Cardiac SPECT Sharpens Its Focus

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 Despite some disadvantages, SPECT imaging can detect severe coronary artery blockages and can offer prognostic information, such as this example of a two-day Tc-Sestamibi normal stress and resting MPI study. Images courtesy of Boca Raton Community Hospital.

Cardiovascular disease remains the No. 1 cause of mortality in the United States, with the lifetime risk of developing coronary artery disease CAD after 40 years of age to be 49 percent for men and 32 percent for women. If echocardiography is the workhorse of cardiovascular imaging for CAD, cardiac SPECT is its partner in the yoke.

In 2003 Medicare payments for myocardial perfusion SPECT exceeded $1 billion, rendering it one of the big ticket items in the healthcare budget, according to Jagat Narula, MD, PhD, editor in chief of the Journal of the American College of Cardiology: Cardiovascular Imaging (JACC:CI).

The reasons for this dramatic utilization of cardiac SPECT imaging can be attributed to a demographic spike of an aging U.S. population as well as a sharp uptick in obesity, and concurrent diabetes in those under age 65.

Approximately 21 million people, or 7 percent of the population in the United States, presents with diabetes, according to the Centers for Disease Control (CDC). Diabetes (concurrently with obesity) has grown in the U.S. for the past 15 years and shows no sign of slowing down; the CDC estimates that the diabetes epidemic is growing by 5 percent annually, says Linda S. Geiss, the chief of the CDC’s diabetes surveillance.

Developers of SPECT equipment and software have kept pace with the modality’s imaging utilization curve. The past several years have seen new developments in both hardware technology and image-processing algorithms that provide substantial reductions in SPECT acquisition time without a sacrifice in diagnostic quality.

At the component level there have been improvements in scintillators and photon transducers as well as a greater availability of semiconductor technology,” wrote Mark T. Madsen, PhD, professor and director of the clinical CT research facility at the University of Iowa in the Journal of Nuclear Medicine (April 2007). “These devices permit the fabrication of smaller and more compact systems that can be customized for particular applications.”

Of particular note has been the development of high-count cardiac SPECT systems that do not use conventional collimation as well as the introduction of hybrid SPECT/CT technology. One high-speed SPECT prototype, utilizing nine pixilated solid-state detector columns, cadmium zinc telluride (CZT) crystals, wide-angle tungsten collimators, region of interest-centric scanning, and increased angular sampling, has shown remarkable benefit for myocardial perfusion imaging (MPI).

According to an international team of researchers from Israel, London and Cedars-Sinai Medical Center in Los Angeles, the technology has recently been shown to provide an 8- to 10-fold increase in sensitivity, coupled with a two-fold improvement in spatial resolution, and has enabled a significant reduction in imaging time and dose of radioisotopes (JACC: CI, March 2008).

Their pilot study demonstrated that high-speed SPECT studies with 4-minutes of stress and 2-minutes of rest acquisition resulted in high-quality images, perfusion abnormality highly correlated to conventional SPECT, and an equivalent level of diagnostic confidence.

The commercial launch of SPECT/CT systems by Philips Healthcare, GE Healthcare and Siemens Healthcare over the past decade has played a strong role in the growing utilization of SPECT for CAD imaging. These hybrid modalities have allowed the direct fusion of morphologic and functional information for physicians, providing even greater clinical clarity and better patient management.

Jean Luc Urbain, MD, PhD, chief and chair of nuclear medicine at the University of Western Ontario, Schulich School of Medicine and Dentistry in London, Ontario, says the facility began its SPECT/CT program with a GE Hawkeye in 2004 and immediately saw utilization of the modality proliferate. “We now have four units installed, and a fifth one coming on-line shortly,” he says.

His practice does quite a bit of nuclear cardiac studies on the units, as many oncology exams as it can, and a fair amount of musculoskeletal imaging.

“We see 3,000 patients for stress tests with MPI a year, mostly on SPECT/CT,” he says. “Cardiology represents about 40 percent of our business annually, oncology about 30 to 40 percent, and the rest is musculoskeletal.”

The department bases its determination of SPECT/CT exam usage on whether the SPECT portion of the study will be enhanced by CT capabilities, Urbain says. “My estimate is about 35 percent of the time SPECT images are enhanced by the CT portion of equipment.”

One of the greatest advantages to using SPECT/CT is that the specificity of the diagnoses from the department has been greatly enhanced, Urbain says. “Now that we have the capability to view the functional and anatomic aspects of the exam together, we are very confident about the localization of what we see. Overall, it has improved tremendously our ability to diagnose accurately.”

This has been most evident in the practice’s MPI studies. Urbain says that they’ve been able to decrease uncertainty in these exams by about 20 to 25 percent by conducting the procedures on SPECT/CT, which has led to better patient management. “It has drastically improved our ability to diagnose, or rule out, ischemia,” he notes.

SPECT/CT has provided another benefit to the facility; by its capability to increase diagnostic certainty, physicians are now better able to provide a diagnosis without conducting additional imaging studies on other modalities. This, in turn, has allowed greater utilization of these technologies for other patients.

Urbain is particularly excited about the possibilities for the future of SPECT/CT in cardiac imaging. He and his colleagues have begun an investigation into the efficacy of SPECT/CT MPI procedures being conducted with coronary CT angiography (CTA) in a single patient session. “We have learned so much over the past six months that we can probably began rewriting textbooks,” he says. “We are seeing things that we never would have suspected.”

Given the capability to design an ideal SPECT/CT system, Urbain would opt for a cardiac dream machine. “From the medical/scientific aspect, I’d like to see a CZT SPECT paired with at least a 64-slice CT in order to do cardiac work,” he says.

Further south, David Davidson, a certified nuclear medicine technologist (CNMT) and clinical coordinator of the nuclear medicine department at the Medical University of South Carolina (MUSC) in Charleston reported that his facility also has experienced a significant increase in cardiac presentations for molecular imaging. So much so that MUSC has dedicated its recently acquired Siemens Symbia TruePoint SPECT/CT systems (one T2 and two T6 models) for deployment at a newly constructed cardiology and digestive diseases hospital.

“We’re performing a lot of myocardial perfusion imaging using Cardiolite [Lantheus Medical Imaging] technetium Tc99m sestamibi,” Davidson says. “Now that we’ve opened up our new facility, we’re really concentrating on our SPECT/CT cardiac imaging.”

He notes that clinicians at the facility have found tremendous benefit to the attenuation correction available with the CT portion of the hybrid technology.

“There are different ways of performing attenuation correction for cardiac studies,” he says. “One is with a sealed source, which is not quite as good as doing it with a CT. The CT portion of the SPECT/CT has better imaging characteristics and is not as limiting as the sealed-source method. In addition, sealed sources have to be replaced every 12 to 18 months due to their half-life, which poses an additional problem. It also just doesn’t have the strength of CT alone for the morbidly obese patient. We feel that the CT method for attenuation correction combined with SPECT is far superior to what we’ve used in the past.”