An Image Quality Study of Medipix2 Single Photon Counting Detector Based on Two Kinds of Flat Field Corrections for Breast Computed Tomography Application

In the framework of a research project for breast Computed Tomography (CT), we studied two different flatfield correction methods for a single photon counting (SPC) semiconductor pixel detector for X-ray CT. The first method is the usual flat field irradiation procedure with a uniform exposure of the detector in air, while the second method uses flat field images obtained by interposing attenuating sheets of material (e.g. Al, PMMA) between the X-ray tube and the detector, in order to account for the beam hardening effect that occurs when imaging a sample in CT. In fact, this effect causes a different detector response than with free-in-air flood irradiations. The experimental setup includes a Medipix2 SPC silicon pixel detector (matrix of 256×256 square pixels of 55 μm pitch) and a micro-focus X-ray tube with a molybdenum anode and a focal spot size of 35 μm. The single photon counting operation of Medipix2 avoids integration of dark current and enables energy discrimination in each pixel, allowing noise rejection and providing high signal to noise ratio. With 13- bit/pixel and a count rate exceeding 105 cps/pixel, the detector permits high flux exposures. We present here two sets of tomographic images of PMMA (1.19 g/cm3) phantoms containing water or high density polyethylene (0.95 g/cm3) as contrast materials. For each set of projections we performed the two types of correction before the tomographic reconstruction. Images are compared in terms of contrast-to-noise ratio.