INCREMENTAL BACKPROJECTION ALGORITHM - MODIFICATION OF THE SEARCHING FLOW SCHEME AND UTILIZATION OF THE RELATIONSHIP AMONG PROJECTION VIEWS

In this paper, a new two-dimensional (2-D) searching flow scheme is pr oposed for the incremental backprojection algorithm [9]. In this schem e, the searching direction in a beam is determined only by the distanc e value of the current pixel to the corresponding ray; no pixels outsi de the beam are involved in the procedure. In addition, the geometrica l relationships between the image grids and the projection rays in dif ferent views are fully exploited to further reduce the searching opera tions. It is found that backprojection of four views, theta,pi/2 - the ta,pi/2 + theta, and pi - theta, can be performed at the same time, an d the searching operation is only required for one of them if the numb er of views is even. Thus, backprojection is accomplished on a quartet -by-quartet basis of views as well as on a beam-by-beam basis. Our app roaches not only minimize the number of searching operations but also possess a simple algorithm structure. Implementation results on an AST -386 computer for the incremental backprojection algorithm by using th is new searching flow scheme and the relationship among projection vie ws show that the processing time can be reduced by a factor of about 2 , compared with the original method.