PET-CT & SPECT-CT: With Fusion Comes Clarity

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Fusion imaging is proving itself in faster diagnosis, better treatment decisions, and better patient outcomes. Both PET-CT and SPECT-CT are growing in applications and volume.

Positron emission tomography combined with computed tomography (PET-CT) is gaining maturity as single photon emission computed tomography combined with computed tomography (SPECT-CT) is just starting to catch on. Industry watchers report higher PET-CT sales of late, but look for SPECT-CT to catch up in the next two to three years.

While some facilities only have the resources for one or the other, others don't see making the choice as a viable option. There was never a question for R. Edward Coleman, MD, director of nuclear medicine at Duke University Medical Center. "The indications and rationale for each are so different," he says. "You can't trade one off for the other." It comes down to applications, applications, applications.

Better vision

The differences between the capabilities of PET-CT and SPECT-CT begin with the type of radioactive materials used in the imaging. It's the basics: Some materials attach themselves to certain kinds of tumors better than others. "The differences between the two depends not on which technology but on which radioactive materials are better suited for a particular diagnosis," says Stephen Scharf, MD, chief of nuclear medicine at Lenox Hill Hospital in New York City. For example, some cancers take up [18]F-flourodeoxyglucose (FDG) much better. FDG and PET-CT go well together for seeking out colon and ovarian cancers with exquisite resolution. FDG does not work as well for patients with prostate cancer."

SPECT detects these radioactive materials, but creates inherently fuzzy pictures, Scharf says. "The materials don't give you the resolution you can get with an x-ray beam."

The challenge is that radiologists can only view those areas of the body that take up the radioactivity. The materials used are detected by standard gamma camera, a technology used since the 1960s. However, about 10 years ago you may recall, researchers discovered that certain radioactive materials emit positrons with an energy so much higher than other materials that you need a different device to detect them - a PET scanner. "PET scanning is not really fundamentally different from anything else in nuclear medicine with the exception of the different materials," says Scharf. By taking the scan that best matches with a particular condition and combining it with CT, clinicians can get the best radioactive image of metabolic or physiological function fused with anatomic imaging.

Growth areas

Duke uses PET-CT primarily for patients with cancer - diagnosing, staging and restaging and therapy monitoring. SPECT-CT is used primarily for neuroendocrine tumors viewed with MIBG, a particular radioisotope. It's also used in imaging parathyroid adenomas and imaging when clinicians want anatomic and metabolic information together.

Although capabilities for oncology will no doubt increase, Scharf says orthopedics and infectious disease are the two areas in which hybrid imaging is poised for growth. He has found that the excellent resolution allows him to pinpoint infection so effectively that he can direct a surgeon to the area.

In one case, a patient had a fever and pain in his shoulder. The orthopedic resident took fluid from the site and had it cultured, but failed to find the root of the problem.

"We did a picture the next day and found the activity right in the shoulder joint," says Scharf. "The activity was in a very small piece of the shoulder." The surgeon went in and found the infection exactly where the PET-CT showed it to be. "We never would have been able to do that with previous equipment."

SPECT imaging is now branching out into new areas as well. Scharf's facility has two surgeons on staff who do a lot of ankle reconstructions on patients having sustained injury or with chronic pain. His team conducts bone scans to look for stress fractures or an abnormality that justifies surgery - to determine whether an ankle fusion would be appropriate. The bones in that area are so small, that it's difficult to get a good idea of what's happening. "SPECT-CT can show activity in the bone, such as a fracture, and lead to more conservative treatment," he says. "It also can show activity between bones, showing that the patient is more amenable to fusion."

Preparing for fusion

Once a facility has made the investment in hybrid imaging, it pays to focus on