Selby BP, Sakas G, Walter S, Groch W-D, Stilla U (2010) A radiometry tolerant method for direct 3D/2D registration of computed tomography data to X-ray images: Transfer function independent registration. In: Deserno TM, Handels H, Meinzer H-P, Tolxdorff T (eds) Bildverarbeitung für die Medizin (BVM 10): 117121.
As exact dose delivery is essential for radiological cancer treatment, image-guided radiotherapy (IGRT) methods are used to estimate corrections of the tumor alignment. This is often done through comparison of a Computed Tomography (CT) to the patient alignment visible in digital radiography images (DRs) acquired from within the treatment device. Digitally Reconstructed Radiography Images (DRRs) computed from the CT are then geometrically registered to the DRs. A problem is that radiometric properties of DRs and DRRs can vary profoundly. If a registration algorithm does not use volumetric CT data directly it is unable to regard deviations of the physical image formation to the simulated image formation. We present a novel method allowing direct DR to CT registration. It is designed to be radiometry tolerant by adapting the simulated X-ray transfer function to observed DR intensities. This is done by solving an overdetermined system of equations given by the histograms along rays through the voxel matrix of the CT. Remaining errors serve as measure for the image dissimilarity, thus minimization in 6 degrees of freedom (DOF) gives the transformation between the images. Thereby higher radiometric tolerance can be achieved, as misalignments can be identified even if DR images are acquired with inappropriate radiation parameters.