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A quantitative theory for the contrast-to-noise ratio (CNR) in differential phase contrast imaging (DPCI) is proposed and compared to that of images derived from classical absorption contrast imaging (ACI). Most prominently, the CNR for DPCI contains the reciprocal of thespatial wavelength to be imaged, the fringe visibility, and a tunable factor dependent on the system geometry. DPCI is thus potentiallybeneficial especially for the imaging of small object structures. We demonstrate CNR calculations for mammography, finding optimal imaging energies between 15 and 22 keV for ACI, and between 20 and 40 keV for DPCI.

Abstract: A pixelated X-ray semiconductor detector (=“direct converter”) is studied which contains an inhomogeneous electric field parallel to the depth axis caused by different concentrations of p- or n-doping. The X-ray energy deposition and charge movement within the detector is modeled in Monte-Carlo simulations which give access to astatistical analysis of electron drift times and current pulse widths for various degrees of static polarization. Integral charges induced on the pixel electrodes are evaluated and put to histograms of spectral detector responses and pulse height spectra (considering energy measurements before and after electronically pulse shaping, respectively). For n-doped semiconductors, the detector performance degrades due to pulse broadening. In contrast, a moderate p-doping can improve the detector performance by generating sharper electron pulses, as long as the detector is not limited by dynamical polarization.Conclusions: We performed Monte-Carlo simulations of energy deposition and charge movement within pixelated photon counting direct conversion detectors made of doped semiconductors of different acceptoror donor concentrations. Induced currents were statistically evaluated with the help of histograms of pulse width and total integral charge (represented before consideration of pulse shaper electronics in spectral responses, and after consideration of pulse shaper electronics in pulse height spectra, respectively). The electrical field close to anode pixels was identified as the main quantity defining the pulse characteristics. For n-doped semiconductors, a weaker electric field near the pixel anodes and longer total electron drift timesare seen, which results in broader pulse widths and a degradation of the spectral responses (by enhanced charge sharing) and pulse height spectra. In contrast, for p-doped semiconductors, a strengthenedelectric field near the anodes and an, on average, reduced total drift time is seen, which results in shorter pulse widths. For moderatep-doping concentrations n_A ≤ 0.9•n_max, an improvement of the spectral response (less charge sharing) and pulse height spectrumis seen. For p-doping concentrations close to the limit still allowing full depletion, n_A > 0.9•n_max, the spectral response und pulseheight spectrum degrade again due to charges which experience long drift times when created in a region with weakened electric field near the cathode. We conclude that using moderately p-doped semiconductor material, n_A ≈ 0.5•n_max, improves detector performance aslong as the detector is not limited by dynamical polarization. In the latter case, it has to be noted that a p-doped semiconductor reduces the maximum count rate at which catastrophic dynamical polarization occurs.

Today’s state-of the art clinical computed tomography (CT) scannersexclusively use energy-integrating, scintillation detector technology, despite the fact that a part of the information carried by the transmitted x-ray photons is lost during the detection process. Roomtemperature semiconductors, like CdTe or CZT, operated in energysensitive photon-counting mode provide information about the energy of every single x-ray detection event. This capability allows novel, promising approaches to selectively image abnormal tissue types like cancerous tissue or atherosclerotic plaque with the CT modality. In thisarticle we report on recent dual K-edge imaging results obtained inthe domain of pre-clinical, energy-sensitive photon counting CT. Inthis approach, the tuning of threshold levels in the detector electronics to the K-edge energy in the attenuation of contrast agents (CA) offers highly specific, quantitative imaging of the distributionof the CA on top of the conventional, morphological image information. The combination of the high specificity of the K-edge imaging technique together with the powerful tool of targeting specific diseases in the human body by dedicated contrast materials might enrich theCT modality with capabilities of functional imaging known from thenuclear medicine imaging modalities, e.g., positron-emission-tomography but with the additional advantage of high spatial and temporal resolution. We also discuss briefly the technological difficulties tobe overcome when translating the technique to human CT imaging andpresent the results of simulations indicating the feasibility of theKedge imaging of vulnerable plaque using targeted gold nano-particles as contrast materials. Our experiments in the pre-clinical domainshow that dual-K edge imaging of iodine and gold based CAs is feasible while our simulations for the imaging of gold CAs in the clinical case support the future possibility of translating the technique to human imaging.