Helical computed tomography has become the preferred patient scanning mode in many clinical applications. A number of comparative studies have been performed to evaluate its performance parameters, such as the noise and the slice sensitivity profile. The objective of this paper is to examine the non-stationary noise characteristics of helical images and demonstrate its impact on the 3D image artifacts. We first derive an analytical equation that relate the interactions between the helical reconstruction scheme and the fan-beam filtered backprojection algorithm. We demonstrate that for many popular helical weights, the nose variation within the field of view of the reconstruction is close to a factor of 3. An adaptive approach to the reduction of the artifacts is proposed.