the study was to determine the changes and reproducibility of FDG uptake in renal medulla by comparing those between before and after diuretic and with renal function tests.

Methods Seventy-six consecutive patients who underwent diuretic PET/CT were retrospectively evaluated. After whole-body PET/CT scan (1st scan), 0.2mg/ kg body-weight of furosemide was administrated 120 minutes after FDG injection. Delayed PET/CT (2nd scan) was performed 180 min after FDG, followed by a contrast-enhanced CT. PET/ contrast-enhanced CT images were evaluated for measurement of maximum uptake in renal medulla (renal-SUVmax) so as not to include the high urinary activity within the regions of interest. To determine the difference in uptakes of renal medulla between before and after diuretics, renal-SUVmax on both scans were statistically compared by the Spearman's rank correlation coefficient and the Bland-Altman analysis. To evaluate the relationship between uptakes of renal medulla and bladder activity or renal function, renal-SUVmax was statistically compared with average bladder activity (bladder-SUVmean) and renal function tests.

Results Renal-SUVmax of 1st and 2nd scan were 5.5±1.3 and 5.2±1.4, respectively. Bladder-SUVmean of 2nd scan (2.0±0.8) was significantly smaller than that of 1st scan (13.8±10.3). Statistically significant correlation was found between renal-SUVmax of 1st and 2nd scan (r= 0.766, p<0.0001). The 95% limits of agreement of renal SUVmax between scans were -0.3±1.8. Renal-SUVmax did not correlate with bladder-SUVmean and renal function tests.

Conclusions ¹⁸F-FDG uptakes in renal medulla show a significant correlation between before and after diuretics and a high reproducibility, regardless of activity in the bladder and the renal functions.