Current research / employment

Short abstract about IMPRS Phd project:

In my thesis a method was developed and successfully tested to reconstruct the concentration and distribution of elements or basis materials in an object through energy resolving detection of X-rays. This method of material reconstruction relies on the difference in the energy dependence of the attenuation coefficients of the basis materials. The photon counting pixel detector Medipix2 was used.

To achive this goal, the energy resolving properties of the Medipix2 were evaluated. A monochromator was set up and the energy response of the detector system to monochromatic irradiation was studied. The effects dominating the energy response behaviour of the detector were successfully modeled in a Monte Carlo simulation which could reproduce the measurements. A complete set of energy response functions of the detector could be simulated in the relevant energy range.

The knowlege of these energy response functions of the detector allowed the development of the spectrum reconstruction. This technique provides the actual spectrum impinging on the detector for every pixel from the distorted measured spectrum through a deconvolution. Two different implemetations of the technique were compared. Multiple X-ray spectra were measured with the Medipix2 and reconstructed successfully with this method.

Different variants of the material recontrution technique were developed. One approach includes an initial spectrum recontruction and a subsequent algebraic material reconstruction step, an other is to calculate the likelihood of a particular combination of basis material concentrations to yield a given measurement and perform a maximum likelihood estimation.

Measurements in a radiography setup with a phantom containing PMMA, aluminum and iodine were done using the Medipix2 detector at different energy thresholds. Material reconstruction was successfully applied with the basis materials PMMA, aluminum and iodine. The resulting material specific images were shown. The Method was transferred to computed tomography. Monte Carlo simulation were carried out to investigate the quantitative features of the method and the inuence of scattered radiation to the method. A CT scan of a mouse with contrast agent iodine was performed. Material Reconstruction was conducted with the basis material water and iodine. The resulting material specific images show that the material reconstruction works very well. The information obtained from material reconstruction can be used to identify the contrast agent and allow to assemble a combined image where contrast agent and bone can be distinguished by color.

Furthermore, a method to enlarge the field of view in computed tomography was developed. It is an extension of the sinogram reection technique and is particularly suitable for two or more small detectors. Measurements and Monte Carlo simulations were done to verify the method.

Current employment: Fraunhofer Gesellschaft

Publications M. Firsching, P. Takoukam Talla, T. Michel, G. Anton, Material resolving X-ray imaging using spectrum reconstruction with Medipix2, NIM A, 591, 19-23, (2008)

T. Michel, G. Anton, M. Böhnel, J. Durst, M. Firsching, A. Korn, B. Kreisler, A. Loehr, F. Nachtrab, D. Niederlöhner, F. Sukowski, P. Takoukam Talla, A fundamental method to determine the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) for a photon counting pixel detector, NIM A, 568(2), 799-802, (2006)