In vivo quantification of pulmonary inflammation in relation to emphysema severity via partial volume corrected (18)F-FDG-PET using computer-assisted analysis of diagnostic chest CT.

Our aim was to quantify the degree of pulmonary inflammation associated with centrilobular emphysema using fluoro-18-2-fluoro-2-deoxy-D-glucose positron emission tomography ((18)F-FDG-PET) and diagnostic unenhanced computed tomography (CT) based image segmentation and partial volume correction. Forty-nine subjects, with variable amounts of centrilobular emphysema, who had prior diagnostic unenhanced chest CT and either (18)F-FDG-PET or (18)F-FDG-PET/CT were selected. Lung parenchymal volume (L) (in cm³) excluding large and small pulmonary vessels, emphysema volume (E) (in cm³) based on a -910HU threshold, fraction of lung emphysema (F=E/L), and mean attenuation (HU) of non-emphysematous lung parenchyma (A) were calculated from CT images using the image analysis software 3DVIEWNIX. Lung uncorrected maximum SUV (USUVmax) was measured manually from PET images on a dedicated workstation. A first level of partial volume correction (PVC) of lung SUVmax to account for presence and degree of macroscopic emphysematous air space was calculated as CSUVmax=USUVmax/(1-F). A second level of PVC of non-emphysematous lung SUVmax to account for the mixture of air and lung parenchyma at the microscopic level was then estimated as CCSUVmax=CSUVmax/(A+1000/1040), assuming that air is -1000HU in attenuation and gasless lung parenchyma is 40HU in attenuation. The correlation of F with USUVmax, CSUVmax, CCSUVmax, % change between CSUVmax and USUVmax (%UC), and % change between CCSUVmax and USUVmax (%UCC) were then tested. The results showed that USUVmax was not significantly correlated with F (r=-.0973, P=0.34). CSUVmax (r=0.4660, P<0.0001) and CCSUVmax were significantly positively correlated with F (r=0.5479, P<0.0001), as were %UC (r=0.9383, P<0.0001) and %UCC (r=0.9369, P<0.0001). In conclusion, the degree of pulmonary inflammation increases with emphysema severity based on (18)F-FDG-PET or (18)F-FDG-PET/CT assessment, but is only detectable when (18)F-FDG uptake is corrected for the partial volume effect based on data provided from diagnostic chest CT images. These results support the notion that pulmonary inflammation plays an important role in the pathophysiology of emphysema. This novel image analysis approach has great potential for practical, accurate, and precise combined structural-functional PET quantification of pulmonary inflammation in patients with emphysema or other pulmonary conditions, although further validation and refinement will be required.