Advanced SPECT and stress-only MPI lower occupational dose

Many studies have maintained that the most advanced SPECT cameras can significantly reduce patient dose, but what about medical staff? A new protocol of myocardial perfusion SPECT imaging combines state-of-the-art detector technology, image reconstruction and stress-only imaging, all precipitating an up to 60 percent drop in radiation exposure to healthcare workers, according to a study published May 30 in the Journal of Nuclear Medicine.

W. Lane Duvall, MD, associate professor of cardiology at Mount Sinai Medical Center in New York City, and colleagues conducted the first study evaluating reductions in occupational radiation exposure during myocardial perfusion imaging (MPI) by comparing exposure to medical staff during two different periods of study--one using conventional SPECT technology during a stress-rest MPI protocol and another using high-efficiency cadmium zinc telluride (CZT) SPECT cameras and iterative image reconstruction implemented with “half-time imaging,” also known as depth-dependent resolution recovery, with conventional cameras and MPI using stress images alone.

“A stress-only imaging protocol can decrease the length of the test, both for the patients and for the laboratory, and results in a lower radiation dose to the patient and the medical staff,” wrote Duvall et al. “A stress-only study can be completed, processed, and read in less than 90 min as opposed to the traditional 3–5 [hours.] The radiation dose can be decreased by 30 percent–60 percent depending on the dose used for stress imaging.”

This is made possible due to the lack of additional value, neither diagnostic nor prognostic, provided by rest imaging during normal SPECT MPI studies, as seen in well-documented studies.

“The benign prognosis of normal results from a stress-only study has now been confirmed in more than 10,000 patients and appears to be no different from that of normal results from a rest–stress study,” wrote the authors.

During the two-phase study, radiation exposure for four nuclear technologists, four nurses and two administrative personnel was tracked by personal dosimeter badges worn on lab coats. Dose was analyzed during two separate periods of research, each lasting one year. The first period was from October 2007 to September 2008, and the second period was after an upgrade in SPECT technology and an increasing number of stress-only MPI procedures, from October 2010 through September 2011. A total of 3,539 patients underwent SPECT during the first period and 3,898 in the second. American Society of Nuclear Cardiology standards for exercise and pharmacology were observed.

About a 40 percent dip in the dose equivalent was seen in all of the tracked employees from the first to the second period of study, or -16.9 and -16.2 mrem. The total activity of Tc-99m used per month during the second period of research dropped 10,746 mCi compared to 7,174 mCi. The mean of patient-administered Tc-99m activity was 36.5 mCi in the first period and 23.8 mCi in the second. About 20 percent more patients, 56 percent vs. 35 percent, received stress-only MPI in the second phase. The new protocol also was found to cut procedure times.

“The combination of the use of high-efficiency SPECT technology and stress-only protocols resulted in a 34.7 percent reduction in mean total Tc-99m administered activity between time periods, with camera technology being responsible for 39.2 percent of the reduction and stress-only protocols for 60.8 percent.”

A noted limitation in the study was the inability to complete a thorough statistical analysis of individual factors related to dose.

“Despite several attempts at statistical modeling, more than 75 percent of the variability in radiation exposure could not be attributed to individual effects, because of confounding variables present in both periods that could not be untangled,” the researchers wrote.

Still, the most important finding of the study was the overall decrease in both staff and patient radiation exposure resulting from high-efficiency SPECT cameras and stress-only MPI protocols. The American Society of Nuclear Cardiology has set the bar at no more than 9-mSv patient exposure for 50 percent of studies by 2014. Adoption of one or both of these improvements is expected to be enough to make the mark.

“Adopting a stress-first approach to imaging is a particularly feasible option for all laboratories, as the expected frequency of normal stress studies using appropriateness criteria for the performance of MPI exceeds 50 percent and the proportion of normal studies in the large published nuclear cardiology cohorts reaches up to 70 percent,” wrote the authors. “Although purchasing new camera technology may not be feasible for all because of its cost, new half-time imaging software for traditional NaI cameras has also been shown to decrease administered activity. New software technology using iterative image reconstruction with resolution recovery improves count statistics and suppresses noise by correctly accounting for formerly suppressed counts.”

Even for those institutions that cannot purchase the most advanced SPECT technology, all major manufacturers offer advanced image reconstruction software for conventional SPECT cameras. This affords those institutions significant dose reduction without the added cost.