[EN] Model-based computed tomography (CT) image reconstruction is dominated by iterative algorithms. Although long reconstruction times remain as a barrier in practical applications, techniques to speed up its convergence are object of investigation, obtaining impressive results. In this thesis, a direct algorithm is proposed for model-based image reconstruction. The model-based approximation relies on the construction of a model matrix that poses a linear system which solution is the reconstructed image. The proposed algorithm consists in the QR decomposition of this matrix and the resolution of the system by a backward substitution process. The cost of this image reconstruction technique is a matrix vector multiplication and a backward substitution process, since the model construction and the QR decomposition are performed only once, because of each image reconstruction corresponds to the resolution of the same CT system for a different right hand side. Several problems regarding the implementation of this algorithm arise, such as the exact calculation of a volume intersection, definition of fill-in reduction strategies optimized for CT model matrices, or CT symmetry exploit to reduce the size of the system. These problems have been detailed and solutions to overcome them have been proposed, and as a result, a proof of concept implementation has been obtained. Reconstructed images have been analyzed and compared against the filtered backprojection (FBP) and maximum likelihood expectation maximization (MLEM) reconstruction algorithms, and results show several benefits of the proposed algorithm. Although high resolutions could not have been achieved yet, obtained results also demonstrate the prospective of this algorithm, as great performance and scalability improvements would be achieved with the success in the development of better fill-in strategies or additional symmetries in CT geometry. / [ES] En la reconstrucción de imagen de tomografía axial computerizada (TAC), en su modalidad model-based, prevalecen los algoritmos iterativos. Aunque los altos tiempos de reconstrucción aún son una barrera para aplicaciones prácticas, diferentes técnicas para la aceleración de su convergencia están siendo objeto de investigación, obteniendo resultados impresionantes. En esta tesis, se propone un algoritmo directo para la reconstrucción de imagen model-based. La aproximación model-based se basa en la construcción de una matriz modelo que plantea un sistema lineal cuya solución es la imagen reconstruida. El algoritmo propuesto consiste en la descomposición QR de esta matriz y la resolución del sistema por un proceso de sustitución regresiva. El coste de esta técnica de reconstrucción de imagen es un producto matriz vector y una sustitución regresiva, ya que la construcción del modelo y la descomposición QR se realizan una sola vez, debido a que cada reconstrucción de imagen supone la resolución del mismo sistema TAC para un término independiente diferente. Durante la implementación de este algoritmo aparecen varios problemas, tales como el cálculo exacto del volumen de intersección, la definición de estrategias de reducción del relleno optimizadas para matrices de modelo de TAC, o el aprovechamiento de simetrías del TAC que reduzcan el tama\~no del sistema. Estos problemas han sido detallados y se han propuesto soluciones para superarlos, y como resultado, se ha obtenido una implementación de prueba de concepto. Las imágenes reconstruidas han sido analizadas y comparadas frente a los algoritmos de reconstrucción filtered backprojection (FBP) y maximum likelihood expectation maximization (MLEM), y los resultados muestran varias ventajas del algoritmo propuesto. Aunque no se han podido obtener resoluciones altas aún, los resultados obtenidos también demuestran el futuro de este algoritmo, ya que se podrían obtener mejoras importantes en el rendimiento y la escalabilidad con el éxito en el desarrollo de mejores estrategias de reducción de relleno o simetrías en la geometría TAC. / [CAT] En la reconstrucció de imatge tomografia axial computerizada (TAC) en la seua modalitat model-based prevaleixen els algorismes iteratius. Tot i que els alts temps de reconstrucció encara són un obstacle per a aplicacions pràctiques, diferents tècniques per a l'acceleració de la seua convergència estàn siguent objecte de investigació, obtenint resultats impressionants. En aquesta tesi, es proposa un algorisme direct per a la recconstrucció de image model-based. L'aproximació model-based es basa en la construcció d'una matriu model que planteja un sistema lineal quina sol·lució es la imatge reconstruida. L'algorisme propost consisteix en la descomposició QR d'aquesta matriu i la resolució del sistema per un procés de substitució regresiva. El cost d'aquesta tècnica de reconstrucció de imatge es un producte matriu vector i una substitució regresiva, ja que la construcció del model i la descomposició QR es realitzen una sola vegada, degut a que cada reconstrucció de imatge suposa la resolució del mateix sistema TAC per a un tèrme independent diferent. Durant la implementació d'aquest algorisme sorgixen diferents problemes, tals com el càlcul exacte del volum de intersecció, la definició d'estratègies de reducció de farcit optimitzades per a matrius de model de TAC, o el aprofitament de simetries del TAC que redueixquen el tamany del sistema. Aquestos problemes han sigut detallats y s'han proposat solucions per a superar-los, i com a resultat, s'ha obtingut una implementació de prova de concepte. Les imatges reconstruides han sigut analitzades i comparades front als algorismes de reconstrucció filtered backprojection (FBP) i maximum likelihood expectation maximization (MLEM), i els resultats mostren varies ventajes del algorisme propost. Encara que no s'han pogut obtindre resolucions altes ara per ara, els resultats obtinguts també demostren el futur d'aquest algorisme, ja que es prodrien obtindre millores importants en el rendiment i la escalabilitat amb l'éxit en el desemvolupament de millors estratègies de reducció de farcit o simetries en la geometria TAC. / Iborra Carreres, A. (2015). Development of a New 3D Reconstruction Algorithm for Computed Tomography (CT) [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/59421 / TESIS

