1. A system for tomographic image reconstruction in a real-time application comprising: (a) an imaging system means for receiving and processing an image; (b) a means for generating tomographic projection data of a subject; (c) a means for acquiring a nonnegative, initial current image of the subject; and (d) a processing means for performing the steps of: (i) receiving tomographic projection data of the subject from the projection data generating means; (ii) processing the subject projection data and the current image using an ordered-subset based algorithm thereby producing a new current image; (iii) outputting the new current image to the imaging system means; and (iv) repeating steps (i)-(iii) until the real-time application is complete.