FDG-PET imaging is a promising imaging technique, which has the potential to overcome many of the shortcomings mentioned previously with regard to radiologic and scintigraphic methodologies. FDG is a diagnostic tracer utilized to measure the metabolic rates of normal and abnormal tissues. Many investigators have noted the affinity of FDG for active inflammatory and infectious disorders, such as sarcoidosis, the abdominal abscess, brain abscess, lung abscess, renal abscess, inflammatory pancreatic disease, lobar pneumonia, asthma, tuberculosis, colitis, sinusitis, myositis, mastitis, vasculitis, deep venous thrombosis, thyroiditis and other infections including those encountered in orthopedic patients.
According to the literature, the most accurate nuclear medicine modality for detecting infection associated with diabetic foot is the labeled WBC method. We hypothesize that FDG-PET imaging has several advantages over this method. Detection of infection by labeled WBC imaging is based upon the assumption that the administered cells will migrate to the sites of infection. Since the majority of the labeled leukocyte preparation consists of neutrophils, inflammatory/infectious processes with a predominantly neutrophilic infiltrate (acute infections) are likely to yield positive results. However, most infections associated with diabetic foot are sub-acute or chronic. Consequently, the dominant inflammatory cells involved are monocytes and lymphocytes. Therefore, labeled leukocytes are unlikely to detect chronic infection since very few monocytes and lymphocytes are labeled. In addition, the previous treatment (antibiotics, etc) can severely reduce the chemotropic effect of bacteria and therefore, fewer leukocytes will migrate to the infectious sites, rendering the labeled leukocyte method ineffective.
In contrast, the uptake of FDG in inflammatory cells reflects "in vivo labeling" of the existing cells at the site of infection soon after the administration of the compound. This would indicate that FDG-PET technique might allow imaging a substantially larger population of cells, which are residing in the area of infection and inflammation. Therefore, in addition to considerable simplification of procedures associated with the labeled WBC method, including the time required to complete the study, this approach may provide higher sensitivity for diagnosing infection in such settings. Furthermore, since FDG uptake does not rely upon leukocyte migration, treatment with antibiotics is less likely to affect its sensitivity in delineating the sites of infection. One possible advantage of the labeled WBC method over FDG-PET imaging is that high serum glucose levels do not appear to have an adverse effect on the test results with the former technique while hyperglycemia is known to decrease tumor cell FDG uptake. However, our preliminary results indicate that high glucose levels do not negatively affect FDG uptake by inflammatory cells. Based on these observations, FDG-PET imaging appears to be an attractive alternative to conventional techniques for the detection of infection.
FDG-PET imaging offers a unique tool for the diagnosis and management of the diabetic foot. Through the establishment of appropriate diagnostic criteria, a PET scan may prove to be highly accurate in localizing deep infections of bone and soft tissue associated with the complicated diabetic foot. By distinguishing these infections from inflammation, it has the potential to become the optimal diagnostic imaging technique with which to diagnose and manage patients with diabetic foot. Therefore, research studies designed to further validate the ability of this technique are essential to achieving this goal.
