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October 16, 2006
Initial SPECT/CT heart studies hint at the possibility of reducing invasive cardiac diagnostic procedures. Since F. Mason Sones, Jr, MD, a pediatric cardiologist working at The Cleveland Clinic in the 1950s, accidentally injected angiography dye into the mouth of a coronary artery instead of the aortic valve, physicians have been using coronary angiography to diagnose coronary artery disease (CAD). In 2004, more than 3 million coronary angiographies were performed to find narrowing caused by plaque, which most often starts gradually but can build up over time to cause a heart attack, cardiac arrest, or sudden death. In the United States, cardiovascular disease is a leading cause of death for both men and women. CAD is the most common cause of cardiovascular disease; as many as 3 to 4 million Americans may have ischemic episodes without knowing it. Angiography is performed under local anesthesia in the angiography suite. Under fluoroscope guidance, a tiny catheter is advanced from the patient’s groin or arm into the opening of the arteries. A small amount of radiographic contrast material is injected into each artery and x-rays are taken to reveal any blockages and their extent. The procedure takes approximately 20 to 30 minutes. Because it is invasive, angiography carries a small risk to patients and is relatively expensive. Physicians have been searching for less invasive and less costly diagnostic procedures and have come close with CT scans of the heart. “The 64-slice CT angiography [CTA] is an x-ray–based technique that can visualize coronary artery morphology and abnormalities with a quality that gets close to conventional coronary angiography,” says Oliver Gaemperli, MD, a nuclear cardiologist at University Hospital Zurich in Switzerland. Physicians also use SPECT to detect oddities in the flow of blood through the heart’s vessels. “Myocardial perfusion imaging using SPECT [single-photon emission computed tomography] is an established method for assessing the physiologic significance of coronary lesions in patients with chest pain,” says Zohar Keidar, MD, PhD, deputy director of the nuclear medicine department at Rambam Health Care Campus in Haifa, Israel. One Noninvasive Exam “The SPECT/CT device can provide—in a single imaging session—an accurate evaluation of cardiac blood vessel narrowing and blood supply to the heart muscles,” says Keidar, who adds that the initial results suggest combining the two technologies provides a more accurate view of the patient’s blood flow to the heart than either imaging test could alone. “Each modality shows a different aspect of coronary artery disease, anatomy of vessel narrowing [CT] vs. blood supply and its effect on the corresponding part of the myocardium, the heart muscle [SPECT],” Keidar says. “The combination of the two modalities in a single device will allow for accurate cardiac assessment in a single noninvasive session.” Gaemperli has also been working on a combined SPECT/CT device with his colleague, Philipp A. Kaufmann, MD, FACC, director of nuclear cardiology at University Hospital Zurich. He says the information provided by SPECT or CT alone is sometimes insufficient to identify the vessel responsible for causing ischemia or a decrease in blood supply. That’s why the combined technique “is pretty exciting.” The Israeli researchers used the Infinia LS SPECT/CT, a noncommercial research device—which was introduced in early 2005—specifically designed for their study. They used the device to assess coronary lesions and determine which should be treated invasively and which could be treated more conservatively. “The fused SPECT/CT images were found to be particularly helpful for correlating the location of the anatomic arterial lesion with the corresponding perfusion defect,” Keidar says. Evaluating Lesions Each lesion found on CTA and SPECT/CT was compared with the corresponding territory on myocardial perfusion SPECT imaging and coronary angiography, which serve as the gold standard, Keidar says. Keidar says it took approximately 35 minutes to perform the SPECT/CT. That time included rest and stress scintigraphy with CT-based attenuation correction and planning and acquisition of CT for calcium scoring and angiography. Since the introduction of the combined device, Keidar says, there have been additional technological developments based in part on the initial clinical results. The research device they used consisted of a 16-slice CT scanner and conventional dual-head SPECT gamma cameras from GE Healthcare. Both need to be upgraded to state-of-the-art imaging technology—64-slice CT or more and advanced, dedicated, faster cardiac cameras—before the technique can have clinical applications, Keidar says. Faster cameras would shorten the acquisition time, he