Accurate physics modeling of a photon counting detector is essential for detector design and performance evaluation, Computer Tomography (CT) system-level performance investigation, material decomposition, image reconstruction. The detector response is complicated because various effects involve, including fluorescence X-rays, primary electron path, charge diffusion, charge repulsion, and charge trapping. In this paper, we will present a comprehensive detector modeling approach, which incorporates all these effect into account.
Dual energy imaging (DE) is a potential alternative to conventional mammography for patients with dense breasts. It requires intravenous injection of contrast agent (CA) and subsequent acquisition of images at two different energies. Each pixel is seen as a vector and is projected onto a two-material basis, e.g. water, CA, to form separate water-equivalent and CA-equivalent images. On conventional detectors, this requires two separate exposures. Spectroscopic detectors allow multiple images from a single exposure by integrating appropriate energy bands. This work investigates the effects of high count rates on quantitative DE imaging using a CdTe spectroscopic detector. Because of its small pixel size (250 μm), a limitation of the detector is charge sharing between pixels, which must be corrected to avoid degradation of the detected spectrum. However, as charge sharing is identified by neighbouring pixels registering a count in a given readout frame, an effective maximum count rate (EMR) is imposed, above which linearity between incident and detected counts is lost. A simulation was used to model detector response of a test object composed of water and iodine, with different EMRs and incident count rates. Using a known iodine thickness of 0.03 cm, and an EMR of 103 s−1 , the reconstructed thickness of iodine was found to be 97%, 74% and 24% of the true value for incident count rates of 100, 1000 and 10000 photons/pixel/s respectively. The simulation was validated by imaging a water-equivalent test phantom containing iodinated CA at different X-ray currents, to determine the optimum beam conditions.
The demand for higher resolution x-ray optics (a few arcseconds or better) in the areas of astrophysics and solar science has, in turn, driven the development of complementary detectors. These detectors should have fine pixels, necessary to appropriately oversample the optics at a given focal length, and an energy response also matched to that of the optics. Rutherford Appleton Laboratory have developed a 3-side buttable, 20 mm x 20 mm CdTe-based detector with 250 μm square pixels (80x80 pixels) which achieves 1 keV FWHM @ 60 keV and gives full spectroscopy between 5 keV and 200 keV. An added advantage of these detectors is that they have a full-frame readout rate of 10 kHz. Working with NASA Goddard Space Flight Center and Marshall Space Flight Center, 4 of these 1mm-thick CdTe detectors are tiled into a 2x2 array for use at the focal plane of a balloon-borne hard-x-ray telescope, and a similar configuration could be suitable for astrophysics and solar space-based missions. This effort encompasses the fabrication and testing of flightsuitable front-end electronics and calibration of the assembled detector arrays. We explain the operation of the pixelated ASIC readout and measurements, front-end electronics development, preliminary X-ray imaging and spectral performance, and plans for full calibration of the detector assemblies. Work done in conjunction with the NASA Centers is funded through the NASA Science Mission Directorate Astrophysics Research and Analysis Program.
This project uses the combination of a spectroscopic detector and a monochromator to produce scatter free images for use in mammography. Reducing scatter is vital in mammography, where typical structures have either low contrast or small dimensions. The typical method to reduce scatter is the anti-scatter grid, which has the drawback of absorbing a fraction of the primary beam as well as scattered radiation. An increase in the dose is then required in order to compensate. Compton-scattered X-rays have lower energy than the primary beam. When using a monochromatic beam and a spectroscopic detector the scattered beam will appear at lower energies than the primary beam in the detected spectrum. Therefore if the spectrum of the detected X-rays is available, the scattered component can be windowed out of the spectrum, essentially producing a scatter free image. The monochromator used in this study is made from a Highly Orientated Pyrolytic Graphite (HOPG) crystal with a mosaic spread of 0.4°±0.1°. The detector is a pixellated spectroscopic detector that is made from a 2 cm x 2 cm x 0.1 cm CdTe crystal with a pixel pitch of 250 μm and an energy resolution of 0.8 keV at 59.5 keV. This work presents the characterisation of the monochromator and initial imaging data. The work shows a contrast increase of 20% with the removal of the low energy Compton scattered X-rays.
Breast lesions and normal tissue have different characteristics of density and molecular arrangement that affect their
diffraction patterns. X-ray diffraction can be used to determine the spatial structure of such tissues at the atomic and
molecular level and Energy Dispersive X-Ray Diffraction Computed Tomography (EDXRDCT) can be used to produce
2-dimensional images of cross sections of the samples. The purpose of this work is to use an EDXRDCT system to find
the limiting visibility for details that simulate breast lesions. Results are presented for EDXRDCT images of samples of
different materials simulating breast tissue contrast and shapes. For simple circular details, the contrast between details and background in the images was measured with the goal of simulating the contrast between real breast tissue components. The limiting visible diameter was measured as a function of detail diameter for different combinations of scanning and geometrical parameters. Images of more complex test objects were assessed in terms of both contrast and accuracy of shape reproduction, evaluating the feasibility of using shape analysis as an additional parameter for lesion identification. The optimum combination of parameters are intended to be applied to the scanning of waxed breast tissue blocks.
Energy dispersive X-ray diffraction (EDXRD) is a technique which can be used to improve the detection and
characterisation of explosive materials. This study has performed EDXRD measurements of various explosive
compounds using a novel, X-ray sensitive, pixelated, energy resolving detector developed at the Rutherford Appleton
Laboratory, UK (RAL). EDXRD measurements are normally performed at a fixed scattering angle, but
the 80×80 pixel detector makes it possible to collect both spatially resolved and energy resolved data simultaneously.
The detector material used is Cadmium Telluride (CdTe), which can be utilised at room temperature
and gives excellent spectral resolution. The setup uses characteristics from both energy dispersive and angular
dispersive scattering techniques to optimise specificity and speed. The purpose of the study is to develop X-ray
pattern "footprints" of explosive materials based on spatial and energy resolved diffraction data, which can then
be used for the identification of such materials hidden inside packages or baggage. The RAL detector is the
first energy resolving pixelated detector capable of providing an energy resolution of 1.0-1.5% at energies up to
150 keV. The benefit of using this device in a baggage scanner would be the provision of highly specific signatures
to a range of explosive materials. We have measured diffraction profiles of five explosives and other compounds
used to make explosive materials. High resolution spectra have been obtained. Results are presented to show
the specificity of the technique in finding explosives within baggage.
This paper presents preliminary work aimed at assessing the feasibility of K-edge subtraction imaging using the
spectroscopic information provided by a pixellated energy-resolving Cadmium Zinc Telluride detector, having an active
area of 20×20 pixels 250 μm in size. Images of a test object containing different amounts of Iodine-based contrast agent
were formed above and below the K-edge of Iodine (33.2 keV) by integrating, pixel by pixel, different windows of the
spectrum. The results show that the optimum integration window for details 1-2 mm in diameter is between 2 keV and 5
keV. Concentrations of down to 50 μg Iodine/ml were detected in a 1-mm diameter tube with an entrance dose of 100