Dissertations / Theses on the topic 'Coded Aperture Imaging'

This thesis studies the coded aperture camera, a device consisting of a conventional camera with a modified aperture mask, that enables the recovery of both depth map and all-in-focus image from a single 2D input image. Key contributions of this work are the modeling of the statistics of natural images and the design of efficient blur identification methods in a Bayesian framework. Two cases are distinguished: 1) when the aperture can be decomposed in a small set of identical holes, and 2) when the aperture has a more general configuration. In the first case, the formulation of the problem incorporates priors about the statistical variation of the texture to avoid ambiguities in the solution. This allows to bypass the recovery of the sharp image and concentrate only on estimating depth. In the second case, the depth reconstruction is addressed via convolutions with a bank of linear filters. Key advantages over competing methods are the higher numerical stability and the ability to deal with large blur. The all-in-focus image can then be recovered by using a deconvolution step with the estimated depth map. Furthermore, for the purpose of depth estimation alone, the proposed algorithm does not require information about the mask in use. The comparison with existing algorithms in the literature shows that the proposed methods achieve state-of-the-art performance. This solution is also extended for the first time to images affected by both defocus and motion blur and, finally, to video sequences with moving and deformable objects.

It is well known that a translating mask can optically encode low-resolution measurements from which higher resolution images can be computationally reconstructed. We experimentally demonstrate that this principle can be used to achieve substantial increase in image resolution compared to the size of the focal plane array (FPA). Specifically, we describe a scalable architecture with a translating mask (also referred to as a coded aperture) that achieves eightfold resolution improvement (or 64: 1 increase in the number of pixels compared to the number of focal plane detector elements). The imaging architecture is described in terms of general design parameters (such as field of view and angular resolution, dimensions of the mask, and the detector and FPA sizes), and some of the underlying design trades are discussed. Experiments conducted with different mask patterns and reconstruction algorithms illustrate how these parameters affect the resolution of the reconstructed image. Initial experimental results also demonstrate that the architecture can directly support task-specific information sensing for detection and tracking, and that moving objects can be reconstructed separately from the stationary background using motion priors. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1996, and Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1996.
Includes bibliographical references (leaves 107-111).
by Li Zhang.
M.S.

This dissertation describes the development of a novel gamma-ray imaging system concept and presents results from Monte Carlo simulations of the new design. Current designs for large field-of-view gamma cameras suitable for homeland security applications implement either a coded aperture or a Compton scattering geometry to image a gamma-ray source. Both of these systems require large, expensive position-sensitive detectors in order to work effectively. By combining characteristics of both of these systems, a new design can be implemented that does not require such expensive detectors and that can be scaled down to a portable size. This new system has significant promise in homeland security, astronomy, botany and other fields, while future iterations may prove useful in medical imaging, other biological sciences and other areas, such as non-destructive testing. A proof-of-principle study of the new gamma-ray imaging system has been performed by Monte Carlo simulation. Various reconstruction methods have been explored and compared. General-Purpose Graphics-Processor-Unit (GPGPU) computation has also been incorporated. The resulting code is a primary design tool for exploring variables such as detector spacing, material selection and thickness and pixel geometry. The advancement of the system from a simple 1-dimensional simulation to a full 3-dimensional model is described. Methods of image reconstruction are discussed and results of simulations consisting of both a 4 x 4 and a 16 x 16 object space mesh have been presented. A discussion of the limitations and potential areas of further study is also presented.

Imaging in nuclear medicine involves the injection of a radioactive tracer into the body and subsequent detection of the radiation emanating from an organ of interest. Single-photon emission computed tomography (SPECT) is the branch of nuclear medicine that yields three-dimensional maps of the distribution of a tracer, most commonly as a series of two-dimensional slices. One major drawback to transaxial tomographic imaging in SPECT today is the rotation required of a gamma camera to collect the tomographic data set. Transaxial SPECT usually involves a large, single-crystal scintillation camera and an aperture (collimator) that together only satisfy a small portion of the spatial sampling requirements simultaneously. It would be very desirable to have a stationary data-collection apparatus that allows all spatial sampling in the data set to occur simultaneously. Aperture or detector motion (or both) is merely an inconvenience in most imaging situations where the patient is stationary. However, aperture or detector motion (or both) enormously complicate the prospect of tomograhically recording dynamic events, such as the beating heart, with radioactive pharmaceuticals. By substituting a set of small modular detectors for the large single-crystal detector, we can arrange the usable detector area in such a way as to collect all spatial samples simultaneously. The modular detectors allow for the possibility of using other types of stationary apertures. We demonstrate the capabilities of one such aperture, the pinhole array. The pinhole array is one of many kinds of collimators known as coded apertures. Coded apertures differ from conventional apertures in nuclear medicine in that they allow for overlapping projections of the object on the detector. Although overlapping projections is not a requirement when using pinhole arrays, there are potential benefits in terms of collection efficiency. There are also potential drawbacks in terms of the position uncertainty of emissions in the reconstruction object. The long-term goal of the research presented is dynamic SPECT imaging of the heart. The basic concepts and tasks involved in transaxial SPECT imaging with pinhole arrays are presented along with arguments for the combination of modular gamma cameras and pinhole arrays. We demonstrate by emulation two methods of tomographically imaging a stationary single object slice and present results for these two systems on object space grids of 10cm x 10cm and 20cm x 20cm.

