Follow-up of acute pyelonephritis: what causes the diffusion-weighted magnetic resonance imaging recovery to lag clinical recovery?

PURPOSE: To analyze with diffusion-weighted magnetic resonance imaging (DW-MRI) the evolution and progress to resolution of acute pyelonephritis (APN) foci over a period of 3 months after onset. METHODS: 30 women (age 22-51 years) with clinical, laboratory (white blood cell and C-reactive protein), and DW-MRI (4b-values 0, 50, 600, 1000 s/mm2) diagnosis of APN were prospectively enrolled. Two double-blinded radiologists evaluated the number of APN foci, and for each of them dimension (D), absolute diffusion coefficient (ADC), and its ratio R to the ADC of unaffected parenchyma. Signature of radiological recovery was focus no longer visible (DW-) and ADC of its site not inferior to the ADC of the unaffected parenchyma, i.e., R ≥ 0.9. Clinical and DW-MRI follow-ups (FU) were performed at 1 and 3 months. RESULTS: At the acute stage (t 0), 187 APN foci were found, with ADC0 = 1.3 ± 0.2 × 10-3 mm2/s, R 0 = 0.65 ± 0.12, and D 0 = 14 ± 7.5 mm. By the 1-month FU (t 1), all patients had no symptoms and physiological laboratory values; despite this, only 80 (43%) foci were solved, increasing to 138 (74%) by at the 3-month FU. The ROC curve (AUC ≥ 0.80) identified R 0 ≤ 0.6 and D 0 > 15 mm as forecast of slow radiologic resolution. About 80% of foci unsolved at 1 month but with R 1 ≥ 0.8 and D 1 ≤ 10 mm reached solution at 3 months. CONCLUSIONS: DW-MRI recovery of APN foci does not always coincide with clinical recovery. The evolution of an APN focus is shaped by its initial values R 0 and D 0. About half of the foci still visible at 1 month reached radiological resolution in the two following months.