CT Venography Performed as Part of a CT Pulmonary Angiogram for Pulmonary Embolism
Abstract & Commentary
Synopsis: CT venography performed as part of a CT pulmonary angiogram for pulmonary embolism is equivalent to ultrasound in detection of lower extremity deep venous thrombosis.
Source: Loud PA, et al. Combined CT venography and pulmonary angiography in suspected thromboembolic disease: Diagnostic accuracy for deep venous evaluation. AJR Am J Roentgenol 2000;174:61-65.
This technique, first described in 1998, entails helical CT pulmonary angiography for the evaluation of pulmonary emboli followed by delayed axial CT examination of the lower extremities, pelvis, and inferior vena cava to detect deep venous thrombosis (DVT). The study describes 71 consecutive patients with suspected pulmonary embolism who had CT pulmonary angiography followed 3.5 minutes after the beginning of contrast injection by axial 5- or 10-mm-thick scans from the upper calves to the diaphragms. All patients had bilateral lower extremity ultrasound (US) examinations performed from the popliteal fossa to the inguinal regions within 12 hours of the CT venogram.
The lower extremity ultrasound was normal in 52 patients, with all patients having negative CT venograms. Eighteen patients had positive US studies for DVT in the femoropopliteal system, with all showing thrombus on CT venography. Six of these patients with positive CT venograms showed extension of thrombus into the iliac veins or inferior vena cava (IVC) not evident on US. One patient with an isolated IVC thrombus on CT venogram had a negative lower extremity US examination. When compared to lower extremity US, the sensitivity and specificity of CT venography were both 100%.
Comment by Jeffrey S. Klein, MD
Since more than 90% of pulmonary emboli originate from lower extremity DVT and the treatment for proximal DVT is similar to that for pulmonary embolism, there is a strong rationale for deep venous imaging in the evaluation of pulmonary embolism. The noninvasive technique described in this paper takes advantage of the normal delayed venous return of contrast-enhanced blood from the lower extremities and provides a noninvasive evaluation of the deep venous system that requires little additional time (5-7 minutes per patient) and radiation exposure.
Although the results of this study appear promising, the routine implementation of this technique awaits the results of large prospective studies investigating the relative use of CT pulmonary angiography in the evaluation of pulmonary embolism. In addition, the optimal type of scan parameters (i.e., collimation, spacing, axial vs helical acquisition), contrast dose, and timing of lower extremity and pelvic imaging following CT pulmonary angiography will vary between different patient populations and has yet to be determined. However, I believe helical CT will prove to have a high negative predictive value for recurrent thromboembolic disease and since the primary prognostic factor for recurrent pulmonary embolism is residual DVT in the proximal veins, the technique of combined CT pulmonary angiography and lower extremity and pelvic venography will supplant the ventilation/perfusion scan and become the examination of choice in the evaluation of suspected pulmonary embolism.