Reconstruction in Cone-Beam Tomography can suffer from artifacts due to geometrical misalignments of the source-detector system. They can be avoided by a complete and precise description of the system. We present a high precision method for the geometric calibration for the PIXSCAN, a small animal X-ray CT scanner demonstrator based on hybrid pixel detectors (XPAD2). The specificities of the XPAD2 detectors (dead pixels, tilts and gaps between modules.. .) made the calibration of the PIXSCAN quite difficult. The method uses a calibration object consisting of a hollow cylinder of polycarbonate on which we positioned four metallic balls. It requires 360 X-ray images (1˚increments). An analytic expression of the 3 image ellipses has been derived. It is used for a least square regression of the 13 alignment parameters after a correction of the internal XPAD2 geometry. Our method is fast and completely automated, achieving a precision of about 30 µm.
A main challenge in the development of new detectors is the achievement of a satisfactory comprehension of the instrument behaviour. We present the simulation work developed to understand and characterize an innovative micro-CT scanner. The PIXSCAN scanner is a photon counting device based on hybrid pixel detectors. Its working principle is expected to improve the contrast for soft tissues and to reduce both the scan duration and the dose absorbed by the animal. A prototype of the scanner, PIXSCAN-XPAD2, has been assembled and studied in order to achieve a proof of principle of the system. Simulations by analytical and Monte Carlo methods of the prototype and of the evaluation phantoms have been developed to ensure a satisfactory comprehension of the data. The Monte Carlo simulation was based on the GATE package. It included the complete simulation of photon propagation in matter, together with the modelling of the source spectrum, the scanner geometry and the sensor response. The analytical simulation is much more approximate, but its merit is the rapidity which permits fast preliminary results. Several figures of merit are studied and show good agreement with real data. Hence, the developed simulations can be used as a valid tool for the estimation of the ultimate PIXSCAN performances, in terms of spatial resolution, contrast measurement and dose reduction.
International audienceBiological research on small animals is constantly demanding for imaging devices with higher performances like combined scanners enabling multiple modality data acquisition. In this context, we consider the concept of using hybrid X-ray pixel detectors (XPAD) with the clearPET to build a prototype of microPET/CT. The system will permit simultaneous acquisition of the anatomical and functional modalities. Thanks to the placement of the XPAD in front of the gamma detectors. It is foreseen to place the X-ray source outside the PET ring, thus involving a non-conventional off-centered circular geometry for the CT data acquisition. In a first step, a series of acquisitions were performed with a microCT, built with the XPAD2 detector. Our work consists first, in the investigation of several CT reconstruction methods for these acquisitions. We hereby compare the reconstruction results obtained with two variants of the Feldkamp and two analytical methods developed to compensate for some artifacts. Second, the reconstruction issue in the off-centered geometry is addressed by testing the same algorithms on numerical simulations. It appears that, if the next generation of XPAD detectors using smaller pixels is needed to obtain high resolution images, the reconstruction method employed, plays an important role and can efficiently improve the quality of the results
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