Rudolph Maria Snoeren

Flat detector ghost image reduction by UV irradiation

We study flat detectors for X-ray imaging performance degradation. In cone beam C-arm-CT, memory effects have a detrimental effect on image quality. Depending on the magnitude and history of irradiance differences, detector sensitivity variations may persist for a long period of time (days) and are visible as rings upon 3D reconstruction. A new method is proposed for reducing memory effects produced in CsI:Tl based Flat Detector X-ray imaging, which is based upon trap-filling by UV-light. For experiments, a commercial detector has been modified such that UV back-lighting is accomplished. A regular LED refresh light array for reducing photodiode temporal effects is interleaved with UV LED sub-arrays of different wavelengths in the near UV range. The array irradiates the scintillator through translucent parts of the detector substrate. In order to assess the efficacy of the method, ghost images are imprinted by well-defined transitions between direct radiation and attenuated or shuttered radiation. As an advantage, the new method accomplishes ghost-prevention, either by (1) continuous trap-filling at image-synchronous UV light pulsing, or (2) by applying a single dose of UV light. As a result, ring artefacts in reconstructed 3D-images are reduced to low levels. An effective wavelength has been found and an equilibrium UV dosage could be set for effective trap-filling. The overall sensitivity of the detector increases at saturated trap-filling. It was found that with optimised detector settings, i.e. optimum saturated trap-filling, the dependence on X-ray irradiation levels is low, so that the usage of the detector and its performance is robust.

A CCD-Based Video Camera For Medical X-Ray Imaging

Using an anamorphic imaging technique, the solid state image sensor can replace the vacuum pick-up tube in medical X-ray diagnostics, at least for the standard quality fluoroscopic application. A video camera is described in which, by optical compression, the circular output window of an image intensifier is imaged as an ellipse on the 3:4 image rectangle of a CCD sensor. The original shape is restored by electronics later. Information transfer is maximized this way: the total entrance field of the image intensifier is available, at the same time the maximum number of pixels of a high resolution CCD image sensor is used, there is also an increase of the sensor illuminance by 4/3 and the aliasing effects are minimized. Imaging descriptors such as Modulation transfer, noise generation and transfer are given in comparison with a Plumbicon-based camera, as well as shape transfer, luminance transfer and aliasing. A method is given to isolate the basic noise components of the sensor. A short description of the camera optics and electronics is given.