Intraoperative cone-beam CT spatial priors for diffuse optical fluorescence tomography

A hybrid system for intraoperative cone-beam CT (CBCT) imaging and continuous-wave fluorescence tomography (FT) has been developed using an image-guidance framework. Intraoperative CBCT images with sub-millimeter spatial resolution are acquired with a flat-panel C-Arm. Tetrahedral meshes are generated from CBCT for finite element method implementation of diffuse optical tomography (NIRFAST). Structural data from CBCT is incorporated directly into the optical reconstruction process using Laplacian-type regularization ('soft spatial priors'). Experiments were performed using an in-house optical system designed for indocyanine green (ICG) fluorescence. A dynamic non-contact geometry was achieved using a stereoscopic optical tracker for real-time localization of a laser diode and CCD camera. Source and detector positions were projected onto the boundary elements of the tissue mesh using algorithms for ray-triangle intersection and camera lens calibration. Simulation studies showed the capabilities of a soft-prior approach, even in the presence of segmentation uncertainties. Experiments with ICG targets embedded in liquid phantoms determined the improvements in the quantification of the fluorophore yield, with errors of 85% and  <20% for no priors and spatial priors, respectively. Similar results were observed with the ICG target embedded in ex vivo porcine loin, with errors of 52% and 12%, respectively. A proof-of-principal animal study was performed in a VX2-tumor in vivo rabbit model using liposomal nanoparticles co-encapsulating contrast for CT (iohexol) and fluorescence (ICG) imaging. Fusion of CBCT and FT reconstructions demonstrated concurrent anatomical and functional delineations of contrast enhancement around the periphery of the buccal tumor. These developments motivate future clinical translation of the FT system into an ongoing CBCT-guided head and neck surgery trial.