Picking the right imaging exam at the right time has always been a challenge. But experts recently offered a blueprint to make the choice easier when it comes to using cardiac PET to assess coronary artery disease, myocardial perfusion, viability, and ventricular function. SNMMI and ASNC this summer published new guidelines for cardiac PET scans. Here is the low down from the physician who led the committee that created the guidelines, Vasken Dilsizian, MD, chief of the division of nuclear medicine at the University of Maryland School of Medicine in Baltimore. While the guidelines aid physicians and technologists in nuclear cardiology, they also help to build awareness for expanded and appropriate use of cardiac PET exams. What do cardiologists need to know about cardiac PET? For starters, the technology represents a significant advancement over SPECT technology in terms of concrete image resolution and quality and lower radiation exposure to patients. It is safe, effective, efficient, equitable, patient-centered and timely, allowing a 30-minute turnaround on results when we need them. So what types of patients benefit most from cardiac PET? When does it trump SPECT? Can it play a role in cardiac disease prevention? And why does it make financial sense? Read on for some expert advice you can start using today.
Why should cardiologists be ordering PET vs. SPECT imaging?
Cardiologists need to look to the current trends in reimbursement and CMS guidelines. CMS has prioritized quality initiatives that include effectiveness. Is the technique effective? Does it have higher diagnostic accuracy than what is being currently applied? Cardiac PET meets that quality measure since its diagnostic accuracy is better than SPECT.
The second quality measure is safety. When it comes to nuclear imaging, cardiac PET offers low radiation exposure. How do we improve our safety for patients, physicians and technologists with regard to radiotracers? With PET imaging, almost all radiotracers for cardiac imaging have shorter half-lives and rapid clearance from the blood, making them much lower radiation exposure to patients, technologists and physicians. So cardiac PET trumps SPECT radiotracers when it comes to safety and radiation exposure as a quality measure.
Third is efficiency. In essence, the technique itself should be short in image acquisition and provide the information in a timely manner. PET certainly, when it comes to image acquisition, is significantly shorter with a 5- to 10-minute acquisition, compared to 20- to 40-minute acquisition with SPECT imaging. This shorter half-life and rapid acquisition allows you to finish a study with rubidium, for example, in 30 minutes from the time the patient goes into the camera to the time the report is being generated compared to minimum 3 to 4 hours with SPECT imaging, if not longer. There is no compromise of quality for shorter time.
The next quality measure is whether it is patient centered. Cardiac PET is patient centered because it can accommodate ill or high risk patients or those with body habitus that may not be appropriate for SPECT imaging. Larger bore cameras can accommodate most patients and we can acquire images with soft tissue attenuation correction that are robust and reproducible, unlike SPECT imaging. There is patient motion, soft tissue, and sub-diaphragmatic visceral activity artifacts with SPECT, while with cardiac PET the soft tissue attenuation is corrected for and patient motion is minimized due to shorter image acquisition time.
The next part involves being equitable: that is, interpreting images independent of patient’s age, race, body habitus and medical condition. [Cardiac] PET does not discriminate, it works for everyone.
And the last measure is timely care. These tests have become very timely in terms of short acquisition time, rapid turnaround of exams and image interpretation. For an emergent patient, we can image and interpret cardiac PET myocardial perfusion images in 30 minutes.
What are the attributes of cardiac PET perfusion that contribute to the high quality of cardiac PET perfusion images?
The main purpose of myocardial perfusion imaging is to assess the state of blood flow in the three main coronary artery vascular territories, which may be reduced due to atherosclerosis, and manifest symptomatically as chest pain or shortness of breath with exertion. Therefore, the closer a radiotracer tracks blood flow at rest and during peak stress, the higher the diagnostic accuracy for detecting coronary artery disease. The current FDA-approved PET radiotracers, rubidium and ammonia, track myocardial blood flow rather well, allowing quantification of regional and global blood flow in absolute terms. When compared to coronary anatomy, the detection of significantly narrowed coronary arteries (sensitivity) or lack of thereof (specificity) is higher with cardiac PET than with SPECT.
Does PET myocardial perfusion imaging contribute to patient radiation exposure? What is the relative risk of exposure to the patient?
