Klaus Jürgen Engel

X-ray scattering in single- and dual-source CT.

For medical imaging applications, such as cardiac imaging, dual-source computed tomography (CT) improves the temporal resolution by the simultaneous use of two cone beams, which acquire twice as many projections as single-source CT does within the same time interval. Besides this advantage, a drawback of such a system is additional x-ray scatter originating from the extra (cross-illuminating) cone beam. In this work, a comparison with single-source CT images is performed under same-dose conditions for two different thorax phantoms, and for different cone beam angles corresponding to a coverage of 20, 40, 80, and 160 mm on the rotation axis (z coverage). As a general result, the HU-magnitude of scatter-induced streak and cupping artifacts scale almost proportional to the illuminated volume. In dual-source CT, cross scatter induces a further factor of almost 2 in the scaling of artifacts in comparison to single-source CT. For all examined systems, the scatter-induced noise reduces the contrast-to-noise ratio (CNR). In the case of an ideal scatter correction, the CNR is reduced even more, but contrast and CNR can be restored by an additional x-ray dose. With a 32-slice single-source CT (z overage of 20 mm) taken as a reference, a corresponding dual-source CT requires 7% more dose to maintain the same CNR. A CT system with a z coverage of 40, 80, and 160 mm requires 8%, 23%, and 54% more dose in a single-source configuration, respectively, and 20%, 47%, and 102% more dose in a dual-source configuration, respectively. In conclusion, a dual-source CT is comparable to a single-source CT with twice the z coverage concerning image degradation by scatter.

Status of Direct Conversion Detectors for Medical Imaging With X-Rays

Imaging detectors for medical X-ray and computed tomography (CT) applications have undergone many improvements and technology changes over time. But most (dynamic) detectors sold in this field still rely on indirect conversion, using scintillators and photodiodes to convert the X-ray quanta ultimately into electrical signals. Direct conversion detectors promise very high spatial resolution and high signal-to-noise ratios. Some direct conversion materials may allow for counting or even energy resolving detection of the X-ray quanta. Based on this, for example spectrally resolving CT systems are becoming an interesting option for the next decade. This contribution highlights the requirements of advanced medical X-ray and CT imaging and reviews examples of status and progress in the field. The emphasis is on the direct conversion sensors for pixelated detectors, but considerations on read-out concepts and on associated challenges such as interconnects will also be presented. Finally, the most burning issues, such as count rate limitations and polarization effects, will be discussed from an application point of view.

Noise properties of grating-based x-ray phase contrast computed tomography

PURPOSE To investigate the properties of tomographic grating-based phase contrast imaging with respect to its noise power spectrum and the energy dependence of the achievable contrast to noise ratio. METHODS Tomographic simulations of an object with 11 cm diameter constituted of materials of biological interest were conducted at different energies ranging from 25 to 85 keV by using a wave propagation approach. Using a Monte Carlo simulation of the x-ray attenuation within the object, it is verified that the simulated measurement deposits the same dose within the object at each energy. RESULTS The noise in reconstructed phase contrast computed tomography images shows a maximum at low spatial frequencies. The contrast to noise ratio reaches a maximum around 45 keV for the simulated object. The general dependence of the contrast to noise on the energy appears to be independent of the material. Compared with reconstructed absorption contrast images, the reconstructed phase contrast images show sometimes better, sometimes worse, and sometimes similar contrast to noise, depending on the material and the energy. CONCLUSIONS Phase contrast images provide additional information to the conventional absorption contrast images and might thus be useful for medical applications. However, the observed noise power spectrum in reconstructed phase contrast images implies that the usual trade-off between noise and resolution is less efficient for phase contrast imaging compared with absorption contrast imaging. Therefore, high-resolution imaging is a strength of phase contrast imaging, but low-resolution imaging is not. This might hamper the clinical application of the method, in cases where a low spatial resolution is sufficient for diagnosis.

Spectral analysis of scattered radiation in CT

In the framework of Spectral Computed Tomography (Spectral CT), scattered X-ray radiation is examined for its spectral composition and spatial distribution by means of Monte Carlo simulations. A reliable material (e.g. bone / contrast agent) separation and quantification requires a precise knowledge of the transmitted X-ray spectrum especially for low energy photons. Unfortunately, for lower energies the primary intensity is increasingly covered by scattered radiation. The detected scattered radiation can be classified into two main categories with respect to their scattering history. The first category contains purely Rayleigh or one-time Compton scattered photons which typically have small scattering angles and an energy spectrum similar to that of the transmitted primary radiation. The second category comprises multiple Compton scattered photons with a spectral composition which is typically softer than that of the transmitted primary photons. In regions of strong beam attenuation (i.e. in the X-ray shadow of a scanned object), the scattered radiation is mainly composed of multiple Compton scattered photons. As a consequence, the spectrally resolved scatter-to-primary ratios strongly increase at low energies. High-quality anti-scatter grids can be used to reduce especially the detection of multiple Compton-scattered photons. A quantitative evaluation of measured photon energies below a certain limit between 30 keV and 50 keV (depending on the phantom geometry and the applied anti-scatter grid) is challenging, since primary photons are superposed by a significantly higher amount of scattered photons.

