Improvement of Image Quality in MDCT by High-Frequency Sampling of x-, y- and z-Direction

摘要

The multi-detector row computed tomography (MDCT) has dramatically increased speed of scanning, and allows high-resolution imaging compared with the conventional single-detector row CT (SDCT). However, use of the MDCT was making use of three-dimensional (3D) volume scanning and four-dimensional (4D) dynamic scanning increase, and made radiation dose to patients increase simultaneously. In addition, in recent years, lung-cancer screening CT (LSCT) is introduced, and low-dose scanning is strongly required to increase the benefit/risk ratio. In this study, high-frequency volume data sampling (over-sampling) method of x-, y- and z-direction was proposed as technique for reduction of image noise in the MDCT and discussed about reduction of radiation dose and improvement of image quality. In this proposed method, volume data are obtained by over-sampling of x-, y- and z-direction and image is obtained by averaging these data. In x- and y-direction, over-sampling is equivalent to obtaining projection data using large matrix size for same scan-field of view (scan-FOV), and in z-direction, equivalent to using thin slice. Normally, when signal with which noise distribution differs are averaged n-times, signal-to-noise ratio (SNR) will increases by factor of n~(1/2). In this method, each pixel value of the image is obtained from n_(x,y)~2, n_z pixels by n_(x,y)-times sampling for x- and y-direction, and n_z-times sampling for z-direction. In other words, SNR of the image increases (n_(x,y)~2 n_z)~(1/2) -times. In this high-frequency data sampling method, it is possible to obtain high-quality image as compared with conventional image. Moreover, by applying to noisy image obtained with low-dose scanning, reduction of radiation dose to patients is possible.