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Computed tomography (CT) pulmonary angiography has a significantly higher sensitivity than standard pulmonary angiography in detecting pulmonary emboli.

Computed tomography (CT) pulmonary angiography has a significantly higher sensitivity than standard pulmonary angiography in detecting pulmonary emboli.

However, a limitation is that during the time between completing CT pulmonary angiography and standard pulmonary angiography, the thrombus can change.

The results of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II study show that thrombi can worsen, improve or even disappear in the time interval between the two diagnostic tests. Plus, angiography itself may cause new thrombi or change in the thrombus under investigation.

Principal investigator in this subanalysis of the PIOPED II data, Dr. Conrad Wittram of Harvard Medical School in Boston, and colleagues evaluated the discordant findings obtained on CT angiography and standard angiography in 226 patients who underwent both tests to evaluate suspected pulmonary embolus.

Of the 226 index cases, 206 had concordant results, according to the report in the September issue of Radiology. Twenty patients, half men and half women, who were a mean of 49-years-old, had discordant findings.

Seven cases were negative for pulmonary emboli and 13 were positive for pulmonary emboli on CT angiography, with the opposite results obtained using standard angiography.

CT angiography had a sensitivity of 87 per cent for the detection of pulmonary embolism, while standard angiography had a sensitivity of 32 per cent.

Standard angiography yielded one false-positive finding and 13 false-negative results. On CT angiography, two patients had false-negative findings. Another four patients had true-negative findings on CT angiography, but were positive on standard angiography.

"We believe that the four cases that were true-negative at CT (angiography) and became positive for thromboembolus by the time of angiography resulted from formation of thrombus between the studies or as a complication of the catheter investigation," Dr. Wittram and colleagues comment.

The mean time between CT and angiography was 40 hours in this study. "Because of the volumes of contrast material used, the tests should ideally be separated by 24 hours and the patient should be well hydrated. Shorter intervals between the two tests will increase the risks of renal damage," Dr. Wittram noted in an interview with Reuters Health.

"The implications are that CT (angiography) can be confidently used as a first line for detecting pulmonary embolism and that conventional angiography, when available, should be used as a problem-solving technique, such as when there is an indeterminate CT (angiography)," Dr. Wittram commented.

"These findings support the paradigm shift in the imaging of pulmonary embolism toward CT pulmonary angiography," the PIOPED II investigators conclude.

Radiology 2007;244:883-889.

• Click here for the research abstract.