Using the Medipix2 detector for photon counting computed tomography

The Medipix2 detector is a photon counting hybrid pixel detector. It consists of 256 by 256 pixels (55×55 μm² each) with a semiconductor sensor layer. Two thresholds enable X-ray imaging with an energy window. Integration of this detector into our setup requires very precise alignment of all devices: Precision of up to 0.05° and 10 μm was realised with special objects. For the verification of the system resolution we constructed phantoms with high contrast at high spatial frequency. The resolution limit of our setup is about 50 μm which is dominated by the relatively large X-ray spot size. Further on we investigated the impact of flatfield correction on ring artifacts and additionally applied a special filtering algorithm for suppression of those. Computed tomography acquisition series with different organic objects have been carried out to prove the system performance for realistic imaging tasks: Using a peanut as well as a newborn mouse produced high quality volume data sets. In addition, we used the quad version of the Medipix2 detector (2×2 ASICS) for CT of larger objects with diameters up to 26 mm. With Monte-Carlo simulations we investigated the interaction of energy weighting with the filtered back projection algorithm if energy resolved data is available: Applying the energy weighting technique before filtering and back projection or afterwards; or with intermixed order for different energy intervals and different kernels. We propose weighting before filtering and recommend smoothing kernels for lower photon energies and high resolution kernels for energy intervals with higher statistics.