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New test reduces time, improves quality of imaging

by Dawn Brazell
Public Relations
The first advantage a cardiac patient is likely to notice is that the test takes less time—one hour as compared to four. The doctors will notice the improved quality in imaging as well.

Leonie Gordon, M.D., Marques Bradshaw, M.D., and Kenneth M. Spicer, M.D., Ph.D., and their cardiology colleagues, will be celebrating both and looking for ways to expand the research opportunities provided by a new type of advanced imaging system at MUSC. The new procedure, called rubidium PET/CT myocardial perfusion scanning, combines Positron Emission Tomography (PET) with Computed Tomography (CT). In less than an hour, this noninvasive imaging technology provides quality images that can help physicians better diagnose and treat coronary artery disease.

Melissa Dutton, a nuclear medicine technologist, likes that the rubidium PET/CT scanner shortens the testing time for patients.

One of the most promising features of the technology is that it delivers more accurate imaging for patients with a body mass index (BMI) of more than 28, for patients weighing 250 pounds or more, or for people who are stocky or large-breasted.

A major advantage is that rubidium PET/CT myocardial perfusion imaging gives the patient a significantly reduced radiation dose, approximately one third of the exposure received from a standard nuclear medicine cardiac perfusion scan.

Spicer said the hospital began rubidium myocardial perfusion imaging in October and MUSC is the one of about a dozen sites in the Southeast to be using this technology.“It’s a much less extensive procedure because the radioisotope has a very short half life—about a minute—half of it goes away every minute. We image very quickly, and can wait five to 10 minutes and repeat the procedure. With normal nuclear medicine imaging of the heart muscle, we use technetium with a six hour half life.”

Having a procedure with improved specificity is extremely helpful as well, he said. Large body size can impair the ability of photons, the particles of energy that come from the radioactive tracer, to register correctly on the imaging camera. PET/CT scanners can minimize this problem. Further, the higher energy level of the radioactive photons emitted from rubidium move through tissue more freely, thus providing better, more accurate imaging, he said.

Another benefit is patients don’t have to remain still as long, so there is less blurring of the images from motion. This newer imaging procedure is just as safe and is not more expensive, two factors important in its effectiveness in treating patients, said Spicer.

Dr. Kenneth Spicer

“One of the greatest frustrations for nuclear medicine physicians and cardiologists occurs when they are confronted with an ambiguous scan,” said Spicer, explaining this too often is the case with patients with a larger body mass. “The ambiguous scan can be caused by excessive tissue scattering of photons, by motion or be a real lesion in the coronary artery that is potentially life threatening to the patient. We have to tell the referring physician that we’re not sure.”

Patients may request the newer imaging procedure and should if they have a larger body mass. “Clearly this would be a better procedure for them.”

PET scanning acquires images by detecting the radioactive tracer rubidium. Special cameras can detect photons being produced in the blood stream by this  radiopharmaceutical and compute a map of the way blood is being distributed to heart tissue. The CT part of the combined imaging system uses rotating X-ray beams to pinpoint normal and abnormal cardiac blood flow. The combined system measures that flow in precise, numerical terms, which is ultimately what is required for optimal treatment, he said.

Spicer said doctors can more accurately measure heart muscle activity with PET scanning than they can with standard nuclear medicine procedures. “That allows us to image actual metabolic activity of the heart muscle cells, which is important in terms of improving our sensitivity for diagnosing serious disease processes.”
Early detection and treatment of incomplete or small partial blockages seen with other, anatomic imaging procedures also has the potential to protect heart muscle, but this can be invasive and expensive, he said. Rubidium PET/CT scans optimally depict not only whether a coronary artery blockage is impairing blood flow to the heart muscle, but also the extent of the decreased flow.

That ability translates into exciting research applications, Spicer said.

When combined with another tracer called FDG, which assesses sugar molecules the heart burns as a fuel, PET/CT produces a clear picture of viable areas of heart muscle and differentiates them from those that are scarred, or even from viable muscle cells that are temporarily stunned. This distinction can help doctors decide which treatments would be the most effective.

This type of scanning, known as Metabolic Imaging, also takes health professionals one step closer to being able to make earlier diagnoses.

There is a progression or stairway of changes that occur as a disease process evolves, Spicer said. “When something goes wrong, it usually produces genetic code changes within a cell. It could be a virus that came in. It could be a chemical shock or even physical injury. But there’s been something that changes the metabolism of the cell. If you’re going to be the most sensitive, you want to catch the changes that occur at that point. We’re not quite that good yet.”

The next step in disease progression is that the biochemical alterations result in changes in the function of the injured cell. For the heart this could mean EKG changes, or heart wall motion decreases. Finally, the shapes of the cell or groups of cells change, he said.

“Typically in radiology we’re imaging the end of the chain—the shape—when the muscle cells die and the heart wall becomes thin or scarred, to an extent that is big enough to see it. That’s obviously very helpful, but it’s not nearly as sensitive as it needs to be. In Metabolic Imaging, we can step down these stairs getting closer and closer to the original event where things started going wrong. That’s where the real forte of PET/CT imaging is.”

Friday, Dec. 15, 2010

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