A growing consensus champions PET as the most effective imaging technique for myocardial perfusion imaging and points to some very clear advantages compared with SPECT for the detection of coronary artery disease (CAD). Such improvements include sharper resolution and the ability to glean meaningful myocardial blood flow reserve and coronary flow reserve measurements. However, whether PET is truly moving in on traditional CT and SPECT territory in clinical practice remains to be seen.
Adoption of PET in recent years has been steady, but slow, and the precision of PET myocardial perfusion imaging (MPI) is contingent on the capability of respective radiotracers to provide a clearer picture of disease.
PET has been considered a first-line and reimbursable option for the diagnosis of CAD since the mid-1990s, but the initial investment and subsequent operating cost is still daunting for small institutions that rely heavily if not solely on SPECT for nuclear stress testing. This is complicated by the need for onsite cyclotrons or contract use of Rubidium-82 (Rb-82) generators, but the promise of a new and potent PET tracer could level the playing field.
Moving a mountain
An estimated 7.2 million people die globally from coronary heart disease each year, according to the World Health Organization. Coronary heart disease is projected to rise from 47 million disability-adjusted life years globally in 1990 to 82 million in 2020. Invasive coronary angiography is by far the most common assessment for CAD. The downstream imaging modality from here is CT, which, unlike invasive coronary angiography, can help determine not only the location of arterial plaque but also its morphology and composition.
The vast majority of physiologic MPI for the detection of CAD is still conducted using SPECT (J Nucl Med 2013;54(2):221-228), but there are very real diagnostic advantages to using PET, despite the challenges the technology faces in its possible path to expansion.
“PET has higher diagnostic accuracy—sensitivity and specificity—than SPECT for the assessment of myocardial perfusion, function and viability,” says Zhonghua Sun, PhD, associate professor of medical imaging in the department of imaging and applied physics at Curtin University of Technology in Perth, Australia. “Myocardial perfusion PET allows assessment of left ventricular function and wall motion, which is of paramount importance for prediction of major cardiac events in coronary artery disease.”
PET has a few features that extend beyond the capabilities of conventional SPECT, including less radiotracer scatter and a greater signal to noise ratio, dual coincidence and higher counts, the oversight of attenuation correction, which is not always or even often used in SPECT, and perhaps most notably, significantly increased spatial and temporal resolution, says Marcelo F. Di Carli, MD, chief of nuclear medicine and molecular imaging at Brigham and Women’s Hospital in Boston.
“The spatial resolution of SPECT after processing is probably in the neighborhood of 12 mm,” says Di Carli. “There are newer SPECT scanners [with cadmium-zinc-telluride detectors] that have superb spatial resolution—about 8 mm. It is 5 and 6 mm with PET, but the problem is that it really depends on the tracer that is used. If Rb-82 is used, the spatial resolution is nowhere near 6 mm.”
The issue with Rb-82 is a diminished number of counts and a higher energy of released positrons. If the energy is too high, the positrons travel through matter longer, resulting in reduced spatial resolution, notes Di Carli.
Even better image quality is available with high-definition PET reconstruction software and other technology enhancements, including ‘‘motion-frozen’’ cardiac images that combine respiratory and ECG gating, corrective software for patient motion and high-definition PET that compensate for distorted data due to circular geometry of PET scanners (J Nucl Card 2012;19[Sup 1]:S38-45).
|Myocardial Perfusion Imaging vs. Coronary CT Angiography: When to Use Which?|
|Potential testing algorithm based on use of stress cardiac SPECT myocardial perfusion imaging.|
|Source: J Nucl Med 2011;52:1079-1086|
Real clinical comparison
The SPARC study, a national registry that includes 40 clinical sites across the U.S., was established for comparative-effectiveness research investigations of SPECT, PET and coronary CT angiography. Researchers found that true clinical performance between SPECT and PET was more similar