The study of computed tomography dose profile width using computed radiography imaging plate

Abstract

The quality control of the CT scanner is usually performed by using the film for the dose profile, but film processing is becoming less available in department of radiology. The purpose of this study is to measure the computed tomography (CT) profile width using computed radiography imaging plate (IP) and compared the profile with the measurement from verification film. The suitable parameters of image plate were selected for optimal exposure. Fuji CR IP and Kodak X-Omat V film were exposed to GE LightSpeed RT to study the relation between pixel value or optical density and exposure at 120 kVp, 10-40 mAs, 10 mm collimator. IP was read by Fuji FCR-XG5000 CR reader. Optical density was read by Vidar DosimertyPro VXR-16. The exposure was measured by Unfors Xi semiconductor detector. Cerrobend block combined with lead plate were used to filter the energy of the x-ray beams. The dose profile widths were measured for 5, 10, 15 and 20 mm for both film and IP. The characteristic curves irradiated with computed tomography scanner plotted in semi- log scales between exposure and pixel value showed linearity for film in the high exposure range. For IP, the pixel value saturated at 20 mAs, so the dose of about 10 mAs was selected to study the width of profile. The full width at half maximum measured by film of 5, 10, 15 and 20 mm. collimator beam were 7.7, 11.7, 17.1, and 20.5 mm, respectively, for the plotted of pixel value. When changing the pixel value to exposure, the full widths at half maximum were 6.9, 11.3, 16.5, and 20 mm, respectively. For IP, CT dose profiles were plotted by pixel value and exposure. By pixel value, the CT dose profile widths were 7.8, 11.6, 16.95 and 20.1 mm at L = 1 S = 5, and by exposure were 7.65, 11.3, 16.5 and 19.95 mm at L = 4 S = 200. IP can be used to measure the CT profile width comparable to film. However, the filter needed to be used to avoid the overexposure of IP in CT scanner. For plotting the profile in pixel value, the best parameter was L = 1 S = 5 because the scatter was taken out and it has high contrast. For the CT dose profile width plotted by exposure, the best parameter is L = 4 S = 200. If S value was increased, the dose profile width was narrower. The deviation from collimator width was less when using exposure instead of pixel values or optical density for both film and IP.