SPECT V/Q Scans

TO THE EDITOR: It was quite depressing to read yet another major collection of papers on the diagnosis and management of pulmonary embolism in Seminars in Nuclear Medicine, volume 38, November 2008, in which only one of the articles, an excellent paper by Paul Roach et al. (1), mentioned SPECT ventilation–perfusion (V/Q) scanning. This standard nuclear medicine imaging technique was virtually ignored by all the other authors, who wrote of planar V/Q imaging exclusively.

No one anywhere in the world would imagine using planar imaging for regional cerebral blood flow studies with ^99mTc-exametazime (Ceretec; GE Healthcare). This is because the superimposition of activity in overlying brain regions would allow only large perfusion defects in the brain to be seen on the planar images. SPECT is used routinely for precise definition of cerebral perfusion defects. This situation is completely analogous to that with V/Q lung scans.

I have been using SPECT V/Q lung scanning on my patients for 9 y. I use about 50 MBq of ^99mTc-(Cyclomedica Austalia Pty Ltd Technegas) for the ventilation studies, with an acquisition time of 15 min, followed by 250 MBq of ^99mTc-macroaggregated albumin for the perfusion studies, with an acquisition time of 10 min.

The reporting algorithm for these studies is simple. Any perfusion mismatch that you can visualize with our resolution can be called pulmonary embolism.

It is time, in 2009, that nuclear medicine physicians in the United States apply the best technology they have when doing V/Q scans for pulmonary embolism, and planar imaging is certainly not it. The old “I'm used to planar scans” approach must be abandoned in the interest of our patients. Who would go back to planar myocardial perfusion imaging or even think of using planar scans for cerebral blood flow?

Although I know that many love the PIOPED probabilistic approach to reporting V/Q scans and feel comforted when they can state confidently that the scan is indeterminate for pulmonary embolism, the referring physicians do not feel this comfort and are referring their patients more often for CT pulmonary angiography despite its significantly higher radiation burden.

The unfortunate issue for us working outside the United States is that although we are using SPECT V/Q imaging and are confident we are not missing clinically significant pulmonary embolism, our referring clinicians read the U.S. literature, which now regards CT pulmonary angiography as the routine screening test for pulmonary embolism. Our lung scan numbers have fallen as a result. The more the U.S. literature shows of complicated decision trees analyzing the probability that pulmonary embolism is present on planar lung scans, the fewer will be the patients who are referred for scans. As I said at the start, it is quite depressing.