SPECT/CT: Defining a Niche in Infection Imaging

SPECT/CT is making notable inroads in imaging infection. It allows the detection of unsuspected infectious foci, especially in areas that are not well investigated by other imaging modalities. It is improving diagnostic performance and treatment management for specific infections such as in bone, the diabetic foot, post-device implantation, fever of unknown origin (FUO), orthopedics and in post transplant patients. A wide cross-section of physicians are starting to take notice—and referring more cases to gain the combined strengths of SPECT and CT to investigate infection.

SPECT/CT is an integral part of infection imaging. “Just like PET/CT, we no longer do SPECT without the CT,” says Jerry Froelich, MD, director, nuclear medicine and molecular imaging at the Imaging Center for the University of Minnesota in Minneapolis.

“I cannot imagine going back to do SPECT without the CT now that I do it every day or all the time,” says Froelich.

SPECT/CT’s imaging advantage

SPECT/CT combines the advantages of molecular imaging to detect infection and anatomical imaging to localize it, says Anne Hitzel, MD, Nuclear Medicine Department, University Hospital of Toulouse, Place du Docteur Baylac, Toulouse, France. The new generation of SPECT/CT scanners with iterative reconstruction allows us to find smaller areas of lower activity that relates to infection or disease, adds Froelich.

SPECT/CT allows the detection of unsuspected infectious foci, especially in areas that are not well investigated by planar scans—such as areas surrounding prothesis or infected areas without abscess, vascular prosthesis or left-ventricular-assist devices, Hitzel says. “We frequently use it to help the surgeons determine what is the next logical step to do with the patient, be it aspiration biopsy or even surgery,” adds Froelich. In pediatric populations, neuroblastomas can be located with mIBG (meta-iodobenzylguanidine) scans as they are not FDG positive and SPECT/CT provides information on where the neuroblastoma is located, while PET/CT cannot, he says.

Evaluation of Diabetic Foot Infection by Dual-Isotope SPECT/CT

Transaxial dual isotope SPECT/CT scan confirms a focus of white blood cell uptake and the diagnosis is soft-tissue infection with no evidence of osteomyelitis. Source: Sherif I. Heiba, MD, Associate Professor of Radiology, Mount Sinai Medical Center, New York City Dual-isotope SPECT/CT is a more highly accurate imaging protocol for evaluating the diabetic foot than Tc-99m hydroxymethylene-diphosphonate (HDP) 3-phase bone (BS) or Indium-111 leukocyte scanning (WBCS) alone, says Sherif I. Heiba, MD, associate professor of radiology, Mount Sinai Medical Center in New York City. Diagnosis of diabetic foot infection is a difficult task and single modality imaging, whether it be a bone scan, MR or CT scan, is not very successful in accurately determining the infection and its precise location, he says. Heiba and his colleagues investigated a method that combined BS/WBCS, and if needed, WBCS/bone marrow scanning (BMS) using SPECT/CT to accurately diagnose/localize infection in a protocol which was published online September 20, 2010, in the Journal of Foot & Ankle Surgery. Blood flow/pool images were obtained followed by WBC reinjection and dual-isotope BS/WBCS planar and SPECT/CT were performed approximately 24 hours later. BMS/WBCS SPECT/CT (step 2 dual isotope) was obtained on the following day when images were suspicious for mid/hindfoot osteomyelitis. The use of SPECT/CT in this protocol improves the diagnostic confidence and helps to discriminate between soft-tissue infection and osteomyelitis, while providing precise anatomic localization. When needed, step 2 dual-isotope SPECT/CT (BMS/WBCS) can yield additional information for a more definitive diagnosis. Combined radioisotope SPECT/CT imaging technique also can beneficially impact diabetic patient management. Heiba’s group has seen encouraging results in an unpublished study involving 200 patients where the combined radioisotope SPECT/CT imaging helps not only in diagnosis, but also the precise management of diabetic foot infection.

Moreover, conventional planar and SPECT images are often unable to precisely locate the site of infection. Hybrid SPECT/CT system delivers the high sensitivity of scinitigraphic technology with the high specificity of CT and overcomes the partial limitations of each technology alone. SPECT/CT has been widely used to detect and localize infections in fever of unknown origin. In the case of bone infection, hybrid SPECT/CT can differentiate soft tissue from bone. For example, 99mTc-hexamethylpropylene amine oxime (HMPAO)-labeled white blood cells imaged using SPECT/CT contributed to the accurate identification of infection in a patient with suspected cranial osteomyelitis in a case report published in the Brazilian Journal of Infectious Diseases in December 2008 by Chiara Bruni et al.

Use of integrated SPECT/CT, in comparison with conventional planar and SPECT scanning, improves diagnostic accuracy and reporter confidence in clinical practice, according to a study published in Nuclear Medicine Communications in December 2006 by Paul J. Roach et al. For diagnosis, SPECT/CT resulted in a minor change in 10 percent of patients and a significant change in 9 percent of patients over planar/SPECT imaging. For the final scan interpretation, there was no change in 44 percent of patients, a minor change in 30 percent and a significant change in 26 percent with the use of SPECT/CT, according to the authors.

Infection imaging’s biggest limitation is the time to appropriately label and image the patient which takes about 24 hours in the case of an indium-111 white blood cell scan or a gallium scan. White blood cells are collected from the patient, exposed to indium, and reinjected back into the patient two hours later. The exam is scheduled between 18 and 24 hours after the white blood cells have been labeled with indium and each study takes 30 to 60 minutes, depending on the size of the area being imaged. Physicians think it is worth the wait. Patients with complex devices can tolerate SPECT/CT better than an MRI. In the case of CT, there also is concern of contrast being given to patients who have infections and are prone to liver or kidney problems. In such cases, SPECT/CT provides the ability to evaluate patients with renal failure without potentially compromising renal function. SPECT/CT with Tc-99m-labeled mercaptoacetyltriglycine (Tc99m-MAG3) can be used to evaluate renal function in renal-transplant patients and to identify kidney failure, while indium-111 labeled white blood cell scans detect infection. Gallium SPECT/CT also can be used to image chronic infections, bone infection of the spine, lung infections and inflammation.

Froelich notes SPECT/CT’s increasing value in infection imaging as well as broadening interest from a wider cross-section of physicians. Applications of SPECT/CT will develop, allowing a more efficient management of infection, with precise diagnosis, and monitoring of response to antibiotic treatment, especially when prosthesis cannot be taken out, predicts Hitzel.