Inside-out liver sign | Radiology Signs

Answer:

The inside-out liver is the reversed pattern of hepatic enhancement produced by hepatic venous outflow obstruction. In the arterial and portal venous phases the caudate lobe and the central perihilar parenchyma enhance strongly while the peripheral liver is patchy and hypoattenuating, and on delayed images the pattern flips, with contrast retained peripherally as the central liver washes out. The central segments enhance early because they retain portal inflow, whereas peripherally the raised sinusoidal pressure stagnates and even reverses portal flow, so contrast arrives late and then lingers. The caudate lobe is protected because it drains by its own short emissary veins directly into the inferior vena cava, and its consequent hypertrophy is seen in about three-quarters of chronic cases. The supporting findings are non-visualisation or narrowing of the hepatic veins, comma-shaped intrahepatic collaterals, a spider-web collateral network at venography, ascites and features of portal hypertension. A common pitfall is the development of benign regenerative nodules, which are typically multiple, small, homogeneously hypervascular with a central scar and, importantly, show no washout, unlike hepatocellular carcinoma, which is a genuine though much less frequent complication. The chief differential is passive hepatic congestion from right heart failure, which shows a dilated inferior vena cava with reflux of contrast and patent hepatic veins, and sinusoidal obstruction syndrome after stem cell transplantation, in which the hepatic veins are also patent.

Why is it called so?

The normal centre-to-periphery gradient of enhancement is inverted, so the liver appears to be enhancing from the inside out.

Pathophysiology

Thrombosis or membranous obstruction of the hepatic veins or of the suprahepatic inferior vena cava, usually in the setting of a myeloproliferative neoplasm with a JAK2 V617F mutation, antiphospholipid syndrome, oral contraceptive use, pregnancy or a hypercoagulable state, raises sinusoidal pressure. The peripheral liver, drained by the obstructed major veins, becomes congested; sinusoidal hypertension opposes portal inflow, which slows and may reverse, so the periphery is perfused predominantly by the hepatic artery and receives contrast late. The caudate lobe, with independent venous drainage, maintains normal portal perfusion, hypertrophies under the increased functional demand and enhances briskly. Chronic congestion leads to centrilobular necrosis, fibrosis and the regenerative nodules that characterise the late disease.

Alternative names: Flip-flop enhancement; reversed enhancement pattern

Other associated named signs: Spider-web sign of intrahepatic collaterals at venography, the nutmeg liver pattern of passive congestion, and caudate lobe hypertrophy

References

1. Camera L, Mainenti PP, Di Giacomo A, Romano M, Rispo A, Alfinito F, et al. Triphasic helical CT in Budd-Chiari syndrome: patterns of enhancement in acute, subacute and chronic disease. Clin Radiol. 2006;61(4):331-7. PMID: 16546463.