Multi-slice CT system refers to the special CT system equipped with a multi-row detector array to simultaneously acquire data at different z locations. A 4-slice CT scanner has been developed. Here, the authors outline the principles of multi-slice helical CT in general and 4-slice helical CT in particular. The multi-slice helical CT has three key components: the preferred helical pitch for the efficient z sampling in data collection; the helical interpolation algorithms to correct for the fast simultaneous patient translation; and the z-filtration reconstruction for further control of the slice profile, image noise and artifacts in reconstruction. Preferred helical pitches for 4-slice CT are chosen for efficient z sampling and better cone-beam artifacts control. The helical reconstruction algorithms are proposed for multi-slice CT in general and for 4-slice helical CT in particular. The z-filtration reconstruction is developed and studied. Slice profiles, noises and artifacts of 4-slice helical CT are studied analytically and experimentally and compared with single slice helical CT. It is concluded that the 4-slice helical CT can provide equivalent image quality with improved z-axis resolution at 2 to 3 times of the volume coverage rate and at much lower mA of the single slice helical CT.