V/Q SPECT protocol provides best practice for pulmonary embolism

A multi-step study has determined that SPECT is a superior method of ventilator-perfusion (V/Q) imaging for the diagnosis of pulmonary embolism (PE) and also has indicated an optimal imaging protocol for its application, according to the second phase of the study published May 1 in the Journal of Nuclear Medicine.

Pierre-Yves Le Roux, MD, a researcher from the Université Européenne de Bretagne, Brest, France, and colleagues have identified a specific method of V/Q SPECT imaging and evaluation for patients with suspected PE for greatest possible diagnostic capability.

“For V/Q SPECT interpretation, a diagnostic cutoff of 1 segmental or 2 subsegmental mismatches seems best for confirming or excluding acute PE,” wrote the authors.

This SPECT study comes on the heels of a former study that compared V/Q SPECT with conventional planar V/Q scintigraphy, which found the former to be more powerful for the detection of PE. This phase sought to breakdown the best diagnostic criteria and involved a comparison of varying protocols that was independently verified by statistical analysis using an algorithm for PE diagnosis. 

A total of 321 patients ages 18 to 95 years with a mean age of 72 were included in the study over a period of 28 months. Sonography was used to verify presence of proximal deep vein thromboembolism in 43 subjects, who were excluded. The remaining 278 patients were referred for V/Q PE imaging. Subjects who were indicated for planar V/Q scintigraphy also underwent V/Q SPECT imaging with injection of Tc-99m macroaggregated albumin and administration of M-81 Kr gas at the same setting with patients staying in a supine position. Resulting scans were interpreted independently by two specialists blind to medical history as well as study objective and patient outcomes. PE was diagnosed in 55 patients, or 20 percent of subjects.

A statistical analysis was conducted using receiver operating characteristic curves based on numbers of mismatch defects and subsegmental mismatch defects. Of all tested protocols, diagnostic performance was at its best with a cutoff of at least 1 segmental or 2 subsegmental mismatches, a criteria that revealed a sensitivity of 0.92 with a 95 percent confidence interval (0.84-1) and specificity of 0.91 with a 95 percent confidence interval (0.87–0.95). Negative V/Q SPECT scans resulted in low, intermediate and high clinical probability of PE at 0.010, 0.037 and 0.119, respectively. Positive V/Q SPECT findings for the same categories of clinical probability were found to be 0.531, 0.814 and 0.939, indicating strong evidence for the diagnostic use of V/Q SPECT for patients with suspected PE.

“Indeed, many studies have suggested that the transition from planar imaging to SPECT may improve the diagnostic performance of V/Q imaging for the diagnosis of PE,” wrote the researchers. “However, only sparse data based on an objective reference test are available, and the criteria used for interpretation varied widely. Before integrating V/Q SPECT into routine practice, three validating steps are needed: the first step was the validation of V/Q SPECT accuracy compared with a validated diagnostic strategy, which was the aim of our previous study; the second step, addressed in this study, is the validation of interpretation criteria dedicated to V/Q SPECT; the last step will be a large management outcome study including V/Q SPECT in the diagnostic algorithm, in which patients with a negative diagnostic work-up would be left untreated and followed up over time.”