Jürgen Durst

On a dark-field signal generated by micrometer-sized calcifications in phase-contrast mammography

We show that a distribution of micrometer-sized calcifications in the human breast which are not visible in clinical x-ray mammography at diagnostic dose levels can produce a significant dark-field signal in a grating-based x-ray phase-contrast imaging setup with a tungsten anode x-ray tube operated at 40 kVp. A breast specimen with invasive ductal carcinoma was investigated immediately after surgery by Talbot-Lau x-ray interferometry with a design energy of 25 keV. The sample contained two tumors which were visible in ultrasound and contrast-agent enhanced MRI but invisible in clinical x-ray mammography, in specimen radiography and in the attenuation images obtained with the Talbot-Lau interferometer. One of the tumors produced significant dark-field contrast with an exposure of 0.85 mGy air-kerma. Staining of histological slices revealed sparsely distributed grains of calcium phosphate with sizes varying between 1 and 40 μm in the region of this tumor. By combining the histological investigations with an x-ray wave-field simulation we demonstrate that a corresponding distribution of grains of calcium phosphate in the form of hydroxylapatite has the ability to produce a dark-field signal which would-to a substantial degree-explain the measured dark-field image. Thus we have found the appearance of new information (compared to attenuation and differential phase images) in the dark-field image. The second tumor in the same sample did not contain a significant fraction of these very fine calcification grains and was invisible in the dark-field image. We conclude that some tumors which are invisible in x-ray absorption mammography might be detected in the x-ray dark-field image at tolerable dose levels.

Noise in x‐ray grating‐based phase‐contrast imaging

PURPOSE Grating-based x-ray phase-contrast imaging is a fast developing new modality not only for medical imaging, but as well for other fields such as material sciences. While these many possible applications arise, the knowledge of the noise behavior is essential. METHODS In this work, the authors used a least squares fitting algorithm to calculate the noise behavior of the three quantities absorption, differential phase, and dark-field image. Further, the calculated error formula of the differential phase image was verified by measurements. Therefore, a Talbot interferometer was setup, using a microfocus x-ray tube as source and a Timepix detector for photon counting. Additionally, simulations regarding this topic were performed. RESULTS It turned out that the variance of the reconstructed phase is only dependent of the total number of photons used to generate the phase image and the visibility of the experimental setup. These results could be evaluated in measurements as well as in simulations. Furthermore, the correlation between absorption and dark-field image was calculated. CONCLUSIONS These results provide the understanding of the noise characteristics of grating-based phase-contrast imaging and will help to improve image quality.

Grating-based darkfield imaging of human breast tissue

Mastectomy specimens were investigated using a Talbot-Lau X-ray imaging set-up. Significant structures in the darkfield were observed, which revealed considerably higher contrast than those observed in digital mammography. Comparison with the histomorphometric image proofs that the darkfield signal correlates with a tumor region containing small calcification grains of 3 to 30μm size.

Analytical model for event reconstruction in coplanar grid CdZnTe detectors

Coplanar-grid (CPG) particle detectors were designed for materials such as CdZnTe (CZT) in which charge carriers of only one sign have acceptable transport properties. The presence of two independent anode signals allows for a reconstruction of deposited energy based on the difference between the two signals, and a reconstruction of the interaction depth based on the ratio of the amplitudes of the sum and difference of the signals. Energy resolution is greatly improved by modifying the difference signal with an empirically determined weighting factor to correct for the effects of electron trapping. This paper introduces a modified interaction depth reconstruction formula which corrects for electron trapping utilizing the same weighting factor used for energy reconstruction. The improvement of this depth reconstruction over simpler formulas is demonstrated. Further corrections due to the contribution of hole transport to the signals are discussed.

Generalised adjoint simulation of induced signals in semiconductor X-ray pixel detectors

The measurement quality of spatial and spectral information in an X-ray photon field is crucial to modern X-ray imaging. Direct converting semiconductor detectors with high spatial resolution and the ability to count single interacting photons, offer the possibility to obtain additional information about the spectral distribution of the X-ray photons. For being able to measure this quantity, the signal shape of the analog signal arriving at the pixel electronic input has to be well understood. The induced signals can be calculated using Ramos formulation. For gaining a complete understanding of the signal shape for different interaction points, the adjoint formulation can be applied to the problem, but up to now all considerations were restricted to sensor layers with undoped material. This is no limitation in the case of silicon, but for materials like CdTe, the doping changes the electric field significantly and therefore the signal shape and timing heavily depend on the material constants. The 3D simulation of the sensor layer of a direct converting semiconductor detector has been carried out with the finite element program COMSOL Multiphysics. The adjoint formulation has been corrected for the doping of the sensor layer. The signals arriving at the input of the electronics have been simulated and can be used for designing the pixel electronics of future detectors.

