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V/Q Imaging Compared with SPECT for Detecting Pulmonary Embolism

October 21, 2009
Written by: , Filed in: Chest Radiology
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For many years, the ventilation-perfusion (V/Q) scan was the cornerstone procedure for diagnosing pulmonary embolism (PE).

More recently, CT pulmonary angiography (CTPA) has become the most frequently used diagnostic technique for pulmonary embolism.

There are limitations, perceived and real, associated with current planar V/Q imaging. For this reason, a number of authors have examined the potential role of SPECT imaging to improve the accuracy of V/Q imaging for diagnosing pulmonary embolism.

In a recent article published in Seminars in Nuclear Medicine, the authors reviewed the advantages and application of SPECT to V/Q imaging for diagnosing pulmonary embolism. They point out that studies in animals as long ago as 25 years found SPECT to be more sensitive for clot detection compared with planar imaging.

Later studies in humans comparing the two methods in the setting of PE found SPECT to be both more sensitive and more specific.

The limited available studies comparing SPECT V/Q imaging with CTPA have found the methods to be highly comparable in accuracy.

Using the criteria of a single mis-matched defect to indicate PE, one study found sensitivity and specificity for SPECT to be 97% and 91% respectively.

SPECT images can also be fused with CTPA results to improve accuracy of this exam in order to accurately diagnose pulmonary embolism.

The authors demonstrate such a case where CTPA was considered to possibly be positive in a smaller peripheral artery, but the fused SPECT perfusion exam was entirely normal, ruling out PE.

Reporting of results from SPECT may follow the general form from the PIOPED studies (Prospective Investigation of Pulmonary Embolism Diagnosis); however, further assessment of the best approach will be needed.

The authors also note that other clinical applications may benefit from the use of SPECT.

For example, functional pulmonary data prior to resection of a single lobe would be better determined with SPECT as opposed to planar imaging.

Conclusions
For diagnosing PE, SPECT V/Q imaging is superior to planar imaging. We should strive to optimize V/Q imaging through implementation of this technique.

Reviewer’s Comments
For patients with inconclusive CTPA results, SPECT perfusion imaging carries the advantage of allowing more direct evaluation of the perfusion pattern in the territory anatomically corresponding to the vessel in question.

Years ago, I had a case where SPECT identified a perfusion defect not seen on planar imaging that was confirmed to be PE by conventional pulmonary angiography; and I do not doubt the better sensitivity of SPECT. On the other hand, do we really know that such a finding has clinical significance?
Author: David Bushnell, MD

Reference:

Roach PJ, Bailey DL, Harris BE. Enhancing Lung Scintigraphy With Single-Photon Emission Computed Tomography. Semin Nucl Med; 2008;38 (November): 441-449

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