Indirect computed tomography venography: a report of vascular opacification.

The clinical utility of indirect computed tomography venography (CTV) of the legs, performed after computed tomography pulmonary angiography (CTPA), to evaluate for deep venous thrombosis (DVT) remains a subject of ongoing debate. Visualization of DVT on CTV requires adequate opacification of venous blood. The objective of this study is to measure the Hounsfield unit (HU) clot-to-blood gradient on CTV and CTPA. Secondary analysis of computed tomography images from a prospective clinical trial was performed. All images were interpreted independently by a radiologist. Using the "region of interest" tool feature of the picture archiving and communication system, observers measured the HU of blood and thrombus on CTPA and CTV, using a structured anatomic approach. Interobserver coefficient of variability (CV%) was examined in all disease-positive cases and in a random sample of 25 disease-negative cases. One hundred seventy-nine patients were enrolled; and 23 had pulmonary embolus (PE) without DVT (13%), one had DVT without PE (0.6%), and 18 had both (10%), leaving 137 (77%) who had no venous thromboembolism (VTE). For the 137 patients with no VTE, the mean (+/-standard deviation) HU values measured for contrast-containing blood with no thrombus were as follows: left and right main pulmonary arteries (MPA), 291+/-73 and 291+/-76; left and right common femoral vein (CFV), 92+/-17 and 88+/-19; and left and right popliteal vein (PV), 87+/-20 and 84+/-20. On CTPA, the HU measured from the filling defects interpreted as acute PE was 5+/-20, suggesting a between-group HU gradient of 250 or 471% for MPA (PE-to-blood). On CTV, the HU for DVT was 63, suggesting a gradient of 27 HU or 43% for CFV (DVT-to-blood) and 23 HU or 37% for PV (DVT-to-blood). Interobserver CV% were: left and right MPA, 5+/-4.0% and 7+/-5.4%; left and right CFV, 7+/-6.8% and 7+/-7.3%; left and right PV, 7+/-8.4%, 6+/-8.2%, and 32+/-27.1% for PE thrombus, and 20+/-30.5% for DVT thrombus. CTV imaging produces a very small HU gradient between clot and blood in leg veins. These findings highlight the importance of ancillary radiological findings in assessing presence of DVT on CTV.