344 chapter 5 Figure 2. Quantitative [18F]FDG-PET/CT assessment and delineation of the VOI for radiomic analysis. Transverse and coronal [18F]FDG-PET/CT (a, b), maximum intensity projection (MIP) (c, d) and low-dose CT (e, f) images of a patient with a solitary, 30 mm Bethesda III thyroid nodule in the right lobe. Visual assessment (a) of the [18F]FDG-PET/CT showed an [18F]FDG-positive index nodule. Quantitative assessment (b) demonstrated a SUVmax of 9.7 g/mL and SUVpeak of 7.0 g/mL of the index nodule, and a SUVmax of 1.6 g/mL in the background of surrounding normal thyroid tissue. Consequently, the SUVmax-ratio and SUVpeak-ratio were 6.1 (9.7/1.6) and 4.4 (7.0/1.6), respectively. For radiomic analysis, VOIs were delineated on the [18F]FDG-PET scans using an isocontour that applies a threshold of 50% of the SUVpeak, corrected for local background (c, d) [511]. Boxing was applied to exclude [18F]FDG-positive tissue surrounding the index nodule and ldCT images were used as a visual reference (e, f). VOIs delineated on the PET images were resampled with a nearest neighbour algorithm to derive the ldCT VOIs. a. b. c. d. e. f. the physiological background [18F]FDG-uptake of the surrounding normal thyroid tissue and that corresponded to the index nodule in size and location, was considered positive. The SUVmax and peak SUV (SUVpeak, defined as the maximum average SUV within a 1 cm3 spherical volume) of the index nodule were semi-automatically measured (Figure 2) [510]. Body weight corrected values were used. The SUVmax-ratio and SUVpeak-ratio were respectively calculated by dividing the SUVmax and SUVpeak of the nodule by the background SUVmax of normal thyroid tissue in the contralateral lobe. [18F]FDG-positive foci in the thyroid that did not correspond to the index nodule in size and location (i.e., thyroid incidentalomas) were not analysed.
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