18FDG uptake associated with CT density on PET/CT in lungs with and without chronic interstitial lung diseases

The dependent-density of computed tomography (CT) images of positron emission tomography (PET)/CT is sometimes difficult to distinguish from chronic interstitial lung disease (ILD) when it accompanies increased F-18-fluorodeoxy-d-glucose ((18)FDG) uptake. Though the possible utility of (18)FDG-PET for the diagnosis of active ILD has been reported, the clinical relevance of mild lung (18)FDG uptake in ILD cases without signs and symptoms suggesting acute progression has not been described. This study aimed to test relationships between (18)FDG uptake and lung density on CT using PET/CT in patients with normal lung as well as clinically stable chronic ILD. Thirty-six patients with normal lungs (controls) and 28 patients with chronic ILD (ILD cases) without acute exacerbation were retrospectively selected from (18)FDG-PET/CT scans performed in examination of malignant neoplasms. Elliptical regions of interest (ROIs) were placed on the lung. The relationships between CT density and (18)FDG uptake between the control and ILD cases were tested. The CT density and (18)FDG uptake had a linear correlation in both the controls and the ILD cases without a difference in their regression slopes, and they were overlapped between the controls and the ILD cases with higher mean values in the ILD cases. Lung (18)FDG uptake was considered to reflect a gravity-dependent tissue density in the normal lung. Though the lung (18)FDG uptake as well as the CT density tended to be higher in chronic ILD patients, it may be difficult to distinguish them in normal dependent regions from those related to chronic ILD in some cases.