FDG uptake in the morphologically normal thymus: comparison of FDG positron emission tomography andCT

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Abstract
The purpose of this study was to evaluate the correlation between fluorine-18 fluorodeoxyglucose (FDG) thymic uptake and a normal appearing thymus on CT. Non-attenuation corrected FDG positron emission tomography (PET) data from 94 young persons (mean age 25.4 years, range 18–29 years) with a normal thymus diagnosed on CT were retrospectively evaluated. No subject had clinical symptoms suggestive of thymus-related disease or mediastinal tumour (follow-up period 6–69 months). PET images were visually assessed and the count ratio between the thymus and the lung (T/L ratio) was calculated. Increased FDG uptake occurred in 32 (34%) subjects. In these 32 cases, the T/L ratio was 2.86±0.49 (range 2.02–3.99). In 86 subjects whose CT images were available to calculate the CT attenuation of the thymus (CAT), the CAT value was −17.5±45.7 HU (range –103.6 HU to 79.9 HU). The T/L ratio correlated with the CAT value (r=0.58). CAT values in subjects with positive PET findings were significantly higher than CAT values i...

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