COMPENSATION FOR DISPLACEMENT OF THE FOCAL POINT IN CONE-BEAM SINGLE-PHOTON EMISSION COMPUTED-TOMOGRAPHY RECONSTRUCTION

This study examined the effects of focal point displacement on image q uality in cone beam single photon emission computed tomography (SPECT) . A new image reconstruction algorithm that accounts for the focal poi nt shift was derived and three shift geometries were investigated. The geometries included a lateral shift with a fixed focal length but off -center focusing, a linear axial shift with a variable focal length th at depends linearly on the distance between a bin of the detector and the center of the detector, and a random axial shift with a randomly v arying focal length. Computer simulation was conducted to evaluate the shift effects with a phantom that was composed of 118 small spherical sources. The results demonstrated that the lateral shift of the focal point was more critical to image quality than was the axial shift. Wi th a 0.64 cm (1 pixel) lateral shift, noticeable artifacts was observe d, while an axial shift resulted in minimal changes in image quality u ntil it reached 8 cm (12.5 pixels). The derived reconstruction algorit hm eliminated most of the artifacts caused by a fixed lateral shift or a linear axial shift of the focal point, but failed to do so for a ra ndom axial shift since the linear distribution assumed in image recons truction did not match the random shift occurred in acquisition of the data. (C) 1997 Elsevier Science Ireland Ltd.