Single Photon Counting X Ray Micro Imaging of Biological Samples
Frallicciardi, Paola Maria (2009) Single Photon Counting X Ray Micro Imaging of Biological Samples. [Tesi di dottorato] (Inedito)
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In this thesis, performed in the framework of the interdisciplinary research of the Italian National Institute of Physics (INFN) and within a collaboration with Prof. S. Pospisil at Czech Technical University in Prague, Institute of Experimental and Applied Physics, we compared the experimental technique of Single Photon Counting (SPC) imaging to the charge integrating Flat Panel (FP) detector imaging, for X-ray biomedical imaging applications. In particular, we investigated the application of SPC detector for the X ray micro-imaging and X-ray volumetric Computed Tomography (CT) technique. The motivation for such a research arises from the potential advantages of the single photon counting technology. In fact, this detection modality allows to have an efficient suppression of the electronic noise, scatter radiation rejection and immunity for afterglow effect of scintillator-based detectors, thanks to a read-out scheme able to discriminate photons with energy above a chosen threshold. This means that, during the exposure, the signal increases but not the noise, leading to excellent values of the image quality parameters such as the Signal-to-Noise Ratio (SNR) and the Contrast-to-Noise Ratio (CNR). In SPC imaging, each interacting photon is counted as one single event, independently of its energy, so that soft X-rays are equally weighted compared to the harder ones. This results into a high Contrast (C) also for low attenuating objects, such as soft tissues in an organism or small biological samples. On the contrary, charge integrating detectors (and FP detector among this class of devices) integrate both signal and noise, and high energy photons bring a larger weight than low energy ones. These high energy photons, however, contribute less to the detectability (SNR) and to the visibility (C) of low contrast samples, since material attenuation generally decreases with increasing energy. Theoretical models and computer simulations   show that energy sensitive detectors and SPC detectors as particular representatives of this class of devices may perform better than charge integrating systems in terms of SNR, for X-ray 2D and 3D imaging. The significance of such result is also related to the possibility of a high image quality for a satisfactory visualization of the sample with a lower radiation dose, because the same image SNR can be achieved with a lower exposure level. The above described aspects of the SPC technology are most important in medical imaging, where the patient absorbed dose and the low contrast image quality for the soft tissues detection are the fundamental parameters to take into account. In fact, the harder task in X-rays imaging is to visualize small and low-attenuating structures in an organism, using X-rays of energies neither too low (because they result in a high absorbed dose) or too high (because they result in loss of contrast for softer tissues). SPC detectors may provide an accurate representation of the beam hardening effect with compared to charge integrating devices . In fact, charge integration decreases the relative weight of the low energy part of the spectrum thus giving less importance to the loss of the soft photons as the beam is transmitted through the sample. On the other hand, SPC devices assign the same weight to all the detected photons, leading to a higher but more correct expression of the beam hardening effect. The aim of this thesis is to experimentally demonstrate the feasibility of planar, real time and tomographic X-ray imaging utilizing an SPC detector in the field of Medical Physics. Since the use of this technology is regarded as an alternative to the more commonly employed charge integrating systems, a comparison with an FP detector, in terms of image quality parameters (SNR, C, CNR) evaluation has also been done. The thesis is organized as follows. In the first chapter, the basic concepts of the SPC technology are described, with a particular attention to the analysis of advantages and drawbacks of its use in Medical Imaging. The CT technique and the more common reconstruction algorithms have also been described in their general features. Finally, an overview of the state of the art of the SPC application in CT is presented. In the second chapter, the experimental systems employed for the experimental part of this work are described, with particular attention to the SPC detector used: the Medipx2 SPC hybrid pixel detector, developed within the Medipix2 European Collaboration (designed at CERN, Geneva, Switzerland) to which University & INFN Napoli belong . The characterization of the measurements setups is presented. Moreover, two kinds of detector pixels efficiency equalizations have been described: the standard Flat Field Correction (FFC) and the Signal to Thickness Calibration (STC)  . In the third chapter are reported the experimental tests and images relative to the application of the Medipix2 SPC detector for planar, tomographic and real-time X-ray imaging of small biological samples. Then, its performance in terms of image quality parameters has been compared to a commercially available FP charge integrating detector used in the same experimental conditions. Moreover, two kinds of detector pixels efficiency equalizations have been compared in terms of image parameters, on images of both phantoms and biological samples.
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