Size dependence of inherent image quality of a 2nd generation dual source CT scanner

Abstract

The fast developing CT technologies have complicated the protocol optimization process. Since there are increasing numbers of free parameters that can be adjusted, it is imperative to know exactly how these parameters affect the image quality. This paper examines the effect of varying dose level, tube voltage, reconstruction methods and the AEC function on the CT image quality across different sizes. A size varying phantom, consisting of five cylindrical tiers, was used. To assess the image quality, task transfer function (TTF), noise power spectra (NPS), contrast, and detectability index was computed and compared across sizes. It was found that from small to large size, detectability increased more as the dose increased. When the CTDIvol increased from 0.7 to 22.6 mGy, the detectability for the smallest size section increased by four fold but increased by eight fold for the largest section. Low tube voltage exhibited superior detectability and contrast for all sections, especially for the small size sections, where detectability doubled for the smallest section when tube voltage decreased from 120 kV to 80 kV. TTF curves showed considerable dependence on size, but more pronounced dependence on reconstruction techniques. In conclusion, small size phantoms were affected very differently from large size phantoms by dose levels, reconstruction methods, and tube voltage selection. Low tube voltage and iterative reconstruction technique can deliver superior image quality for small patients. Due to beam hardening and substantially increased noise at low tube voltage for large patients, high tube voltage is still recommended for large patients to retain image quality.