|Gordon DePuey, MD, division of nuclear medicine, St. Luke’s-Roosevelt Hospital|
Considered to be a critical diagnostic tool in imaging patients with suspected or confirmed coronary artery disease (CAD), single photo emission computed tomography (SPECT) is holding its own against emerging cardiovascular imaging modalities such as cardiac CT and MRI and offers considerable advantages that cannot be ignored.
Nuclear medicine utilization (excluding PET procedures) continues to be dominated by cardiovascular applications, which have grown to 60 percent of procedures in 2006 from 54 percent in 2002, according to Lorna Young, senior director, market research, IMV Medical Information Division. IMV estimates that 60 percent of 15.2 million patient visits in the United States in 2006 were for cardiovascular studies, such as cardiac perfusion, echocardiography and SPECT.
SPECT is a critical test for the assessment of patients with suspected CAD and one of the most commonly ordered tests for patients presenting with suspected coronary artery stenosis, confirms U. Joseph Schoepf, MD, associate professor of radiology and cardiology, the Medical University of South Carolina (MUSC) in Charleston.
SPECT has increasingly come under fire from proponents of cardiac MRI and CT, but Schoepf believes the attacks are not entirely justified. “[SPECT] is fairly entrenched in the diagnostic workup of patients with CAD—people commonly refer to that test whenever managing patients with the disease,” he says. “We also know that one of the most valuable pieces of information derived from SPECT is patient prognosis, which is extremely important in patient management. If a patient presents with no signs of myocardial ischemia on SPECT, we have incremental prognostic value regarding the incidence of cardiovascular events—value that even exceeds that of an invasive catheter angiogram.”
Gordon DePuey, MD, division of nuclear medicine, St. Luke’s-Roosevelt Hospital, New York, N.Y., is another believer in SPECT’s negative prognostic value.
“From virtually every study published on nuclear imaging, we know that the more extensive, the more severe the perfusion abnormality, the higher likelihood of morbidity and mortality, so we can rely very well on the prognostic ability of SPECT and its ability to risk stratify patients,” he says. “Our sensitivity in detecting coronary disease is not all that great—perhaps about 90 percent—meaning we miss about 10 percent. As it turns out, though, in patients where we miss disease and the scan is negative, the prognosis and risk of patients is still very, very low.”
DePuey says that despite the other modalities like stress echocardiography and CT coronary angiography that are suitable for cardiac imaging, SPECT continues to prove its value as an important tool in diagnosing CAD. “SPECT is really giving us hemodynamic-significant information necessary for effective patient management,” he says.
“Myocardial perfusion SPECT imaging has considerable advantages over stress echo,” says DePuey. He says that there are advantages to stress echo including the ability to look not only at stress-induced left ventricular dysfunction associated with coronary disease, but also other abnormalities like valvular disease. While echo has great advantages in young patients and young women who we do not want to expose to radiation unnecessarily, where SPECT’s great advantage is that, compared to exercise echo, it is “probably more accurate,” he says. “[SPECT’s] accuracy is similar to dobutamine echo, which can be technically inadequate due to hyper-expansion of the lungs, except SPECT has better inter-observer agreement and better precision,” says DePuey.
Schoepf adds that the role of SPECT as the “premier modality” to assess myocardial perfusion will remain untouched by the introduction of CT coronary angiography (CTCA). The two cannot really be pitted against each other, he says.
“[SPECT and CTCA] evaluate two very different aspects of coronary artery disease,” says Schoepf. “CTCA is specifically an anatomical and morphological test with the goal of detecting coronary arteries with stenosis. Myocardial perfusion imaging with SPECT is a completely different animal—its emphasis is on function. It has very little morphological information to it. One test looks at coronary artery obstruction or stensosis and the other looks at hemodynamic significance—both form a cornerstone method of imaging patients with suspected or confirmed CAD,” he says.
Proving its business value
Aside from the diagnostic value that SPECT continues to prove, it also provides significant business value compared to other imaging studies. “This is a changing environment because reimbursement changes can happen at the stroke of a pen with CMS [Centers for Medicare & Medicaid Services], but currently, reimbursement for SPECT is pretty good,” says DePuey.
DePuey says it is important to remember that “we have a huge installed base of myocardial perfusion SPECT cameras” in the outpatient setting and most of that has been in private cardiologists’ offices. “Considering the current economic climate, it is doubtful that these outpatient centers are going to throw away their nuclear cameras and invest in 64-slice CT or more expensive cardiac MR—perhaps if money were no object and reimbursement was excellent, then maybe we would be moving more rapidly over to CTCA, but it is not. The reality is that SPECT is well established and is here to stay for a long time.” According to a 2007 report from IMV Medical Information Division, while SPECT/CT is nipping at the heels of SPECT, comprising 10 percent of cameras installed in 2007, dual-head SPECT cameras are “the most preferred camera type being considered, comprising more than two-thirds of the planned cameras.”
Stephen Weiss, MD, operates a private cardiology practice, West Side Cardiology, in New York, N.Y. Weiss has been performing in-house SPECT studies for about three years to detect ischemic heart disease.
For Weiss, the “most important aspect of SPECT ‘holding it’s own’ is that it has stood the test of time with an enormous wealth of clinical data to support its use as a diagnostic and prognostic tool for ischemia detection, assessment of myocardial infarction size, and risk stratification following a myocardial infarction. It is safe and reliable because it is reproducible and accurate, and it is relatively cost-effective given the alternatives,” he says.
Weiss says the developments within SPECT imaging add to the wealth of information on how to use it, what its limits are and how to apply to the right patient. “It is not the right test for every patient and we know, thanks to the clinical and historical information backing it up, who should get it for what test,” he says.
“For a number of years, things really plateaued and weren’t very interesting. Things have changed recently and it is a very exciting time for nuclear cardiology, and for SPECT,” says DePuey. New cameras are being developed that use new detector technology – instead of using sodium iodide, they use cadmium zinc telluride detectors which DePuey says have “much, much better energy resolution.”
“This should provide us better image quality and presumably better diagnostic accuracy—it will be a huge advancement in terms of diagnostic accuracy, but also patient throughput, lab efficiency and cost efficiency,” he says.
Also, DePuey says that new software is being developed to correct for loss of resolution when imaging patients via SPECT. An inherent problem with SPECT is that “the deeper you go, the more resolution you lose, and of course the heart is a deep structure.” New software methods are incorporating resolution recovery to correct for that loss with depth.
These advancements do not mean that SPECT won’t be supplanted in the future by another modality, according to Weiss, but that currently, it is still an important tool.
|SPECT: The Deep Dive|