Let’s place the entire discuss of radiation exposure in the proper clinical context. All diagnostic nuclear medicine imaging studies expose patients and technologists to low levels of radiation. The goal is to acquire high-quality images for accurate interpretation of the disease in question at the lowest reasonably achieved radiation dose, a concept termed ALARA. Yes, there is radiation exposure to consider but it is as low as possible for each patient and our diagnostic accuracy is far higher with cardiac PET than achievable with SPECT.
What types of patients benefit from cardiac PET?
There are two major applications of cardiac PET: 1) myocardial blood flow assessment to detect coronary artery disease, and 2) myocardial metabolism assessment (with the glucose analogue FDG, fluorodeoxyglucose, that looks at glycolysis) to detect viable myocardium in dysfunctional left ventricular regions, and more recently the inflammatory phase of cardiac sarcoidosis and cardiac device infections. The assessment of myocardial glucose utilization with FDG is a unique application with PET, which cannot be performed with SPECT. Thus, cardiac PET expands the application of cardiac imaging beyond myocardial perfusion and function.
The types of patients who may benefit from PET are those who are at high risk for major cardiac events and for more accurate identification of multi-vessel CAD, particularly when planning for coronary artery revascularization. Relative visual interpretation with SPECT may underestimate the presence and extent of multi-vessel disease. The additional quantitative assessment of absolute hyperemic myocardial blood flow and flow reserve with PET, with its robust attenuation correction algorithm, improves the diagnostic accuracy for detecting single or multi-vessel CAD, and provides incremental prognostic value to visual or semi-quantitative regional radiotracer uptake assessment with SPECT.
Assessment of fractional flow reserve (FFR) across indeterminate coronary artery lesions in the cardiac catheterization laboratory has changed the way interventional cardiology is practiced today. Absolute quantification of myocardial blood flow with PET can provide similar information as FFR, noninvasively, avoiding an unnecessary coronary angiography procedure in a patient if revascularization is not indicated.
Are there any restrictions with regard to patients that we should be aware of?
There are no restrictions or technical limitation in terms of patients undergoing cardiac PET studies, with the exception of the table weight limit for a particular vendor. There are some patient groups, such as diabetic patients, in whom dietary preparations, oral hypoglycemic medications or insulin may impact the quality of FDG-PET images, particularly if the proper blood glucose maintenance guidelines and procedures are not followed. Both diabetic and non-diabetic patients will have high quality PET images as long as the clinician follows the proper dietary guidelines and the protocols.
Why is quantitative myocardial blood flow obtained from cardiac PET so important?
Let’s compare it to SPECT so we understand the importance. When we read SPECT images, we're looking for blood flow equivalence by assessing the distribution of the radiotracer uptake in the myocardium in relation to the typical distribution of the three main vascular territories: LAD, RCA and circumflex artery. Thus, with SPECT, we visually interpret reduction in radiotracer uptake in one vascular territory compared to the others, which is in “relative” rather than in “absolute” terms. We don't know exactly how significantly blood flow is decreased in a particular region or vascular territory. All we know is that one area of the myocardium appears visually worse compared to another area, or compared to a reference region. The problem with the latter is that it assumes that the reference region is normal, when in fact it is possible that all three vascular territories are hypoperfused and abnormal. This can be clarified if myocardial blood flow is assessed quantitatively in all regions, including the reference region.
With SPECT imaging, we tend to underestimate the presence of three-vessel coronary artery disease. We are very good in identifying the most hypoperfused myocardial region, which in the absence of prior history of myocardial infarction corresponds to the culprit coronary artery lesion that may be responsible for the patient’s symptoms. That is not a bad thing. PET can quantify reduction in myocardial blood flow in absolute terms in all three vascular territories, rather than simply identifying the culprit region, thereby providing a more comprehensive assessment of the presence and extent of coronary artery disease. The latter information has implications from detection and treatment of coronary artery disease to prediction and prevention. Recent studies have shown that aggressive medical therapy and/or risk factor modification and prevention have become effective alternative treatment options for coronary artery disease.
What are the appropriate clinical indications for cardiac PET perfusion imaging?
Back in the 1990s when I worked at NIH, we only used PET in patients with large body habitus, in whom robust soft tissue attention and scatter correction improved the diagnostic accuracy of the test, and when SPECT imaging was equivocal, say due to extensive patient motion artifacts or poor image counts. These two indications have not changed much in 2016 when it comes to insurance companies giving pre-approval for myocardial perfusion PET studies, but it needs to. Even though PET and PET/CT cameras are now prevalent in almost every major hospital, driven by imaging oncology patients, the technique is significantly underutilized in cardiac patients. This is one of the reasons why the members of ASNC and SNMMI wrote the recently published joint position statement on the clinical indications for myocardial perfusion PET.
