Picking the best cardiac imaging test
"Coronary disease is a growing area for cardiac imaging. The trend we are seeing is that both anatomic and structural imaging play a very important role in certain patient populations. And physiologic imaging is also very relevant for patient populations that are higher risk," DiCarli explained. "So the way the clinical evidence is shaping up, for low and intermediate risk patients, CT seems to be a very effective test to exclude significant coronary disease and help clinicians assess the burden of atherosclerosis. It is very difficult to challenge CT on its negative predictive value as a rule-out test for coronary disease."
For patients who have more coronary disease, or known coronary disease, these patients are better suited for physiological imaging, such as nuclear or MRI, DiCarli said.
Patients with more advanced cardiovascular disease often have higher amounts of calcium in their coronary vessels, making CT more difficult to interpret.
"In these patients, functional imaging for the detection of ischemia and ventricular function starts to become become more relevant and can potentially lead to better decisions," Beanlands said.
The best cardiac centers will have a mix of CT, MR, SPECT and PET to offer the best imaging modality to answer specific questions is specific patients. Over the past decade, there has been a reduction in the number of SPECT exams, but an increase in the number of cardiac CT, MRI and PET exams. The use of echocardiography is still the most widely used cardiac imaging modality and is considered the frontline imaging test for most patients, he said.
CT is considered an anatomical test that can show very detailed images of the anatomy, but until recently, it did not offer functional information. Even if a coronary has a clear blockage, CT cannot offer a physiological assessment of whether the blockage is flow limiting or not. At least, that was the case until multiple studies validated fractional flow reserve CT (FFR-CT), which uses computational fluid dynamics to estimate the coronary blood flow past a lesion or series of narrow vessel segments. FFR-CT was also included in the 2021 chest pain guidelines. However, FFR-CT has the barriers of access and cost. While a growing number of centers are using it, the overall number is still relatively small.
"Cardiac MRI is also a wonderful option for imaging coronary disease. MRI also can quantify perfusion and patients with microvascular disease," DiCarli said.
In addition, he added, MRI has the best soft tissue delineation of any of the cardiac imaging modalities. However, it has certain limitations, including high costs, potentially limited access and the longer exam times.
PET vs. SPECT, according to two imaging experts
Beanlands, a former president of the American Society of Nuclear Cardiology (ASNC), said single photo emission computed tomography (SPECT) is the most widely used nuclear imaging technology in both the U.S. and around the world.
"It's been a go-to technology for many, many years, but it has pitfalls that make it more challenging," he explained. "People are developing technologies to offer quantification of flow, which has some advantages."
Beanlands also said that clinical studies to validate SPECT quantification software are still ongoing, but that technology is already in clinical use with positron emission tomography (PET). PET in many ways is seen as a better technology, with clearer imaging, use of fused CT for attenuation correction, the CT also adds anatomical imaging, and it has the ability to calculate coronary flow reserve. But PET adoption has been limited because of the cost for these systems and the required change in radiotracers and workflow.
"PET is emerging more and more and is becoming widely used, because it has the advantage of this flow quantification, which really can distinguish the patients who are high risk and low risk. It has the potential to risk stratify patients with that important piece of information," Beanlands explained. "And not only for coronary disease, but for what we call microvascular disease."
He said PET may play an increasing role to better quantify the extent of these microvascular diseases, like myocardial ischemia with no obstructive coronary arteries (INOCA), or myocardial infarction with non-obstructive coronary arteries (MINOCA).
For years these patients, particularly women who have a higher prevalence of these conditions, where brushed off and told the chest pain was just in their head, because there was no clearly seen obstruction inside their coronaries, Beanlands said.
Another advantage associated with PET is that the rubidium radiotracer used only has a 75 second half life, so the exams are very faster compared to SPECT.
"I think PET is ahead of SPECT," Beanlands said. "Don't get me wrong, SPECT is useful and you want to be using it in certain patients. But, PET as significant advantages pretty much in all areas."
DiCarli agreed PET is a better imaging modality and more accurate. However, while it may be better, he said access to PET is still limited, so SPECT will likely remain the workhorse nuclear technology for years to come.
He added that PET also has a new imaging agent in trials to improve perfusion imaging that does not require an on-site cyclotron, or the use of an expensive rubidium generator. DiCarli said the agent can be produced by commercial cyclotrons off-site and sent as unit doses to the hospital. This will lower the cost of ownership for PET systems and accelerate wider adoption. He said it is possible the new agent may see FDA clearance in the next two years.
"That will help open access in ways that we have not been able to realize with the currently approved FDA radiopharmaceutical," DiCarli said.
Cardiac inflammation imaging
Another trend in cardiac imaging is the area of visualizing inflammation in the heart. This includes amyloidosis, sarcoid, pericarditis, myocarditis and other inflammatory disorders.
"We have seen over the last fives years a tremendous growth in the referrals for those patients," DiCarli explained. He added that imagers also have learned how to use the complimentary advantages of PET and MRI together for a more accurate diagnosis.
Beanlands said inflammation also plays a role in atherosclerosis, so being able to image it may play a future role in how these patients are managed.