Transferrin is a protein involved in the transport of iron. Transferrin was labeled with 68Ga as [68Ga]apotransferrin. The tracer was found to be able to detect Staphylococcus aureus infection in a rat model within an hour of injection .
Nonspecific Tracers Used in Infection to Discriminate Infection from Malignancy
Synthesis of DNA can be imaged by [18F]FLT. During an infection, microorganisms are constantly growing with actively synthesizing nucleic acids. The growth of Staphylococcus aureus in rabbit was imaged using [18F]FLT. However, nucleic acid formation is not limited to bacteria growth and makes the tracer just as nonspecific as [18F]FDG [79, 80]. In another study using a model with Yersinia enterocolitica, [18F]FLT was not useful in assessing bacterial proliferation .
Other clinically available PET tracers have been used in the evaluation of different aspects a particular infection. In tuberculosis, for example, [18F]NaF used to evaluate calcification of tuberculous granulomas in mice, the complex lipid covering was evaluated by [11C]choline or [18F]fluoroethylcholine. The use of these tracers is limited to the pathology of a particular infection and lacked specificity for the infection .
Conclusion and Future Perspective
PET imaging of infection has gained prominence over the last decade. [18F]FDG has been found useful in many site‐specific infections. It is likely to be validated in more clinical situations in the future. The introduction of PET/MRI into clinical practice may open a new chapter in infection imaging, especially where soft‐tissue definition is essential. The use microorganism‐specific tracers are being explored and the presence of longer acting PET tracer such as 64Cu and 89Zr has increased the possibilities especially in the labeling of antibodies. A lot of the tracers are at the preclinical stage of development. A lot more research is needed for the clinical application of these tracers. The search for an ideal PET tracer is still ongoing. The ideal tracer should be able to distinguish infection from inflammation, cheap, easy to prepare, and not require handling of blood products. A tracer should be able to detect both resistant and susceptible species.
Another major clinical hurdle is biofilm formation making bacteria not sensitive to anti‐bacterial agents as these agents are not able to penetrate into biofilm. Recent research on adaptive biofilm‐targeted agents can trigger development of a completely new class of imaging agents in the future.
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