In an acute stroke parts of the human brain undergo subtle physiological changes, which are often visible as hypo- or hyperdense regions in Computed Tomography (CT) images. In the case of ischemic stroke usually an edema develops due to undersupplied cells forming regions of a core (dead tissue) and a penumbra (salvable tissue). For stroke diagnosis and outcome control it is very important to know the location and size of these different kinds of damaged tissue.
We have modelled the changes in elemental composition of brain tissue in different phases of an ischemic stroke. Influence of a number of factors on the absolute Hounsfield units is investigated as possible causes of intra- and interpatient variation. The modeled pathological changes are included in different software brain models. Subsequently we have simulated X-ray images of these brain models acquired by dual energy Cone Beam Computed Tomography (CBCT). Our modelling is based on a combination of analytical and Monte-Carlo methods. As an example of spectral processing virtual monoenergetic images are reconstructed from the simulated projections.
Simulated images are intended to optimize acquisition parameters for clinical studies beforehand and to develop new image processing algorithms to enhance the diagnostic value. As example a water map is calculated to better visualize the formation of an edema after ischemic stroke.
We report on the modeling, characterization, benchmarking, and optimization of an interventional cone beam CT system based on a dual layer X-ray detector by means of physics based simulations.
By Monte Carlo methods, we log the interaction and dose deposition (i.e. signal generation) of X-ray photons in the dual layer geometry, including scattering processes and fluorescence photon emission. From the spatial dose distribution inside the detection volume, we derive typical detector properties like X-ray spectral responses, detective quantum efficiencies 𝐷𝑄𝐸(0), and noise characteristics for particular detector layouts.
We apply these results in subsequent full system simulations to generate 3D imaging scans of dual layer spectral projections, for custom virtual phantoms containing inserts of e.g. blood sediment or iodine with different concentrations. These simulated images are used to calculate key performance indicators of the imaging system, like e.g. receiver operating characteristic based analysis of material separation capabilities.
Computed tomography (CT) imaging of the thorax is a common application of CT in radiology. Most of these scans are performed with a helical scan protocol. A significant number of images suffer from motion artefacts due to the inability of the patients to hold their breath or due to hiccups or coughing. Some images become nondiagnostic while others are simply degraded in quality. In order to correct for these artefacts a motion compensated reconstruction for non-periodic motion is required.
For helical CT scans with a pitch smaller or equal to one the redundancy in the helical projection data can be used to generate images at the identical spatial position for multiple time points. As the scanner moves across the thorax during the scan, these images do not have a fixed time point, but a well-defined temporal distance inbetween the images. Using image based registration a motion vector field can be estimated based on these images. The motion artefacts are corrected in a subsequent motion compensated reconstruction. The method is tested on mathematical phantom data (reconstruction) and clinical lung scans (motion estimation and reconstruction).
With the emergence of energy-resolved x-ray photon counting detectors multi-material spectral x-ray imaging has been made possible. This form of imaging allows the discrimination and quantification of individual materials comprising an inspected anatomical area. However, the acquisition of quantitative material data puts strong requirements on the performance capabilities of a given x-ray system. Scattered radiation is one of the key sources of influencing the quality of material quantification accuracy. The aim of the present investigation was to assess the impact of x-ray scatter on quantitative spectral CT imaging using a pre-clinical photon counting scanner prototype. Acquisitions of a cylindrical phantom with and without scatter were performed. The phantom contained iodine and gadolinium inserts placed at various locations. The projection data was then decomposed onto a water-iodine-gadolinium material basis and reconstructed. An analysis of the resulting iodine and gadolinium material images with and without scatter was conducted. It was concluded that, at an SPR level of up to 3.5%, scatter does not compromise material quantification for all investigated gadolinium concentrations, but for iodine a substantial underestimation was observed. The findings in this study suggest that scatter has a lower impact on K-edge material imaging in comparison with material imaging not featuring a K-edge.
Flat field calibration methods are commonly used in computed tomography (CT) to correct for system imperfections.
