SPECT/CT’s Role in Post-Transplant Infection Imaging

Timely and accurate diagnosis and treatment of infection in post-transplant organ patients is critical to patient survival and the prevention of organ rejection. When a patient presents with infection symptoms and physicians cannot immediately ascertain the location, they turn to SPECT/CT imaging as a technique of last resort to pinpoint infection at the source—and ultimately improve patient management for post-transplant recipients.

Organ transplantation has continued to evolve since the first successful kidney transplant in 1954 between living identical twins. Over the last 20 years, organ transplants and longer survival rates for recipients have increased due to breakthroughs in tissue typing, immunosuppressant drugs and medical imaging. Challenges still remain, however, to effectively ensure a recipient heals correctly, particularly regarding suppressing and preventing infection.

Approximately 56 people receive an organ transplant in the United States every day. From Jan. 1, 1988, to Jan. 31, 2008, there have been 424,064 organ transplants performed in the U.S. Of that, more than half of the transplants performed were kidney transplants, followed by liver and heart. 

No matter what type of organ is transplanted, post-surgery is a critical time as the road to recovery is paved with additional medication, hospital visits and essentially a lifetime medical treatment plan, due to the immune system’s reaction to the drug. Even with a good match, the organ is susceptible to infection and possibly rejection.

Depending on the time that has elapsed post-transplant, recipients are vulnerable to a host of bacterial or viral infections. The risk of infection is determined by a range of sources, including latent viruses to pathogens within the community or hospitals. Since patients also are immunosuppressed, muting the capability to ward off viruses, extra care and management is required to monitor infection during the post-transplant period.

Typically, other tests, such as CT are more commonly ordered to search for possible infection sites. Since CT is the most readily available scan in a hospital, when there is a question of infection, many times it can explain what is going on with the patient, according to Jerry Froelich, MD, director, nuclear medicine and molecular imaging at the Imaging Center for the University of Minnesota, Fairview, a large tertiary care hospital with an extensive history in organ transplants and cancer treatment. But molecular imaging, specifically SPECT/CT imaging, is an effective tool for physicians when the source of the infection in unknown.

“Nuclear medicine is great normally when you can’t find the source of the infection,” says Froelich. In mid-February 2008, the department installed a Symbia T6 TruePoint SPECT·CT system from Siemens Healthcare. “SPECT, however, can take anywhere from eight to 24 hours, and in today’s rapid medicine market, this is a very long time to wait for an answer,” he added.

The nuclear medicine section at the university performs more than 6,000 studies per year, including cardiac imaging and stress tests; scintigraphic evaluation of the skeletal system for cancer, infection, or inflammation; physiologic testing of the gastrointestinal and urinary tract; specialized tumor imaging and detection; and endocrinologic testing, lung scanning, and sentinel node detection. In the first six weeks after going live with SPECT/CT, the department imaged 220 patients. 

Normally, nuclear medicine images are 2D images of a 3D environment—the body. Trying to triangulate using planar imaging is not always easy and can involve a lot of imprecision and guess work. “SPECT allows us to do a 3D map of where the activity of interest is, giving us the sensitivity needed to find infection,” he says.

According to Froelich, SPECT/CT localizes small abscesses related or adjacent to the transplanted organ, but when imaging is unable to reveal such abscesses, the next logical place to look is in the chest, the rest of the body including the extremities, and sometimes even the bones.

Successful SPECT/CT imaging for infection not only relies on knowing where to look for possible sources of infection, it also is reliant upon the kinds of radiopharmaceuticals used. Indium-labeled white blood cells (In111-WBC) are a common preference due to their high sensitivity to infection, Froelich says. They are good for patients who might be on antiobiotics or some form of drug management, which can mute the immune response—they are very good at finding abnormalities in patients. “With Indium white blood cells, we can see more precise localization,” he notes.

By using a highly specific agent like In111-white blood cells imaging that will almost exclusively go to the sites of infection, SPECT/CT demonstrates precisely where the WBCs are localizing and then, based on the CT registration, further define what is infected, for example, an abscess, bowel wall or transplanted organ. “Planar In111-WBC images—like a painting of a forest—can show abnormal uptake within the forest but SPECT/CT can also show you which tree [is infected],” he says.

Typically, after injection of In111-WBC, the first scan is made after six to eight hours and then the physician waits another 18 to 24 hours before performing the second scan.

Less commonly used in SPECT imaging is technetium white blood cells which are quick—usually providing an answer in two to four hours—but they have higher background activity and if the infection is slower or the patient is on antibiotics, they are less likely to find the infection. “So Indium-labelled gives us more time to find the infection for patients on immunosuppression therapy or antibiotics,” Froelich says.

SPECT/CT is one of many tools used in infection imaging; ultrasound is another tool, typically used to look at renal transplants, and CT, which allows a physician to see if something anatomically is going on that should not be there. With SPECT, symptoms are correlated with localization and specificity. “You have a patient with symptoms of infection, I have localization with SPECT and based on uptake, we can tell more accurately if something is abnormal,” he adds. “We can diagnose and track responsive therapy more accurately.”

Post-transplant imaging is more a technique of last resort, Froelich adds, noting that SPECT/CT is called in when CT or ultrasound cannot find the source of the infection. But the combination of the SPECT and CT data offers the most efficient way to determine where the infection is and then precisely localize it. In terms of imaging volume for organ recipients’ post-transplant scans for infection, the medical center usually performs approximately 20 scans per year. “But in every case where we find an infection, it always changes patient management,” he says.

Regardless of what kind of organ transplant is performed, the fact remains that monitoring infection post-transplant is critical to the patient’s survival and to prevent possible organ rejection. SPECT/CT, while not always used as the first line of defense in monitoring patients post-transplant, plays a vital role in identifying the source of infection when more frequently used methods of infection detection cannot.