Both PET and CT are excellent cardiac imaging modalities, but does their individual value hold up when combined in a hybrid scanner? The tentative answer seems to be no, with a few exceptions.
One of the benefits of a hybrid PET/CT test is acquiring the coronary artery calcium (CAC) score along with perfusion information, without the need for contrast and with an average of 1 mSv of radiation exposure. Another benefit for larger practices is the ability to utilize the scanner in a more comprehensive manner: half the day for PET imaging and the other half for CT imaging. While smaller practices might not be able to justify the expense of a hybrid scanner, usually upwards of $1.5 million, it does offer slightly higher resolution and slightly faster throughput.
While hybrid scanners have found a niche delivering perfusion and calcium information, it is rare for them to be used to acquire simultaneous CT angiography data. “At Cedars-Sinai Medical Center, we have a dedicated cardiac PET/CT scanner that is used for simultaneous rest/stress myocardial perfusion imaging and coronary calcium scanning. Simultaneous PET myocardial perfusion and coronary CT angiography studies are not performed, since it is not known prior to testing with either of these examinations if the other test will be indicated,” says Daniel S. Berman, MD, director of cardiac imaging and nuclear cardiology at Cedars-Sinai.
Likewise, the dedicated cardiac PET/CT scanner at Brigham and Women’s Hospital in Boston is utilized for simultaneous perfusion and calcium imaging, but rarely—in less than 5 percent of PET patients—to acquire simultaneous CT angiography data, partly due to concerns about radiation exposure, says Sharmila Dorbala, MD, director of nuclear cardiology. “The combined studies are not recommended unless we have a very specific clinical question that is best answered by both studies,” Dorbala says.
K. Lance Gould, MD, executive director of Weatherhead P.E.T. Center for Preventing and Reversing Atherosclerosis at the University of Texas Medical School in Houston, and colleagues have published protocols to lower the radiation dose of PET/CT imaging to be within the range of 7 to 8 mSv, compared with 3 to 5 mSv for a conventional PET study (J Nucl Med 2008; 49:738–745). Researchers reconstructed the resting perfusion images using post-stress cine CT attenuation data, thus reducing the CT radiation dose by 50 percent.
“Although we can achieve these lower doses with our protocols, the commonly used standard protocols, which include calcium scoring and/or a CT angiogram, may give patient doses of 15 to 20 mSv,” he says.
Vasken Dilsizian, MD, a professor of medicine and radiology at the University of Maryland Medical Center in Baltimore, agrees that one of the two tests should be sufficient to answer the clinical question. “In certain cases where the patient is diabetic or has a lot of calcification or when anatomy is difficult to assess, a hybrid PET/CT could be used to evaluate anatomy, calcium and perfusion,” he says.
While there has been some transition to PET imaging due to the SPECT isotope shortage and the better PET reimbursement, a wholesale switch to PET or PET/CT is not likely in the near future mainly because of the larger number of installed SPECT cameras, SPECT’s less expensive price tag and the recent technological advancements in SPECT imaging that significantly decrease imaging times, while increasing resolution. However, when cardiologists want to use PET, they will most likely rely on a dedicated PET scanner, rather than a hybrid model. But medical imaging is a fast moving field and these preferences can change in a heartbeat depending on reimbursement, technology or patient protocols.