PET/CT: Diagnosing Cardiac Disease—Adding Function to Form

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  A Rubidium-82 myocardial rest and stress perfusion study on Philips GEMINI TF, courtesy of University Hospitals’ Case Medical Center in Cleveland, Ohio.

The physiological nature of PET/CT is proving useful in the detection of coronary artery disease (CAD), providing answers to questions of cardiac function while determining cardiac risk to reduce the percentage of complications and deaths.

PET/CT can be seen as a gold standard for assessing the presence of CAD, or more specifically, for assessing risk of suffering complications of CAD such as myocardial infarction or cardiac death.

In looking at regional myocardial perfusion at rest and stress, if both images are normal, no more is evidence of CAD; if normal at rest, abnormal at stress, ischemia is implied; when both are abnormal, it is very likely the patient has had an infarct or there is a scar ring and the muscle is very likely dead, says James K. O’Donnell, MD, director of nuclear medicine at University Hos-pitals’ Case Medical Center in Cleveland, Ohio, who utilizes Philips’ Gemini TF PET/CT.

A major strength of PET/CT is the ability to combine measures of biology or physiology with CT morphology, according to Frank M. Bengel, MD, director of cardiovascular nuclear medicine at Johns Hopkins Medical Institutions in Baltimore. Not only can CT reveal the anatomy of the coronary arteries or the morphology of the heart, but the PET component can provide additional information such as myocardial perfusion that cannot be extracted from CT, he says.

With higher spatial resolution, attenuation correction and tracers that are more physiologic, photon per photon, PET is more accurate than SPECT. A test that is more accurate could lead to avoiding subsequent follow-up costs that might be associated with unnecessary diagnostic or therapeutic procedures.

“It looks as though cardiac PET studies have somewhat increased specificity and sensitivity compared to SPECT,” says Daniel S. Berman, MD, chief of cardiac imaging and nuclear cardiology at the S. Mark Taper Foundation Imaging Center at Cedars-Sinai Medical Center in Los Angeles. “We think that this is probably most true in patients who have had a previous SPECT study that was unable to reveal if the finding was due to something real or due to soft-tissue attenuation.”

Utilizing PET would be advantageous, for example, in large breasted women in whom artifact is expected. “It is known that breast attenuation artifact is quite common with SPECT and can be corrected with PET/CT,” adds Berman.

The classic PET mismatch

Cardiac PET/CT has proven effective in determining cardiac viability in patients with known CAD and cardiac events. In a patient who has had an infarct and a blockage in the LAD artery, a viability study will confirm whether to bypass, stent or perform surgery. A resting perfusion scan with FDG will reveal if a segment of the heart is not getting enough oxygen but is still viable, as it will shift from fatty acid to glucose metabolism, signifying it might be ischemic but it is still alive, says O’Donnell.

In patients with known CAD, determining a patient’s risk using PET/CT will determine how closely they need to be managed prior to surgery, he adds.

PET/CT in evolution

In the future, a key methodology could be to use more specific molecular targeted tracers to identify more specific components of heart disease, and then fuse a very specific regional PET signal with the high-resolution CT, to localize the PET signal very well, adds Bengel.

Additionally, a new Fluorine-18 labeled tracer for PET-based myocardial perfusion imaging in phase 1/2 clinical trials has the potential to provide even higher resolution than the current PET tracer, Rubidium-82. “This new agent would result in improved image resolution and since it has a long enough half life, it could be used conveniently with exercise—potentially meaning tests can be done with exercise only,” Berman concludes.