Ultrasound is a well-established tool for medical imaging. It is non-invasive and relatively inexpensive, but the severe attenuation caused by propagation through tissue limits its effectiveness for deep imaging. In recent years, the ready availability of fast, inexpensive computer hardware has facilitated the adoption of signal coding and compression techniques to counteract the effects of attenuation. Despite widespread investigation of the topic, published opinions vary as to the relative suitability of discrete-phase-modulated and frequency-modulated (or continuous-phase-modulated) signals for ultrasonic imaging applications. This thesis compares the performance of discrete binary-phase coded pulses to that of frequency-modulated pulses at the higher imaging frequencies at which the effects of attenuation are most severe. The performance of linear and non-linear frequency modulated pulses with optimal side-lobe characteristics is compared to that of complementary binary-phase coded pulses by simulation and experiment. Binary-phase coded pulses are shown to be more robust to the affects of attenuation and non-ideal transducers. The comparatively poor performance of frequency-modulated pulses is explained in terms of the spectral characteristics of the signals and filters required to reduce side-lobes to levels acceptable for imaging purposes. In theory, complementary code sets like bi-phase Golay pairs offer optimum side-lobe performance at the expense of a reduction in frame rate. In practice, misalignment caused by motion in the medium can have a severe impact on imaging performance. A novel motioncompensated imaging algorithm designed to reduce the occurrence of motion artefacts and eliminate the reduction in frame-rate associated with complementary-coding is presented. This is initially applied to conventional sequential-scan B-mode imaging then adapted for use in synthetic aperture B-mode imaging. Simulation results are presented comparing the performance of the motion-compensated sequential-scan and synthetic aperture systems with that of simulated systems using uncoded and frequency-modulated excitation pulses.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2008.
Includes bibliographical references (p. 50).
Improvised explosive devices (IED) pose a very serious threat to civilians and military forces around the world, and new technologies must be developed for the early detection of these objects. Because of the high concentrations of low atomic number material such as nitrogen and hydrogen present in these explosives, x-ray backscattering provides a viable method of collecting information about these targets by analyzing their shape. Furthermore, a coded aperture used in conjunction with dynamic reconstruction algorithms offers high sensitivity and resolution even while the target is moving towards the detector. This paper describes a lab-based system that simulated a source-target-detector arrangement to be utilized in a radiation detecting vehicle in order to test dynamic reconstruction methods. Using a 225 kVp x-ray tube as the source, a medical CT-system camera fitted with a drill mask of 50% fill factor as the detector, and both radioisotope sources and low Z backscatter targets, images were acquired and reconstructed. The geometry of the experimental setup was optimized to reduce background noise from air scatter and environmental sources, as well as to prevent incident photons from directly reaching the detector from the x-ray tube. Measurements of a Co-60 point source and Co-57 area source with high activity generated high contrast images for which the shapes of the sources were clearly resolved. Acquisitions with varying target-detector distance of low Z materials, including a filled water jug and a four inch thick polyethylene arrow, produced lower contrast images in which the shapes were not as easily distinguished. The radioisotope tests were a proof of principle for dynamic reconstruction and the backscatter targets provided much insight on methods for improving the lab system, including the addition of steel behind the target, the narrowing of the detector energy window, and reassessment of the x-ray cone-beam.
by Tiffany Lee.
S.B.

Coded-aperture imaging without detector motion can be used to reconstruct three-dimensional radionuclide distributions in the context of nuclear medicine. This approach offers several advantages over the rotating gamma-ray camera systems presently employed in the clinic. These advantages include improved sensitivity, potentially better spatial resolution, and the capability of doing dynamic studies. There are two problems associated with the coded-aperture approach, however. First, the data is "multiplexed", which refers to the fact that many line integrals of the source distribution are combined together and not measured individually, so that information is lost. Second, the number of resolvable detector elements is typically an order of magnitude less than the number of object elements to be reconstructed, so that the reconstruction problem is underdetermined. Consequently, the reconstruction is not unique. By using various types of a priori information in forming the reconstruction, however, it is possible to augment the incomplete data set. Two algorithms are presented to reconstruct objects from their coded-image projections and various types of a priori information. The first, a Monte Carlo algorithm, is a flexible and computationally efficient approach using the a priori knowledge of positivity and nearest-neighbor correlation. This algorithm is used to qualitatively explore the effect of the data-taking geometry on reconstruction performance. The second algorithm is a linear estimator incorporating as a priori knowledge completely general first- and second-order statistical information about the object class to be reconstructed. The linear-estimator formalism also provides a minimum-variance expression for system optimization. This linear algorithm is used to explore the effects of correct and incorrect a priori information on reconstruction performance, and to quantitatively investigate reconstruction quality with respect to data-taking geometry.

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Electrical and Computer Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

