Influence of exposure parameters on image quality in single-slice computed tomography (CT)

Computed tomography presents a significant source of the population exposure to ionizing radiation. Selection of CT parameters allowing to obtain good image quality at reasonably low dose was the aim of the presented study. Studies were performed using a Picker PQ 2000 unit. Starting from routine ADULT BRAIN procedure,the influence of changed current-voltage parameters on image quality was investigated using the Catphan 424 phantom. The following image characteristics were analyzed: contrast level and size of the smallest detectable object for low-contrast objects, as well as spatial resolution for high-contrast objects. Dosimetric evaluation of exposure was based on the value of CTDI vol, derived from the radiation dose and DLP measurement product when a HEAD phantom was used. The nominal parameters of routine ADULT BRAIN protocol were as follows: axial mode with 120 kVp, 150 mA, 1.5 s, slice width 10 mm, table feed 10 mm.Tube current varying from 30 mA to 200 mA; voltages of 120 kVp and 100 kVp were selected.The reduced tube current - time product from routine 236 mAs to 100 mAs was found as the optimal parameter, causing no decrease of the contrast level,whereas the diameter of the smallest detectable object was only slightly increased, with no change of high-contrast resolution. The corresponding reduction of CTDI vol from 36 mGy to 16 mGy was obtained. More significant effect (CTDI vol = 8 mGy) was achieved by simultaneous lowering of high voltage 120 kVp to 100 kVp and tube current to 50 mA; under such conditions, high-contrast resolution was even better. The presented phantom studies indicate that modifications of routine ADULT BRAIN protocol offer the possibilities of significant dose reduction without loss of image quality; these results will be verified clinically. (author)