Unfortunately, they cannot be applied in energy-resolving CT when using bow-tie filters owing to spectral distortions
imprinted by the filter. This work presents a novel semi-analytical calibration method for photon-counting spectral CT
systems, which is applicable with a bow-tie filter in place and efficiently compensates pile-up effects at fourfold
increased photon flux compared to a previously published method without degradation of image quality. The achieved
reduction of the scan time enabled the first K-edge imaging in-vivo. The method employs a calibration measurement with
a set of flat sheets of only a single absorber material and utilizes an analytical model to predict the expected photon
counts, taking into account factors such as x-ray spectrum and detector response. From the ratios of the measured x-ray
intensities and the corresponding simulated photon counts, a look-up table is generated. By use of this look-up table,
measured photon-counts can be corrected yielding data in line with the analytical model. The corrected data show low
pixel-to-pixel variations and pile-up effects are mitigated. Consequently, operations like material decomposition based
on the same analytical model yield accurate results. The method was validated on a experimental spectral CT system
equipped with a bow-tie filter in a phantom experiment and an in-vivo animal study. The level of artifacts in the resulting
images is considerably lower than in images generated with a previously published method. First in-vivo K-edge images
of a rabbit selectively depict vessel occlusion by an ytterbium-based thermoresponsive polymer.
Clinical CT applications such as oncology follow-up using iodine maps require accurate contrast agent (CA)
quantification within the patient. Unfortunately, due to beam hardening, the quantification of CA materials like iodine in
dual energy systems can vary for different patient sizes and surrounding composition. In this paper we present a novel
method that handles this problem which takes into account properly the CA energy dependent attenuation profile. Our
method is applicable for different dual energy scanners, e.g. fast kVp switching or dual layer detector array and is fully
compatible with image domain material analysis. In this paper we explain the concept of so called landmarks used by our
method, and give the mathematical formulation of how to calculate them. We demonstrate by scans of various phantom
shapes and by simulations, the robustness and the accuracy of the iodine concentration quantification obtained by our
Image noise is an important aspect when optimizing CT system parameters. For example in multi-energy CT the
statistical error in material images is of high interest. It has been shown that the statistical error of individual
line integrals can be quantified by the Cramér-Rao lower bound, but it is not straightforward to estimate the
noise in the corresponding material images in a time-efficient way, since for an exact calculation a large number
of projections has to be taken into account. This paper introduces a way to approximate the image noise by using
noise estimates of only a few line integrals. The method is exemplified using simulated dual energy CT data, and
the results are shown to closely correspond with Monte Carlo outcomes. The method provides a fast alternative
to Monte Carlo simulations in order to calculate the image noise depending on various system parameters, which
is very helpful, e.g., for system design.
In scintillating detectors x-rays are converted to luminescent photons with a time delay. The corresponding time
resolution of the detector can have - in contrast to usual
multi-slice CT - a deteriorating effect in new CT concepts with
multiple sources illuminating one detector, because x-ray intensities measured here in consecutive projections
correspond to the absorption along paths through very different regions of the object. A new analytical description of
these effects is presented and a correction algorithm is derived. It is also shown that the detector time delay and its
correction can lead to a noticeable increase of image noise.
Energy-resolved fan-beam coherent scatter computed tomography (CSCT) is a novel X-ray based tomographic imaging
method revealing structural information on the molecular level, namely the momentum-transfer dependence of the
coherent scatter cross-section. Since the molecular structure is the source of contrast a very good material discrimination
and possibly also medical diagnosis of structural changes of tissue can be achieved with this technique. For the design of
a medical or baggage inspection CSCT-scanner acquisition speed is of particular importance. Several performance
improvements for CSCT were investigated. The multi-slit fan beam collimator and multi-line scatter detector allow
increasing the detected photon flux without compromising angular resolution. Analysis of the noise in reconstructed data
leads to the possibility to adjust scan time to the size of the objects to be analyzed. Improved energy resolution of the
detector improves momentum-transfer resolution such that angular resolution becomes the limiting factor. Overall, the
implemented improvements now enable the real-world application of CSCT.
Coherent Scatter Computer Tomography (CSCT) is a novel x-ray imaging method revealing structural information on the molecular level. More precisely, the momentum transfer dependence of the coherent scatter cross section of the object material in each voxel of an object slice under investigation is determined. Compared to other x-ray diffraction techniques very large objects can be investigated which allows to apply the technique in medical imaging, material analysis or baggage inspection. The ratio of multiple scattered radiation over single scatter increases with object size. For large objects multiple scatter can become the dominant contribution. Since this part of the measured radiation cannot be reconstructed correctly, artifacts in the resulting images occur. The amount of multiple scattered radiation in CSCT and its dependence on the object size and material have been investigated by means of Monte Carlo simulations. A method to correct for multiple scattered radiation in energy-resolved CSCT is introduced. The benefit of this correction method to the quality of reconstructed data is demonstrated.
