Background: Computed tomography (CT) pulmonary angiography is up to 90 percent sensitive and 95 percent specific for diagnosing pulmonary embolism, and has largely replaced traditional ventilation-perfusion (V/Q) scintigraphy for this purpose. However, up to 24 percent of patients with acute pulmonary embolism have relative contraindications to CT pulmonary angiography. For instance, CT pulmonary angiography's high radiation dose (10 to 50 mGy) to breast tissue of premenopausal and pregnant women can exceed the levels shown to increase cancer risk, and the associated intravenous contrast media can cause complications from contrast allergy or renal impairment. Single-photon emission computed tomography (SPECT) V/Q scintigraphy has the potential to use much less radiation (e.g., 0.28 mGy), and maintains a similar level of accuracy as CT pulmonary angiography in detecting pulmonary embolism. Miles and colleagues compared SPECT V/Q scintigraphy with CT pulmonary angiography to determine the best method for diagnosing pulmonary embolism.

The Study: The authors conducted a prospective observational study of patients 50 years and older with clinically suspected pulmonary embolism. Participants were scheduled to have CT pulmonary angiography and both traditional and SPECT V/Q scintigraphy within 24 hours of presentation. Deidentified scans were reviewed by two radiologists (CT pulmonary angiography) or nuclear medicine subspecialists (traditional and SPECT V/Q scans, presented separately). Exclusion criteria included renal insufficiency and inability to complete CT pulmonary angiography and V/Q scanning within 24 hours.

Results: A total of 83 patients received both CT pulmonary angiography and V/Q scanning, but four of these CT pulmonary angiography scans were nondiagnostic. In the remaining 79 patients, CT pulmonary angiography and SPECT V/Q scans were similar regarding the presence or absence of pulmonary embolism 95 percent of the time. None of the four patients who had discordant CT pulmonary angiography and SPECT V/Q results were clinically diagnosed with pulmonary embolism, and they remained in good health three months later. When compared with the reference diagnosis using clinical data and CT pulmonary angiography results, SPECT V/Q scintigraphy had a sensitivity of 83 percent and a specificity of 98 percent in diagnosing pulmonary embolism, with positive and negative predictive values of 95 and 94 percent, respectively.

Conclusion: The authors conclude that SPECT V/Q scintigraphy is a viable alternative to CT pulmonary angiography for diagnosing pulmonary embolism, and uses substantially less radiation. They recommend SPECT V/Q scintigraphy for patients in whom CT pulmonary angiography is contraindicated, such as those with allergy, renal impairment, diabetes mellitus, or critical illness, and in women who have proliferating breast tissue.