Transient Hepatic Attenuation Difference

Transient hepatic attenuation difference (THAD) is an area of enhancement seen during the hepatic arterial phase of dynamic CT that is caused by an increase in arterial flow. This increase may be primary or in response to decreased portal flow.

Primary causes include hypervascular lesions (the sump effect), inflammation of adjacent organs (e.g., gallbladder, pancreas), or an aberrant hepatic arterial supply.

Decreased portal flow, in turn, can be caused by portal or hepatic vein thrombosis, compression by focal lesions, long-standing biliary obstruction, parenchymal trauma, and arterioportal shunts.

Transient hepatic attenuation difference is classified into four types based on morphology:

• Lobar multisegmental: All or almost all segments of one hepatic lobe is involved. Usually caused by a hypervascular lesion that sucks in extra blood to the whole segment. The shape is not triangular shape and a straight border sign (see below) is not seen.
• Sectorial: Triangular wedge- or fan-shaped areas with at least one straight border that separates the area of increased attenuation from adjacent normally attenuating liver. Caused by increased arterial flow in response to a portal venous obstruction. A malignant lesion may cause sectorial THAD by compression or infiltration of a portal vein branch. Benign lesions causing sectorial THAD are usually subcapsular. Alternatively, a focal liver abscess can cause portal hypoperfusion by the spread of inflammatory mediators.
• Polymorphous: Have various shapes and sizes without a straight border. Present as areas of irregular enhancement around or lateral to an injury. The polymorphous pattern may be caused by an aberrant blood supply, inflammation, physical parenchymal injury (contusion, percutaneous biopsy), chemical parenchymal injury (e.g., ethanol injection for tumor), extrinsic compression by ribs or stretched diaphragmatic pillars, and radiofrequency ablation.
• Diffuse: Involves the entire hepatic parenchyma. This pattern is caused by a generalized blood flow obstruction before, at, or after the level of sinusoids, with resultant portal hypoperfusion. The appearance depends on the level of obstruction.
  • Before: For example, portal vein thrombosis, cirrhosis. Portal flow is inadequate for the periphery of the liver, resulting in a central-peripheral appearance (peripheral subcapsular hepatic enhancement with relative low attenuation of the central perihilar area.
  • At: For example with dilatation of the biliary tree from choledocholithiasis or pancreatic cancer. The appearance is a peribiliary pattern of transient hepatic attenuation difference.
  • After: For example, Budd Chiari syndrome, right heart failure. Results in generalized central lobular enhancement with a marbled appearance.

References

• Colagrande S, Centi N, La Villa G, Villari N. Transient hepatic attenuation differences. AJR Am J Roentgenol. 2004 Aug;183(2):459-64.
• Itai Y, Moss AA, Goldberg HI. Transient hepatic attenuation difference of lobar or segmental distribution detected by dynamic computed tomography. Radiology. 1982 Sep;144(4):835-9.
• Itai Y, Murata S, Kurosaki Y. Straight border sign of the liver: spectrum of CT appearances and causes. Radiographics. 1995 Sep;15(5):1089-102.