Nonstationary noise characteristics of the helical scan and its impact on image quality and artifacts

Helical computed tomography has replaced the conventional step-and-shoot CT in many clinical applications. Because of its clinical importance, a number of comparative studies have been performed to evaluate its performance parameters, such as the noise and the slice sensitivity profile. In this paper, the nonstationary noise characteristics of helical images are carefully examined. Their impact on the low contrast detectability and three-dimensional (3-D) image artifacts is assessed. An analytical equation is derived to relate the interactions between various helical reconstruction schemes and the fan-beam filtered backprojection algorithm. We demonstrate that for many popular helical weights, the noise variation within the field of view of the reconstruction is close to a factor of 3. Several approaches to overcome the undesired characteristics are proposed.