In the setting of acute pyelonephritis, the areas of abnormally reduced enhancement typically involve a complete wedge of renal parenchyma, extending from medulla peripherally to the capsule. The imaging appearance is thought to correspond to a combination of oedema and ischaemia.
By contrast, segmental renal infarcts may result in wedge shaped areas of abnormal renal parenchymal hypoenhancement with relative sparing of the cortex. This occurs because the blood supply to the outer aspect of the cortex is derived from perforating branches of the renal capsular artery which is an early branch of the renal artery. As such, when a branch of the renal artery is occluded (by thromboembolism, dissection etc) perfusion may be preserved to a thin rim (2-4 mm) of cortex which enhances normally.
Unfortunately the cortical rim sign is only seen in approximately half of renal infarcts.
A "reverse cortical rim sign", with relative hypoenhancement of the renal cortex and normal enhancement of the renal medulla, is a finding of renal cortical necrosis. It may occur in the setting of renal transplant rejection, intravascular haemolysis, shock, and as an obstetric complication 3.
- 1. Zagoria RJ. Genitourinary radiology, the requisites. Mosby Inc. (2004) ISBN:0323018424. Read it at Google Books - Find it at Amazon
- 2. Lin E, Garg K, Escott E et-al. Practical differential diagnosis for CT and MRI. Thieme Medical Pub. (2008) ISBN:1588906558. Read it at Google Books - Find it at Amazon
- 3. Dyer RB, Chen MY, Zagoria RJ. Classic signs in uroradiology. Radiographics. 2004;24 Suppl 1 (suppl 1): S247-80. Radiographics (full text) - doi:10.1148/rg.24si045509 - Pubmed citation