Cardiac PET: When It’s Better, Stronger, Faster

At Brigham and Women’s Hospital, like many hospitals and medical centers across the country, cardiac PET is emerging as the go-to imaging modality for a growing list of clinical scenarios. That’s largely because, as laid out in the updated ASNC/SNMMI guidelines for cardiac PET, the burgeoning modality has the inherent ability to do something traditional cardiac SPECT cannot do: assess myocardial blood flow (MBF).

Quantification of MBF “may provide diagnostic and prognostic information earlier than visual interpretation of relative radiotracer uptake, which is a fundamental disadvantage of the conventional SPECT technique,” Dilsizian et al pointed out in the guideline.

Sharmila Dorbala, MD, MPH, Brigham and Women’s director of Nuclear Cardiology and a member of the committee that wrote the guidelines, says SPECT may “still remain the workhorse, but PET is clearly advantageous in specific clinical scenarios.”

She believes cardiac PET deserves more promotion among ordering and interpreting physicians to ensure that it comes to mind whenever it could and should stand out as the right test for the right patient.

“We don’t want to perform SPECT in a patient who would derive more benefit from PET,” Dorbala says.

Why PET?

In its overview, the ASNC/SNMMI guideline committee wrote that that cardiac PET images offer diagnosticians, among other pluses, better spatial and temporal resolution than SPECT. Hence cardiac PET’s rise as a frequently surer fit, the authors noted, with the quality- and value-based payment requirements set by the Centers for Medicare & Medicaid Services.

In fact, the guideline committee commented, PET myocardial perfusion imaging is:

“effective (high diagnostic accuracy), safe (low radiation exposure), efficient (short image acquisition times), and patient-centered (accommodates ill or higher risk patients, as well as those with large body habitus), providing equitable and timely care.”

“We carefully select patients who could benefit from PET,” says Dorbala, who teaches cardiovascular imaging at Harvard Medical School and works at Brigham and Women’s Carl J. and Ruth Shapiro Cardiovascular Center. “Primarily those are patients who undergo pharmacologic stress testing, those who are unable to exercise and those who have high-risk coronary disease.”

And that’s just for starters.

“The other group that may be specific to tertiary care medical centers, is cardiac transplant. We specifically schedule a cardiac PET in individuals with prior cardiac transplantation so that we can quantify MBF.”

Further, Dorbala finds cardiac PET useful in younger patients, not least because of its lower radiation dose, as well as patients with complex congenital heart disease and early coronary atherosclerosis.

“We also offer exercise PET using 13N-ammonia,” she adds. “We use that with the hope of providing a physiological stress test with the least amount of radiation but the best image quality.”

Dorbala then ticks off other scenarios in which cardiac PET is increasingly indicated, including cardiac sarcoidosis and cardiovascular infections.

“In individuals with prosthetic valves, who come in with fevers, suspected endocarditis—we generally start with an echocardiogram and sometimes a transesophageal echocardiogram. But echocardiography may or may not show much information,” she says. “In those cases, we are performing FDG-PET imaging to look for infection around, for example, a prosthetic valve, prosthetic vascular material, or pacemaker/other implanted devices.”

There’s still more. Cardiac PET is also indicated to check myocardial viability, Dorbala says, in patients with heart failure from severe coronary artery disease and for whom physicians are considering bypass surgery.

There are a lot of comorbidities in some of these patients, she adds, and the risk vs. benefit ratio is often unclear.

“If it’s not a straightforward coronary revascularization, the surgeons want to make sure that the risk of surgery is worth it,” Dorbala says. “We will sometimes perform a viability scan with PET to establish that the jeopardized myocardium is still alive and not scarred, for example.”

Sparking awareness

Turning back to the question of how to raise awareness of cardiac PET’s virtues, Dorbala reflects on how far the modality has come since her group started using it about 12 years ago. “It’s a challenge. It’s not been an overnight change of practice,” she says, adding that, initially, they started with changing inpatient pharmacologic stress patients from SPECT to PET.

“We contacted the physicians and discussed that the PET scan offers better quality and more accurate images,” Dorbala says. “We would ask if it was okay for us to switch the pharmacologic stress SPECT scan to a pharmacologic stress PET scan. We started with inpatients first because it was logistically feasible as we did not have to deal with insurance preauthorization.”

In making the ask, the team emphasized the quick turnaround times for accurate results—30 to 45 minutes.

Today more physicians know about cardiac PET, but old habits die hard. So, on a daily basis, Dorbala and colleagues have an imaging physician, a fellow in training, review every nuclear cardiology test request. Fellows check the patient’s EMR, take notes and try to match the test requested with the clinical question.

“If it seems the patient would benefit from PET myocardial perfusion study, but the test is ordered as a SPECT, then we would typically make a phone call to the referring doctor, discuss the specifics of the case and, if appropriate, suggest that PET may be better,” Dorbala says. “If they agree, the order gets changed. It’s a truly consultative approach.”

That’s clearly not much different than 12 years ago, but Dorbala places her hopes in incremental adjustments rather than sweeping cultural change.

She believes radiotracer availability and its attendant costs were an issue then and remain one now, but limited awareness of PET is proving a difficult hurdle to clear. 

“All cardiologists are trained in SPECT, but not all are trained in PET,” Dorbala says, adding that this probably explains the tendency for physicians to continue ordering SPECT as the initial test.

Improving outcomes

If cardiac PET succeeds in helping heart doctors help heart patients get better outcomes, it will owe not only to the modality’s clinical utility but also it cost effectiveness. For more on the latter, see “Cardiovascular PET Will Become Mainstream in the Changing World of Value Imaging” by Gary V. Heller, MD, PhD, in this e-newsletter.

For Dorbala, the quantitative MBF capability is a “first-among-equals” clinical advantage of cardiac PET over cardiac SPECT in the scenarios she’s run through and more.

“It adds another layer for diagnostic certainty, beyond regular SPECT perfusion imaging,” she says. “It gives us a lot more certainty in calling a normal scan normal.”

“A lot of institutions have PET scanners, especially for oncology applications. Why aren’t they using it for cardiac?” she wonders aloud.

Looking forward, she believes the good working relationship her imaging group has with referring physicians will help advance cardiac PET as the right imaging for the right patient, perhaps more often than not.

With the referring physicians, “if they realize that all patients who require pharmacologic stress benefit from a PET, that may be building awareness,” says Dorbala. “At the same time, we don’t just want to forget about exercise SPECT and send everyone for a PET scan. That would not be desirable either.

“We want to use PET optimally in the right patient group so that the diagnostic accuracy is maximized and patients benefit.”