Technical Note: MTF determination from a star bar pattern image

PURPOSE: A procedure to obtain the MTF of an image system from a star bar pattern image is presented. The results obtained are compared to the standard analysis of an edge image (as described in the norm IEC 62220-1). METHODS: The images used are artificially generated following a procedure that simulates sampling, blurring, and noise present in a real phantom image. The MTF from a star bar pattern is obtained from the analysis of circular scans over the star pattern image and concentric to it. The radius of the scanning circumference is proportional to the spatial frequency of the curve arising from the scan, and the amplitude of this curve is directly related to the system MTF for that frequency. An oversampling procedure is also followed to minimize the effect of noise in the image. In order to investigate the MTF in a particular direction, the image analysis is restricted to an angular sector centered around that direction. RESULTS: The MTFs calculated following this procedure show a very good agreement with those obtained following the standard IEC 62220-1 for low noise levels. As the noise increases, however, the star bar method is more accurate than the edge method. CONCLUSIONS: The procedure presented provides a good method to investigate the MTF in any spatial direction. Finally, as the whole star bar pattern image is used for MTF computation, the high signal to noise ratio guarantees a good immunity against noise.