Determining the effectiveness of the RF ablation usually involves follow-up computed tomography (CT) scans. CT imaging provides an assessment of tissue changes following an ablation, which can manifest as central cavitation of the lesion with decreased tissue density measured by a decrease in Hounsfield units (Hu). As complementary modality, positron emission tomography (PET) can be used to determine tissue viability within the lesion, which can be useful in the follow-up of these patients. In particular, patients with questionable response to RFA as determined by CT scan can benefit from PET scanning, by determining the presence of residual disease. The patient is injected with a radio-labeled sugar (typically 18 Fluoro-deoxyglucose or FDG) prior to the PET scan and viability is determined based upon the uptake of the FDG by cells, since FDG is taken up by cells with high metabolic activity, such as tumors, infection, or inflammation. In a small pilot study, monitoring with PET noted no viable tissue within the boundaries of the thermal lesions induced in pulmonary nodules.