Abstract
The purpose of this study is to investigate the nonlinearity in digital X-ray images as a feasibility study for a noise reduction process using a mathematical model, which realizes an accurate digital X-ray image system. To develop this mathematical model, it is important to confirm whether the system is linear or nonlinear. We have verified the nonlinearity of the digital X-ray image system through analysis of computed radiography (CR) images by using the method of surrogate, a type of statistical test of nonlinearity, applied the Wayland test. In the method of surrogate, we use the Fourier transform surrogate method. The Wayland test can be used for evaluating the complexity of the orbit of a signal aggregate called the attractor reconstructed in a high-dimensional phase space, setting the delay difference on the coordinate axes from the original data using a nonlinear statistical parameter called the translation error. The nonlinearity is determined by statistically comparing the translation error of the original data with that of the surrogate data. X-ray images are obtained under different conditions to investigate the effects of various tube voltages-50 and 80kV-and dose settings-2 and 10 mAs. We extract 30 line profiles from both directions of the X-ray images as original data. One is the direction vertical to the X-ray tube (X-direction), and the other is the direction horizontal to the X-ray tube (Y-direction). In the Y-direction, nonlinearity is found at all voltage and dose settings. On the other hands, nonlinearity is only found at 10 mAs and 80 kV in the X-direction. On the basis of this result, the nonlinearity is indicated by decrease in fluctuation of the X-ray photon. Therefore, the factors of nonlinearity exhibit a comprehensive variation of pixel values produced by the digital X-ray image system contained the X-ray imaging.
