Cortical rim sign (kidneys)
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The cortical rim sign describes the thin, viable rim of subcapsular cortex seen on contrast-enhanced CT or MRI in major renal vascular compromise including:
renal artery obstruction from embolism, thrombosis or dissection
In the setting of lobar nephronia / 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 edema and ischemia.
By contrast, segmental renal infarcts may result in wedge-shaped areas of abnormal renal parenchymal hypoenhancement with relative sparing of the cortex where 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 and it may be partial or total depending on the level of vascular occlusion.