American Association of Physicists in Medicine, 38th Annual Meeting, Philadelphia, Pennsylvania, 21-25 July. Paper TU-C4-04.
The modulation transfer function (MTF) of radiographic systems is frequently evaluated by measuring the system's line spread function (LSF) using narrow slits. A slit phantom is difficult to align in the radiation beam, and corrections must be made for its finite width. We have developed a technique for determining the presampled MTF using a sharp attenuating edge phantom. The phantom is made of a 250-µ m-thick lead plate laminated between two thin sheets of acrylic. The edge of the phantom is milled to form a straight surface with a 90 degree edge angle and subsequently polished with diamond grits. The phantom is then placed on the image plane and aligned such that the edge is perpendicular to the x-ray beam using an adjustable phantom holder and a laser pointer. The acquired edge image is processed to obtain the presampled MTF by performing: 1) conversion of the digital data to a linear scale, 2) Hough transformation to find the line that best describes the straight edge, 3) projection of the gray level data along the edge into a one-dimensional trace with 0.1 pixel bins, 4) smoothing of the edge spread function (ESF), 5) differentiation of the ESF to obtain the LSF, and 6) Fourier transformation of the LSF. The method was applied on an upright chest digital radiography system (Fuji 9501 CR system) using a 115 kVp unfiltered x-ray beam. The measured MTF was 0.1 at 3.27 cycles/mm vertically and 2.85 cycles/mm horizontally. The results were reproducible. This method provides a quick and accurate measurement of the presampled MTF of a digital system that can be implemented easily in a clinical setting. When combined with measures of exposure and the noise power spectra (NPS), it permits field measurement of the detective quantum efficiency (DQE) of these systems.