The wedges of reduced enhancement seen in the setting of acute pyelonephritis represent oedema and ischaemia which involves the whole wedge of renal parenchyma, from medulla to the capsule. This sign, seen as a result of cortical necrosis, may also be seen in other conditions e.g. renal transplant rejection, intravascular haemolysis, shock, and as an obstetric complication 3.
In segmental infarcts, 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.
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- 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