The BAT Slew Survey (BATSS) is the first wide-field survey of the hard X-ray sky (15–150 keV) with a slewing coded aperture imaging telescope. Its fine time resolution, high sensitivity and large sky coverage make it particularly well-suited for detections of transient sources with variability timescales in the \(\sim 1 sec–1 hour\) range, such as Gamma-Ray Bursts (GRBs), flaring stars and Blazars. As implemented, BATSS observations are found to be consistently more sensitive than their BAT pointing-mode counterparts, by an average of 20% over the 10 sec–3 ksec exposure range, due to intrinsic systematic differences between them. The survey’s motivation, development and implementation are presented, including a description of the software and hardware infrastructure that made this effort possible. The analysis of BATSS science data concentrates on the results of the 4.8-year BATSS GRB survey, beginning with the discovery of GRB 070326 during its preliminary testing phase. A total of nineteen (19) GRBs were detected exclusively in BATSS slews over this period, making it the largest contribution to the Swift GRB catalog from all ground-based analysis. The timing and spectral properties of prompt emission from BATSS GRBs reveal their consistency with Swift long GRBs (L-GRBs), though with instances of GRBs with unusually soft spectra or X-Ray Flashes (XRFs), GRBs near the faint end of the fluence distribution accessible to Swift-BAT, and a probable short GRB with extended emission, all uncommon traits within the general Swift GRB population. In addition, the BATSS overall detection rate of 0.49 GRBs/day of instrument time is a significant increase (45%) above the BAT pointing detection rate. This result was confirmed by a GRB detection simulation model, which further showed the increased sky coverage of slews to be the dominant effect in enhancing GRB detection probabilities. A review of lessons learned is included, with specific proposals to broaden both the number and range of astrophysical sources found in future enhancements. The BATSS survey results provide solid empirical evidence in support of an all-slewing hard X-ray survey mission, a prospect that may be realized with the launch of the proposed MIRAX-HXI mission in 2017.
Physics

Il lavoro della presente tesi è stato svolto nell'ambito di un progetto avente come scopo principale lo sviluppo di una nuova tecnica di imaging per la ricostruzione di tracce di particelle in un rivelatore ad argon liquido. Si vuole sfruttare la luce di scintillazione emessa dall'argon per realizzare immagini di tali tracce. A questo fine, è necessario utilizzare un dispositivo di imaging, composto da un sistema ottico e da un sensore. Per quanto riguarda il sistema ottico, si è optato per una maschera ad apertura codificata, mentre come sensori si intende utilizzare matrici di silicon photomultiplier (SiPM) con un elevato numero di canali. In particolare, il lavoro di questa tesi si è focalizzato sulla caratterizzazione delle matrici di SiPM impiegate. Grazie a tale sistema, sono state effettuate le prime misure che hanno indicato la fattibilità di questa tecnica innovativa per la ricostruzione di immagini.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001.
Includes bibliographical references (p. [251]-255).
Coded Aperture Imaging is a technique originally developed for X-ray astronomy, where typical imaging problems are characterized by far-field geometry and an object made of point sources distributed over a mainly dark background. These conditions provide, respectively, the basis of artifact-free and high Signal-to-Noise Ratio (SNR) imaging. When the coded apertures successful in far-field problems are used in near-field geometry, images are affected by extensive artifacts. The classic remedy is to move away from the object until a far-field geometry is restored, but this is at the expense of counting efficiency and, thus, of the SNR of the images. It is shown in this thesis that the application to near-field of a technique originally developed to mitigate the effects of non-uniform background in far-field applications results in a considerable reduction of near-field artifacts. This result opens the way to the exploitation in near-field problems of the favorable SNR characteristics of coded apertures: images comparable to those provided by state-of-the-art imagers can be obtained in a shorter time or while administering a lower dose to patients. Further developments follow when the SNR increase is traded for better resolution at constant time and dose.
(cont.) The main focus of this work is on a coded aperture camera specifically designed for high-resolution single-photon planar imaging with a pre-existing gamma (Anger) camera. Original theoretical findings and the results of computer simulations led to an optimal coded aperture that was tested experimentally in phantom as well as in-vivo studies. Results include, but are not limited to, 1.66-mm-resolution images of 99mTc-labeled blood and bone agents in a mouse. The theoretical bases for extension to sub-millimeter resolution and higher-energy isotopes are also laid and a candidate aperture capable of 0.96-mm resolution proposed. Potential applications are in small-animal imaging, pediatric nuclear medicine and breast imaging, where increased resolution can result in earlier diagnosis of disease. The last Chapter of the thesis extends the ideas developed to the design of a coded aperture suitable for CAFNA (Coded Aperture Fast Neutron Analysis), a contraband detection technique that has been under development at MIT for a number of years.
by Roberto Accorsi.
Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering; and, Harvard-MIT Division of Health Sciences and Technology, 1998.
Includes bibliographical references (leaves 149-154).
by Li Zhang.
Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 117-118).
Standoff detection of improvised explosive devices (IEDs) is a continuing problem for the U.S. military. Current X-ray detection systems cannot detect explosives at distances above a few meters and with a source-detector system moving in relation to the target. The aim of this study is to determine the feasibility of a large-area, Coded-Aperture Imaging (CAI) system using X-Ray backscatter as the source of radiation. A moving source-detector system required development of a new reconstruction technique, dynamic reconstruction (DR), which continually back-projects detected events on an event-by-event basis. This research imaged multiple low-Z (polyethylene and water-filled), area targets with backscattered X-rays using standard medical imaging equipment, coded aperture masks with ideal bi-level autocorrelation properties, and dynamic reconstruction (DR). Lower fill factor apertures were the primary metric investigated because contrast was shown to be inversely related to the mask's percentage of open area. This study experimentally determined the optimal mask fill factor, gamma camera imaging protocols, and experimental geometry by examining the resulting effects on image quality. Reconstructed images were analyzed for Contrast-to-noise ratio (CNR), Signal-to-noise Ratio (SNR), resolution, sharpness, the uniformity of the background (artifacts). In addition to changing the fill factor, additional methods of improving the contrast included changing the experimental geometry, reducing the X-ray tube filtration, and widening the X-ray source's cone beam (FOV).
(cont.) 14 studies were performed that found 25% fill factor mask reconstructions had the highest average CNR (14.7), compared to 50% and 12.5% fill factor (CNRs 8.50 and 6.9, respectively) with a system resolution of 25 mm at the target. Thus, this study's techniques confirmed that large-area, low fill factor coded apertures could successfully be used, in conjunction with dynamic reconstruction, to image complex, extended scenes at 5 meters with capabilities of up to 50 meters or more.
by Jayna T. Bell.
S.M.

