Helical CT Pulmonary Angiography in the Detection of Pulmonary Embolism
Abstract & Commentary
Synopsis: The detection of deep venous thrombosis by indirect CT venography following CT pulmonary arteriography can have significant diagnostic benefit in the diagnosis of pulmonary thromboembolic disease.
Source: Cham MD, et al. Deep venous thrombosis:
Detection by using indirect CT venography. Radiology 2000; 216:744-751.
Since its initial description by remy-jardin and colleagues in 1992, there has been considerable interest in the use of helical CT pulmonary angiography in the detection of pulmonary embolism. As the speed of helical scanners continually increases, the ability to obtain a greater number of more thinly collimated enhanced scans at a more optimal phase of intrarterial enhancement will progressively improve the ability of helical CT to detect emboli, particularly at the segmental and subsegmental level. However, it is clear from well- performed outcome studies that it is the status of the deep veins of the legs and pelvis that determines outcome in patients, with suspected pulmonary thromboembolic disease (PTD). In fact, most clinical algorithms for the evaluation of patients with suspected pulmonary embolism include some form of imaging of the deep venous system (usually duplex sonography) either because of symptoms of deep venous thrombosis (DVT) or those with inconclusive ventilation-perfusion lung scans or negative or inconclusive CT pulmonary angiograms. The rationale for evaluation of the deep venous system is that serial negative examinations for DVT portends a favorable clinical outcome and therefore, allows safe withholding of anticoagulant therapy. With the increased use of CT pulmonary angiography in the direct evaluation of the pulmonary arteries, the potential benefit of assessing the status of the deep venous system by performing indirect CT venography of the lower extremities and pelvis immediately following CT pulmonary angiography becomes obvious.1 This study seeks to determine the feasibility of performing indirect CT venography following CT pulmonary angiography and its effect on the diagnosis of pulmonary thromboembolic disease by detecting DVT.
This prospective multi-institutional study enrolled 541 patients over a 10-month period who underwent combined CT pulmonary angiography followed by indirect CT venography for the evaluation of suspected pulmonary thromboembolism. The CT pulmonary angiograms were performed using helical scanners with a collimation of 3 mm at variable pitch and a bolus of 140 mL of 300 mg% nonionic contrast injected at a rate of 3 mL/sec through an arm vein. The chest scans started after a delay of 28 seconds. The CT venograms were obtained helically beginning 120 seconds after completion of the CT pulmonary angiogram using a helical acquisition with 10 mm collimation. The venographic scans were obtained from the iliac crests to the popliteal fossae. The examinations were evaluated for the quality of both the angiographic and venographic components and for presence or absence of pulmonary emboli or deep venous thrombi. One hundred sixteen patients also underwent lower extremity ultrasonography (US).
While the vast majority of CT angiograms were rated as good or excellent (95%), 23% of the venographic studies were rated as fair or poor. Indirect CT venography detected DVT in 45 patients, 16 of whom had no emboli on CT pulmonary angiography. The finding of DVT in these patients increased the diagnosis of thromboembolic disease by 18% over the 17% of patients with evidence of pulmonary emboli. All patients who had DVT on US had positive CT venograms, and an additional four patients who had negative ultrasound exams had DVT on CT venography.
Comment by Jeffrey S. Klein, MD
This study, although hampered by certain design limitations, builds upon previous work that shows the feasibility of performing indirect CT venography following CT pulmonary angiography for the evaluation of pulmonary thromboembolic disease. The incremental increase in detection of thromboemboli provided by this technique will clearly prove useful as the study requires only an additional three minutes to perform following the CT angiographic study and relies only upon the contrast administered for the angiographic phase of the examination. The additional radiation exposure is cause for some concern, particularly in young and pregnant patients, and this will need to be addressed by reduction in exposure parameters and increases in helical pitch. The relatively high rate of fair or poor CT venographic studies is somewhat concerning given the nearly universal availability of high-quality duplex US of the deep veins. Some of the difficulties in obtaining reliably high quality CT venograms is likely due to variable left ventricular output and aortic and lower extremity arterial and venous circulation times. However, it is likely that with further advances in helical CT software that allows for assessment of time/density relationships after contrast administration, the technical difficulties of performing high quality indirect CT venography will be solved. The next several years will likely see this technique used in clinical trials that determine the safety of withholding anticoagulant therapy following a combined CT pulmonary angiogram and indirect pelvis and lower extremity venogram that is negative for thromboemboli.
Reference
1. 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.
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