Cardiac PET/CT Fills in Gaps Left by SPECT

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 Daniel Berman, MD, director of cardiac imaging and nuclear cardiology, Cedars-Sinai Medical Center

As SPECT imaging migrates to private physicians offices, PET/CT provides hospitals an opportunity to recapture some of that business.

Cardiac imaging is in a state of evolution with traditional techniques giving way to advanced imaging tools that promise to improve diagnosis and treatment and also help hospitals grow their cardiac imaging services. High-resolution PET/CT is one such technology. Cedars-Sinai Medical Center in Los Angeles, Calif., is one of a handful of institutions leading the charge in cardiac imaging by deploying the latest 64-slice hybrid imaging technology in its cardiology department.

Cedars-Sinai Medical Center ranks among the country’s best, and its commitment to leading-edge research and excellence in the non-invasive cardiac imaging program is recognized worldwide. Performed in the S. Mark Taper Foundation Imaging Center at Cedars-Sinai, the center’s non-invasive cardiac imaging services include nuclear cardiology with PET and SPECT, cardiac MRI, and dual-source cardiac CT.

Daniel Berman, MD, director of cardiac imaging and nuclear cardiology and a world-renowned expert in nuclear cardiology and cardiac imaging, is the chief architect of Cedars-Sinai’s cardiac PET/CT program. He discusses the shortfalls of current cardiac imaging tools and peers into current applications as well as the future promise of high-resolution cardiac PET/CT.

Q: For years, SPECT has been the tried-and-true nuclear cardiology cardiac imaging tool. Does SPECT fully meet cardiac imaging needs?

A: No. In fact, the drawbacks to cardiac SPECT are well-known and widespread. The most troublesome drawback is that SPECT often underestimates myocardial perfusion abnormalities. It’s possible and not uncommon for patients with advanced multi-vessel disease to have a balanced reduction in blood flow. Uniform reduction in blood flow can be interpreted as normal myocardial perfusion. In these cases, the SPECT study may be read as normal, so cardiologists may miss some patients with advanced disease.

Q: Clearly, there is room for improvement. Does high-resolution cardiac PET/CT improve on SPECT imaging?

A: Hybrid PET/CT brings cardiac imaging new opportunities; its advantages fall into several categories.

For starters, image quality is clearer. Systems like ours, the Siemens Biograph 64, offer higher resolution than SPECT cameras with the added benefit of very effective attenuation correction. Both sensitivity and specificity are higher than SPECT. These advantages translate into fewer inconclusive results, which may translate into fewer follow-up studies, optimizing both the physician’s and patient’s time and enabling cardiologists to begin any necessary interventions in a timelier manner.

In addition, unlike SPECT, which does not allow image acquisition during exercise-induced stress, cardiologists can image patients during stress with PET/CT. With SPECT, the assessment of function is performed after stress when stress-induced abnormalities of ventricular function may have resolved. With PET/CT, on the other hand, physicians can compare vasodilator stress to rest and thus view and analyze the effects of stress on the heart muscle.

Finally, PET/CT provides the potential to calculate blood flow and determine whether or not blood flow is diminished in a particular region of the heart. SPECT does not allow cardiologists to measure myocardial blood flow in specific regions.

Q: PET/CT can provide additional diagnostic information and benefit patients with uniform reductions in blood flow. Are there other patient populations that may benefit from PET/CT rather than SPECT studies?

A: Absolutely. SPECT tends to produce attenuation artifacts in certain patients, particularly obese patients and large-breasted women. PET/CT provides excellent attenuation correction, so it is well-suited to these patients. I believe that certain Cedars-Sinai patients will see an immediate benefit with the deployment of our PET/CT scanner: patients with an equivocal nuclear scan and initial studies for obese and large-breasted patients.

Q: Hospitals need to consider questions beyond the clinical advantages of technologies such as PET/CT. In the current fiscal climate, any investments should be cost-justified. What are the economic, throughput or competitive advantages to adding cardiac PET/CT to a facility’s imaging portfolio?

A: Hospitals have seen a significant portion of myocardial perfusion SPECT imaging migrate to private physician offices. PET/CT provides hospitals with an opportunity to recapture some of that business and differentiate themselves from private practices by offering a more sophisticated exam.

In addition, there are throughput and volume advantages associated with cardiac PET/CT. It takes several hours to complete a myocardial perfusion SPECT study. A high-resolution cardiac PET/CT scan on our Biograph PET/CT, on the other hand, can easily be completed in 30 minutes, so sites can increase their volume while offering a more complete scan.

Q: Cardiac PET/CT is in its infancy. How do you envision technology and applications evolving over the next few years?

A: The use of PET to provide a measure of absolute blood flow will move from the research realm into routine clinical practice. Hybrid, high-resolution PET/CT images provide cardiologists additional information about perfusion and anatomy that we never had in the past.

Another application that shows promise is the use of PET/CT to measure endothelial dysfunction to identify early coronary disease and determine patients at high-risk for a coronary event.

Similarly, in the future, cardiologists may turn to high-resolution PET/CT scanners to characterize activity associated with coronary artery disease and vascular disease. Current imaging techniques limit cardiologists to a static anatomic image of stenosis; however, we know that a 50 percent blockage filled with inflammatory cells may be more likely to rupture and lead to a heart attack than a 70 to 80 percent stable stenosis. The hitch is identifying inflammatory activity of coronary plaque. PET provides a tool to image glucose activity, which may identify rupture-prone plaque. Currently, researchers at Cedars-Sinai Medical Center and Massachusetts General Hospital are undertaking a research project that uses PET/CT to analyze the degree of inflammatory activity and consequent risk for a coronary event.

82Rb stress and rest study showed infero-lateral wall ischemia at stress with normalization of perfusion at rest, suggesting reversible left circumflex territory ischemia. Image courtesy of Emory Crawford Long. 

Q: There’s a great deal of research in the clinical realm. What about on the tracer front? Can we expect to see new tracers for cardiac PET/CT?

A: Right now, rubidium-82 is the primary isotope used in cardiac PET/CT; however it isn’t ideal, particularly in terms of its resolution characteristics. One current research focus is fluorine-labeled tracers. A fluorine-labeled agent currently in Phase 1 development provides several advantages. It offers improved uptake characteristics in terms of its properties and flow. The fluorine-labeled agent provides a linear uptake relationship and remains in the heart longer than rubidium-82, so it offers flexibility by uncoupling the tracer injection and the scan. This will allow the performance of exercise PET scans currently not feasible with rubidium-82.

The fluorine-labeled compound also could open the cardiac PET/CT market to smaller centers that might not have the volume to justify a dedicated cardiac PET/CT camera. Because it offers some flexibility in terms of time, such sites could slip a cardiac case among the oncologic PET/CT caseload.

Q: Hospitals across the country are expressing increasing interest in cardiac PET/CT. Can you offer any advice in terms of implementing a solution?

A: Clearly, the first step is raising the funds to purchase the scanner. For large sites, cardiac PET/CT can be cost-effective because it offers a shorter imaging time than cardiac SPECT. Sites can increase patient throughput with only a slight increase in staff and may be able to increase utilization of their PET/CT scanner. It’s my understanding that cardiac PET/CT becomes cost-effective with a minimum of four patients a day; our system easily accommodates more than double the four patient threshold.

It’s important to remember that PET/CT requires a larger room than cardiac SPECT. In fact, the space requirement is double that of a nuclear cardiology suite. The other element is staff training. Nuclear cardiologists require training in terms of interpretation, and technologists need to be licensed in both nuclear and CT modalities.