SPECT vs. PET, Which is Best?
If the case went to court for whether positron emission tomography (PET) or single, photon emission computed tomography (SPECT) is better suited for myocardial perfusion imaging, experts say the current technology and developments on the horizon would result in a hung jury.
“This has been an ongoing debate for a long time – which one is better,” said Diwakar Jain, M.D., FACC, FRCP, FASNC, professor of medicine, director of nuclear cardiology, Drexel University College of Medicine in Philadelphia. “It is difficult to give an answer because they both have their strengths and weaknesses.”
Technology for both modalities is also advancing rapidly, said Daniel S. Berman, M.D., FACC, director of nuclear cardiology/cardiac imaging, Cedars-Sinai Medical Center in Los Angeles. “It’s a very hard question to answer,” he said “We are in a time of a lot of change in diagnostic cardiac imaging.”
More research is needed to compare SPECT and PET, said Peter F. Faulhaber M.D., associate professor of radiology, Case Western Reserve University, director of clinical PET, University Hospitals Case Medical Center in Cleveland, OH. He said the jury is still out in his mind. In addition to testing which modality is better for perfusion imaging, he said research also needs to provide more cost analysis and consider patient throughputs. Both components are needed to convince CMS to provide reimbursements.
“CMS will hold us to the costs,” he said. “They will say, ‘Yeah, you have a great tool, but what works best?’”
Pros and Cons
The main positives of SPECT are that it’s much more available and widely used and much cheaper than PET. A SPECT gamma camera costs $400,000 to $600,000, while a PET-CT scanner can cost around $2 million. While PET is more expensive in terms of purchasing equipment, SPECT radio tracers also have half-lives of up to six hours, allowing a lot of imaging time, while PET tracers only have a half-life of about 75 seconds. SPECT radio tracers are also much cheaper and more abundant than PET tracers, Dr. Jain said.
“SPECT is the most well-established modality and is the most widely used,” said Dr. Jain, noting SPECT has been is common use since the early 1990s. “SPECT is not going to go away.”
In 2007, there were 15.9 million SPECT procedures performed, which included 8.3 million nuclear cardiology procedures. In comparison, there were a total of about 1.6 million PET procedures performed in 2007.
However, SPECT has issues, including long scan times and low-resolution images prone to artifacts and attenuation. Some artifacts can easily be misidentified as perfusion defects. SPECT also does not provide a quantifiable estimate of the blood flow, whereas PET does, experts say.
SPECT issues have been partially resolved by technological progress, cutting scan times with triple-headed cameras, improved cameras, computer-aided image enhancement and visual tracking systems to monitor and compensate for patient movement during long scans.
“PET is better in many respects,” Dr. Faulhaber said. “With PET you have less artifacts.”
He said this is especially important when imaging larger patients, because the more tissue gamma rays pass through, it causes attenuation issues.
Other positives of PET are higher spatial resolution and the capability to perform quantitative measurements at the peak of stress and speed. Dr. Faulhaber said PET scan times of 30-40 minutes mean more patients can be processed in a day, instead of the two or more hours it takes for a SPECT scan.
Dr. Jain said PET is not rapidly replacing SPECT because it does have some drawbacks.
“PET is better, but not so much better than SPECT,” Dr. Jain said. “If a new technology is decidedly better it will replace the old technology quickly.”
Dr. Faulhaber agreed, saying PET is only “a little bit better” than SPECT imaging.
Adding CT to SPECT and PET
The incorporation of a CT scanner with a gamma camera for combined SPECT/CT or PET/CT imaging is designed to give anatomic sign posts to clinicians so they can accurately locate and identify the effected tissue in a nuclear image. CT data can also help correct attenuation and be used for calcium scoring. However, these hybrid machines come with a high price tag, and Dr. Jain said it has not been established yet if SPECT/CT is better than SPECT alone.
“The added advantages of SPECT/CT do not out-weigh the increased costs,” Dr. Jain said. “It is hard to justify the costs.”
