Dynamic abdominal computed tomography: "top-down" compared with "bottom-up" imaging.

Dynamic contrast-enhanced computed tomography (CT) of the liver is usually performed from the top down, and there is usually less opacification of the top slices than the more inferior slices. The authors reasoned that reversing the direction of scanning would allow more time for the parenchyma at the top of the liver to become enhanced and would result in better opacification of the parenchyma and the hepatic veins. To test this hypothesis they assigned 32 patients to either a "top-down" or a "bottom-up" group; each patient then underwent unenhanced and contrast-enhanced CT. The method of intravenous administration of contrast agent was identical in the two groups and consisted of rapid injection of 135 mL of iohexol by a power injector; scanning was initiated 30 seconds after the injection was started. Attenuation measurements were obtained at specified areas in the liver and in the hepatic and portal veins. The degree of enhancement was calculated at each site and was compared between groups. The upper portion of the liver showed significantly greater enhancement in the bottom-up sequence than in the top-down sequence (123% and 22% respectively; p < 0.01); the entire liver showed excellent enhancement in the bottom-up sequence. In addition, the hepatic veins showed significantly greater conspicuity in the bottom-up sequence (p < 0.01). The authors conclude that the dynamic bottom-up method yields excellent hepatic enhancement and vascular opacification and is therefore superior to the standard top-down method.