Issues in quantification of registered respiratory gated PET/CT in the lung

Objectives Lung density changes during respiration which affects PET/CT quantification in the lung. We investigate the relation between density changes measured in CT, FDG concentration changes measured in PET and local volume changes obtained from the registration deformation field.

Methods CINE CT and FDG PET data were acquired from patients suffering from lung cancer with solitary pulmonary nodules. These data were respiratory gated into 6 gates using the amplitude of the respiratory signal from an external device (RPM). Gated PET data were reconstructed with matched gated CT for the attenuation correction. After segmenting the lungs on the gated CT images, we performed an affine plus a non-rigid registration (NiftyReg with the Normalized Mutual Information objective function) of gated CT to a reference gate, which was chosen to correspond to an inhalation breathing state (i.e largest lung volume). From each registration, the local volume change (i.e Jacobian determinant) was measured from estimated deformation fields. ROIs were drawn on the lung away from edges, obvious artefacts and blood vessels. Regions where density changed less than 5% were excluded.

Results The registration was visually judged to be of very good quality. The density and the PET tracer concentration were not conserved after registration. The average difference in density (PET concentration) between the registered image and the reference is 10% (15%), but this reduced to 5% (8%) after correcting for local volume change. The density and PET concentration are linearly related with a positive slope and a non-zero intercept (r² between 0.6 and 0.9).

Conclusions We have shown that using the Jacobian underestimates the correction needed to preserve both density and PET concentration. The non-zero intercept in the density-activity relationship suggests a lack of local conservation of PET activity. In the future, we will investigate if this could be clarified by taking the PET tracer in blood into account.

Research Support This work is supported by funding from GSK, Fibrosis DPU, GlaxoSmithKline R&D and EPSRC. This project is supported by researchers at the National Institute for Health Research, University College London Hospitals Biomedical Research Centre.