Heart & Vascular Institute Physician eNewsletter - Fall 2011
The wide availability of CT coronary angiography for assessing coronary artery disease (CAD) introduces an interesting question concerning cardiovascular imaging. What is the best way to manage patients: stress myocardial perfusion imaging (MPI) to assess physiologic criteria; or CT coronary angiography to identify anatomic criteria?
"These are two very different tests that address very different questions," says Rory Hachamovitch, MD, Section of Cardiovascular Imaging, Department of Cardiovascular Medicine, Cleveland Clinic. The popular CT test—despite its aesthetic appeal, magazine-like images, non-invasive nature and the argument that it’s "the most accurate test out there"—could result in overestimating the severity of CAD. "Nothing is perfect," Hachamovitch says, noting that many agree CT angiography is not as accurate as once perceived.
That’s because of a negative conflict value, Hachamovitch explains.
"By CT angiogram, if a patient does not display atherosclerosis in his or her arteries, CAD has not started—so you can tell a lot by a negative test," he says. However, a positive test "takes the shine off of the appeal" of CT angiography. "We are realizing what this test can and cannot do."
On the left hand side of the panel are two images from a normal CTA study. No calcification or plaques are present in the coronary arteries. The two panels on the right show an example of an abnormal CTA study. The left anterior descending artery has severe proximal with both calcified and uncalcified plaque present (white arrow).
Specifically, CT can identify plaque, its composition and texture—whether plaque is smooth or irregular. This allows a radiologist or physician to identify various cardiac risks associated with the type of plaque presented in the image. CT indeed answers the question: Does my patient have atherosclerosis? "CT angiography can keep a doctor from sending a patient to the cardiac catheterization lab unnecessarily, and a physician can get more definitive answers non-invasively—in that role, CT does a good job," Hachamovitch says.
However, CT angiography does not answer the question: At what stage is the coronary artery disease? It cannot measure the functional significance of coronary stenosis or whether individual plaque is likely to be the site of a future acute coronary event.
"If you know the disease is there and the question is: Is there enough disease present?, then perfusion imaging is the answer," Hachamovitch says.
Historically, stress MPI was performed through SPECT imaging, and as PET scanning caught on in the past few years—its advantages being the ability to gather more accurate information with less radiation exposure to patients—PET was adopted as a physiological-based approach. While SPECT is a well-established modality, PET has the same accuracy for detecting CAD with better image quality and the ability to quantify absolute myocardial blood flow. This has been useful in identifying 3-vessel CAD. Plus, a flow rate can serve as a helpful prognostic marker for adverse cardiac events: It could predict risk of heart attack.
Ultimately, one test is not better than the other. And, it is possible that integrating anatomical and physiological imaging can maximize the strengths of both tests. CT angiography can be used as an initial test to exclude CAD. And, MPI can identify the best therapeutic approach: medical vs. possible revascularization. Borderline results from either test can be further investigated with complementary imaging.
"While CT angiography will not miss high-risk CAD, it can fail to determine the best therapeutic approach—but by only using a physiological approach (PET MPI), there is the possibility of false-positive and false-negative findings," Hachamovitch says.
"There is no perfect test," he continues. "But these tests can be used in combination, possibly using CT angiography as the initial test and PET MPI as a secondary test for those who present with CAD. It’s important for physicians to make this decision on a case by case basis."