A hybrid pixel detector based on the concept of simultaneous charge integration and photon counting will be presented.
The second generation of a counting and integrating X-ray prototype CMOS chip (CIX) has been operated with different
direct converting sensor materials (CdZnTe and CdTe) bump bonded to its 8x8 pixel matrix. Photon counting devices
give excellent results for low to medium X-ray fluxes but saturate at high rates while charge integration allows the
detection of very high fluxes but is limited at low rates by the finite signal to noise ratio. The combination of both signal
processing concepts therefore extends the resolvable dynamic range of the X-ray detector. In addition, for a large region
of the dynamic range, where counter and integrator operate simultaneously, the mean energy of the detected X-ray
spectrum can be calculated. This spectral information can be used to enhance the contrast of the X-ray image. The
advantages of the counting and integrating signal processing concept and the performance of the imaging system will be
reviewed. The properties of the system with respect to dynamic range and sensor response will be discussed and
examples of imaging with additional spectral information will be presented.
The detective quantum efficiency (DQE) is regarded as a suitable parameter to assess the global imaging performance of an x-ray detector. However, residual signals increase the signal-to-noise ratio and therefore artificially increase the measured DQE compared to a lag-free system. In this paper, the impact of lag on the DQE is described for two different sources of lag using linear system models. In addition to the commonly used temporal filtering model for trapping, an increase of the dark current is considered as another potential source of lag. It is shown that the assumed lag model has a crucial impact on the choice of an adequate lag estimation method. Examples are given using the direct conversion material PbO. It turns out that the most general approach is the evaluation of the temporal noise power spectrum. A new algorithm is proposed for the crucial issue of robustly estimating the power spectrum at frequency zero.
A flat X-ray detector with lead oxide (PbO) as direct conversion material has been developed. The material lead oxide, which has a very high X-ray absorption, was analysed in detail including Raman spectroscopy and electron microscopy. X-ray performance data such as dark current, charge yield and temporal behaviour were evaluated on small functional samples. A process to cover a-Si TFT-plates with PbO has been developed. We present imaging results from a large
detector with an active area of 18 × 20 cm2. The detector has 1080 × 960 pixels with a pixel pitch of 184 μm. The linearity of detector response was verified. The NPS was determined with a total dark noise as low as 1800 electrons/pixel. The MTF was measured with two different methods: first with the analysis of a square wave phantom and second with a narrow slit. The MTF at the Nyquist frequency of 2.72 lp/mm was 50 %. We calculated first DQE values of our prototype detector plates. Full size images of anatomic and technical phantoms are shown.
Integrated dose sensing in Flat Detectors allows a during pulse control of the X-ray illumination without the need for external dose sensing devices. Standard designs of Flat Detectors do not allow during pulse dose sensing since the information is collected from the pixels only in the read-out phase after the X-ray illumination. This paper introduces a special detector plate design for obtaining dose sensing information directly from the X-ray detector while the X-ray pulse is being applied. This dose sensing information is read at a lower spatial resolution than the actual X-ray image but with a sub-millisecond temporal resolution. The dose sensing operates without any additional radiation burden on the patient and without attenuation of the image information. Experimental results from a small area (4x4 cm2) detector are presented, including an analysis of noise, linearity and cross-talk.
Flat X-ray detectors require a systematic calibration and correction of image artifacts. Based on an analysis of the physics of the image generation chain, this work presents a unified framework for the correction of these artifacts. Algorithms for the correction steps are presented, including a new method for the calibration and correction of the intertwined offset, gain, and non-linearity as well as an improved method for the interpolation of defects, where the interpolation direction is chosen based on a novel method. Experiments using a hand phantom without and with a wire, imaged on a flat detector, demonstrate that line artifacts in Digital Subtraction Angiography (DSA) applications due to differences in non-linearity between adjacent amplifiers are significantly reduced by applying the non-linearity, offset, and gain correction in the correct order, as proposed in this work. For the defect interpolation investigations, we used medical images of angiographic image subtraction sequences, containing small vessels. Artificial clusters of pixel defects were added to these images and subsequently corrected. The experimental verification clearly demonstrates the robustness and superior performance of the new interpolation scheme, especially for clusters of defects.
Flat X-ray detectors based on CsI:Tl scintillators and amorphous silicon photodiodes are known to exhibit temporal artefacts (ghost images) which decay over time. Previously, these temporal artefacts have been attributed mainly to residual signals from the amorphous silicon photodiodes. More detailed experiments presented here show that a second class of effects, the so-called gain effects, also contributes significantly to the observed temporal artefacts. Both the residual signals and the photodiode gain effect have been characterized under various exposure conditions in the study presented here. The results of the experiments quantitatively show the decay of the temporal artefacts. Additionally, the influence of the detector's reset light on both effects in the photodiode has been studied in detail. The data from the measurements is interpreted based on a simple trapping model which suggests a strong link between the photodiode residual signals and the photodiode gain effect. For the residual signal effect a possible correction scheme is described. Furthermore, the relevance of the remaining temporal artefacts for the applications is briefly discussed for both the photodiode residual signals and the photodiode gain effect.