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Transient hepatic attenuation differences

Transient hepatic attenuation differences or THAD lesions refer to areas of parenchymal enhancement visible during the hepatic artery phase on helical CT. They are thought to be a physiological phenomenon caused by the dual hepatic blood supply. Occasionally they may be associated with hepatic tumours such as a hepatocellular carcinoma 2


Thought to be due a localised mismatch in hepatic arterial versus portal venous blood supply where often there is an relative increase in hepatic arterial supply therefore giving a higher attenuation to the affected region. 

Four pathogenetic mechanisms have been described:

  • directly by a siphoning effect (lobar multisegmental shape)
  • portal hypoperfusion (sectorial shape) due to portal branch compression or infiltration
  • thrombosis resulting in a portal branch blockade
  • flow diversion caused by an arterioportal shunt

Compensatory relationships between two liver sources of blood supply so that arterial flow increases when portal flow decreases as a result of communication among the main vessels, sinusoids, and peribiliary venules that open in response to autonomic nervous system and humoral factors activated by liver demand for oxygen and metabolites.

Radiographic features

Areas of parenchymal enhancement visible during the hepatic artery phase that are caused by the dual hepatic blood supply.

See also

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