Negli ultimi decenni, una delle più importanti scoperte riguardanti i neutrini è stata il fenomeno delle oscillazioni, che implica una massa del neutrino non nulla. Il loro studio può fornire risposte a molte domande aperte riguardo la natura dei neutrini. DUNE, un esperimento long-baseline sui neutrini, è attualmente in fase di costruzione ed avrà come obiettivo principale quello di misurare alcuni parametri delle oscillazioni ancora ignoti, come il segno di Δm_{13}^2, che definisce l’ordinamento delle masse dei neutrini, ed il valore della fase di violazione CP, δCP . In questa tesi, è effettuato uno studio preliminare di un nuovo sistema di imaging, in grado di operare in presenza di intensi flussi di particelle. Questo sistema sfrutterà la luce di scintillazione Vacuum Ultra Violet (VUV) emessa dall’Argon liquido per fornire una ricostruzione completa delle tracce. La fattibilità di questa tecnica è studiata confrontando due diversi sistemi ottici: una camera basata sulla Coded Aperture Technique ed una tradizionale basata sulle lenti.

Le présent travail a pour but le développement d’une caméra gamma à masque codé permettant d’estimer la position tridimensionnelle (3D) des sources radioactives. Cela est d’un intérêt considérable dans le cadre d’un grand nombre d'applications, de la reconstruction de la forme 3D des objets radioactifs aux systèmes de réalité augmentée appliqués à la radioprotection. Les caméras gamma portables actuelles ne fournissent que la position angulaire relative des sources gamma à localiser, c'est-à-dire qu'aucune information métrique concernant les sources n’est disponible, comme par exemple leur distance par rapport à la caméra. Dans cette thèse, nous proposons principalement deux approches permettant d’estimer la position 3D des sources. La première approche consiste à étalonner la caméra gamma avec un capteur de profondeur à lumière structurée. La seconde approche permet d'estimer la distance source-détecteur par une méthode d’imagerie gamma stéréoscopique. Pour aligner géométriquement les images obtenues par la caméra gamma, le capteur de profondeur, et la caméra optique, une procédure d'étalonnage n’utilisant qu’une seule source ponctuelle radioactive a été conçue et mise en œuvre. Les résultats expérimentaux démontrent que les approches proposées permettent d'obtenir une précision inférieure au pixel, tant pour l’erreur de reprojection que pour la superposition des images gamma et optiques. Ces travaux présentent également une analyse quantitative de la précision et de la résolution relatives à l’estimation de la distance source-détecteur. De plus, les résultats obtenus ont validé le choix de la géométrie du modèle sténopé pour les caméras gamma à masque codé
A coded aperture gamma camera for retrieving the three-dimensional (3-D) position of radioactive sources is presented. This is of considerable interest for a wide number of applications, ranging from the reconstruction of the 3-D shape of radioactive objects to augmented reality systems. Current portable γ-cameras only provide the relative angular position of the hotspots within their field of view. That is, they do not provide any metric information concerning the located sources. In this study, we propose two approaches to estimate the distance of the surrounding hotspots, and to autonomously determine if they are occluded by an object. The first consists in combining and accurately calibrating the gamma camera with a structured-light depth sensor. The second approach allows the estimation of the source-detector distance by means of stereo gamma imaging. To geometrically align the images obtained by the gamma, depth, and optical cameras used, a versatile calibration procedure has been designed and carried out. Such procedure uses a calibration phantom intentionally easy to build and inexpensive, allowing the procedure to be performed with only one radioactive point source. Experimental results showed that our calibration procedure yields to sub-pixel accuracy both in the re-projection error and the overlay of radiation and optical images. A quantitative analysis concerning the accuracy and resolution of the retrieved source-detector distance is also provided, along with an insight into the respective most influential factors. Moreover, the results obtained validated the choice of the geometry of the pinhole model for a coded aperture gamma camera

