Figure 1. In the upper pole of the left kidney, there is a hypoenhancing lesion that is markedly hypermetabolic (SUV_max 7.3). Persistent uptake (SUV_max 5.7, not shown) was shown on the delayed scan.

Figure 2. A follow-up scan performed after antibiotic treatment 3 weeks later shows resolution of the previously seen FDG-avid lesion in the upper pole of left kidney.

Figure 3. Hypermetabolic lesion in the upper pole of the left kidney (A), resolved on follow-up (B).

Acute lobar nephronia, or acute focal bacterial nephritis, is a focal non-liquefactive bacterial infection in the kidney with a mass-like appearance on imaging. The appearance may mimic a neoplasm, with mass-like morphology and high FDG-avidity. Whilst urinary cultures may be negative, the clinical features and mass resolving with antibiotic treatment suggest the diagnosis.

Localisation of septic foci is often difficult. PET/CT has demonstrated great utility in pyrexia of unknown origin. 18-F-FDG accumulates in infection and inflammation through overexpression of glucose transport protein 1 (GLUT 1) subtype in the stimulated macrophages, neutrophils and lymphocytes. The utility of PET/CT is expected to improve diagnostic capability in PUO, with the modality contributing to diagnosis in up to 70% patients (not revealed through history-taking, laboratory tests and simple imaging procedures). As occurred in oncologic imaging, the combined PET/CT modality has been shown to improve specificity of diagnosis compared to FDG-PET alone.