Imaging properties of the Medipix2 system exploiting single and dual energy thresholds

Low noise, high resolution, and high dose efficiency are the common requirements for most X-ray imaging applications. The dose efficiency is especially important for medical imaging systems. We present the imaging performance of the Medipix2 readout chip bump bonded to a 300 mum thick Si detector as a function of the detection threshold, a free parameter not available in conventional charge integrating imaging systems. Spatial resolution has been measured using the modulation transfer function (MTF) and it varies between 8.2 line-pairs/mm and 11.0 line pairs/mm at an MTF value of 70%. An associated measurement of noise power spectrum (NPS) permits us to derive the detective quantum efficiency (DQE) which can be as a high as 25.5% for a broadband incoming spectrum. The influence of charge diffusion in the sensor together with threshold variation in the readout chip is discussed. Although the Medipix2 system is used in photon counting mode with a single threshold in energy, the system is also capable of counting within a given energy window as narrow as ~1.4 keV. First measurements and images using this feature reveal capabilities that allow identifying fluorescence and other sources of disturbance