<p dir="ltr">X-ray computed tomography (CT) based on photon counting detectors (PCD) extends standard CT by counting detected photons in multiple energy bins. PCD data can be used to increase the contrast-to-noise ratio (CNR), increase spatial resolution, reduce radiation dose, reduce injected contrast dose, and compute a material decomposition using a specified set of basis materials [1]. Current commercial and prototype clinical photon counting CT systems utilize PCD-CT reconstruction methods that either reconstruct from each spectral bin separately, or first create an estimate of a material sinogram using a specified set of basis materials and then reconstruct from these material sinograms. However, existing methods are not able to utilize simultaneously and in a modular fashion both the measured spectral information and advanced prior models in order to produce a material decomposition. </p><p dir="ltr">We describe an efficient, modular framework for PCD-based CT reconstruction and material decomposition using Multi-Agent Consensus Equilibrium (MACE). Portions of this dissertation appear in [2]. Our method employs a detector proximal map or agent that uses PCD measurements to update an estimate of the path length sinogram. We also create a prior agent in the form of a sinogram denoiser that enforces both physical and empirical knowledge about the material-decomposed sinogram. The sinogram reconstruction is computed using the MACE algorithm, which finds an equilibrium solution between the two agents, and the final image is reconstructed from the estimated sinogram. Importantly, the modularity of our method allows the two agents to be designed, implemented, and optimized independently. Our results on simulated data show a substantial (2-3 times) noise reduction vs conventional maximum likelihood reconstruction when applied to a phantom used to evaluate low contrast detectability. Our results with measured data show an even higher reduction (2-12 times) in noise standard deviation. Lastly, we demonstrate our method on a Lungman phantom that more realistically represents the human body. </p>

La quête du détecteur parfait, sans bruit, capable de détecter des photons uniques dans le visible et l’infrarouge, et ultimement de déterminer leur énergie est le graal de la détection. Pour arriver à ce but, de nombreux scientifiques ont développé des dispositifs depuis plusieurs dizaines d’années, et les astronomes ont toujours été à la pointe en ce domaine. En ce sens les imageurs à amplification semblent être la voie la plus rapide et la plus prometteuse pour atteindre ce but ultime. Ainsi après un bref historique de l’état de l’art sont exposés les systèmes à comptage de photons (IPCS) développés au LAM, qui ont été utilisés sur les télescopes ESO 3m60, OHP 1m93 ou encore WHT 4m20. Sont ensuite abordés les dispositifs imageurs intégrés à amplification comme les EMCCD (Electron multiplying charge coupled devices) dans le visible, avec quelques exemples de leur utilisation en astronomie. C’est la technologie qui, appliquée aux senseurs de front d’onde, aura permis conjointement à d’autres développements l’avènement des optiques adaptatives extrêmes comme celle de l’instrument VLT-SPHERE ou encore de SUBARU-SCExAO. Pour finir les e-APD (electron initiated avalanche photodiode) dans l’infrarouge seront abordés. Les e-APD ont cette propriété très intéressante d’être des amplificateurs quasi parfaits, et ont une capacité à détecter l’énergie des photons, des propriétés qui seront développées et analysées. Nous finirons par les perspectives et les progrès que nous sommes en droit d’attendre dans les prochaines années. / The quest for the perfect, noiseless detector, capable of detecting unique photons in the visible and infrared, and ultimately determining their energy is the grail of detection. To achieve this goal, many scientists have developed devices for several decades, and astronomers have always been at the forefront in this area. In this sense amplification imagers seem to be the fastest and most promising way to achieve this ultimate goal. Thus, after a brief history of the state of the art are exposed the photon counting systems (IPCS) developed at LAM, which were used on ESO telescopes 3m60, OHP 1m93 or WHT 4m20. Imaging integrated imaging devices such as Electron Multiplying Charge Coupled Devices (EMCCDs) are then discussed in the visible, with some examples of their use in astronomy. It is the technology that, applied to the wavefront sensors, has jointly enabled other developments the advent of extreme adaptive optics such as the VLT-SPHERE or SUBARU-SCExAO. To finish the e-APD (electron-induced avalanche photodiode) in the infrared will be discussed. E-APDs have this very interesting property of being almost perfect amplifiers, and have an ability to detect photon energy, properties that will be developed and analyzed. We will end up with the prospects and the progress that we are entitled to expect in the coming years.

<p>Medical X-ray imaging is an important tool in diagnostic radiology. The ionising-radiation dose to the patient is justified by the clinical benefit of the examination. Nonetheless, detectors that operate at even lower doses and provide more information to the radiologist are desired. A hybrid pixel detector has the potential to provide a leap in detector technology as it incorporates a more advanced signal-processing capability than currently used detectors.</p><p>The DIXI digital detector is a hybrid pixel detector developed for X-ray imaging. It consists of a readout chip and a semiconductor sensor. The division in two parts makes it possible to optimise each part individually. The detector is divided into square pixels with a size of 270 x 270 μm2. DIXI has the ability to count single photons and every readout pixel has two embedded counters to allow the acquisition of two images close in time. A discriminator enables the selection of photons with energies above a preset threshold level.</p><p>The readout chip Angie has been developed and its performance has been evaluated in terms of noise, threshold variation and capability to perform energy weighted counting. Silicon sensors have been fabricated, and a control system for DIXI has been designed and built. An electroless process for deposition of Ni/Au bumps on the chip and sensor has been optimised as a preparation for the assembly of a complete detector, which is being assembled by flip-chip bonding using anisotropic conductive film.</p><p>A simulation library for the DIXI detector has been set up and results on the image quality are reported for different exposures and working conditions. A theoretical model for hybrid pixel detectors based on the cascaded linear system theory has been developed. The model can be used to investigate and optimise the detector for different detector configurations and operating conditions.</p>

No description available.

