Infectious conditions may include tuberculosis, cryptococcosis, paragonimiasis and others.
Tuberculosis is a caseating granulomatous disease which can involve any organ. Intense 18F-FDG uptake is usually noted in active tuberculous lesions involving the lung parenchyma.[13,14] This is attributed to the presence of a large number of activated inflammatory cells which have a high glycolytic rate. However, a 'cold' abscess due to Mycobacterium tuberculosis shows only moderate peripheral and low central 18F-FDG activity because it is not accompanied by an inflammatory reaction. Therefore, the 18F-FDG uptake by tuberculous lesions varies according to the grade of inflammatory activity.[13,15] Tuberculoma is one of the most well-known diseases that shows increased 18F-FDG uptake. In HIV-negative patients suffering from tuberculosis, 18F-FDG PET images reveal more extensive involvement when compared with diagnostic contrast-enhanced CT scan.
The nonenhanced CT images in PET/CT may add morphological information useful in defining the nature of the lung lesions. 18F-FDG PET may help in determining the activity in the lesions and the extent of disease, in guiding the biopsy, and in assessing the response to therapy (Fig. 1).[17,18]
Patient with histology-proven tuberculosis
18F-FDG PET/CT scan in a patient who presented with fever, lethargy and palpable lymph nodes. Upper left (axial CT), upper right (axial PET) and lower left (axial fused PET/CT) panels show FDG avid and enlarged bilateral axillary (yellow arrow) and mediastinal (red arrow) nodes. Lower right panel (MIP PET image) shows additional abnormal FDG activity in bilateral cervical nodes (green arrow). Biopsy revealed tuberculosis. FDG, fluorodeoxyglucose; MIP, maximum intensity projection.
Tuberculous lymphadenopathy also shows high 18F-FDG uptake. A common dilemma faced during oncologic workup with 18F-FDG PET/CT is the presence of 18F-FDG-avid mediastinal or hilar nodes (Fig. 2) in cases of extrathoracic malignancies (e.g. carcinoma of the colon, renal cell carcinoma, or carcinoma of the cervix). In these entities, isolated mediastinal nodal metastases are uncommon, and in view of its increasing prevalence, tuberculosis should be considered in the differential diagnosis and histopathological confirmation is needed in most cases.
Patient with histology-proven tuberculosis
18F-FDG PET/CT scan in a patient with previously treated lymphoma, who presented with fever. Upper left (axial CT), upper right (axial PET), lower left (axial fused PET/CT) and lower right (MIP) panels show a 1.9-cm subcarinal node (red arrow) and bilateral hilar nodes (yellow arrow). Biopsy of the subcarinal node revealed tuberculosis. FDG, fluorodeoxyglucose; MIP, maximum intensity projection.
Apart from 18F-FDG, 111In-octreotide has been used for imaging tuberculosis. 68Ga-DOTATATE, a PET tracer with a much higher affinity for SSR-2 receptors, can be used to differentiate active tuberculosis from metastatic disease. To our knowledge there is no published report on the role of 68Ga-citrate in tuberculosis.
Cryptococcosis is an infection caused by the inhalation of the yeast-like fungus Cryptococcus neoformans. This fungus reproduces by budding and forms round, yeast-like cells. Pulmonary lesions are characterized by intense granulomatous inflammation and appear as clustered nodules. There are reports showing high 18F-FDG uptake in two-thirds of patients which may simulate malignant lesions and display varying degrees of resolution on post-treatment 18F-FDG PET scans.[22•,23] Fungal infection may be more of a problem in immunocompromised patients, and caution should be exercised when a patch of activity is noted in the lung on the 18F-FDG PET/CT scan.
Pulmonary paragonimiasis is a disease caused by a lung fluke common in south Asia. Once ingested by humans, by eating infected crabs or crayfish, the larvae excyst in the small intestine, penetrate the intestinal wall, and then penetrate the diaphragm and pleura and enter the lungs. The major target organs for the larvae are the lungs followed by the brain. Watanabe et al. reported one case of paragonimiasis mimicking lung cancer which showed high 18F-FDG uptake (SUV 4.7 at 1 h postinjection, which rose to 6.2 at 2 h). Although the exact causes of 18F-FDG accumulation have not yet been proven, it is likely that inflammatory cells including eosinophilic infiltration, active inflammatory responses, and viable worms cause high 18F-FDG uptake.
Pneumocystis infections can also cause high 18F-FDG uptake. These conditions have not been well documented to date. However, in one report by Lorenzen et al., patients with fever of unknown origin who presented with high 18F-FDG uptake on PET imaging were finally proved to have Pneumocystis carinii pneumonia.
Positive results on 18F-FDG PET should be interpreted cautiously when evaluating single pulmonary nodules, especially in patients with predisposing factors for nontuberculous Mycobacterium infections.
Curr Opin Pulm Med. 2011;17(3):197-205. © 2011 Lippincott Williams & Wilkins
Cite this: Combined PET and X-ray Computed Tomography Imaging in Pulmonary Infections and Inflammation - Medscape - May 01, 2011.