Today, any myocardial perfusion study that requires pharmacologic stress should and can easily be done with PET. Absolute myocardial blood flow quantification with PET may identify coronary artery disease earlier and monitor progression or regression of disease with risk factor modification, medical therapy or revascularization. For example, absolute myocardial blood flow will detect early coronary artery vasculopathy in patients who have undergone cardiac transplantation, where the disease is diffuse rather than focal. Relative radiotracer uptake assessment with SPECT will miss those patients, and quantitation will identify them better. Any time you feel that visual interpretation or relative radiotracer uptake may underestimate a specific disease process; you may consider quantitative PET imaging. I also feel strongly about using PET in younger patients who are at higher lifetime accumulated radiation risk.
Why do you think that cardiac PET myocardial perfusion imaging is really not more widely utilized?
There are a number of reasons, number one being education. There is a lack of awareness of the benefits that cardiac PET brings. There also is skepticism of any new technology. We are not urging the use of newer and more expensive technology but rather exams that are better for patients.
How can cardiac PET assist in preventing disease and ultimately save money?
Cardiac PET is useful as U.S. healthcare transitions from simply detecting disease at a late stage and treating it with expensive procedures and care. This allows us to be a lot more responsible and do risk factor modification, i.e. controlling cholesterol, blood pressure, smoking, dietary and life habits, so that we can prevent the disease. This will have a major impact on the healthcare dollar, hopefully reducing the high costs of the late stages of disease. The best way to do it is actually to spend a lot more time in the prevention world. The advantages of PET over SPECT is that it quantifies blood flow in absolute terms. We don’t have to wait until symptoms like chest pain or shortness of breath are present. We can get ahead of a disease process. The fact that PET can quantify blood flow in absolute terms is like measuring your blood sugar level, your blood potassium or sodium level. So we're actually quantifying things and therefore we'll be able to detect early phases of decreasing regional and global myocardial blood flow, before we have the patient become symptomatic. We can combine common sense diet, lifestyle changes, and risk factor modification and monitor progression or even regression of arterial sclerosis in response to the medical therapy, and lifestyle change, which we couldn't do with SPECT imaging.
I've been a proponent of this for about 20 years now. The transition to PET from SPECT is not only desirable, but inevitable. PET opens up new paths for imaging and widens the existing indications while remaining innovative and current in medicine with new reagents and the ability to quantify.
What should physicians expect from a final cardiac PET report?
The final cardiac PET report should consist of the visual interpretation of relative radiotracer content in the five left ventricular myocardial regions and three vascular territories, which should be superior to SPECT on the basis of higher count rate images, reliable attenuation and scatter correction, high spatial and temporal resolution. In addition, data on absolute quantitative hyperemic blood flow and flow reserve values should be interpreted as confirming and verifying the visual finding or providing incremental information that wasn't identified visually. The concluding remarks should be in terms of visual and quantitative blood flow interpretation of the data, regional and global functional information, and if relevant, prognostic implications.
Some physicians right now are not comfortable in interpreting the quantitative blood flow data because it was not part of their educational and board certification training. That's where some of the professional society’s come in in offering education at meetings and in publications. Take them up on it. This can really make a difference for your patients.
Read the ASNC imaging guidelines/SNMMI procedure standard for positron emission tomography (PET) nuclear cardiology procedures.
- Dilsizian V, Bacharach SL, Beanlands SR, Bergmann SR, Delbeke D, Dorbala S, Gropler RJ, Knuuti J, Schelbert H, Travin M. ASNC Imaging Guidelines/SNMMI Procedure Standard for Positron Emission Tomography (PET) Nuclear Cardiology Procedures. J Nucl Cardiol 2016 Oct;23(5):1187-226.
- Bateman TM, Dilsizian V, Beanlands RS, DePuey EG, Heller GV, Wolinsky DG. ASNC/SNMMI Position Statement on the Clinical Indications for Myocardial Perfusion PET. J Nucl Med 2016 Oct;57(10):1654-1656.