**5.4 Tc-99m labelled Anti-granulocyte antibody: Hybrid SPECT/CT imaging**

(Horger et al., 2003) showed that SPECT/CT changed the interpretation of radioimmunoscintigraphy with Tc-99m labelled anti-granulocyte antibodies in 28% of suggestive foci evaluated in 27 patients in whom relapsing post-traumatic osteomyelitis was

Radionuclide Infection Imaging: Conventional to Hybrid 89

granulomatous disease. Increased F-18 FDG uptake in these tissues is attributed to the increased glucose consumption through the hexose monophosphate shunt, which is the main energy source in chemotaxis and phagocytosis. The respiratory burst or the phagocytes activation results in increased F-18 FDG uptake. Marked F-18 FDG uptake is seen in neutrophils during acute phase of inflammation while during the chronic phase it is the macrophages and polymorphonuclear leukocytes that take up the tracer. Therefore, in cases of sterile inflammation it is mainly the neutrophils and macrophages that take up F-18 FDG. The mechanism of F-18 FDG uptake in infectious and inflammatory process is the same as in malignancy with metabolic trapping of the F-18 FDG-6-phosphate that cannot be further metabolised as it is not a substrate for the glucose 6-phosphatase isomerase enzyme. However, as the level of glucose 6-phosphatase remains the same in inflammatory cells as opposed to tumour cells where they are decreased, the F-18 FDG washes out from the inflammatory cells in due course. Further the numbers of GLUT (glucose transporter

The normal distribution of F-18 FDG includes the brain, myocardium, and the genitourinary system with variable uptake seen in the stomach, bowel and the bone marrow. Increase F-18 FDG activity can be seen in the spleen in patients with infection and presumably reflects the

The patients are advised to fast for several hours before imaging. This reduces the F-18 FDG uptake in normal tissues. Moreover, it reduces the competition for glucose transporters. The physical activity of the patient is limited prior to injection and this reduces the F-18 FDG uptake in the striated muscles. Some centers also administer benzodiazepines 30-60 minutes prior to injection to reduce the brown fat and muscle uptake. Patient is routinely injected 370-550 MBq of F-18 FDG intravenously and laid to rest in a comfortable bed. Imaging is usually done at 60 minutes post injection, however, some centers may extend the uptake period to 90 minutes or may acquire images twice at different times (dual point) particularly in cases of granulomatous processes such as tuberculosis. Whole body imaging is preferred in cases where a focus of infection is to be investigated. F-18 is cyclotron produced radioisotope. The physical half-life of F-18 is 110 minutes. The principal gamma photons

The indications for F-18 FDG PET for imaging infection are not different from those discussed previously, however, in particular investigation for the site of infection or ascertaining the cause in FUO, vasculitis, HIV-AIDS, infected prostheses, as well as osteomyelitis, diabetic foot infections, sarcoidosis and tuberculosis have been studied. In case of FUO, the sensitivity and specificity of F-18 FDG PET has been observed to be 84- 93% and 86-90% respectively. In most studies, it has helped in the management of about 35- 37% cases. The reported PPV is 87% and the NPV 95%. Negative F-18 FDG PET makes it very unlikely that a morphologic origin of the fever will be identified. Infective endocarditis

F-18 FDG accumulation has been observed in certain conditions resulting in vasculitis. These include giant cell arteritis, Takayasu arteritis, polymyalgia rheumatica, aortitis/periaortitis, infectious vasculitis and unspecified large vessel vasculitis. Inflammation of the

receptors) are less in inflammatory cells when compared with tumour cells.

increased glucose usage by spleen in the setting of an infectious process.

**6.2 F-18 FDG PET: Imaging protocols and pre-requisites** 

produced are of 511 KeV energy generated by positron emission.

that can be a source of FUO has also been studied with F-18 FDG PET.

**6.3 F-18 FDG PET: Clinical utilities and applications** 

suspected. In another recent study (Graute V et al., 2010) concluded that SPECT/CT substantially improves the utility of imaging with Tc-99m labelled anti-granulocyte antibodies for diagnosis and localization of suspected joint infections and provide information on the extent of the infection. We have found that SPECT/CT imaging not only helps anatomical localization of the infectious site but also provides lesion characterization and extent of involvement of the infectious process (Figure 10).

Fig. 10. A 25-year old female referred for evaluation of osteomyelitis in the left middle finger. Tc-99m MDP bone scan show hyperaemia, increased pool activity and intense linear increased tracer uptake in the left 3rd proximal phalanx extending up to the mid of middle phalanx. Tc-99m Sulesomab images show increased uptake at the same site confirming osteomyelitis. Further correlative SPECT/CT images show evident cortical distortion with low attenuation changes.
