Effect of window function on noise power spectrum measurements in digital x-ray imaging

Noise power spectrum (NPS) is an important parameter of x-ray imaging systems. Accurate and precise measurement of NPS is crucial in the system characterization. One method to improve the accuracy of the NPS measurement is to implement window functions, which fall off gradually at the edge of the images, before the Fourier transform of the noise intensity. Such window functions can reduce frequency leakage caused by sharp intensity change at the edge of an image. Three window functions, Bartlett, Hann and Welch, were studied for their effects on the NPS measurement using two different digital x-ray imaging systems. The noise power spectra masked by window functions were also compared with that of no window (or square window) masking. Our results showed that the window functions reduced the high-frequency contribution to the noise power spectrum, hence improving the accuracy of its measurement. The relative errors of NPS using different window functions relative to NPS using Welch window function showed that the square window had the most fluctuation.