Novel X-ray detectors for medical imaging

A number of different imaging systems are in use in X-ray medical diagnosti cs (e.g. digital radiography or computer tomography). The design goal of th ese imaging systems is to optimally use the information contained in X-ray quanta that have passed through the patient. The best image quality, as wel l as the minimisation of the X-ray dose applied to the patient are of prime importance. We report about innovations for novel detectors which reduce t he X-ray dose and improve the image quality simultaneously. Advances in thin film electronics have permitted the development of large a -Si:H imaging arrays to design flat panel solid state detectors (short FD, up to 45 x 45 cm2) for both digital radiography and fluoroscopy. The propos ed detector consists of a CsI:TI needle shaped scintillation crystal layer (thickness: 450 mu m, needle diameter similar to 10 mu m) in front of an a- Si:H-panel. The Detective Quantum Efficiency (DQE) is about 65 % (at spatia l frequency zero) and spatial resolution is 2.8 lp/mm at 20% of the MTF and 6.2 lp/mm at 4 %. In computed tomography (CT), a new generation of linear detector array cons ists of Gadolinium Oxysulfid (GOS) ceramic scintillator elements, glued ont o photodiodes. Important criteria for the selection of the detector materia l are good absorption of the incident X-rays (alpha > 95 %) and high effici ency of conversion of the absorbed radiation energy to an electrical signal . A very short decay time to extremely low levels of afterglow is an advant age for the very short: scanning times in CT. One gets a DQE (at 0 mm(-1)) about 80 %. The next step toward dose reduction could be implemented by the application of monochromatic instead of polychromatic X-rays. This would additionally improve the DQE and thus enhance image duality. In addition, with the appli cation of monochromatic X-rays, scattered radiation could be suppressed to a large extent by energy-selective single photon measurement, without loss of unscattered photons. At present, large area detectors in particular suff er from image quality losses, if no scattered radiation (multiline) grid is used.