Innovations in system design, new radioisotopes and attenuation correction are what's getting noticed in SPECT. But, the biggest advancement thus far has been hybridization thanks to the coupling with CT and the attenuation correction it brings. SPECT-CT scanners are hitting the market, bringing SPECT into the limelight and out of the shadows of PET.
Single photon emission computed tomography (SPECT), a nuclear medicine imaging procedure introduced in the early 1980s, is routine in almost every hospital today and has applications in cardiology, neurology and oncology. Growth in the technology has been stable over the years, with new equipment options being single head cameras, dual-head systems and variable angle cameras that perform both nuclear cardiology and general radiology exams.
Digirad Corp.'s mobile Cardius-3, triple head camera system, designed exclusively for cardiac imaging applications, fits in a room as small as eight-by-seven feet. "You can roll it out of the office and use this system in another office very easily," says Howard Lewin, MD, FACC, a nuclear cardiologist/nuclear medicine physician at Cardiac Imaging Associates in Los Angeles, who has been evaluating the system. "The other advantage of this camera is because of its triple head design, it acquires images more quickly without substituting quality."
The lion's share
While SPECT has applications in neurology and oncology, cardiac SPECT represents the lion's share of exams. An IMV census reported that 18.4 million SPECT studies were performed in 2002, with 10 million of these being cardiology. At that time there were 12,700 SPECT systems installed at 6,700 sites, according to IMV.
Cardiac SPECT is non invasive and provides clinicians with very accurate, three-dimensional pictures of the heart's structures and function. Nearly all cardiac patients undergo a SPECT scan as part of their diagnostic work-up for detection and evaluation of heart disease. SPECT scans are increasingly used to monitor patients following bypass surgery. Cardiac SPECT scans are particularly important in the diagnosis of heart disease in women because other tests such as the EKG and exercise stress tests often fail to detect women with heart disease.
"The bulk of nuclear cardiology is perfusion imaging," says Frank DiFilippo, PhD, director of nuclear imaging physics in the Department of Molecular and Functional Imaging at the Cleveland Clinic Foundation in Cleveland, Ohio. "We are able to determine the perfusion not just under base line or rest conditions, but also when the patient is under stress. The patient will either undergo a treadmill exam or a pharmacological stress exam in which the effect of exercise is simulated and a tracer is injected. The way it distributes into the heart reflects the relative amount of blood that is reaching under those conditions at the time of injection. Even though SPECT imaging is done a half-hour after the stress, the distribution is preserved. We show the relative distribution of blood flow at the different base line and stress conditions."
The Cleveland Clinic's arsenal of gamma cameras includes Siemens' e.cam dual-headed SPECT camera. Siemens recently introduced c.cam, a SPECT system specific for outpatient and office-based nuclear cardiology practices, which features a reclining chair that lets patients of any size sit back comfortably during exams.
"SPECT is a cornerstone now with any patient with heart disease and the patient will have a stress test one way or another," says Frans Wackers, MD, PhD, director of nuclear cardiology at Yale University School of Medicine.
"The heart is a hollow organ," Wackers continues. "The disadvantage of the old technique, planar imaging, was that you would have superimposition. This would make it difficult to distinguish various abnormalities. Sometimes the abnormality could be masked by the superimposition. With tomographic imaging, you acquire images around the patient. With computer reconstruction, you can reconstruct 3D localization of the radioactivity in the heart. Then you can look at slices of the heart and look at much more detail to determine particular problems with blood flow."
The nuclear medicine department at Yale School of Medicine uses a GE Hawkeye SPECT-CT system. On the market for several years, GE has 500 Hawkeye systems installed globally. The Hawkeye consists of a dual-headed gamma camera with an x-ray tube and row of detectors housed within the gantry.