notes. Keidar says theirs are the first results of cardiac SPECT/CT in a clinical setting of which he is aware. Keidar says more clinical studies are needed. “What we need now are studies in larger patient populations in order to prove the clinical role of this emerging cardiac hybrid imaging modality and justify its widespread use,” he says. Avoiding Diagnostic Procedures The Swiss researchers studied 100 consecutive patients ranging in age from 33 to 89 with suspected or known CAD, performing electrocardiographic gated myocardial perfusion imaging and 64-slice CTA on each. They analyzed 399 coronary arteries and 1,386 coronary segments, including 12 bypass grafts. The cardiac CT scans took only roughly 5 to 7 seconds. However, it required the placement of an IV line and multiple low-dose scans for orientation and planning, so it took approximately 20 minutes for each patient, Gaemperli says. The SPECT half of their study was a bit more time-consuming. “We performed routinely a one-day protocol with one scan after pharmacological stress and one scan at rest,” Gaemperli says. The pharmacological stress procedure was performed first and took approximately 20 minutes; each scan took roughly 20 to 30 minutes. “For a whole exam, we calculate roughly two hours,” Gaemperli says. The researchers used a commercially available radioactive tracer, 99mTc-tetrofosmin, at rest and during cardiac pharmacological stress. Functional Information “Conversely, we found that a normal myocardial perfusion SPECT does not exclude the presence of subclinical coronary artery disease, which may become significant as age progresses and for which cardiovascular risk modification may be warranted,” he notes. The results, Gaemperli says, underline the value of a combined assessment of coronary morphology (the visualization of coronary plaque) and function (blood supply to the heart muscle), which may easily and efficiently be accomplished with future hybrid SPECT or PET/CT scanners. However, Gaemperli does not believe the new hybrid devices will ever fully replace conventional coronary angiography. He believes invasive coronary angiography will continue to have a role in the diagnosis and treatment of CAD because it allows the physician to immediately perform treatment procedures should the catheterization identify a severely blocked artery. Several procedures, including balloon angioplasty and stenting, may be used to open the artery and be performed while the patient still is sedated and in the angiography suite. Gaemperli expects that conventional coronary angiography will remain the diagnostic method and treatment of choice in emergency situations where vessels are occluded suddenly and a quick revascularization (restoration of the blood flow to the heart muscle) is necessary and life-saving. Also, he expects coronary angiography to be the first choice for patients with a very high likelihood of obstructive CAD and who likely will need a revascularization procedure. Still, he says, the significant mortality inherent in invasive coronary angiography and high cost of the invasive procedure should not be neglected. Of the more than 3 million invasive coronary angiographies performed in 2004, a large portion were purely diagnostic, he says. “No revascularization procedure was performed,” he notes. Gaemperli says his team is focusing on finding noninvasive tools that can provide the same morphological information as coronary angiography while avoiding its risks and costs. Such a Holy Grail is more likely now, he says, thanks to the advances in CT technology, especially in the last six years. The new generation of CT scanners, he says, has enhanced spatial and temporal resolution “that in our view may be an excellent alternative to invasive coronary angiography in diagnosing patients with a low to intermediate pretest probability for CAD.” Looking at Limitations Some referring physicians are also concerned about the high radiation exposure with SPECT/CT—it is two to five times higher than conventional coronary angiography. Others don’t see that as an onerous concern. “All these limitations are the focus of ongoing research,” Gaemperli says. The next step is to improve the resolution of the CT scanner, Gaemperli says. He and Kaufmann are currently evaluating the feasibility and reproducibility of SPECT/CT image fusion, an important prerequisite for any further research with hybrid imaging. “We have to evaluate whether hybrid imaging really has an additive value in the cardiac nuclear field compared to separate image analysis from stand-alone scanners,” he says. “This information will be important for those involved with the development of hybrid scanners with high-end CT devices.” — Beth W. Orenstein is a freelance medical
writer who resides in Northampton, Pa.
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