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

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 optimizing patient flow and workflow. "All sites must be aware of patient workflow," says Patrick J. O'Day, global PET-CT product manager for GE Healthcare. "No facility will ever fully utilize the throughput capabilities of hybrid imaging equipment without the proper consideration for scheduling, staff and facilities for tracer injection and biological uptake waiting."

And hybrid imaging offers several opportunities for best practices, says Jonathan Frey, director of molecular imaging marketing for Siemens Medical Solutions. That may include modified patient workup, such as including both modalities in the patient workup for some indications, rather than just one modality or the other. A facility might also be able to use to locate the two technologies in the same area.

"From a patient flow standpoint, PET-CT also has offered opportunities, in increasing the patient throughput and volume [PET-CT is a much faster scan that PET alone], in facility layout [because of the increased volume potential, more uptake rooms are required to support one scanner], and in novel approaches to scheduling [scheduling PET-CT patients as well as CT-only patients on the same scanner]," says Frey.

How the facility is adding the CT capabilities is a key factor in how hybrid imaging affects flow. "If CT is used for attenuation correction only, then there is no real change in workflow," says Deepak Malhotra, director of marketing for Philips Medical Systems nuclear medicine. He sees "in most cases, CT is used to either localize the PET or SPECT findings through a low dose CT scan and/or it is used to do a full diagnostic CT scan, which is complementary to the diagnostic PET or SPECT scan. If a diagnostic CT is ordered, a [radiologist] will most likely need to read the CT scan, thereby requiring close cooperation and coordination between the nuclear medicine physician [who reads PET or SPECT] and the radiologist [who reads diagnostic CT] for reading and reporting."

Storing those scans

Another big consideration for facilities moving to hybrid imaging is image storage and communication. Physicians need to access the images, and often, view prior images for comparison. "Since there is no DICOM standard for coregistered/fused image datasets yet, it is important that end-users are able to store both the PET-SPECT and CT datasets in one archiving system," says Philips' Malhotra. The PACS should be capable of storing both types of images.

"Secondly, it also may be beneficial to store secondary capture of fused images used for diagnosis by the reading physician for future reference," he continues. "Also, hospitals need a large archive or PACS device to be able to store several years of data and have the ability to quickly bring back images from several months ago to monitor effectiveness of therapy or progression of disease."

Siemens' Frey agrees that PACS is essential to successful hybrid imaging storage. "Our primary advice is to ensure that sites have and use a PACS, especially if they plan to use the CT for diagnostic purposes. While PET and SPECT images are relatively small, CT images can create a considerable volume of images." Most radiology departments are accustomed to these larger data sets, and have usually implemented a PACS solution before the implementation of hybrid technology. "However, nuclear medicine departments are less-often accustomed to PACS solutions, and can otherwise be surprised by the data set size demands," he says.

More options for image storage are on the way, says GE's O'Day. The company already offers CD/DVD archive media and tools that archive complete patient hybrid exams. An automated network transfer utility that allows DICOM-compliant exam transfer to multiple destinations including remote workstations and PACS in the healthcare enterprise is another option. "Our best advice to customers is to have a plan based on anticipated patient volumes for the flow and storage of patient image data in advance of equipment arrival," says O'Day.

But hybrid imaging results in more images for each patient exam and increased volume driven by the productivity advances of CT attenuation correction and scanner technology improvements. "This is an area of current focus and product development," says O'Day. "We want to provide computer-assisted reading tools ensuring similar productivity gains in the reading room."

Duke's Coleman advises against PACS - for the moment. "Hybrid imaging files are not large compared to those from multidetector CT and MRI, so facilities that have these technologies that already have a lot of data output are going to have data storing mechanisms that only get better and better. Currently, PACS are not well-designed to display PET-CT and SPECT-CT images, and companies are working hard to correct that."

Effective reads

"Everybody thinks they can read a CT scan, and very few think they can read a fusion study," says Scharf. "Few [physicians] are comfortable with both." Nuclear medicine physicians often are not expert in CT scanning and they use the CT scan to help them figure out where things are rather than what they are, he says. Meanwhile, radiologists look at a CT scan, see an abnormality and use the CT scan to determine whether it's important.

Today's residents are learning both. Scharf reads all of his studies with a radiology resident who can help him find abnormalities. The Society of Nuclear Medicine has spent time and energy educating nuclear medicine specialists on the fundamentals of CT scanning.

The technology has improved faster than everyone's ability to deal with it, says Scharf. That raises difficult, but important, questions. "The better the image quality, the more likely you're going to have readable images," he says.  That makes things complicated. For example, cardiologists wanted the software to segment out the lungs from their scans. "They didn't want to see any problems in the lungs. Theoretically, anytime you take an image, you are responsible for anything on it." Better images show more problems and no one has figured out how to manage that conundrum.

"The better the technology gets, the harder the questions," Scharf says. But overall, fusion brings success in diagnosis.