AN ANALYTICAL RECONSTRUCTION ALGORITHM FOR MULTIFOCAL CONVERGING-BEAMSPECT

Multifocal converging-beam collimation has been suggested for cardiac SPECT imaging to increase sensitivity over the heart without truncatio n of the activity distribution in the chest. In this study, an analyti cal reconstruction algorithm is derived for multifocal fan-beam and mu ltifocal cone-beam tomography. In the algorithm, the projection data a re differently weighted and filtered, depending on the distance from t he detector. For a given image point, the set of filtered data corresp onding to the distance between this point and detector is backprojecte d to determine the pixel value of the point. Thus, the backprojection is done only once at each projection view. To evaluate the algorithm, simulation studies are performed using a 3D Defrise slab phantom witho ut considering photon attenuation and scatter, detector response, and statistical noise. Reconstructed images demonstrate that reasonable qu ality can be achieved with a modest focal-length variation rate and wi th a small radius of rotation. A collimator with a focal length increa sing quickly near its centre provides better quality in the image regi on distant from the central plane of the cone geometry, but produces m ore severe artifacts at the centre of the reconstructed image, compare d to a collimator with an initially slowly varying focal length.