Techniques to improve the accuracy and to reduce the variance in noise power spectrum measurement

Several techniques to increase the accuracy and to reduce the variance of the noise power spectrum (NPS) measurement for digital X-ray imaging systems are investigated. These techniques include: (1) averaging the outputs from subblocks of the entire image; (2) averaging the two-dimensional NPS data along a circular route centered on the origin of spectral domain; and (3) masking a window function on each subblock before Fourier transforms. Techniques (1) and (2) are used mainly to reduce the variance of the NPS measurement. Technique (3) serves to improve the accuracy of the final result. Experiments with two different charge-coupled device-based X-ray imaging systems demonstrated that the precision and accuracy of the NPS measurement could be significantly improved using these techniques. The impact of the image partition for averaging is discussed and the corresponding NPS estimations are presented for the number of subblocks ranging from 4 to 64. The effect of masking on the NPS is also studied using four different window functions.