Oncology was the first clinical arena to exploit the advantages of the fusion of positron emission tomography and computed tomography images in diagnosing and staging cancers, following treatment effects and differentiating between normal and malignant tissues. Some experts assert that PET/CT enables earlier diagnosis of some forms of cancer.
As the technology evolved from software co-registration of images obtained on separate PET and CT systems to combined hardware via hybrid PET/CT scanners, and the CT component gained capabilities and speed with the advent of multidetector design, other clinicians began to explore the value that fused images could afford their patients.
Cardiology adopts the hybrid approach
Kenneth D. Henson, MD, FACC, director of cardiovascular imaging at Imaging for Life in Sarasota began utilizing the PET component of their biograph from Siemens Medical Solutions for cardiac patients in December 2005. The scanner was installed primarily for oncology applications, but Henson took advantage of the time that the equipment was available to other clinicians. He uses the CT component of the biograph solely for attenuation correction for their PET studies, while using the Siemens Somatom Sensation 64-slice scanner for CT angiography.
Regarding the PET/CT studies, he explains they use the biograph scanner as a means to follow patients with existing coronary disease, those who have had bypass surgery or stent placement or those with former myocardial infarctions. “I use this as a technology to look for recurrent ischemia.”
Additionally, Henson notes that the radiation exposure from a PET scan is a fraction of that produced by a single photon emission computed tomography (CT) study, while accuracy is enhanced and the number of unnecessary cardiac catheterizations is reduced.
These PET/CT tests are all performed sequentially:
- an “at rest” CT for attenuation correction
- a PET scan following injection of Rubidium-82
- injection of a pharmacologic stressor
- PET stress acquisition, and
- a second attenuation correction with CT after the stress PET is acquired.
This sequence generally takes between 35 and 40 minutes to complete.One of their technical concerns involves misregistration of the images if motion occurs. “Not only is there a potential for patient movement, but there also can be hemodynamic changes that affect the position of the diaphragm.”
The radiopharmaceuticals used in cardiac imaging are different than the fluorodeoxyglucose (FDG) used for oncology. Cardiac PET scans usually employ Rubidium-82 because it is related to potassium used by the muscles of the heart. “Because it is a potassium analog, you get less interference from the isotope in other organs or the diaphragm.”To perform these scans, the institution must have access to a Rubidium generator, which means they must either have the volume of patients to support the costs, or share the generator with another institution or other clinicians. A Rubidium generator can be used for 28 days.
How you slice it
As health centers install PET/CT scanners with multidetector CT components, the increased slice count offers improved spatial resolution of anatomic structures beneficial to diagnostic activities.
Marcelo Di Carli, MD, FACC, FAHA, chief of nuclear medicine/PET and co-director of cardiovascular imaging at Brigham and Women’s Hospital and associate professor of radiology at Harvard Medical School in Boston, describes the increased functionality of their GE Healthcare 16-slice Discovery LS for three major cardiac activities. “The test needs to not only diagnose the disease at hand, but also be able to risk stratify, and guide physician management.”
Vasken Dilsizian, MD, FACC, FAHA, professor of medicine and radiology at the University of Maryland School of Medicine and director of nuclear cardiology and cardiac PET at the University of Maryland Medical Center is using a Philips Medical Systems Gemini 16-slice PET/CT system (installed in October 2003) and concurs about the benefits of PET/CT in diagnosing and managing the care of their patients with cardiac disease.
“Our practice in cardiology is to use both [imaging modalities] synergistically,” says Dilsizian. Given the preference of avoiding unnecessary invasive cardiac catheterization procedures, nuclear medicine perfusion imaging has become the frontier for diagnosing patients with chest pain.
This synergistic approach capitalizes on the strengths of each imaging technique to provide unique and complementary data to the physician. CT provides anatomic information, while PET affords a different view with the physiology in a given situation. Additionally, the CT component provides attenuation correction to further enhance the PET images.
For diagnostic purposes, clinicians must determine whether or not there is physical obstructive disease in the coronary arteries and whether or not the heart muscle has sustained damage. By using CT and PET in tandem, both pieces of the puzzle fit together. Imaging results inform both risk stratification and direct treatment decisions.
The patient with multi-vessel disease serves as an illustrative example.
Di Carli explains that while nuclear perfusion imaging may be effective in identifying patients who have coronary disease, it may be unable to recognize patients with multi-vessel disease. “We can tell who has disease, but not how much disease is present, and we tend to underestimate the presence of three-vessel disease.” That is where CT often provides accurate definition of the anatomic extent of the problem.
Since PET is ideal for reviewing the effect of blood vessel obstruction, it can be used as the first-line image for diagnosing patients with chest pain. “If we use a nuclear perfusion study as a gatekeeper to screen patients who will need subsequent invasive coronary angiography, then this serves us quite well,” says Dilsizian.
