The multi-slice CT scanner refers to a special CT system equipped with a multiple-row detector array to simultaneously collect data at different slice locations. The multi-slice CT scanner has the capability of rapidly scanning large longitudinal (z) volume with high z-axis resolution. It also presents new challenges and new characteristics. In this paper, we study the scan and reconstruction principles of the multi-slice helical CT in general and the 4-slice helical CT in particular. The multi-slice helical computed tomography consists of the following three key components: the preferred helical pitches for efficient z sampling in data collection and better artifact control; the new helical interpolation algorithms to correct for fast simultaneous patient translation.