A number of different imaging systems are in use in X-ray medical diagnostics (e. g. digital radiography or computer tomography). The design goal of these imaging systems is to optimally use the information contained in X-ray quanta that have passed through the patient. The best image quality, as well 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 the 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 cm 2) for both digital radiography and fluoroscopy. The proposed detector consists of a CsI:T1 needle shaped scintillation crystal layer (thickness: 450 gm, needle diameter -10 gm) in front of an a-Si:H-panel. The Detective Quantum Efficiency (DQE) is about 65 % (at spatial 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 consists of Gadolinium Oxysulfid (GOS) ceramic scintillator elements, glued onto photodiodes. Important criteria for the selection of the detector material are good absorption of the incident X-rays (a > 95 %) and high efficiency of conversion of the absorbed radiation energy to an electrical signal. A very short decay time to extremely low levels of afterglow is an advantage for the very short scanning times in CT. One gets a DQE (at 0 mm -~) 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 quality. In addition, with the application 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 suffer from image quality losses, if no scattered radiation (multiline) grid is used.