Nonlinear Polychromatic and Noise Artifacts in X-Ray Computed Tomography Images

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Abstract
The variance of the image noise in computed X-ray transmission tomography (CT) due to quantum noise was first approximated a nonlinear function of X-ray attenuation. Beam hardening in CT was also nonlinear. Computer simulations and numerical results showed that both nonlinear dependencies had quite similar effects on image quality. The 2-dimensional distribution of the noise variance in a CT image was a weighted superposition of images obtained by backprojecting integer powers of the noiseless projection data corresponding to the scanned object. The streak-like pattern in the image noise due to the anisotropic nature of the noise cross-correlation function and how these nonlinear phenomena affect noise filtering and tissue characterization using statistical parameters were discussed.