Low energy dosimetry with photon counting pixel detectors such as Medipix

The use of a semiconductor photon counting pixel detector like the Medipix detector [1] [2] in a dosimeter offers the possibility to take the photon energy dependence of the personal dose equivalents especially in the low energy range below 50 keV into account. Furthermore the measuring range can be extended down to low photon energies of about 10 keV. In this contribution we restrict our considerations to the medical diagnostic energy range from 10 to 150 keV. Due to the fact that the sensitive area of the Medipix detector is relatively large with (1.41 cm)2 while the sensitive area of one pixel is small with (55 mum)2, it is able to measure very low dose rates with high statistical precision while still processing each photon even at high dose rates. In this contribution we explain a method to determine personal dose equivalents from photon counted data, present measurement results of the air-kerma for different X-ray qualities and show simulation results of the performance of a dosimeter based on a hybrid photon counting pixel detector. We outline the advantages and perspectives of using a photon counting pixel detector in a dosimeter.

Pulse-shape discrimination of surface events in CdZnTe detectors for the COBRA experiment

Abstract Events near the cathode and anode surfaces of a coplanar grid CdZnTe detector are identifiable by means of the interaction depth information encoded in the signal amplitudes. However, the amplitudes cannot be used to identify events near the lateral surfaces. In this paper a method is described to identify lateral surface events by means of their pulse shapes. Such identification allows for discrimination of surface alpha particle interactions from more penetrating forms of radiation, which is particularly important for rare event searches. The effectiveness of the presented technique in suppressing backgrounds due to alpha contamination in the search for neutrinoless double beta decay with the COBRA experiment is demonstrated.

Low Energy Dosimetry With Photon Counting Pixel Detectors Such as Medipix

Hybrid semiconductor photon counting pixel detectors like the Medipix detector have several advantages for an use in X-ray dosimetry. The noiseless photon counting principle allows to monitor low photon energies down to 3.5 keV. Due to the small pixel size (55 mum in case of Medipix2) dosimetry at very high dose rates is possible still processing each photon individually. The large amount of pixels in combination with the possible thickness of the sensor layer enables dosimetry at very low dose rates. A method has been developed to determine personal dose equivalents from the number of counts in energy deposition intervals measured with a semiconductor photon counting pixel detector, despite the strong influence of charge sharing effects among pixels. We tested the method experimentally by reconstructing the air kerma free in air for different qualities of X-radiation in the energy range below 150 keV with an accuracy better than 4%. We show that the response of a dosimeter based on a hybrid photon counting pixel detector can fulfill the IEC type testing requirements. The statistical precision is high due to the thickness and the large area of the sensor layer. We estimate that a dosimeter based on the Medipix detector will be able to cope with dose rates of more than approximately 57 Sv/h for mathdot Hp (0.07) or 19 Sv/h for mathdot Hp(10) . We outline the advantages and perspectives of using this kind of detector in a dosimeter in comparison to standard active personal dosimeters.

Phase-unwrapping of differential phase-contrast data using attenuation information

Phase-contrast imaging approaches suffer from a severe problem which is known in Magnetic Resonance Imaging (MRI) and Synthetic Aperture Radar (SAR) as phase-wrapping. This work focuses on an unwrapping solution for the grating based phase-contrast interferometer with X-rays. The approach delivers three types of information about the x-rayed object - the absorption, differential phase-contrast and dark-field information whereas the observed differential phase values are physically limited to the interval (-π, π]; values higher or lower than the interval borders are mapped (wrapped) back into it. In contrast to existing phase-unwrapping algorithms for MRI and SAR the presented algorithm uses the absorption image as additional information to identify and correct phase-wrapped values. The idea of the unwrapping algorithm is based on the observation that at locations with phase-wrapped values the contrast in the absorption image is high and the behavior of the gradient is similar to the real (unwrapped) phase values. This can be expressed as a cost function which has to be minimized by an integer optimizer. Applied on simulated and real datasets showed that 95.6% of phase-wraps were correctly unwrapped. Based on the results we conclude that it is possible to use the absorption information in order to identify and correct phase-wrapped values.

Increasing the darkfield contrast-to-noise ratio using a deconvolution-based information retrieval algorithm in X-ray grating-based phase-contrast imaging

A novel information retrieval algorithm for X-ray grating-based phase-contrast imaging based on the deconvolution of the object and the reference phase stepping curve (PSC) as proposed by Modregger et al. was investigated in this paper. We applied the method for the first time on data obtained with a polychromatic spectrum and compared the results to those, received by applying the commonly used method, based on a Fourier analysis. We confirmed the expectation, that both methods deliver the same results for the absorption and the differential phase image. For the darkfield image, a mean contrast-to-noise ratio (CNR) increase by a factor of 1.17 using the new method was found. Furthermore, the dose saving potential was estimated for the deconvolution method experimentally. It is found, that for the conventional method a dose which is higher by a factor of 1.66 is needed to obtain a similar CNR value compared to the novel method. A further analysis of the data revealed, that the improvement in CNR and dose efficiency is due to the superior background noise properties of the deconvolution method, but at the cost of comparability between measurements at different applied dose values, as the mean value becomes dependent on the photon statistics used.