Intravascular hematocrit effect in cross-sectional CT imaging.

Venous fluid-fluid levels were seen on CT or MR of three patients being evaluated for possible thrombosis. In all three cases, further investigation (by ultrasonography, venography, or surgery) revealed the cause of the fluid-fluid level to be slowly flowing blood without thrombosis. On CT done after intravenous contrast medium administration, the nondependent half of the vessel was denser than the dependent half. In an attempt to confirm this MR and CT picture and, particularly, to explain the dense upper half on CT, we conducted an in vitro study. Our study confirmed that when blood with diatrizoate sodium is not clotted and allowed to settle, a similar fluid-fluid level is observed by CT imaging. Our results indicate that an intravascular fluid-fluid level seen on cross-sectional images represents slowly flowing blood, not thrombosis.