| Popis: |
In order to evaluate and characterize the spatial resolution properties of a direct digital radiography (DDR) system based on a four CCD array detector, we measure the modulation transfer function (MTF) of the imaging system from its edge spread function (ESF). Different from the traditional algorithm for fine sampling of the ESF of an edge or a slit image with a slight angle to the image coordinate, in this paper we propose a new iterative algorithm. The novel algorithm finely samples the ESF along the column almost parallel to the edge, but not the row perpendicular to the edge. The samples of the first column and the second column are combined and averaged to get a segment of fine-sampled ESF. Then the segment of fine-sampled ESF is further combined with the next column and averaged to get a slight longer segment of ESF, and so forth. A complete fine-sampled ESF can be obtained after repeating the procedure for all columns of the edge image. An adaptive filtering is used for smoothing the ESF so that the sharpness at the edge of ESF can be protected. The MTF of the DDR is obtained from a series of image processing and data processing. The image processing includes the automatic determination of the angle of the edge to the pixel array of the edge image and fine sampling of edge spread function (ESF); data processing includes smooth filtering of ESF, numerical differential and fast Fourier transformation (FFT). |
