Introduction A limitation of renal cortical scintigraphy is the poten- tial difficulty in distinguishing acute pyelonephritis from renal scarring. Patel et al. [1] proposed criteria to dis- tinguish between acute pyelonephritis and scarring from the appearance of the cortical defect, which is useful in the acute phase. However, the most reliable means of differentiating a scar from acute pyelonephritis is by serial renal cortical scintigraphy. Transient defects indicate acute pyelonephritis, while persistent defects represent scars [2, 3]. Defects that persist can either be from scars present before the first renal cortical scinti- gram or from acute pyelonephritis that has evolved into a new scar. These can only be distinguished if an earlier study was performed before the episode of acute pye- lonephritis. It is often recommended that there be a delay of at least 3 months following urinary tract infection (UTI) to allow resolution of parenchymal inflammation before ORIGINAL ARTICLE Pediatr Radiol (2002) 32: 849–852 DOI 10.1007/s00247-002-0784-6 Michael R. Ditchfield Dianne Summerville Keith Grimwood David J. Cook Harley R. Powell Robert Sloane Terrance M. Nolan John F. de Campo Time course of transient cortical scintigraphic defects associated with acute pyelonephritis Received: 3 September 2001 Accepted: 22 May 2002 Published online: 3 August 2002 Ó Springer-Verlag 2002 Presented at IPR, Paris, May 2001 M.R. Ditchfield (&) Æ D. Summerville D.J. Cook Æ J.F. de Campo Department of Radiology, Royal Children’s Hospital, Melbourne 3052, Australia E-mail: ditchfim@cryptic.rch.unimelb.edu.au Tel.: +61-3-93455237 Fax: +61-3-93455284 K. Grimwood Æ R. Sloane Æ T.M. Nolan Department of General Paediatrics, Royal Children’s Hospital, Melbourne, Australia H.R. Powell Department of Nephrology, Royal Children’s Hospital, Melbourne, Australia K. Grimwood Æ T.M. Nolan Department of Paediatrics, University of Melbourne, Melbourne, Australia Abstract Background: Acute pye- lonephritis is distinguished from re- nal scarring using repeat cortical scintigraphy. The defects of acute pyelonephritis resolve, while those of scars persist. Objective: To deter- mine the duration of reversible cortical defects following acute pye- lonephritis and the time interval required to differentiate infection from scars. Materials and meth- ods: An observational prospective study of 193 children (386 kidneys) aged less than 5 years following their first proven urinary tract infection (UTI). Renal cortical scintigraphic defects were detected in 112 (29%) kidneys within 15 days of diagnosis. Of these, 95 underwent repeat renal cortical scans 2 years after the UTI, including 50 with additional scans performed within 2–6 months of in- fection. Results: Of the 50 kidneys undergoing a second renal cortical scan within 2–6 months of the first UTI, 22 (44%) had persistent defects. A third scan was performed on 17 (77%) kidneys after 2 years, by which time defects had resolved in another 8 (47%) kidneys. The predictive value of defects detected within 2–6 months of UTI repre- senting scars is 53% (95% CI 28, 77). Overall, nine (18%) kidneys with initial renal cortical abnormal- ities had permanent defects. In the 45 kidneys undergoing a second cortical scan more than 6 months after the UTI, 11 (24%) had persis- tent defects. None of the 95 kidneys undergoing serial scans developed new or larger defects. Conclu- sions: Renal scars may not be reliably diagnosed by cortical scintigraphy performed within 6 months of UTI because the inflammatory lesions may not have fully resolved. Keywords Kidney Æ Pyelonephri- tis Æ Scar Æ Scintigraphy