The use of myocardial perfusion imaging (MPI) has spiked over the past four decades and along with it individual patient dose. A comprehensive effort to reduce that dose could reduce unnecessary procedures and reduce the risk of downstream cancers, according to a meta-analysis published in the April issue of the Journal of Nuclear Medicine.
With increasing procedure numbers came an estimated 10-fold increase in nuclear cardiology-related radiation exposure spanning from the early 1980s until 2006. In response, industry leaders have spurred multiple initiatives focused on appropriate-use criteria and technological development to reduce radiation dose.
Sharmila Dorbala, MD, an assistant professor at Harvard Medical School and a specialist in nuclear cardiology at Brigham and Women’s Hospital in Boston, and colleagues collected current literature on MPI dose reduction centering on patient selection and other practical means to see which techniques measure up to keep individual dose down.
The methods reviewed by Dorbala and her team include modeling preset appropriate use criteria, procedure optimization and post-procedural checks and balances.
“Most of the approaches listed can be implemented currently,” Dorbala told Health Imaging. “We hope that careful scrutiny of radiation dose for medical imaging may over time reduce the lifetime radiation dose for the general patient population.”
Prioritizing patient selection
The researchers reviewed current industry-vouched appropriate use criteria for MPI, last updated in 2009, and evaluated general adherence and limitations. The most recent criteria are based on the median scores of 15 experts in the field and MPI indications range along a spectrum from “rarely appropriate” to “appropriate.”
Although the frequency of procedures deemed appropriate by current criteria probably differs throughout the country for a variety of reasons, one study reviewing six UnitedHealthcare sites showed that 85.6 percent of studies were considered appropriate upon review. However, efforts to reduce procedures that would be considered rarely appropriate have shown inconsistent gains. On the bright side, decision support tools (DST) such as those built into electronic order-entry systems could provide a solid means of reducing rarely appropriate use of MPI. Dorbala and colleagues specifically mentioned the use of an AUC-DST, which led to a decrease in the frequency of rarely appropriate tests from 22 percent to 6 percent eight months after deploying the tool in one study. This technique will be mandated as soon as January 2017 for both SPECT and PET MPI as a prerequisite to reimbursement.
Once MPI is deemed appropriate and slated for a patient, there are further steps to help reduce an individual’s dose. Both the Society of Nuclear Medicine (SNMMI) and the American Society of Nuclear Cardiology (ASNC) have explicit recommendations for reducing dose at the point of procedure. The latter has provided the industry with methods that could bring individual patient dose down to 9 mSv or less for as much as half the patient population. These methods include smart radiotracer selection and the adoption of modern imaging protocols, applying image-reconstruction techniques and using, whenever possible, high-tech dose-reduction software, 3D modes, and novel collimator designs to fit the bill. If done right, all of these methods should not threaten the quality and usefulness of MPI.
In terms of radiotracer selection, Dorbala and colleagues noted that Tc-99m is superior to Tl-201 for SPECT due to better imaging quality, shorter half-life and lower per-procedure dose. The researchers indicated that Rb-82 and N-13 ammonia PET were even better choices than Tc-99m SPECT for reducing radiation dose.
Protocols including stress-first or stress only testing has shown some benefit, according to the analysis, as well as the use of state-of-the-art cadmium zinc telluride or thallium-activated cesium iodide detectors. Some researchers have posited that the application of novel hardware and software allows clinicians to cut patient dose in half. However Dorbala et al mentioned that not many studies have assessed half-dose MPI using novel software in prospective studies.
Digging deeper for lower dose
After the procedure has taken place, there are several quality controls that can help keep patients’ dose in check. Dorbala and colleagues wrote that “accurate, clear, and unambiguous report and timely communication to the referring physician will reduce repeat, layered testing.” In addition, quality reviews can track what dose is being administered on a consistent basis to make sure the numbers remain as low as possible, and regular scanner optimization also can keep patient dose to a minimum.
“By implementing these practices, the field [of nuclear cardiology] will be able to deliver outstanding image quality with minimal imaging-related radiation dose,” said Dorbala.
This is just the tip of the iceberg for Dorbala and her colleagues. “This is a review paper summarizing the existing studies and our recommendations,” she explained. “Several follow-up studies are planned by our group as well as by other groups active in this research.”