PET/CT Moves Beyond Oncology

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hiit040602Oncology 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