RESUMO
Because of the characteristics of the tomographic inversion problem, iterative reconstruction techniques often suffer from poor convergence rates-especially at high spatial frequencies. By using preconditioning methods, the convergence properties of most iterative methods can be greatly enhanced without changing their ultimate solution. To increase reconstruction speed, spatially invariant preconditioning filters that can be designed using the tomographic system response and implemented using 2-D frequency-domain filtering techniques have been applied. In a sample application, reconstructions from noiseless, simulated projection data, were performed using preconditioned and conventional steepest-descent algorithms. The preconditioned methods demonstrated residuals that were up to a factor of 30 lower than the assisted algorithms at the same iteration. Applications of these methods to regularized reconstructions from projection data containing Poisson noise showed similar, although not as dramatic, behavior.
RESUMO
SPRINT II is a stationary detector ring tomograph designed for brain imaging. Eleven two-dimensional sodium iodide camera modules that use maximum-likelihood position logic are arranged in a 50-cm-diameter ring with a scintillator packing fraction of 96%. A 34-cm-diameter rotating lead aperture ring containing either 10 or 12 slits is used for in-plane collimation, while the z-axis collimator is constructed of parallel lead foil rings. The field of view is 22 cm in diameter by 12 cm long. Sensitivity is 10 count/s/muCi for an on-axis (99m)Tc point source and 8500 count/s/muCi/cm(3) for 19.8-cm-diameter by 6.2-cm-long cylindrical source. Longitudinal resolution is 10 mm FWHM, and in-plane resolution varies from 8 mm FWHM on-axis to 5 mm FWHM at a radius of 9 cm. Performance results are presented.
