Advances in SPECT: The Boost from CT
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."
Adding CT
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. The x-ray system is used to acquire low-resolution CT scans of the patient, which are then used for attenuation correction of the corresponding SPECT data. The CT images, which typically take 5 to 10 minutes to acquire, also can be used for anatomical localization of the SPECT images.
The system represents the newer generation of multi-modality nuclear medicine scanners gaining clinical recognition. Siemens and Philips Medical Systems both offer hybrid SPECT-CT scanners, as SPECT-CT is seen as the next wave in nuclear medicine, particularly in nuclear cardiology. A SPECT camera with a multislice CT hybrid imaging module acquires co-registered SPECT and CT images, individual SPECT and CT images and attenuation-corrected nuclear medicine images.
The Cleveland Clinic recently installed Siemens' Symbia TruePoint SPECT-CT scanner that debuted last year. The system integrates Siemens e.cam SPECT with its CT technology. Siemens now has three models: Symbia T for CT-based attenuation correction and functional anatomical mapping; Symbia T2 that has a diagnostic dual-slide CT; and the Symbia T6 that utilizes a six-slice CT system.
At the Cleveland Clinic, DiFilippo says the SPECT-CT scanner is being used for several different types of exams, such as cardiac perfusion studies, parathyroid imaging cases, oncology studies, octreoscan studies and brain perfusion studies. "We have done a large variety [of exams] but I would not say that any of those specific cases have dominated on that camera because we are still learning as to what is the best use of this technology and what type of clinical scans are most likely to benefit."
Is SPECT-CT better?
SPECT is a low cost exam as well as a low maintenance install, so making the convincing argument to spend $2 million on a SPECT-CT scanner when most dual-head cameras cost in the range of $300,000 to $400,000 is a little difficult. Proponents stress its clinical outcomes and clinical relevance, especially CT-based attenuation correction and tumor localization.
"The convincing argument for SPECT-CT is huge because the CT can help eliminate the two most important issues with SPECT: attenuation and lack of anatomical information," says Markus Lusser, vice president, global sales and marketing, of Siemens' nuclear medicine division. "Attenuation has been addressed by various companies with several concepts, but it is generally accepted that CT-based attenuation correction is the best possible way to correct for the patient specific attenuation. Transmission source-based attenuation correction has been limited to cardiology applications and has had only very minimal success. CT-based attenuation correction is so accurate that researchers are already applying it to other clinical applications including dose calculations for radionuclide therapy planning.
"The second common problem with SPECT is that the more specific the radioactive tracer, the harder the images become to read because the lack of any kind of anatomical reference," he continues. "To be able to overlay the SPECT image on a multislice CT image and being able to exactly pinpoint where the lesions are is of tremendous value for therapeutic decisions."
Many clinicians agree that CT-based attenuation correction is the biggest advancement to hit SPECT. "By overlaying the SPECT image on top of a CT image, you really get the best of both worlds," say DiFilippo. "You get the specific molecular or functional information from a nuclear scan and you get the anatomic detail of the CT exam. Not only can you see a great deal of contrast from a radiotracer, you know better where it is located.
"The other benefit of the CT data is that you get more quantitative, accurate SPECT data through attenuation and scatter correction," DiFilippo continues. "The SPECT images are not usually considered quantitative because of the limitations of attenuation and how that impacts the reconstructed image. But if you have the information from the CT, you can calculate the attenuation during image reconstruction and correct images accordingly."
DiFilippo acknowledges that cost will hinder the widespread adoption of SPECT-CT. "Gamma cameras are very widespread because they provide a pretty low cost technique," says DiFilippo. "SPECT-CT is a much more costly device, and I think that is going to be one of the factors that limits its acceptance. The performance is going to make it worth its while in many types of procedures. It is going to be interesting to see how the market plays out, as to whether many people adopt it right away or a fewer number of people adopt it at the very beginning."
