The liver has a dual blood supply (70% portal vein, 30% hepatic artery) with compensatory relationships between the two inflows: arterial flow increases when portal flow decreases. This flow occurs as a result of communications among main vessels, sinusoids, and peribiliary venules that open in response to nervous and humoral factors. Transient hepatic attenuation differences are areas of parenchymal enhancement visible during the hepatic arterial phase on helical CT [1,2]. Because of the wide diffusion of hepatic arterial phase evaluation, transient hepatic attenuation differences are now rather frequent. In a previous study from our group, the differences were identified on 130 (13%) of 988 helical CT scans of the liver [3]. Transient hepatic attenuation differences have been associated with a large variety of liver disorders [1-3]. Our article aims to show the range of these arterial phenomena in a comprehensive diagnostic organization correlating morphology with etiology and pathogenesis.
Transient hepatic attenuation differences can be classified according to morphology, etiology, pathogenesis, and association with focal lesions [3]. According to morphology, they can be organized into four groups: lobar multiseg-mental, sectorial, polymorphous, and diffuse.
Lobar multisegmental differences involve all or almost all segments of one hepatic lobe and are usually caused by a primary increase in arterial inflow and therefore follow arterial distribution.
Sectorial differences follow the portal dichotomy, appearing as triangular wedge- or fan-shaped areas with at least one "straight border" sign (a clear separation line from the normally attenuating parenchyma) that occurs because of the strict connection between the territory downstream portal obstruction and the arterial reaction.
Polymorphous differences usually do not follow the portal dichotomy and show various shapes and sizes without a straight border sign.
Diffuse differences involve the entire hepatic parenchyma and may assume a patchy, central peripheral, or peribiliary pattern on the basis of the location of the portal blockade.