A METHOD FOR MEASURING THE PRESAMPLED MTF OF DIGITAL RADIOGRAPHIC SYSTEMS USING AN EDGE TEST DEVICE

The modulation transfer function (MTF) of radiographic systems is freq uently evaluated by measuring the system's line spread function (LSF) using narrow slits. The slit method requires precise fabrication and a lignment of a slit and high radiation exposure. An alternative method fur determining the MTF uses a sharp, attenuating edge device. We have constructed an edge device fl om a 250-mu m-thick lead foil laminated between two thin slabs of acrylic. The device is placed near the dete ctor and aligned with the aid of a laser beam and a holder such that a polished edge is parallel to the x-ray beam. A digital image of the e dge is processed to obtain the presampled MTF. The image processing in cludes automated determination of the edge angle, reprojection, sub-bi nning, smoothing of the edge spread function (ESF), and spectral estim ation. This edge method has been compared to the slit method using mea surements on standard and high-resolution imaging plates of a digital storage phosphor (DSP) radiography system. The experimental results fo r both methods agree with a mean MTF difference of 0.008. The edge met hod provides a convenient measurement of the presampled MTF for digita l radiographic systems with good response at low frequencies. (C) 1998 American Association of Physicists in Medicine.