Cortical rim sign | Radiology Signs

Answer:

The cortical rim sign is a thin peripheral band of enhancing renal cortex, typically 2 to 4 mm thick, surrounding a non-enhancing kidney or a non-enhancing wedge of renal parenchyma. It indicates renal infarction and is produced because the outermost cortex survives on supply from capsular collateral vessels while the main renal arterial territory is occluded. The sign is not immediate: it takes roughly 8 hours to several days to develop, so its absence in the first hours after an acute occlusion does not exclude infarction, and the primary finding at presentation is simply a wedge-shaped or global absence of enhancement with a sharply defined margin. It is seen with global infarction from renal artery embolism in atrial fibrillation or endocarditis, from dissection extending into the renal artery, from trauma with intimal injury, and with segmental infarcts from small emboli or vasculitis such as polyarteritis nodosa. The main differential on a contrast-enhanced study is acute pyelonephritis, which produces striated nephrographic wedges that are less sharply demarcated, with perinephric stranding, clinical infection and no cortical rim, and renal vein thrombosis, which shows an enlarged kidney with a delayed dense nephrogram and thrombus in the vein. Management depends on the cause and on the time from onset, and CT angiography defines the arterial anatomy.

Why is it called so?

The surviving outer cortex forms a thin bright rim around the infarcted, non-enhancing kidney.

Pathophysiology

The renal parenchyma is supplied almost entirely by end arteries derived from the renal artery, which is why occlusion produces sharply demarcated infarction with no collateral rescue of the bulk of the tissue. The outer 2 to 4 mm of cortex, however, also receives blood from the capsular arteries, which arise from the inferior adrenal, gonadal and lumbar arteries and anastomose in the renal capsule. When the main arterial supply is lost, this capsular circulation maintains perfusion of the subcapsular cortex alone. The rim takes time to become visible because it depends on collateral vessels dilating and on the infarcted parenchyma losing its enhancement, and it disappears later as the infarcted kidney atrophies and scars.

Alternative names: Rim nephrogram; cortical rim nephrogram

Other associated named signs: Striated nephrogram of pyelonephritis, the flip-flop enhancement of renal vein thrombosis and the spotted nephrogram

References

1. Wong WS, Moss AA, Federle MP, Cochran ST, London SS. Renal infarction: CT diagnosis and correlation between CT findings and etiologies. Radiology. 1984;150(1):201-5. PMID: 6689761.