La Tesi di Dottorato di Ricerca, svolta all’IASF CNR/INAF di Roma sotto la supervisione del dott. Enrico Costa, contiene lo studio delle prestazioni scientifiche dello strumento SuperAGILE. SuperAGILE è il monitor a raggi X di AGILE, missione su satellite dell’ASI composta da due strumenti, sensibili rispettivamente nelle bande di energia 15-40 keV e 30 MeV-50 GeV, il cui lancio è previsto per la fine del 2005. SuperAGILE è uno strumento a maschera codificata, con rivelatore a microstrip di silicio e maschera di tungsteno. Oggetto della mia Tesi di Dottorato è lo studio delle prestazioni scientifiche di SuperAGILE e delle più importanti criticità dello strumento: misura dell’uniformità delle prestazioni del circuito XAA1.2 dell’elettronica di lettura, della sua stabilità termica e della stabilità per variazioni della tensione di alimentazione, studio dell’interazione dei raggi cosmici nel circuito con misure sperimentali e stima del flusso aspettato in orbita, misura delle prestazioni scientifiche del modello da volo di SuperAGILE e, infine, studio dell’effetto della disuniformità di soglia sulle immagini. Le misure dell’uniformità di prestazioni dell’XAA1.2, della stabilità termica (tra –20° C e +40° C) e della stabilità per variazioni della tensione di alimentazione si effettuano con una scheda di acquisizione dedicata e stimolando il circuito per mezzo di un generatore di impulsi di tensione all’interno della scheda. Dalle misure si trova una variazione dei segnali di indirizzo dell’XAA1.2 (fondamentali per ricostruire le immagini delle sorgenti in Cielo) sulla scala dei 10° C. Lo studio dell’effetto dell’interazione dei raggi cosmici nel circuito, non progettato per applicazioni spaziali, riguarda il latch-up (aumento delle correnti di alimentazione che può danneggiare il chip per surriscaldamento) e il SEU (variazione di un bit nella memoria con perdita di programmazione) e l’effetto della dose assorbita sulla linearità e sul consumo. La misure sperimentali sono state effettuate irraggiando l’XAA1.2 con ioni (da 16O a 197Au) all’esperimento dedicato SIRAD dell’acceleratore Tandem nei Laboratori Nazionali INFN di Legnaro presso Padova. Al variare del LET, che misura l’energia per unità di lunghezza rilasciata dalle particelle cariche nel silicio, si misura la sezione d’urto di latch-up e SEU. Ad intervalli regolari si effettuano misure di linearità con l’impulso di calibrazione per studiare l’effetto della dose. Stimando il flusso di ioni in orbita con il codice CREME96 e usando un modello approssimato che tiene conto della spallazione dei protoni, ho trovato che il tasso aspettato di latch-up e SEU in orbita è minore di un evento per tutta la durata di AGILE e che l’effetto della dose assorbita è trascurabile. La Tesi di Dottorato contiene anche la caratterizzazione del modello da volo di SuperAGILE, che consiste nel misurare con l’impulso di carica la linearità e il rumore dell’elettronica di lettura dopo il montaggio degli XAA1.2, dopo la procedura di burn-in (accensione della scheda programmata in configurazione nominale all’interno di un forno a 75° C per 240 ore consecutive) e dopo l’integrazione del rivelatore. Dalle misure si trova che il burn-in non produce variazioni di prestazioni e che, dopo il montaggio del rivelatore, il rumore dell’elettronica di lettura è pari a circa 7.5 keV FWHM mentre l’energia di soglia è circa 19 keV. Il rumore dell’elettronica è stato misurato anche tramite l’acquisizione di sorgenti di raggi X (241Am, 57Co, 109Cd e righe di fluorescenza del Ba) e i valori trovati sono in buon accordo con le misure con l’impulso di carica. La Tesi contiene anche la discussione delle principali problematiche affrontate durante la scrittura di programmi per l’analisi dei dati raccolti in laboratorio. A causa del gran numero di pixel del rivelatore di SuperAGILE, infatti, la linearità e il rumore (sia con impulso di carica che con sorgenti di raggi X) devono essere stimate in modo automatico, senza l’immissione di parametri dall’utente. Infine, ho stimato l’effetto della disuniformità dell’energia di soglia sulle immagini di SuperAGILE generando immagini del fondo sul rivelatore, applicando diversi modelli di disuniformità di soglia e decodificando le immagini del cielo. Mentre l’attuale livello di uniformità di soglia non è sufficiente per poter osservare sorgenti deboli con integrazioni dell’ordine di 106 s, l’equalizzazione fine digitale delle soglie del circuito, tramite un DAC a 3 bit, permette di ottenere una uniformità sufficiente per poter effettuare osservazioni per 106 s.
The Ph.D. Thesis, performed at IASF CNR/INAF in Rome under the supervision of dr. Enrico Costa, contains the study of the scientific performances of the SuperAGILE instrument. SuperAGILE is the X-ray monitor of AGILE, satellite-borne mission of ASI whose payload is composed of two instruments, sensitive in the 15-40 keV and 30 MeV-50 GeV energy bands respectively, and whose launch is foreseen in late 2005. SuperAGILE is a coded aperture instrument with silicon microstrip detector and tungsten coded mask. Topic of my Ph.D. Thesis is the study of the SuperAGILE scientific performances and criticalities: measurement of the performances uniformity of the XAA1.2 front-end electronic circuit, of its thermal stability and of its stability toward supply voltage variations, study of the cosmic rays interaction in the front-end circuit with experimental measurements and estimate of the expected flux in orbit, measurements of the scientific performances of the SuperAGILE flight model and finally study of the impact of the threshold non uniformity on the images. The measurements of the performances uniformity of the XAA1.2, of its thermal stability (between –20° C and +40° C) and of the stability toward supply voltage variations are performed using a dedicated acquisition board feeding the chip with a pulse generator contained in the board. From the measurements a variation of the XAA1.2 address signals (used to reconstruct the images of the sources in the Sky) on the 10° C scale is found. The study of the effect of the cosmic rays interaction in the XAA1.2 chip, that is not designed as a radiation hard component for space applications, concerns the latch-up (sudden increase of the supply currents that can damage the chip due to overheating) and the SEU (bit flip in the memory registers with loss of chip configuration) and the effect of the absorbed dose on the linearity and power consumption. The measurements have been performed with ions irradiation (from 16O to 197Au) at the SIRAD facility of the Tandem accelerator in the Laboratori Nazionali INFN in Legnaro near Padova. With different values of LET, a measure of the energy released per unit length by the charged particles in silicon, the latch-up and SEU cross-section values are measured. During the irradiation linearity measurements using the test pulse generator are performed in order to study the total dose effect. Evaluating the ions flux in orbit with the CREME96 code and using an approximated model to take into account the proton spallation, I have found that the expected latch-up and SEU rate in orbit is less than one event during all the AGILE duration and that the total dose effect is negligible. My Ph.D. Thesis contains also the characterization of the SuperAGILE flight model, performed measuring the linearity and the noise of the front-end electronics after the XAA1.2 integration, after the burn-in procedure (by supplying the board in nominal configuration inside an oven at 75° C for 240 hours long) and after the detector integration. From the measurements I have found no performance degradation after the burn-in procedure. After the detector integration the noise in the front-end electronic is about 7.5 keV FWHM while the energy threshold is about 19 keV. The noise in the front-end electronic has been measured also using X-ray sources (241Am, 57Co, 109Cd and Ba fluorescence lines) and the measured values are in good agreement with the test pulse measurements. My Thesis contains also the discussion of the most important topics in the development of data analysis programs. Because of the big number of the SuperAGILE detector pixels, linearity and noise (using both test pulse generator and X-ray sources) need to be estimated automatically, without requiring the user to provide specific parameters. Finally, the Thesis contains an estimate of the threshold non uniformity on SuperAGILE images by means of background detector images generation applying different non uniformity threshold models. By decoding the resulting Sky images I have found that, while the nominal threshold uniformity does not allow to observe faint sources with exposures of order 106 s, the uniformity level obtained with the digital fine threshold equalization (3 bit DAC), allows expose for 106 s long.

