Transient hepatic attenuation difference

Liver has a unique dual blood supply, which comes from the hepatic artery (25%) and the portal vein (75%).
Transient hepatic attenuation difference (THAD) reflects a change in the normal dual blood supply of the liver in which the affected site appears as an area of high attenuation on the arterial phase images and returns to normal on portal venous phase images. This is typically secondary to enhanced, redistributing arterial flow to a hepatic segment or lobe, usually due to decreased portal or hepatic venous flow and resulting in an arterioportal shunt (APS) (1). Perfusion alterations are produced by increases in arterial flow through transsinusoidal, transvasal, transtumoral, and especially transplexal (peribiliary) routes to compensate for the diminished portal venous flow and by decreased dilution of the contrast material by the nonenhanced portal venous flow (2).
THAD may be lobar, segmental, or subsegmental and is usually peripheral and triangular or wedge shaped, has a straight margin, and contains normal vessels (2).
The most common cause of such perfusion disorders are portal vein obstruction; other causes include liver cirrhosis, hepatic neoplasms, hepatic trauma, hereditary hemorrhagic telangiectasia (HHT), hepatic vein obstruction, steal phenomenon by hypervascular tumors, inflammatory changes, aberrant blood supply, hepatic parenchymal compression, and other causes (3)