Nuclear imaging plays a big role in diagnosing and monitoring heart conditions, but there have been an increasing number of calls for modernization of these imaging systems due to radiation doses being higher than necessary. The American Society of Nuclear Cardiology (ASNC) says a high percentage of cardiac nuclear imaging systems are 10-25 years old and cannot provide optimal care without replacement or upgrades.
This was a topic in sessions at the 2023 ASNC meeting last fall chaired by Rob deKemp, PhD, associate professor of medicine, engineering and physics with the University of Ottawa in Canada. He spoke with Cardiovascular Business about a key study on lowering PET myocardial perfusion imaging (MPI) dose and new technologies that are now available.
Weight-based rubidium-82 myocardial perfusion imaging
DeKemp discussed a study presented by Manchester University focused on the use of weight-based rubidium-82 myocardial perfusion PET imaging. Traditionally, a constant dosing approach has been employed, with the same dose injected for all patients. However, deKemp said there is now a shift toward weight-based dosing, emphasizing that this can significantly reduce radiation exposure.
"It's becoming more common today to use some kind of weight-based dosing and giving dose in proportion to weight. For example, if you gave 740 megabecquerel (MBq) to a 100 kilogram patient, that would be 7.4 MBq per kilo. And what Manchester's study showed was that you can scale that down from a 100 kilograms all the way down to patients of 40 or 50 kilograms at that dose. A 50 kilogram patient then really only needs half the dose compared to a 100 kilogram patient, and they found that image quality was maintained and the perfusion defects were exactly the same as they were using 20 millicuries (mCi) for everybody," deKemp explained.
There are two vendors that offer rubidium generators for PET imaging and both offer weight-based dosing. In the past, some older PET systems were limited and every patient had to get the same dose.
"With the newer instrumentation now giving dose by weight, it is really completely feasible, and I would say appropriate, especially in the smaller patients. There is really no reason to be giving them a 20 mCi dose when 10 mCis is completely adequate and the images were almost indistinguishable, as shown by the Manchester group," deKemp explained.
PET myocardial perfusion imaging offers lower doses than SPECT
DeKemp emphasized that the current rubidium-82 generators used for perfusion imaging exams are an ideal tracer for achieving low radiation doses due to the ultra-short half-life of just 75 seconds. This does not allow much time for the tracer to deposit dose in the body. So even the 20 mCi dose in a 100-kilogram patient has minimal radiation exposure. By implementing proportional dosing by weight, smaller patients can receive significantly lower doses, deKemp said.
He said the new PET radiotracer everyone is excited about is flurpiridaz, an F-18 tracer being submitted for final FDA review. The AURORA pivotal trial compared it with the current standard of care, SPECT with the radiotracer technetium-99m. The results showed flurpiridaz significantly exceeded prespecified threshold criteria for the detection of CAD and was superior to Tc-labeled SPECT MPI for the detection of CAD in the overall population and in women and obese patients.
"The benefit with the F-18 tracers is, although they're half-life is longer, you can give less radiation dose and simply extend the imaging time to get the diagnostic quality needed. So I think flurpiridaz as a tracer will be very similar to the other PET tracers," deKemp said.
Technological advances for nuclear imaging dose reduction
A decade ago, there was a lot of talk about the higher radiation doses required for cardiac computed tomography angiography (CCTA) imaging, raising concerns in general about radiation levels used in medical imaging. But as CT doses have dropped significantly, nuclear cardiology has been left with the highest doses used in cardiac imaging, especially at imaging centers using older technology. This has led ASNC to encourage members to modernize their labs.
One of the biggest advancements in lowering nuclear imaging dose has been the introduction of digital detectors to replace older photo-multiplier tube (PMT) technology. These can image patients faster or allow lower doses. SPECT cadmium zinc telluride (CZT) detectors can reduce SPECT doses in half.
"There's been a focus on dose reduction that came about into the public eye as a result of wider use of CT for angiography. I think that helped bring light to some relatively high dose procedures using SPECT imaging. So thallium, for example, is really not recommended any longer because of the relatively high doses," deKemp said.
CZT SPECT systems are less expensive than PET systems and SPECT still makes up a large percentage of MPI exams in cardiology. He also said older SPECT systems can be upgraded with virtual attenuation correction and iterative image reconstruction software to improve image quality and help lower dose.
On the PET side, deKemp said digital detectors have improved time-of-flight (TOF) imaging, further enhancing PET's image clarity even further. In newer scanners, deKemp said dose reduction can be achieved partly due to larger axial field of view, and partly due to better time of flight.
"You're getting more bang for your buck out of each recorded count, so you can potentially lower the dose that way," he said.
New image remonstration and AI technology to reduce dose
DeKamp said there are also software upgrades for older SPECT systems that offer better image quality with iterative reconstruction and collator response modeling to improve image quality without the need to increase dose. "There's really no reason, I think, not to be using that in today's environment," deKemp said.
Additionally, he said artificial intelligence (AI) algorithms are now available to enable virtual attenuation correction on SPECT systems without integrated CT to significantly increase the quality of images. AI is also being developed to enable very low-dose nuclear imaging by enhancing low-count images.
"I would be crazy not to mention some of the AI image reconstruction methods that I think do really have promise now to be able to generate diagnostic quality images with potentially order magnitude lower counts. We'll look forward to validation of some of those advanced techniques," deKemp said.