ALGORITHM FOR IMAGE-RECONSTRUCTION IN MULTISLICE HELICAL CT

Efforts are being made to develop a new type of CT system that can sca n volumes over a large range within a short time with thin slice image s. One of the most promising approaches is the combination of helical scanning with multi-slice CT, which involves several detector arrays s tacked in the z direction. However, the algorithm for image reconstruc tion remains one of the biggest problems in multi-slice CT. Two helica l interpolation methods for single-slice CT, 360LI and 180LI, were use d as starting points and extended to multi-slice CT. The extended meth ods, however, had a serious image quality problem due to the following three reasons: (1) excessively close slice positions of the complemen tary and direct data, resulting in a larger sampling interval; (2) the existence of several discontinuous changeovers in pairs of data sampl es for interpolation; and (3) the existence of cone angles. Therefore we have proposed a new algorithm to overcome the problem. It consists of the following three parts: (1) optimized sampling scan; (3) filter interpolation; and (3) fan-beam reconstruction. Optimized sampling sca n refers to a special type of multi-slice helical scan developed to sh ift the slice position of complementary data and to acquire data with a much smaller sampling interval in the z direction. Filter interpolat ion refers to a filtering process performed in the z direction using s everal data. The normal fan-beam reconstruction technique is used. The section sensitivity profile (SSP) and image quality for four-array mu lti-slice CT were investigated by computer simulations. Combinations o f three types of optimized sampling scan and various filter widths wer e used. The algorithm enables us to achieve acceptable image quality a nd spatial resolution at a scanning speed that is about three times fa ster than that for single-slice CT. The noise characteristics show tha t the proposed algorithm efficiently utilizes the data collected with optimized sampling scan. The new algorithm allows suitable combination s of scan and filter parameters to be selected to meet the purpose of each examination. (C) 1998 American Association of Physicists in Medic ine. [S0094-2405(98)00204-1].