We investigate the nonlinearity in digital X-ray images to determine the feasibility of a noise reduction process using a
mathematical model, which realizes an accurate digital X-ray imaging 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 imaging system
through an analysis of computed radiography (CR) images by using the method of surrogate, a statistical test of
nonlinearity, and 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 using a nonlinear statistical parameter called the translation error.
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 80
kV--and dose settings--2 and 10 mAs. We extract 30 profiles from both directions, the directions vertical (V-direction)
and horizontal (H-direction) to the X-ray tube. In the H-direction, nonlinearity is found at all voltage and dose settings.
On the other hand, nonlinearity is found only at 10 mAs and 80 kV in the V-direction. Hence, it can be concluded that
nonlinearity is indicated by a decrease in the quantum mottle, and the factors of nonlinearity exhibit the comprehensive
variation produced by the digital X-ray imaging system.