SPECT-CT in the ED
Two Philips' SKYLight gantry-free nuclear cameras and one Philips' Precedence SPECT-CT unit are in use at the Baptist Miami Cardiac and Vascular Institute in Miami, Fla. Another Precedence SPECT-CT will soon be added to the nuclear medicine laboratory, gearing up their hybrid imaging capabilities to include two SPECT-CT systems and two PET-CT systems.
"We use the SKYLight for all aspects of nuclear medicine," says Jack Ziffer, MD, PhD, director of cardiac imaging at the institute. "It is predominately used for body imaging. We also can use it for cardiac applications, including gated blood pool studies and myocardial perfusion studies."
The SPECT-CT scanner is in an evolutionary stage as clinicians try to determine the best environment for the system. "Right now the system is being used for many aspects of tumor imaging, such as suspected parathyroid adenomas and octreotide scans," details Ziffer. "We recently started using it for cardiac applications. That's going to be growing a lot. We see it being used pretty extensively for evaluating patients from the emergency department in different contexts."
Ziffer believes that CT will make SPECT better. "With routine SPECT, the problems of false positives and false negatives are largely driven by the patients shape," says Ziffer. "For example, in a woman, the left breast may absorb protons on the way out of the heart. [Clinicians] then see areas of abnormal perfusion that had nothing to do with the heart. It was just a technical factor of the patient's breast blocking the photons so they were not detectable. It mimics an abnormal area of perfusion in the heart.
"With CT, we can use very low doses of radiation to image the body, map how much more soft tissue there is, and map for attenuation and start to make the SPECT study much more accurate," Ziffer says.
The most important future application of the SPECT-CT system at Baptist will be to evaluate patients in the emergency department that present with symptoms of chest pain. "That would be a very frequent type of study, perhaps performed 10 times a day," says Ziffer.
"Most SPECT studies that we do are normal and we are left with that unsatisfying sense of not knowing what caused the chest pain," Ziffer continues. "By adding CT, I think in most patients we are going to determine life-threatening causes of chest pain very quickly, such as pneumonia, pericarditis, aortic aneurysms and rib fractures. Those patients who are abnormal with SPECT, we may eventually provide the roadmap for the invasive cardiologists to know what the culprit vessel is. This may increase the urgency of getting patients to the cardiac catheterization lab and/or recognizing that something may be a smaller abnormality that is better treated medically."
Conclusion
"I think SPECT has not gotten a real lot of attention since nuclear medicine has been overshadowed by PET," says Cleveland's DiFilippo. "Also, advancements in SPECT have been really incremental over the years. Certainly SPECT-CT is a quantum leap more so than incremental advancement, so it is going to get a lot of attention and deservedly so. I think the future is very bright because SPECT and PET are unique compared to other modalities because the tracer really gives the diagnostic information. There is always the potential for new radioisotopes to be invented and they are constantly being developed. Overall, I think SPECT is going to undergo a reawakening as more attention is focused on SPECT-CT. It's hard to say what next major advance will be, whether it will come from instrumentation, radiochemistry or image analysis techniques." Whichever way, the future is bright.
New technologies are speeding up myocardial perfusion scanning times
UltraSPECT's Xpress cardiac package is designed to reduce myocardial perfusion SPECT acquisition time by 50 percent without degrading image quality. Xpress.cardiac employs UltraSPECT's wide-beam reconstruction technology to boost throughput and reduce patient motion artifacts.
"The most important aspect of the software is that patients are going to be scanned in half of the time," says Salvador Borges-Neto, MD, professor of radiology, associate professor of medicine, and co-director of nuclear cardiology at Duke University Medical Center in Durham, N.C. "Nuclear cardiology is competing with other imaging modalities for the evaluation of patients with ischemic heart disease. The ability to decrease the time that the patient will be in the laboratory is certainly a huge plus."
Borges-Neto says that Duke is in the process of finishing a study using UltraSPECT software that determines its potential to reduce scanning times and improve efficiency of nuclear cardiology laboratory. The study also is looking at the image quality of the studies, which Borges-Neto says are of excellent quality in comparison to SPECT studies not using the software.
UltraSPECT announced FDA 510(k) clearance for the software package in June.