Transient hepatic attenuation differences
Transient hepatic attenuation differences (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.
THADs are 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 thereby giving a higher attenuation to the affected region.
Four pathogenic 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.
Differences in hepatic arterial blood flow can be primary or secondary (i.e. compensatory) 4:
- primary: focal hypervascular lesions, variant hepatic artery anatomy, adjacent inflammation (e.g. cholelithiasis, pancreatitis)
May be seen as areas of parenchymal enhancement usually visible only during the hepatic artery phase that are caused by the dual hepatic blood supply.
- transient hepatic intensity differences (THID): the equivalent finding at MRI
- 1. Colagrande S, Centi N, Galdiero R et-al. Transient hepatic intensity differences: part 1, Those associated with focal lesions. AJR Am J Roentgenol. 2007;188 (1): 154-9. doi:10.2214/AJR.05.1368 - Pubmed citation
- 2. Kim HJ, Kim AY, Kim TK et-al. Transient hepatic attenuation differences in focal hepatic lesions: dynamic CT features. AJR Am J Roentgenol. 2005;184 (1): 83-90. AJR Am J Roentgenol (full text) - Pubmed citation
- 3. Colagrande S, Centi N, Galdiero R et-al. Transient hepatic intensity differences: part 2, Those not associated with focal lesions. AJR Am J Roentgenol. 2007;188 (1): 160-6. doi:10.2214/AJR.05.1367 - Pubmed citation
- 4. Colagrande S, Centi N, La Villa G et-al. Transient hepatic attenuation differences. AJR Am J Roentgenol. 2004;183 (2): 459-64. doi:10.2214/ajr.183.2.1830459 - Pubmed citation