CT density is closely related to FDG uptake, and low CT density coupled with high FDG uptake suggests a change in thyroid tissue composition, disease process and thyroid function of patients whose thyroid shows diffuse FDG uptake on FDG PET/CT, according to a study published in the July issue of the American Journal of Roentgenology.
You Mie Han, MD from the department of radiology, college of medicine, Hallym University Hangang Sacred Heart Hospital in Seoul, South Korea, and colleagues enrolled 112 patients in the study, who all underwent PET/CT and a thyroid function test between August 2007 and December 2008.
The 56 study patients enrolled had visible 18F-FDG uptake in the thyroid, whereas the 56 control subjects had no visible FDG uptake. Maximum standardized uptake values (SUVmax), Hounsfield units, thyroid-stimulating hormone levels, free thyroxine levels and visual assessments of thyroid CT densities (grades 0–2) were evaluated by Han and colleagues.
The researchers found a strong negative relationship between Hounsfield units and SUVmax values, whereas Hounsfield units were found to have weak correlation with free thyroxine level and thyroid-stimulating hormone level.
CT density reflects iodine concentration, whereas FDG uptake reflects cellular glucose metabolism. The thyroid density on CT was found to be closely related to FDG uptake. “Low CT density coupled with high FDG uptake suggests that there is a certain change in thyroid tissue composition. This change may be an inflammatory cell infiltration or a glandular hyperplasia,” noted Han and colleagues.
The researchers concluded that CT density could be used in combination with FDG uptake for determining disease status and thyroid function on PET/CT images.