Markus Firsching

Quantitative Material Reconstruction in CT with Spectroscopic X-ray Pixel Detectors -- a Simulation Study

Modern photon counting X-ray detectors offer energy resolving capabilities for every single photon interacting in the sensor layer. This provides a new dimension of information which could and should be used to improve image quality. As the energy dependance of X-ray attenuation is a characteristic for the material, one can gain information on the composition of the object by analysis of the attenuation as a function of energy. One possibility is the material reconstruction, where the areal densities of chosen basis materials are obtained in projective geometry. For instance, this method could be used not only to distinguish and identify contrast agent from other highly absorbing regions, but also to gain quantitative information on the specific material. We have successfully applied this technique to computed tomography with Monte-Carlo simulated data and investigated the properties of this approach. Material selective images are presented and image quality properties are evaluated. The Signal Difference to Noise Ratio (SDNR) in the material-reconstructed and in the conventional CT images are compared.

Multi-energy X-ray imaging as a quantitative method for materials characterization.

IO N X-ray imaging is an established method for the characterization of material samples. Two-dimensional, radiographic methods as well as three-dimensional computed tomography are used very successfully in many fi elds. However both usually have a drawback: They provide only qualitative information rather than quantitatively exact physical properties. Thus a priori information and experience in evaluating the images are necessary to interpret the results. In this paper we present multi-energy X-ray imaging as a quantitative method for material characterization that is applicable for twoand three-dimensional analysis. The method we developed can be used in a classical “dual energy” approach as well as with newest generation “ spectral”, i.e. energy resolving, X-ray detectors. In X-ray imaging, three quantities of the object defi ne the attenuation of X-rays and thereby image contrast: the atomic number, the density and the thickness of the object that must be penetrated. For radiographic images, thickness and density can be united to the integral along the X-ray path through the object, resulting in the extinction, i.e. the exponent of Lambert-Beer’s law (see Equation 1 ). Computed tomography directly provides the attenuation coeffi cient depending on the atomic number and the density only. In reality usually more than one element is present, but the attenuation coeffi cient of a compound object is simply the linear combination of its constituents. Generally, the attenuation coeffi cients depend on both the energy of the X-ray photons and the type of material. If an object is imaged using different X-ray spectra or using an energy resolving detector, information on the type of material becomes available. Those so called “dual energy” techniques have been known since the mid-70s [ 1 ] and are established in medical imaging.

First measurements of material reconstruction in X-ray imaging with the medipix2 detector

In this paper we present first successful measurements with the Medipix2 detector in a projective geometry setup. This provides a new domain of information which could and, especially in medical imaging, should be used to improve image quality and reduce patient dose.

Strategies for efficient scanning and reconstruction methods on very large objects with high-energy x-ray computed tomography

X-ray computed tomography (CT) is an established tool for industrial non-destructive testing purposes. Yet conventional CT devices pose limitations regarding specimen dimensions and material thicknesses. Here we introduce a novel CT system capable of inspecting very large objects (VLO) like automobiles or sea freight containers in 3-D and discuss strategies for efficient scanning and reconstruction methods. The system utilizes a 9 MeV linear accelerator to achieve high penetration lengths in both dense and high-Z materials. The line detector array has an overall length of 4 meters. The presented system allows for reconstruction volumes of 3.2 meters in diameter and 5 meters in height. First we outline the general capabilities of high energy CT imaging and compare it with state of the art 450 kV X-ray systems. The imaging performance is shown based on experimental results. The second part addresses the problem of considerably higher scanning times when using line detectors compared to area detectors. Reducing the number of projections considerably causes image artifacts with standard reconstruction methods like filtered back projection (FBP). Alternative methods which can provide significantly better results are algebraic reconstruction techniques (ART). One of these is compressed sensing (CS) based ART which we discuss regarding its suitability in respect to FBP. We could prove the feasibility of inspecting VLOs like complete automobiles based on experimental data. CS allows for achieving sufficient image quality in terms of spatial and contrast resolution while reducing the number of projections significantly resulting in faster scanning times.

Simple solutions for spectroscopic, photon counting X-ray imaging detectors

The central idea of our approach is to use standard imaging sensors for industrial optical imaging as X-ray sensors. We use them as a simple solution for a spectroscopic, single photon counting X-ray detector with a reduction of the pixel size by a factor of almost 10 compared to commercially available photon counting X-ray detectors. In principle, each of the sensors photodiodes can act as a direct converting X-ray sensor pixel. We compare the acquisition of images in integrating and photon-counting mode and notice a much better spatial resolution in photon-counting mode compared to integrating- mode. Using the benefits of direct detection we gather spectroscopic information of the incident photons. Gray value and energy deposition are correlated linearly. Energy resolutions down to 700 eV are by limitation to single event clusters. Furthermore detection efficiency, multiplicity, DQE(0), MTF and the radiation hardness are investigated.

Reconstruction of incident X-ray spectra using the Medipix2 detector

The single photon counting pixel detector Medipix2 has been used in a variety of applications. Because of its spectral capability, the detector could be used as spectrometer. However the measured spectrum is not the real incident spectrum as the Medipix suffers from charge sharing because of its small pixel size. This implies that the energy information is smeared out heavily. Knowing the measured spectrum and the responses to monoenergetic exposure we applied some deconvolution methods to determine the incoming spectrum.