L'imagerie gamma est une technique qui permet la localisation spatiale de sources radioactives. Les différentes applications de cette technique couvrent les phases de démantèlement des installations nucléaires ou de gestion des déchets nucléaires, mais aussi la radioprotection ou la sécurité intérieure. L'utilisation de caméras gamma permet de réduire la dose reçue par les opérateurs, et, par conséquent, de respecter le principe ALARA. Il existe deux techniques d’imagerie permettant la localisation de radioéléments émetteurs gamma : l’imagerie à masque codé et l’imagerie Compton. L’imagerie à masque codé utilise la modulation spatiale du flux de photons gamma incidents par collimateur multi-trous placé entre la source et le détecteur. Elle présente l’avantage d’être extrêmement performante pour des émetteurs gamma « basses énergies », aussi bien en matière de sensibilité, qu’en matière de résolution angulaire. L'imagerie Compton, quant à elle, repose sur l’utilisation de la mécanique de diffusion Compton. L'énergie déposée pendant le processus de diffusion déterminera l'angle de diffusion, et les positions des interactions détermineront la direction des rayons gamma entrants. La position de la source radioactive peut ainsi être limitée à un cône. Si plusieurs cônes sont utilisés, alors la position où le plus grand nombre de cônes se chevauchent correspond à la position de la source radioactive. Une des limitations de cette technique concerne la localisation des émetteurs gamma « basses énergies », pour lesquels la résolution angulaire est fortement dégradée allant jusqu’à l’impossibilité complète de trouver la position. L’objectif de ces travaux est de développer un prototype d’imageur hybride associant les techniques d’imagerie à masque codé et d’imagerie Compton, afin de tirer profit des avantages de chacun des types d’imagerie. Les différents travaux menés, autour du détecteur pixellisé Timepix3, mais aussi en matière de développement d’algorithmes mathématiques, ont permis de proposer deux prototypes d’imageurs hybrides. Les résultats obtenus à l’issue de ces travaux de recherche ont permis de valider expérimentalement les performances d’un des prototypes d’imageurs et d’illustrer l’intérêt d’un système hybride
Gamma imaging is a technique that allows the spatial localization of radioactive sources. The various applications of this technique cover decommissioning phases of nuclear facilities, nuclear waste management applications, but also radiation protection or Homeland Security. Using gamma camera reduces the dose received by operators and consequently contributes to the respect of the ALARA principle. There are two imaging techniques for the localization of gamma ray emitters: coded aperture imaging and Compton imaging. Coded aperture imaging relies on the spatial modulation of the incident gamma-ray flux by a multi-hole collimator placed between the detector and the radioactive source. It has the advantage of being extremely efficient for « low energy » gamma-ray emitters in terms of sensitivity and angular resolution. On the other hand, Compton imaging is based of the Compton scattering kinematic. The energy deposited during the scattering process will determine the scattering angle, and the positions of the interactions will determine the direction of the incoming gamma-ray. The position of the radioactive source can thus be limited to a cone. If several cones are used, then, the position where the greatest number of cones overlap corresponds to the position of the radioactive source. One limitations of this technique concerns the location of « low energy » gamma-ray emitters, for which the angular resolution is strongly degraded until it is completely not localizable. The objective of this work is to develop a prototype of hybrid imager that combines coded aperture and Compton imaging techniques in order to take advantage of each type of imaging. The different studies carried out, around the Timepix3 pixel detector, but also in the development of mathematical algorithms, have led to propose two prototypes of hybrid imager. The results obtained from this research work made it possible to validate experimentally the performance of one of the imager prototypes, and to illustrate the interest of a hybrid system

