document: Modelling tomographic cone-beam projection data from a polyhedral phantom Analytical phantoms are used to generate projection data for testing reconstruction accuracy in computed axial tomography . A circular source locus -LRB- equivalent to rotating specimen with a fixed source -RRB- provides insufficient data for ` exact ' reconstruction in cone-beam transmission tomography , thus phantom data are useful for studying the consequent errors and also for investigating alternative scanning loci and reconstruction techniques . We present an algorithm that can compute phantom cone-beam projection data from a phantom comprising geometrically defined polyhedra . Each polyhedron is defined as a set of polygons enclosing a volume of fixed linear attenuation coefficient . The algorithm works by projecting each polygon in turn onto the modelled detector array , which accumulates the product of source to polygon intersection distance -LRB- for the rays intersecting each detector element -RRB- , linear attenuation coefficient and sign of projected polygon area -LRB- indicating whether rays enter or exit the polyhedron at this face -RRB- . The phantom data are rotated according to the projection angle , whilst the source location and detector plane remain fixed . Polyhedra can be of simple geometric form , or complex surfaces derived from 3D images of real specimens . This algorithm is illustrated using a phantom comprising 989 238 polygons , representing an iso-surface generated from a microtomographic reconstruction of a piece of walrus tusk
doc_bio_tags: O O O O O O O B I O O O O O O O O O O B I O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O B I
extractive_keyphrases: polyhedral phantom reconstruction accuracy computed axial tomography alternative scanning loci geometrically defined polyhedra linear attenuation coefficient microtomographic reconstruction walrus tusk
abstractive_keyphrases: tomographic cone-beam projection data cone-beam transmission tomography reconstruction software accuracy x-ray attenuation cumulative pixel array interpolation geometry file