We used the Monte Carlo code EGSnrc to simulate electron energy loss profiles as well as angle resolved x-ray spectra for metal-layer/substrate combinations in the primary electron energy range of 60-160 keV. We were furthermore able to separate the bremsstrahlung fraction originating in the substrate from that of the metal layer. The simulations were accompanied by experimental investigations. High-energetic electrons of 60-160 keV were directed onto 1-2 μm thin tungsten layers on top of 500 μm diamond substrates. The spectra were recorded by an energy resolved detector positioned in backward direction. We compared the experimental data with the simulation results and found good agreement. An enhanced monochromaticity in backward direction however, as expected from thin film theory, has not been observed due to the influence of the substrate.
For the first time, a reconstruction technique based on filtered back-projection (FBP) using curved 3D back-projection lines is applied to energy-resolved coherent-scatter projection data. Coherent-scatter computed tomography (CSCT) yields information about the molecular structure of an object. It has been shown that the relatively poor spectral resolution due to the application of a polychromatic X-ray source can be overcome, when energy-resolved detection is used. So far, the energy-resolved projection data, acquired with a CSCT scanner, are reconstructed with the help of
algebraic reconstruction techniques (ART). Due to the computational complexity of iterative reconstruction, these methods lead to relatively long reconstruction times. In this contribution, a reconstruction algorithm based on 3D FBP is introduced and applied to
projection data acquired with a demonstrator setup similar to a multi-line CT scanner geometry using an energy-resolving CdTe-detector. Within a fraction of the computation time of algebraic reconstruction methods, an image of comparable quality is generated when using FBP reconstruction. In addition, the FBP approach has the advantage, that sub-field-of-view reconstruction becomes feasible.
This allows a selective reconstruction of the scatter function for a region of interest. The method is based on a high-pass filtering of the scatter data in fan-beam direction applied to all energy channels. The 3D back-projection is performed along curved lines through a volume defined by the in-plane spatial coordinates and the wave-vector transfer.
This paper presents simulated and measured spectra of a novel type of x-ray tube. The bremsstrahlung generating principle of this tube is based on the interaction of high energetic electrons with a turbulently flowing liquid metal separated from the vacuum by a thin window. We simulated the interaction of 50-150 keV electrons with liquid metal targets composed of the elements Ga, In, Sn, as well as the solid elements C, W and Re used for the electron windows. We obtained x-ray spectra and energy loss curves for various liquid metal/window combinations and thicknesses of the window material. In terms of optimum heat transport a thin diamond window in combination with the liquid metal GaInSn is the best suited system. If photon flux is the optimization criteria, thin tungsten/rhenium windows cooled by GaInSn should be preferred.
Energy-resolved fan beam coherent scatter computed tomography (CSCT) is a novel X-ray based imaging method revealing structural information on the molecular level of tissue or other material under investigation with high resolution of the momentum-transfer dependent coherent scatter cross-section. Since the molecular structure is the source of contrast a very good material discrimination and possibly also medical diagnosis of structural changes of tissue can be achieved with this technique. Poor spectral resolution as found in previous work due to the application of a polychromatic X-ray source can be overcome when energy-resolved detection is used. In this paper experimental results on phantoms using an energy-resolving CdTe-detector are shown. With the present setup the spatial resolution was found to be 4.5 mm (FWHM) and a spectral resolution of 6% was achieved. Applications of this technique can be found in medical imaging, material analysis and baggage inspection.
A novel type of electron-impact x-ray source based on the interaction of energetic electrons with a turbulently flowing liquid metal target is presented. The electrons enter the liquid through a thin (several microns thick) window, separating the liquid from the vacuum region in which the cathode is situated. Several electron window materials including diamond, tungsten and molybdenum were tested in combination with the liquid metal GaInSn. Satisfactory agreement has been obtained between the predictions of thermal transport models and the measured dependence of the loadability on fluid velocity. The liquid metal technology appears to represent a significant improvement in continuous loadability relative to stationary anode x-ray tubes.
A summary is given of methods to manipulate the polychromatic radiation emitted from electron impact x-ray sources so as to generate a (quasi-)monochromatic beam. These methods include: differential attenuation of bremsstrahlung, differential reflection of x-rays from monochromating crystals, production of fluorescence x-rays from secondary targets and geometrical enhancement of characteristic radiation. Typical values for some of the parameters which characterize (quasi-)monochromatic sources i.e. monochromaticity, energy bandwidth and source radiance are presented. A brief description is given of some radiological techniques which either necessitate or benefit from monochromatic radiation. With the help of a figure of merit for monochromatic x-ray sources, the suitability of the candidates mentioned above for these techniques is assessed.