Single photon counting system for mammography with synchrotron radiation

Digital imaging systems for medical applications must be based upon highly efficient detectors to ensure low patient dose. This is considerably important, especially in mammography, because the high sensitivity of the breast to radiation. A mammoraphy system must also provide high spatial and contrast resolution to be able to detect important structures related to breast malignancies. The work performed and described in this thesis is the development of a readout system for a detector optimised for clinical mammography with synchrotron radiation. The detector called PICASSO (Phase Imaging for Clinical Application with Silicon detector and Synchrotron radiatiOn) is developed mainly for the mammography station of the SYRMEP beamline. The detector described in this work is based on Silicon microstrip sensors that are illuminated edge-on. The incoming beam impinging the detector is parallel to the strips of its sensors. This configuration permits high detection efficiency in the energy range that is of interest for mammography. Moreover, the Silicon sensors also allow direct conversion of X-rays. The readout electronics of the Picasso detector works on single-photon counting mode. That is, only signals from photons that are equal or greater than a pre-set threshold are counted, and low freqency noise are automatically rejected. The visibility of small details, normally valuable in mammograms, are maximised because the system is quantum limited, ie, the quality of the image is limited only by the intrinsic fluctuation of the detected photons. Picasso has four layers, each containing three detector modules. The layers are grouped into pairs and arranged one in front of the other along the beam of propagation. The pairs are controlled separately but are working in parallel. The system is a modular detector that implements a read-out system with MYTHEN II ASICs, an embedded Linux-based controller board and a Scientific Linux acquisition workstation. The developed system architecture and its characteristics will be presented. Preliminary imaging tests were perfomed and results with the new system will be presented. Standard mammographic phantoms were imaged and good quality images were obtained at doses comparable with what is delivered in conventional full field mammographic systems. The whole system was able to sustain fast acquisition speeds up to 10ms/frame and runs stable until a breast-equivalent length acquisition is accomplished. A delay between frame of 150μs and delay between controllers of around 750μs is achieved. Phase-contrast imaging has revolutionized the face of mammography with synchrotron radiation in the last ten years as the first clinical phase has been successfully implemented in our facility. This initial step made use of commercial screen-film system producung promising results. Thanks to the coherence and monochromaticity of light coming from synchrotron sources that edge-enhancement in the image is achieved due to phase effects. The compatibility of the Picasso detector to phase-contrast imaging with other novel techniques has also been evaluated in line with this project. Phase-contrast was well demonstrated with the system, details of which will be fully described.