J. Giersch

ROSI: an object-oriented and parallel-computing Monte Carlo simulation for X-ray imaging

Abstract In the field of X-ray imaging, Monte Carlo simulation is an important tool. It gives the possibility of understanding experimental results and it allows the construction of virtual imaging setups with predictions of their quality. For these reasons, we developed the Roentgen Simulation (ROSI) which is based on the object-oriented C++ class library GISMO. The interaction algorithms are based on the established EGS4-code and its current LSCAT-extension. ROSI introduces random variables for modelling physical parameters by a given random distribution, e.g. the source position or the direction and energy of the photons to be emitted. It is possible to run ROSI in parallel on a local computer network (Beowulf cluster) to obtain simulation data in shorter time. Finally, it has an easy-to-use interface. We will present the concept of ROSI and demonstrate its flexibility by an example.

Modulation transfer function of a selenium-based digital mammography system

Digital mammography systems with detectors based on amorphous selenium exhibit outstanding spatial resolution characterized by the modulation transfer function (MTF). We measured the detector behavior of the Siemens Mammomat Novation/sup DR/ with 70 /spl mu/m pixel pitch and compared the results to analytical evaluations based on Monte Carlo simulations. Experimentally, the MTF of the mammography system is obtained from the images of a lead bar pattern or an edge phantom using different X-ray spectra. The simulations take into account all relevant X-ray interactions in the selenium layer. The resulting line-spread function is transformed to the MTF. Even at the Nyquist frequency (i.e., 7.14 mm/sup -1/), the measured MTF is well above 45% and thus fairly close to its theoretical limit (64%). The MTF shows a few percentage points of low-frequency drop, which can be explained in part by the presence of scattered radiation. The simulations allow the features observed to be explained. The detector investigated provides excellent spatial resolution and appears well suited for high-end mammography.

Threshold characterisation of the Medipix1 chip

Abstract The Medipix1 chip is a hybrid pixel detector working in photon counting mode and has the ability to put a discriminating threshold on each photon signal. To increase the image quality the threshold voltage can be electronically fine tuned with a 3 bit threshold adjust for each pixel. This electronic adjustment can only equalise the inhomogeneities of the electronics, not those of the physical frontend (conversion material and bump bonds). The remaining noise of the threshold has an obvious impact on the image quality. To optimise the uniformity over the whole chip we did threshold scans with a cadmium X-ray source analysing every pixel. With those scans we were able to create a bit mask including the physical frontend. Comparing the results of the different methods we can judge the quality of the electronically generated mask and draw conclusions about the properties of the physical frontend. Furthermore, simulations have been carried out for comparison with the measured data.

Absorbers for medical x-ray detectors with optimum spatial resolution: a simulation study

The requirements for medical X-ray detectors tend towards higher spatial resolution, especially for mammography. Therefore, we have investigated common absorber materials with respect to the possible intrinsic limitations of their spatial resolution. Primary interaction of an incident X-ray quantum is followed by a series of processes: Rayleigh scattering, Compton effect, or the generation of fluorescence photons and subsequent electrons. Lateral diffusion of carriers relative to their drift towards the electrodes also broadens the point-spread function. One consequence is that the spatial resolution of the detector, expressed in terms of the modulation transfer function (MTF), is reduced. Monte Carlo simulations have been carried out for spectra with tube voltages of 28-120 kV using the program ROSI (Roentgen Simulation) based on the well-established EGS4 algorithm. The lateral distribution of deposited energy has been calculated in typical materials such as Se, CdTe, HgI2, and PbI2 and used to determine the line spread function. The complex absorption process is found to determine the spatial resolution of the detector considerably. The spectrum at energies closely above the K-edge of the absorber material tends to result in a reduced MTF. At energies above 50 keV, electron energy loss increasingly reduces spatial resolution in the high frequency range. The influence of fluorescence is strongest in the 5-20 lp/mm range. If a very high spatial resolution is required, a well-adapted semiconductor should be applied.

Large-scale images taken with the Medipix1 chip

Abstract Two methods to acquire large-scale X-ray images with high spatial resolution using the Medipix1 detector chip are presented. The Medipix1 chip is a pixelated, photon counting X-ray detector which was developed within the framework of the Medipix Collaboration. 1 Since the lateral dimensions of the Medipix1 chip are only 10.88 mm ×10.88 mm , larger images are acquired by tiling, which also helps to minimise the influence of defective pixels on the image quality. Measurements on the modulation transfer function (MTF) and the detective quantum efficiency (DQE) of the detector were performed and first results are given. The MTF of the Medipix1 chip is quite close to the theoretical limit. The DQE is lower than expected and has to be investigated further.

Design and material considerations for a sensitive scintillation detector as absorber for a resolution Compton-Camera

The primary energy of the photons and the angle under which they are detected by the absorption detector after the Compton scatter process in the scatter detector are the main parameters of a Compton-Camera determining the demands on the scatter- and the absorption detector. The design of a high resolution Compton-Camera results in using gamma energies higher than 400 keV and small scatter angles. Due to the forward scattering a very fast trigger signal is needed for coincidence measurements. In order to get a good efficiency in the absorption detector at such high energies a material with good stopping power is much-needed. For the design and the estimation of the performance of an absorption detector the detailed information of interaction branching ratios and radii in which the energy is deposited during an interaction is mandatory. Several simulations with ROSI have been done in order to optimize and estimate the expected performance of a fast 3D-position sensitive scintillation detector.