Mammography screening aims to reduce the number of breast cancer deaths by early detection of the disease, which is one of the leading causes of deaths for middle aged women in the western world. The risk from the x-ray radiation in mammography is relatively low but still a factor in the benefit-risk ratio of screening. The characterization and optimization of a digital mammography system is presented in this thesis. The investigated system is shown to be highly dose efficient by employing a photon counting detector in a scanning multislit geometry. A novel automatic exposure control (AEC) is proposed and validated in clinical practise. The AEC uses the leading detector edge to measure the transmission of the breast. The exposure is modulated by altering the scan velocity during the scan. A W-Al anode-filter combination is proposed. The characterization of the photon counting detector is performed using the detective quantum efficiency. The effect of the photon counting detector and the multislit geometry on the measurement method is studied in detail. It is shown that the detector has a zero-frequency DQE of over 70\% and that it is quantum limited even at very low exposures. Efficient rejection of image-degrading secondary radiation is fundamental for a dose efficient system. The efficiency of the scatter rejection techniques currently used are quantified and compared to the multislit geometry. A system performance metric with its foundation in statistical decision theory is discussed. It is argued that a photon counting multislit system can operate at approximately half the dose compared to several other digital mammography techniques. / QC 20100825

Medical X-ray imaging is an important tool in diagnostic radiology. The ionising-radiation dose to the patient is justified by the clinical benefit of the examination. Nonetheless, detectors that operate at even lower doses and provide more information to the radiologist are desired. A hybrid pixel detector has the potential to provide a leap in detector technology as it incorporates a more advanced signal-processing capability than currently used detectors. The DIXI digital detector is a hybrid pixel detector developed for X-ray imaging. It consists of a readout chip and a semiconductor sensor. The division in two parts makes it possible to optimise each part individually. The detector is divided into square pixels with a size of 270 x 270 μm2. DIXI has the ability to count single photons and every readout pixel has two embedded counters to allow the acquisition of two images close in time. A discriminator enables the selection of photons with energies above a preset threshold level. The readout chip Angie has been developed and its performance has been evaluated in terms of noise, threshold variation and capability to perform energy weighted counting. Silicon sensors have been fabricated, and a control system for DIXI has been designed and built. An electroless process for deposition of Ni/Au bumps on the chip and sensor has been optimised as a preparation for the assembly of a complete detector, which is being assembled by flip-chip bonding using anisotropic conductive film. A simulation library for the DIXI detector has been set up and results on the image quality are reported for different exposures and working conditions. A theoretical model for hybrid pixel detectors based on the cascaded linear system theory has been developed. The model can be used to investigate and optimise the detector for different detector configurations and operating conditions.

In this thesis, the development and implementation of fluorescence lifetime imaging microscopy that integrates time correlated single photon counting (TCSPC) and a confocal microscope will be described. The TCSPC method has high detection efficiency, with a time resolution limited only by the transit time spread of the detector, and directly delivers the decay functions in the time domain. TCSPC can also be used to obtain images that indicate the fluorescence resonance energy transfer (FRET) effect between critical fluorophores, an important method distinguish the difference between binding and co-localization. Estimation of distances between RET fluorophore pairs can also be established. Additionally, the effects of ion concentration, oxygen concentration, pH value, ..etc. can also be revealed.

No description available.

Advances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs. / text

The goal of this research is to contribute to the development of MARS spectral CT and to demonstrate the feasibility of molecular imaging using the technology. MARS is a newly developed micro CT scanner, incorporating the latest spectroscopic Medipix photon counting detector. I show that the scanner can identify both drug markers and stenosis of atherosclerosis labelled with non-toxic nanoparticles. I also show that spectral computed tomography using Medipix x-ray detectors can give quantitative measurements of concentrations of gold nanoparticles in phantoms, mice and excised atheroma. The characterisation of the Medipix2 assemblies with Si and CdTe x-ray sensors using poly-energetic x-ray sources has been performed. I measure the inhomogeneities within the sensors; individual pixel sensitivity response; and their saturation effects at higher photon fluxes. The effects of charge sharing on the performance of Medipix2 have been assessed, showing that it compromises energy resolution much more than spatial resolution. I have commissioned several MARS scanners incorporating several different Medipix2 and Medipix3 cameras. After the characterization of x-ray detectors and the geometrical assessment of MARS-CT, spectral CT data has been acquired, using x-ray energies that are appropriate for human imaging. The outcome shows that MARS scanner has the ability to discriminate among low atomic number materials, and from various concentrations of heavy atoms. This new imaging modality, used with functionalized gold nanoparticles, gives a new tool to assess plaque vulnerability. I demonstrated this by using gold nanoparticles, attached to antibodies, which targeted to thrombotic events in excised plaque. Likewise, the imaging modality can be used to track drugs labelled with any heavy atoms to assess how much drug gets into a target organ. Thus the methodology could be used to accelerate development of new drug treatments for cancers and inflammatory diseases.