Hybrid approach pulls together ischemia imaging

August 1, 2007

Angiography may be the gold standard for documenting coronary artery disease, but it cannot predict the physiological relevance of coronary stenoses. Multislice coronary CT angiography is equally unable to determine if a lesion seen on imaging is causing ischemia. Deciding when to intervene and revascularize is thus problematic, particularly when patients have intermediate stenoses.

Angiography may be the gold standard for documenting coronary artery disease, but it cannot predict the physiological relevance of coronary stenoses. Multislice coronary CT angiography is equally unable to determine if a lesion seen on imaging is causing ischemia. Deciding when to intervene and revascularize is thus problematic, particularly when patients have intermediate stenoses.

The answer to this dilemma could be adding perfusion imaging to CT-based assessments of coronary vasculature, according to Prof. Dr. Philipp Kaufmann, director of nuclear cardiology at the University Hospital Zurich. Functional nuclear imaging could help doctors decide whether an observed coronary lesion is causing ischemia or not, he told delegates at this year's European Congress of Radiology in Vienna.

"Perfusion is what is telling us most about prognosis. It is clear that we have to look at function," he said.

Kaufmann and his colleagues first attempted to add perfusion information to images of vascular anatomy by taking data from a four-slice CT system and a PET scanner. Data from the separate scanners were fused using a dedicated software package. The coronary arteries could hardly be seen in these initial images, however, because of the relatively low spatial resolution of the four-slice CT.

Visualization of the coronaries has improved steadily in line with advances in multislice CT technology. Many institutions, University Hospital Zurich included, now have access to combined PET/CT scanners, though such systems are not essential for this application, Kaufmann said. His group often acquires CT data for fusion from a stand-alone scanner, but they have moved on from four-slice CT to a 64-slice system.

The necessary functional information could alternatively be provided by SPECT. Kaufmann's group has investigated the importance of SPECT/CT when assessing low-risk patients with suspected myocardial infarction. An image from this work was chosen as the U.S. Society of Nuclear Medicine's 2006 Image of the Year.

"Integrated perfusion imaging and CT has a lot of potential," he said. "I am not telling you to do SPECT or to do PET. I am not telling you that you need a hybrid scanner. The advantages are that you get anatomy and function together. This means that you get convincing presentations, which is very important for referring doctors."

Having both sets of data in a single image reveals more than a side-by-side comparison would, Kaufmann said. Experience suggests that patients with multiple lesions and complex coronary artery disease benefit most from the combination of perfusion and anatomy. Three-D views of the myocardium, its perfusion, and the coronary artery tree can eliminate uncertainties over the relationship between perfusion defects and diseased coronary arteries in watershed regions.

He recommends that the image fusion be performed by expert practitioners who are aware of the technique's limitations as well as its potential. This is a more important issue than questioning which department they are based in, Kaufmann said. He urged radiologists and nuclear physicians to forget their disagreements and stick together as "imaging doctors."

"Image fusion for cardiac imaging may have had a difficult start, but it will come," he said. "The most important point is that our teams are fusing. In our hospital, radiology, nuclear cardiology, cardiovascular surgery, and cardiology are all working together. I think that is what makes us the winning team."