To achieve the American Society of Nuclear Cardiology’s (ASNC) 2014 goal of having the total radiation exposure for patients referred for SPECT/PET myocardial perfusion imaging (MPI) to be equal to or lower than 9 mSv in 50 percent of studies, nuclear cardiologists must focus on appropriate patient selection and assessing protocols, radiotracers and imaging systems, said Manuel D. Cerqueira, MD, chairman of the department of nuclear medicine at the Cleveland Clinic, during a webinar Feb. 17 sponsored by the ASNC.
There are two different effects of ionizing radiation—deterministic and stochastic. Nuclear medicine does not see deterministic effects, which usually involve very high doses of radiation (3 to 5 Gy) that can produce tissue damage or skin reddening. These effects are more of a problem within electrophysiology or cardiac catheterization.
Nuclear medicine sees more of the stochastic effects, which are theoretically produced by any radiation exposure and whose effects are cumulative and could result in genetic defects and malignancies, Cerqueira said.
Physicians should use lower radiation levels for younger patients because they will have a higher lifetime cancer risk, he offered.
But which imaging exam puts patients at the greatest risk? Nuclear MPI, said Cerqueira. In fact, MPI has an average effective dose of 15.6 mSv and holds the largest contribution of cumulative effective dose.
Looking at cardiac imaging specifically, Cerqueira said a previous study showed that in patients 18 to 34 years old, for every 1,000 patients, 7.1 would actually get some sort of imaging study and 4.5 had MPI. By the time a patient is 55 to 65, 82 out of 1,000 would have an MPI study.
But do patients benefit from MPI, which accounted for over 80 percent of radiation for men and women aged 18 to 34 years old? He said some of the patients did not have a likelihood for coronary disease and the MPI exam would most likely not be beneficial.
These types of studies, however, are making patients aware of radiation exposure, which is a good thing, but it also can create a fear of testing, Cerqueria added. Questions that are being asked include: Are patients getting exposed? What are the risks of exposure? Do patients benefit from these types of studies? Do they need them?
ASNC is still attempting to find a strategy that defines the optimal risk-benefit ratio for stress MPI.
An ASNC information statement outlined recommendations for reducing radiation exposure when performing nuclear cardiology studies. According to Cerqueira, performing one-day rest-stress studies will expose patients to about 11 mSv of radiation, however, two-day studies, which use two doses, will increase radiation doses significantly.
In addition, he said that use of dual-isotopes could expose patients to almost 30 mSv of radiation.
“Radiation levels from PET tracers are less than from longer lived SPECT isotopes like thallium or from dual-isotope protocols,” said Cerqueira. “You have to keep this in mind when deciding what study to do.”
Radiation reduction could be achieved if three aspects are carried out:
- appropriate patient selection
- assessment of protocols, radiotracers and imaging systems
- using the existing technology to make them better.
Appropriate use criteria
Appropriate use criteria should be applied if there are alternative modalities with a comparable diagnostic accuracy without radiation. These modalities should be considered seriously in younger patients. Cerqueira offered that MPI is the most helpful in patients with intermediate CAD risk, those requiring prognostic and management information and those with persistent and unexplained symptoms.
Protocols, radiotracers and imaging systems
The clinical indications and physical structure of each patient should be reviewed and the best combination of radiotracers and protocols selected using the following guidelines: use radionuclides with shorter half-lives such as Tc-99m for SPECT MPI or PET imaging with rubidium-82 (Rb-82); perform stress-only testing, which cuts time and radiation exposure; and use weight-based dosing.
Cerqueira said that iterative reconstruction is strongly recommended because it can reduce dose; however, filtered back projection, a commonly used strategy, is not recommended because of the minimum dose reduction