Nuclear cardiology as it should look in the twenty-first century

Abstract

The currently used Tc-99m-based myocardial perfusion imaging (MPI) protocols were developed over 25 years ago. Considering the length of the tests (4 hours), consequent inconvenience to the patients, and radiation doses ([12 mSV), the decline in competiveness with other non-invasive modalities used for the diagnosis of coronary artery disease (cardiac CTA and stress echocardiography) seen in recent years becomes apparent. For SPECT MPI to remain relevant, the field must evolve which requires new approaches to the old problems. Recent technological advances (new software and advent of high-efficiency cameras) present the field an opportunity to emerge into the twenty-first century reinvigorated. European practices (mostly performed and regulated by non-cardiologists) are ahead of the United States in one important respect: the stress portion of the test is done first and the rest portion is done selectively only if needed. In this issue of the Journal, Sharir et al from Assuta Medical Center in Tel Aviv tested a combination of a stress-first protocol and low tracer doses using a highefficiency SPECT camera for its diagnostic accuracy. They evaluated 284 patients without known CAD undergoing stress-first imaging, 208 of which underwent coronary angiography. Diagnostic performance using automated quantification of the stress perfusion images was compared in patients who underwent a ‘‘half-dose’’ 5 mCi stress/15 mCi rest protocol to those who had a ‘‘standard dose’’ 10 mCi stress/30 mCi rest protocol during different time periods. The very low-stress dose (5 mCi) used in this work is below the 10 mCi low-dose stress-first protocol used with conventional Na-I cameras (even with iterative reconstruction time software) and is made possible by the high-efficiency SPECT camera hardware employed. As expected, sensitivity, specificity, and accuracy were acceptable and similar in the low and standard tracer dose groups. In 2010, an ASNC Information Statement on recommendations for reducing radiation exposure suggested that by 2014 on average a total radiation exposure of B9 mSv could be achieved in 50% of MPI studies. Determining whether this goal has been met in general clinical practice is problematic, but one cannot argue that we do not have the tools to easily achieve this goal. High-efficiency SPECT cameras, new iterative reconstruction time software on conventional SPECT cameras, as well as a stress-first approach to most patients can all achieve this goal individually and in combination. The reduction in effective dose to the patient which was seen in the current study was impressive (Figure 1). The average doses of 10.2 and 30.3 mCi in the standard dose protocol results in 11.7 mSv of exposure which is reduced by 74% to 3 mSv when a stress-only study is performed. When high-efficiency SPECT technology is added by cutting the administered activity in half, further reduction in effective dose by 42% is achieved. The dose stress-rest protocol of 5 and 15 mCi results in 6.9 mSv of exposure which is cut to 1.7 mSv if a stressonly study is performed. Thus, a stress-only study with standard doses or a stress-rest or stress-only with doses will achieve the ASNC 2010 goals. Previous papers focused on the selection of patients with expected normal stress images for a stress-first protocol. The current paper extends the stress-first protocol to all patients without known CAD. Even in the cohort that See related article, pp. 11–20