2.3. Image analysis and semi-quantitative evaluation

Visual and semi-quantitative analysis of 18 F-FDG PET/CT scans were performed. All 18 F-FDG PET/CT scans in our study population were reviewed by two nuclear medicine physicians. Any suspicious 18 F-FDG avid thyroid lesion in 18 F-FDG PET/CT was evaluated and correlated with histopathology result, recorded and tabulated. In this study, a suspicious thyroid lesion was defined as either an increased thyroid F-18 FDG uptake on PET images or a focal thyroid lesion on CT images regardless of F-18 FDG uptake. A focal F-18 FDG uptake was defined as a localized uptake occupying less than a single entire thyroid lobe while an uptake involving at least a whole thyroid lobe was analyzed in this study under the category of diffuse uptake. All included patients had at least one thyroid lesion/nodule on CT and no patients with morphologically normal thyroid glands were included.

Images were analyzed for SUVmax, uptake pattern (focal or diffuse) and non-enhanced CT tissue density in mean Hounsfield units (HU) [measured using a circular ROI at the center of the suspicious F-18 FDG avid thyroid lesion, and whenever possible corresponding to the site of highest SUVmax and avoiding areas of gross calcification].