Nuclear medicine and diabetic foot infections

Up to 25% of the diabetic population is at risk for developing a pedal ulcer. These ulcers serve as a portal of entry for osteomyelitis and overlie more than 90% of diabetic pedal osteomyelitis cases. The diagnosis of osteomyelitis often is overlooked, and imaging studies are an essential part of the evaluation. The most commonly performed radionuclide tests are bone and labeled leukocyte imaging. Focal hyperperfusion, focal hyperemia, and focal bony uptake on the 3-phase bone scan comprise the usual presentation of osteomyelitis. Many conditions to which the diabetic population with foot problems is prone, however, mimic osteomyelitis, and the test is sensitive but not specific. Consequently, the bone scan often is used as a screening test or to facilitate localization of activity on labeled leukocyte images. Because of its high sensitivity and prevalence of positive results, its value as a screening test is questionable. Investigations comparing labeled leukocyte imaging alone to labeled leukocyte plus bone imaging, demonstrate only marginal improvement for the combined study. Thus, it is time to reevaluate the role of the bone scan in diabetic foot infections. Labeled leukocyte imaging is the radionuclide procedure of choice for evaluating diabetic pedal osteomyelitis. Sensitivity and specificity range between 72% and 100%, and 67% and 98%, respectively. Although intraindividual comparisons are few, the accuracy of the test is similar, whether the leukocytes are labeled with (99m)Tc or (111)In. Labeled leukocytes accumulate in uninfected neuropathic joints, and marrow scintigraphy may be needed to determine whether infection is present. Alternatives to labeled leukocyte imaging include in vivo methods of labeling leukocytes, radiolabeled polyclonal IgG, and radiolabeled antibiotics. The results obtained have been variable and none of these agents is available in the United States. There are few data available on single-photon emission computed tomography/computed tomography. It probably will be useful in the mid and hind foot; in the distal forefoot, given the small size of the structures, its value is less certain. Data on (18)F-fluorodeoxyglucose positron emission tomography and positron emission tomography/computed tomography are limited and inconclusive, and further investigation is needed.