Another issue holding back SPECT/CT is the lack of CMS reimbursement. Dr. Faulhaber said this is because CMS is not yet convinced SPECT/CT has any advantage over cheaper modalities or PET/CT. Dr. Berman said the higher cost and lack of reimbursement might help determine which technology is adopted by some hospitals.
Dr. Faulhaber said SPECT cameras alone are effective in cardiac imaging and the CT image only gives an anatomical guide to the surrounding tissue. He said this is more important in oncology studies to locate the exact location of a tumor.
“With the heart it is less important because I know what I am looking at,” Dr. Faulhaber said. “SPECT/CT comes in very helpful for imaging other parts of the body.”
SPECT/CT as a cardiac modality is still very much in its infancy, Dr. Berman explained. He said there is a big question if SPECT/CT or PET/CT will become commonly used cardiac technologies, or if one will be able to gain a significant edge over the other.
“SPECT/CT has not even begun to take off as an imaging modality,” Dr. Berman said, because the technology is so new and expensive.
Due to the high cost, the only companies manufacturing PET/CT and SPECT/CT machines so far are GE, Philips and Siemens. Dr. Jain said he is familiar with all of them and they are pretty much the same in terms of function and quality.
Travis Chong, a medical imaging research analyst for Frost & Sullivan, said the latest advances in the hybrid modalities include GE Healthcare's Volumetrix Suite, a set of quantitative tools designed to expand capabilities of a SPECT/CT system. With these tools, automatic registration and 3D image rendering is integrated into the workflow. In addition, Chong said the software can help cut acquisition times in half.
In its newest gamma camera, IQ·SPECT, Chong said Siemens offers the latest technology in dedicated-cardiac cameras that accelerates workflow through a redesigned collimator. The SMARTZOOM collimator design breaks away from the traditional fixed 90-degree dual-head design and instead employs what Siemens refers to as a cardio-centric orbit such that the heart is maintained within the collimator's magnification area throughout the entire study. Additionally, calcium scoring with SPECT/CT cameras can be performed under one minute.
What the Future Holds
There are advancements being made in both PET and SPECT imaging that could help boost either technology. Dr. Berman said in PET it is the development of the radiotracer fluorine-18 (F-18), which has a half-life of about two to two and a half hours. He said this would be a big boost for PET, eliminating one of its biggest disadvantages. Dr. Berman also said the new agent would become the radiotracer of choice for PET and PET/CT imaging.
Lantheus Medical Imaging Inc. and FluoroPharma both have ongoing testing of F-18 tracers for cardiac imaging.
In SPECT, Dr. Berman said there is development on a new generation of smaller, less expensive gamma cameras dedicated to cardiac imaging. He said these cameras are being developed by CardiArc, GE, Spectrum Dynamics, DigiRad, Philips and Siemens.
“In the future there will be small dedicated cameras for SPECT for cardiac imaging,” Dr. Berman said. “There is an increasing trend toward smaller cameras for doctors’ offices.”
Once the price and size makes these cameras more attractive to private cardiac practices, there may be a movement of cardiac nuclear imaging away from dedicated imaging centers and radiology departments to the doctors’ offices, Dr. Berman predicts.
To challenge the move to smaller, cheaper SPECT cameras, Chong said Positron Corp. is developing a PET dedicated cardiac camera that will be “economically priced” and is speculated to cost about $600,000 - considerably less than an average PET/CT system. With a small footprint and lower costs, the PET camera is expected to directly compete with the mobile dedicated-cardiac SPECT cameras within the primary-care physician and cardiologist office setting. Frost & Sullivan says this area represents the largest and most lucrative untapped market segment in the cardiac perfusion imaging industry.
All three of the doctors agreed the technology is rapidly changing not only with improvement to both PET and SPECT, but also with advancements in CT-angiography, MRI and cardiac ultrasound.
“The field of cardiac imaging is just changing so drastically and it is making the predictions more difficult,” Dr. Berman said.