The physical properties of visible light and its interaction with matter create obstructions the human eye cannot explore. High energy radiation has been used as an alternative to visible light to penetrate these concealed regions and reveal their contents. However, traditional imaging techniques require a two-sided apparatus with a radiation source and a detector on opposite sides of the concealed object. One-sided imaging of concealed objects is made possible by a technique called backscatter imaging, utilizing high energy radiation. However, the signal produced by backscatter imaging is inherently weak, which makes in- terpretation di cult. One of the most promising techniques for recovering the weak signal is the coding and decoding provided by Coded Aperture Imaging (CAI). The purpose of this study was to create and test a coded aperture imaging system using backscattered x-rays. This would enable one-sided imaging of concealed objects and demonstrate whether a portable imaging system was feasible. The results obtained from conducting a computer simulation, visi- ble light experiments, and x-ray experiments proved that the process works, however, the x-ray ux levels required were too high for a portable system, based upon the current equipment available at UOIT.
UOIT

This thesis describes three computational optical systems and their underlying coding strategies. These codes are useful in a variety of optical imaging and spectroscopic applications. Two multichannel cameras are described. They both use a lenslet array to generate multiple copies of a scene on the detector. Digital processing combines the measured data into a single image. The visible system uses focal plane coding, and the long wave infrared (LWIR) system uses shift coding. With proper calibration, the multichannel interpolation results recover contrast for targets at frequencies beyond the aliasing limit of the individual subimages. This thesis also describes a LWIR imaging system that simultaneously measures four wavelength channels each with narrow bandwidth. In this system, lenses, aperture masks, and dispersive optics implement a spatially varying spectral code.

Dissertation

Current x-ray technologies provide security personnel with non-invasive sub-surface imaging and contraband detection in various portal screening applications such as checked and carry-on baggage as well as cargo. Computed tomography (CT) scanners generate detailed 3D imagery in checked bags; however, these scanners often require significant power, cost, and space. These tomography machines are impractical for many applications where space and power are often limited such as checkpoint areas. Reducing the amount of data acquired would help reduce the physical demands of these systems. Unfortunately this leads to the formation of artifacts in various applications, thus presenting significant challenges in reconstruction and classification. As a result, the goal is to maintain a certain level of image quality but reduce the amount of data gathered. For the security domain this would allow for faster and cheaper screening in existing systems or allow for previously infeasible screening options due to other operational constraints. While our focus is predominantly on security applications, many of the techniques can be extended to other fields such as the medical domain where a reduction of dose can allow for safer and more frequent examinations. This dissertation aims to advance data reduction algorithms for security motivated x-ray imaging in three main areas: (i) development of a sensing aware dimensionality reduction framework, (ii) creation of linear motion tomographic method of object scanning and associated reconstruction algorithms for carry-on baggage screening, and (iii) the application of coded aperture techniques to improve and extend imaging performance of nuclear resonance fluorescence in cargo screening. The sensing aware dimensionality reduction framework extends existing dimensionality reduction methods to include knowledge of an underlying sensing mechanism of a latent variable. This method provides an improved classification rate over classical methods on both a synthetic case and a popular face classification dataset. The linear tomographic method is based on non-rotational scanning of baggage moved by a conveyor belt, and can thus be simpler, smaller, and more reliable than existing rotational tomography systems at the expense of more challenging image formation problems that require special model-based methods. The reconstructions for this approach are comparable to existing tomographic systems. Finally our coded aperture extension of existing nuclear resonance fluorescence cargo scanning provides improved observation signal-to-noise ratios. We analyze, discuss, and demonstrate the strengths and challenges of using coded aperture techniques in this application and provide guidance on regimes where these methods can yield gains over conventional methods.

The emerging theory of compressed sensing has been nothing short of a revolution in signal processing, challenging some of the longest-held ideas in signal processing and leading to the development of exciting new ways to capture and reconstruct signals and images. Although the theoretical promises of compressed sensing are manifold, its implementation in many practical applications has lagged behind the associated theoretical development. Our goal is to elevate compressed sensing from an interesting theoretical discussion to a feasible alternative to conventional imaging, a significant challenge and an exciting topic for research in signal processing. When applied to imaging, compressed sensing can be thought of as a particular case of computational imaging, which unites the design of both the sensing and reconstruction of images under one design paradigm. Computational imaging tightly fuses modeling of scene content, imaging hardware design, and the subsequent reconstruction algorithms used to recover the images.

This thesis makes important contributions to each of these three areas through two primary research directions. The first direction primarily attacks the challenges associated with designing practical imaging systems that implement incoherent measurements. Our proposed snapshot imaging architecture using compressive coded aperture imaging devices can be practically implemented, and comes equipped with theoretical recovery guarantees. It is also straightforward to extend these ideas to a video setting where careful modeling of the scene can allow for joint spatio-temporal compressive sensing. The second direction develops a host of new computational tools for photon-limited inverse problems. These situations arise with increasing frequency in modern imaging applications as we seek to drive down image acquisition times, limit excitation powers, or deliver less radiation to a patient. By an accurate statistical characterization of the measurement process in optical systems, including the inherent Poisson noise associated with photon detection, our class of algorithms is able to deliver high-fidelity images with a fraction of the required scan time, as well as enable novel methods for tissue quantification from intraoperative microendoscopy data. In short, the contributions of this dissertation are diverse, further the state-of-the-art in computational imaging, elevate compressed sensing from an interesting theory to a practical imaging methodology, and allow for effective image recovery in light-starved applications.

