Characterization of a Single Photon Counting Imaging System by Transfer Function Analysis

A method to quantitatively evaluate the performances of a radiographic detection system consists in measuring the contrast, noise and modulation transfer functions. These functions have been evaluated for a digital radiographic system based on a single photon counting pixel detector. The X-ray detector is a Silicon sensor with one side segmented in a matrix of 256 by 256 square contacts with a pitch of 55 mum. The active area is about 2 cm². The sensor is connected to the Medipix2 read-out chip by bump-bonding. As X-ray source we have used a tube for general radiography. To reproduce the conditions of a radiographic examination, a 4 cm thick lucite block positioned above the detector has been used to simulate a tissue sample. To study the Contrast Transfer Function we have measured the contrast of an 1 mm thick lead slab with respect to the background. To evaluate the scattering contribution from the lucite, the measurements have been performed with and without a collimator placed at the beam exit. To assess the efficiency and noise transfer properties, we have measured the Detective Quantum Efficiency (DQE) of the detector as a function of the tube voltage. The Modulation Transfer Function has been measured applying the slit method for different conditions of tube voltage and energy threshold