Towards direct conversion detectors for medical imaging with X-rays

Imaging detectors for medical X-ray and Computed Tomography (CT) applications have undergone many improvements and technology changes over time. But most (dynamic) detectors sold in this field still rely on indirect conversion, using scintillators and photodiodes to convert the X-ray quanta ultimately into electrical signals. Direct conversion detectors promise very high spatial resolution and high signal-to-noise ratios. Some direct conversion materials may allow for counting or even energy resolving detection of the X-ray quanta. Based on this, for example spectrally resolving CT systems are becoming an interesting option for the next decade. This contribution highlights the requirements of advanced medical X-ray and CT imaging and shows examples of status and progress in the field. The emphasis is on the direct conversion sensors for pixelated detectors, but considerations on read-out concepts and on associated challenges such as interconnects will also be presented. Finally, the most burning issues, such as count rate limitations and polarization effects, will be discussed from an application point of view.

Comparison of Pixelated CdZnTe, CdTe and Si Sensors With the Simultaneously Counting and Integrating CIX Chip

CIX is a direct converting hybrid pixel detector designed for medical X-ray imaging applications. Its key feature is the simultaneous counting and integrating of absorbed X-ray quanta, which offers, among other benefits, a large dynamic range as well as the opportunity to calculate the average photon energy of the absorbed spectrum. In this work several different Si, CdTe and CdZnTe sensors are characterized and their suitability as sensor materials for a combined counting and integrating system is assessed. The measurements indicate that temporal stability is an issue for the tested CdTe samples, both on short (ms) and long (s) timescales. Furthermore, the homogeneity of the detector response is addressed and interpreted in terms of lateral polarization. The influence of charge sharing and X-ray fluorescence in the sensor materials will be characterized and their impact on the average photon energy reconstruction will be illustrated through simulations and measurements.

Combined effects of pulse pile-up and energy response in energy-resolved, photon-counting computed tomography

The very high x-ray flux rates employed in todays human computed tomography (CT) scanners in order to keep scanning times at a conveniently low level constitute the most challenging obstacle to the advent of clinical, photon-counting (spectral) CT. Even with most sophisticated, application-specific, energy-discriminating, photon-counting readout electronics, designed for room-temperature semi-conductor sensors like CdTe or CZT, the effects of spectral degradation due to pulse pile-up, i.e., count rate losses and gains will have to be taken into account in a clinical setting. The energy registered in a first-order pile-up event (superposition of two pulses) depends strongly on the energies of the two primaries involved, the difference in their arrival times and the spectral detector response behavior. We present an analytic model for the number of expected counts in binned photon-counting detectors, which is based on work by Wielopolski and Gardner and takes into account the combined effects of a spectral detector response function and 1st order pulse pile-up. The analytic model is validated by means of Monte-Carlo simulations and is applied to a simulation of a clinical spectral CT scenario in the context of K-edge imaging of a high-atomic number element as a contrast material. The artifacts in the reconstructed single-bin images and their manifestation in material-decomposed images are discussed and interpreted in terms of gains and losses of counts due to pile-up. Finally, we discuss the shortcomings of the model like the limitation to 1st order pile-up and the inherent restriction of the Wielopolski-Gardner model to peak pile-up.

Study of Charge Diffusion in a Silicon Detector Using an Energy Sensitive Pixel Readout Chip

A 300 μm thick thin p-on-n silicon sensor was connected to an energy sensitive pixel readout ASIC and exposed to a beam of highly energetic charged particles. By exploiting the spectral information and the fine segmentation of the detector, we were able to measure the evolution of the transverse profile of the charge carriers cloud in the sensor as a function of the drift distance from the point of generation. The result does not rely on model assumptions or electric field calculations. The data are also used to validate numerical simulations and to predict the detector spectral response to an X-ray fluorescence spectrum for applications in X-ray imaging.

Pulse Temporal Splitting in Photon Counting X-Ray Detectors

We performed Monte-Carlo simulations of X-ray interactions and charge transport within a photon counting cadmium zinc telluride (CZT) detector. We study various physical processes affecting the width and shape of detected pulses and find that the energy resolution depends on the pulse width variance. One impact factor (and main topic of this work) is the temporal splitting of pulses due to the simultaneous creation of two or more charge clouds by K-fluorescence within the same pixel. Measured energies are underestimated if the timing constants of detector and electronics are below or in the order of the arrival time differences of the charge clouds. Pulse temporal splitting is of minor relevance if other effects like charge sharing or polarization by hole trapping are present which cause a stronger degradation of the pulse height spectrum.

Impact of CT detector pixel-to-pixel crosstalk on image quality

In Computed Tomography (CT), the image quality sensitively depends on the accuracy of the X-ray projection signal, which is acquired by a two-dimensional array of pixel cells in the detector. If the signal of X-ray photons is spread out to neighboring pixels (crosstalk), a decrease of spatial resolution may result. Moreover, streak and ring artifacts may emerge. Deploying system simulations for state-of-the-art CT detector configurations, we characterize origin and appearance of these artifacts in the reconstructed CT images for different scenarios. A uniform pixel-to-pixel crosstalk results in a loss of spatial resolution only. The Modulation Transfer Function (MTF) is attenuated, without affecting the limiting resolution, which is defined as the first zero of the MTF. Additional streak and ring artifacts appear, if the pixel-to-pixel crosstalk is non-uniform. Parallel to the system simulations we developed an analytical model. The model explains resolution loss and artifact level using the first and second derivative of the X-ray profile acquired by the detector. Simulations and analytical model are in agreement to each other. We discuss the perceptibility of ring and streak artifacts within noisy images if no crosstalk correction is applied.