Dissertation

Coded apertures have the potential to increase imaging e ciency in nuclear medicine without degrading resolution, but near- eld artifacts are present even under idealised aperture and imaging conditions. The purpose of this work is to reduce artifacts prior to reconstruction, and to work towards coded apertures that are clinically useful. A ray-tracing simulator was developed. Far- eld conditions produce nearperfect images, but the simulation of distributed sources under idealised near- eld conditions results in the presence of artifacts. Three core concepts are introduced in this thesis: a novel rotatable array of identical limited- eld-of-view coded apertures; the use of high-resolution aperture projections; and the deliberate and counter-intuitive use of thin, highly transparent aperture material. An array of identical limited- eld-of-view coded apertures, which can be rotated so as to implement an existing artifact-reduction technique, was simulated. The artifacts that exist for a single coded aperture under idealised conditions are removed. This novel technique remains e ective when realistic near- eld conditions are introduced into the simulation. However, realistic apertures increase artifacts due to nite pinhole widths and nite thickness of the aperture material. To address the pinhole width problem, high-resolution patterns, in which the smallest hole corresponds to a projection area of 1 1 detector pixels, o er the best trade-o between e ciency and resolution despite the partial volume e ect. The nite aperture thickness problem is addressed by another novel concept; viz. the deliberate reduction in material thickness, which results in a highly transparent coded aperture. Simulation shows that this counter-intuitive approach diminishes collimation e ects. The implementation of any or all of these three core concepts, however, reduces count statistics. An ultra-near- eld geometry, which would ordinarily result in severe artifacts, can theoretically be used to maintain count statistics, without altering either patient dose or image acquisition time. This was veri ed by simulation. ii ABSTRACT A prior-state-of-the-art 1 mm thick tungsten coded aperture, and a deliberately highly transparent 100 m tungsten foil coded aperture, were constructed for use with a dual head gamma camera. Phantom studies of Technetium-99m point, line, syringe and printed distributed sources were performed. The experimental acquisitions veri ed the simulation results, for both the prior-state-of-the-art coded aperture and the novel high-transparency coded aperture. The results and arguments presented in this thesis point to the potential for these three core developments to produce high-quality coded aperture images in a fraction of the time that is taken for a collimator acquisition. The limiting factor appears to be the poor count statistics that result from the low sensitivity of current gamma cameras; a situation which looks set to change given current research trends.

One of the challenges facing clinical practice today is intra-operative margin detection in breast conserving surgeries (BCS) or lumpectomy procedures. When a surgeon removes a breast tumor from a patient during a BCS procedure, the surgically excised tissue specimen is examined to see whether it contains a margin of healthy tissue around the tumor. A healthy margin of tissue around the tumor would indicate that the tumor in its entirety has been removed. On the other hand, if cancerous tissue is at the surface of the specimen, that would indicate that the tumor may have been transected during the procedure, leaving some residual cancerous tissue inside the patient. The most effective intra-operative real-time margin detection techniques currently used in clinical practice are frozen section analysis (FSA) and touch-prep cytology. These methods have been shown to possess inconsistent accuracy, which result in 20% to 30% of BCS patients being called back for a repeat BCS procedure to remove the residual tumor tissue. In addition these techniques have been shown to be time-consuming--requiring the operating room team to have to wait at least 20 minutes for the results. Therefore, there is a need for accurate and faster technology for intra-operative margin detection.

In this dissertation, we describe an x-ray coherent scatter imaging technique for intra-operative margin detection with greater accuracy and speed than currently available techniques. The method is based on cross-sectional imaging of the differential coherent scatter cross section in the sample. We first develop and validate a Monte Carlo simulation of coherent scattering. Then we use that simulation to design and test coherent scatter computed tomography (CSCT) and coded aperture coherent scatter spectral imaging (CACSSI) for cancerous voxel detection and for intra-operative margin detection using (virtual) clinical trials. Finally, we experimentally implement a CACSSI system and determine its accuracy in cancer detection using tissue histology.

We find that CSCT and CACSSI are able to accurately detect cancerous voxels inside of breast tissue specimens and accurately perform intra-operative margin detection. Specifically, for the task of individual cancerous voxel detection, we show that CSCT and CACSSI have AUC values of 0.97 and 0.94, respectively. Whereas for the task of intra-operative margin detection, the results of our virtual clinical trials show that CSCT and CACSSI have AUC values of 0.975 and 0.741, respectively. The gap in spatial resolution between CSCT and CACSSI affects the results of intra-operative margin detection much more than it does the task of individual cancerous voxel detection. Finally, we also show that CSCT would require on the order of 30 minutes to create a 3D image of a breast cancer specimen, whereas CACSSI would require on the order of 3 minutes.

These results of this work show that coherent scatter imaging has the potential to provide more accurate intra-operative margin detection than currently used clinical techniques. In addition, the speed (and therefore low scan duration: 3 min) of CACSSI, along with its ability to automatically classify cancerous tissue for margin detection means that coherent scatter imaging would be much more cost-effective than the clinical techniques that require up to 20 minutes and a trained pathologist. With the cancerous voxel detection accuracy of a 0.94 AUC and scan time of on the order of 3 minutes demonstrated for coherent scatter imaging in this work, coherent scatter imaging has the potential to reduce healthcare costs for BCS procedures and rates of repeat BCS surgeries. The accuracy for CACSSI can be considerably improved to match CSCT accuracy by improving its spatial resolution through a number of techniques: incorporating into the CACSSI reconstruction algorithm the ability to differentiate noise from high frequency signal so that we can image with higher frequency coded aperture masks; implementing a 2D coded aperture mask with a 2D detector; or acquiring additional angles of projection data.

Dissertation