The role of PET/CT in cardiac treatment decisions
“The CT contribution to PET is being able to tell us the presence of left main disease or proximal three-vessel coronary disease,” says Di Carli. “The PET contribution to CT’s assessment of risk is by defining the extent and severity of myocardial ischemia which is a powerful predictor of death in myocardial infarction. There is synergism between the two tests to improve our risk stratification.”
Dilsizian adds that PET has the advantage of providing information about absolute blood-flow quantification through less scatter and higher “counts” than SPECT is capable of demonstrating. “You’ll be able to say, in quantitative terms, whether a person has two-vessel disease or one-vessel disease, and not just rely on visual interpretations which give opportunities for inter-observer variability. That’s the reason we believe PET provides added value to interpretation.”
Although perfusion imaging has its place in revealing the extent of cardiac damage, it is ineffective in providing specifics about the nature of arthrosclerosis within the blood vessels. When decisions must be made about whether to perform an invasive revascularization procedure, or to manage the patient medically, the fused images from PET and CT help to guide treatment decision-making.
Di Carli explains the example of a patient with normal perfusion, but a calcium score of 1,000 is a patient who would be treated medically, but more aggressively than a patient with normal perfusion and a calcium score of 0. “The goals of LDL [cholesterol] reduction would be different in a patient with a normal perfusion…who would not need to go to the cath lab for revascularization because there is no flow-limiting stenosis.” However, if there were a lot of atherosclerosis, they would need to slow its progression and perhaps attempt regression with a goal of an LDL level of 70 mg per deciliter if the calcium score were high.
He asserts that not every patient would be a candidate for a hybrid imaging study. Taking the entire universe of patients with suspected disease, the most appropriate individuals to image in this way would be those with a low to intermediate likelihood of coronary artery disease. In general, those patients are younger, and are those in whom the rate of yield of normal CT is very high, he says.
Di Carli offers one caveat to the adoption of widespread use for PET/CT in cardiac management: much of this information has not been proven in clinical trials. He and several colleagues are in the process of conducting a broad-based study that was slated to begin accruing 3,700 patients last month. The Study of Myocardial Perfusion and Coronary Anatomy Imaging Roles in Coronary Artery Disease (SPARC) is a prospective, multi-center observational study designed to evaluate the clinical value of stress perfusion (SPECT or PET) imaging, noninvasive CT angiography (CTA) and combined perfusion-anatomy (PET/CT) studies. For more information, visit the website: www.sparctrial.org.
Dilsizian notes that there is literature available regarding the use of SPECT/CT imaging, and he believes these techniques hold promise because they may decrease the number of invasive procedures required. Nuclear medicine cardiac perfusion imaging has become the frontier for diagnosing patients with chest pain.
Given the attenuation correction enabled by CT to say nothing of additional clinical information from the anatomic data, it is intuitive that the combination of these imaging modalities would prove valuable in a number of clinical settings.
PET/CT in neurology
Not only the field of cardiology has been impacted by the use of hybrid PET/CT scanners. In neurologic settings, some centers have found the combination of these imaging studies to hold promise.
Neil B. Horner, MD, chief of neuroradiology for the Atlantic Neuroscience Institute of Atlantic Health System at Overlook Hospital in Summit, N.J., has been using a GE Healthcare Discovery LS scanner since March 2002. Overlook was the first institution in New Jersey to install the Discovery LS, and among the first eight in the nation. This neuroscience center provides highly specialized services for the 1.6 million people covered in the Atlantic Health Plan.
He explains that he believes PET/CT is the first step in a new era of imaging: the fusion of anatomic and physiologic images. They have used this technology in diagnosis and treatment of patients with brain tumors, those with Alzheimer’s disease and those who suffer from seizure disorders.
“We use it in patients with Alzheimer’s Disease who have a decrease in metabolism in certain areas of the brain, particularly in the temporal lobes,” Horner says. “We find that fusion imaging — whether you’re talking about PET/CT or PET/MR — is better than PET alone because we can look at anatomic areas in detail as well, not just the decrease in physiology, and have more of a global view.”
They have a well-established epilepsy program in their center with dedicated epileptologists, and they use PET as well as SPECT for the metabolic aspects of imaging. Sometimes they want to gather data during the time a patient is experiencing a seizure. Depending on the specific requirements of the exam, they may prefer to employ a SPECT radiopharmaceutical over one used in PET.
Horner anticipates that in the future, with PET/CT as the basic fused study, they will add MR fusion to the equation to provide anatomic analysis of brain tissue. “The take-home message for PET/CT and PET/MR is that these tests add another dimension to the diagnosis of neurologic diseases.”
The use of PET/CT has been widely accepted within the realm of oncology care for many years. The ability of PET radiotracers to highlight metabolically active tissue has proven invaluable in managing those patients. Now cardiologists and neurolo-gists employ different radiotracers to provide detailed information for their patients as well, and the value of fused imaging that joins anatomic information from CT or MR with PET is growing in acceptance across clinical areas.