In Review - News from the 2004 Meeting of the RSNA

CTA becomes gold standard for pulmonary embolism

Scan replaces all other techniques used in ER as diagnostic tool for investigating chest pain

By: Jane Lowers

Whether it scans in four slices or 16, CT is the method of choice for detecting pulmonary emboli. Even four-slice technology boasts a negative predictive value greater than 99%, better than ventilation/perfusion studies or conventional pulmonary angiography, according to researchers from Stanford University.

CT is fast replacing all other techniques in the emergency room, not only because of its accuracy but because it can help rule out pneumonia, pneumothorax, aortic dissections, and other possible causes of chest pain and shortness of breath.

Differences among four-, eight-, and 16-slice scanners appear marginal so far, but few data are available to solidify any claims of superiority for 16, except in speed. The Stanford researchers reported lung scan times at 26.6 seconds for four-slice, 9.25 seconds for eight-slice, and 5.45 seconds for 16.

Dr. Alessandro Napoli and colleagues reviewed CT studies for 1240 consecutive patients referred for suspected PE. Overall, 20% of cases were deemed positive, with 41% of emboli found in the segmental arteries, 27% in lobal regions, 17% in subsegmental arteries, and 15% on the main branches.

While there was no statistically significant difference in detection rates among the three generations of scanners, readers said eight- and 16-slice studies yielded more and clearer diagnostic data. The 16-slice scanner may eventually prove better at finding subsegmental emboli, but only a small number of cases were included in the study.

If a CT study is negative, it is generally safe to forgo further examination. Researchers at the University of South Carolina conducted a metastudy of 14 published papers that included 3283 patients who had undergone CTA to evaluate suspected PE, had negative findings, and had stopped anticoagulation therapy based on those findings. Fourteen developed fatal PE, 23 demonstrated nonfatal embolism, and 52 showed evidence of deep venous thrombosis during follow-up.

The studies produced a 95% to 100% negative predictive value, with an average of 99.1%. NPV for mortality was 99.4%. Whether the CTAs were performed on single-slice or multislice CT made no significant difference. By comparison, conventional pulmonary angiography has an NPV of 98.4%, and V/Q studies average 88%.

The number of CTA referrals for suspected PE has been increasing steadily for several years at Brown University, according to Dr. William Mayo-Smith, a professor of diagnostic imaging. In the 24 months preceding an upgrade from single-row to 16-row CT, radiologists read an average 16.5 studies per month, though the numbers were growing. The figure jumped to 55.2 per month in the six months after installation, an increase of 234% in PE studies, compared with an increase of only 14% in overall CT studies performed.

As a result, radiologists are uncovering more PEs per month-an average 2.7, up from 1.3 before the new scanner-although the number found as a percent of total studies performed has decreased. V/Q studies and conventional pulmonary angiography referrals have dropped to nearly zero.

"Referring physicians are ordering more studies, but we're also diagnosing more," said lead investigator Dr. Jay Donohoo. "It's not clear if we are overutilizing now or if we were underdiagnosing before."

CT's ability to scout for other possible causes of chest pain while investigating PE may account for its growing popularity, Mayo-Smith said.

"We're able to take a chest pain case and tease out the cause much more accurately than before," he said.