Myocardial perfusion imaging (MPI) remains the most common modality used to determine the severity of known or suspected coronary artery disease, but an increasing emphasis on reducing radiation exposure has prompted a push away from the traditional rest-stress procedure and toward stress-first or stress-only MPI. While eliminating the rest component decreases radiation exposure, this new way of imaging hasn’t been widely implemented.
Although stress-first or stress-only SPECT saves a considerable amount of time—imaging takes only a few minutes, compared with a few hours for rest-stress imaging—perhaps the bigger impetus behind its rise in MPI prominence was its ability to drastically cut down on radiation dose.
“The main driver has been the increasing scrutiny in the medical world about radiation that the patient receives as part of imaging,” says Karthik Ananthasubramaniam, MD, of Henry Ford Hospital in Detroit. “A few studies have indicated that a substantial amount of radiation comes from imaging, and about a quarter comes from SPECT imaging.” The standard technetium-99 (Tc-99m) rest-stress test lasts about four hours and delivers a radiation dose of 12 to 15 mSv.
The American Society of Nuclear Cardiology (ASNC) established a goal of achieving radiation exposure of 9 mSv or less in half of imaging studies. Guidelines also recommend stress-only SPECT imaging as the method that requires the lowest radiation dose. To improve the accuracy of these studies, ASNC suggests attenuation correction.
Patient & staff considerations
Both patients and staff experience considerably less radiation exposure from stress-only or stress-first imaging, according to recent studies. One study found that mortality rates among patients with a normal SPECT who underwent stress-only imaging were similar to patients with a normal SPECT who needed rest images as well (2.57 percent vs. 2.92 percent), but the stress-only group received 61 percent less radiation (J Am Coll Cardiol 2010; 55:221-230).
Similarly, a 2013 study compared radiation dosimeter readings from 10 full-time radiology department employees who followed a stress-first protocol at two time periods—before the use of high-efficiency SPECT and after introducing the new technology (J Nucl Med 2013; 54:1251-1257). The researchers found a 40 percent reduction in radiation dose among all the staff members, and during the period after the introduction of the high-efficiency SPECT, the monthly activity of Tc-99m decreased.
Overall, they noted that both high-efficiency SPECT and a stress-only modality led to a 34.7 percent decrease in average total radiation administered between the time periods. There was a 60 percent increase in the proportion of stress-only studies.
At Henry Ford Hospital, patients scheduled for stress tests are triaged to determine who is suitable for stress-first or stress-only imaging. “We try to predict whether stress images will be normal,” Ananthasubramaniam says.
If it’s likely that a patient’s images will be normal, the imaging team starts with stress-first imaging. If the images are not normal or if there is not a clear call for normality, that patient will undergo a rest-stress study.
Stress-only imaging appears to be efficient and appropriate for identifying patients at risk or at low risk. Stress-only imaging with attenuation correction in 1,383 patients with symptoms of MI found a low rate of cardiac events (0.7 percent annual event rate) in patients with normal stress-only scans (J Nucl Card 2013; 20:27-37). Without attenuation correction, interpretation revealed that 58 percent had abnormal images, but after correction, 83 percent of these were reclassified as normal. Of the patients who had abnormal stress images after correction, 63 percent had rest testing and the other 37 percent were managed clinically without additional rest imaging.
The development of cadmium zinc telluride cameras about five or six years ago meant that image quality would not suffer with lower radiation doses. “The new cameras are more efficient,” says Milena J. Henzlova, MD, PhD, of Mount Sinai Medical Center, a co-author of the radiation dose study.
Stress-only Radiation Exposure
Average annual radiation exposure by staff exposure before and after the introduction of stress-only protocols
Source: J Nucl Med 2013; 54:1251-1257
Barriers largely financial
The modality has not yet been widely implemented, particularly among private practices. The main reasons, experts say,