Radiografisk bilddiagnostisering innebär högteknologiska metoder och är ett område inomsjukvården som utvecklas i snabb takt. Med hjälp av olika modaliteter erbjuds detaljeraddiagnostik av patologi och skador på hela kroppen. Yrket som röntgensjuksköterska är mångfacetterat och det är viktigt att snabb kunnaöverblicka situationer och samtidigt skapa en bra relation till patienten under det korta mötet. Den vanligaste metoden för undersökning av multipelt myelom är idag konventionell röntgenvilket kan genomföras med så kallad slätröntgen eller alternativt med konventionellgenomlysning. Syftet med studien är att undersöka och jämföra stråldoser från konventionellgenomlysning med värden från lågdos datortomografi, då vissa sjukvårdsområden har lågdosDatortomografi som en alternativ metod för undersökning av multipelt myelom. Studien är kvantitativ och datainsamlingen har utförts genom mätning av stråldoser med hjälpav fantomdocka i både konventionell genomlysning och datortomografi. Jämförandestråldoser från tidigare utförda undersökningar i både konventionell genomlysning samtdatortomografi är hämtade från PACS. Studiens resultat tyder på att det finns skillnader i stråldoser mellan metoderna, både vid egnamätningar och jämförande värden från tidigare utförda undersökningar. Röntgen meddatortomografi ger en högre stråldos med en faktor på ca 2,9 jämfört med konventionellgenomlysning.

Röntgensjuksköterskans huvudområde är radiografi som innefattar bland annat användning av joniserande strålning. CT undersökningar av urinvägssystemen och dess funktion ökar. CT ger en högre stråldos till patienter, jämfört med konventionell röntgen, vilket kan medföra olika sorters skador hos den som bestrålats. ALARAprincipen skall användas för att minska joniserande strålning så mycket som möjligt. Studien är utförd på Höglandsjukhuset i Eksjö där de har implementerat lågdosprotokoll på CT-urografi när det gäller att kontrollera funktionen av urinvägarna. Syftet med studien är att jämföra stråldoser vid konventionell urografi med lågdos CT urografi. Studien är kvantitativ och retrospektiv. Data har insamlats från PACS och RIS. Jämförelse av stråldoser har gjorts mellan patienter som gjort urografier med konventionell röntgen och patienter som gjort lågdos CT. Patienterna har matchats i avseende på kön, vikt och längd. Resultatet visar att lågdos CT-urografi gav 2,5 gånger mer stråldos till patienten än konventionell urografi. Detta var ett rimligt resultat som stämmer med andra studier inom området.

<p>The work presented in this dissertation focuses on evaluation of absolute quantification accuracy in dedicated breast SPECT-CT. The overall goal was to investigate through simulations and measurements the impact and utilization of various correction methods for scattered and attenuated photons, characterization of incomplete charge collection in Cadmium Zinc Telluride detectors as a surrogate means of improving scatter correction, and resolution recovery methods for modeling collimator blur during image reconstruction. The quantification accuracy of attenuation coefficients in CT reconstructions was evaluated in geometric phantoms, and a slice-by-slice breast segmentation algorithm was developed to separate adipose and glandular tissue. All correction and segmentation methods were then applied to a pilot study imaging parathyroid patients to determine the average uptake of Tc-99m Sestamibi in healthy breast tissue, including tissue specific uptake in adipose and glandular tissue. </p><p>Monte Carlo methods were utilized to examine the changes in incident scatter energy distribution on the SPECT detector as a function of 3D detector position about a pendant breast geometry. A simulated prone breast geometry with torso, heart, and liver was designed. An ideal detector was positioned at various azimuthal and tilted positions to mimic the capabilities of the breast SPECT subsystem. The limited near-photopeak scatter energy range in simulated spectra was linearly fit and the slope used to characterize changes in scatter distribution as a function of detector position. Results show that the detected scatter distribution changes with detector tilt, with increasing incidence of high energy scattered photons at larger detector tilts. However, reconstructions of various simulated trajectories show minimal impact on quantification (<5%) compared to a primary-only reconstruction.</p><p>Two scatter compensation methods were investigated and compared to a narrow photopeak-only windowing for quantification accuracy in large uniform regions and small, regional uptake areas: 1) a narrow ±4% photopeak energy window to minimize scatter in the photopeak window, 2) the previously calibrated dual-energy window scatter correction method, and 3) a modified dual-energy window correction method that attempts to account for the effects of incomplete charge collection in Cadmium Zinc Telluride detectors. Various cylindrical phantoms, including those with imbedded hot and cold regions, were evaluated. Results show that the Photopeak-only and DEW methods yield reasonable quantification accuracy (within 10%) for a wide range of activity concentrations and phantom configurations. The mDEW demonstrated highly accurate quantification measurements in large, uniform regions with improved uniformity compared to the DEW method. However, the mDEW method is susceptible to the calibration parameters and the activity concentration of the scanned phantom. The sensitivity of the mDEW to these factors makes it a poor choice for robust quantification applications. Thus, the DEW method using a high-performance CZT gamma camera is still a better choice for quantification purposes</p><p>Phantoms studies were performed to investigate the application of SPECT vs CT attenuation correction. Minor differences were observed between SPECT and CT maps when assuming a uniformly filled phantom with the attenuation coefficient of water, except when the SPECT attenuation map volume was significantly larger than the CT volume. Material specific attenuation coefficients reduce the corresponding measured activity concentrations compared to a water-only correction, but the results do not appear more accurate than a water-only attenuation map. Investigations on the impact of image registration show that accurate registration is necessary for absolute quantification, with errors up to 14% observed for 1.5cm shifts. </p><p>A method of modeling collimator resolution within the SPECT reconstruction algorithm was investigated for its impact on contrast and quantification accuracy. Three levels of resolution modeling, each with increasing ray-sampling, were investigated. The resolution model was applied to both cylindrical and anthropomorphic breast phantoms with hot and cold regions. Large volume quantification results (background measurements) are unaffected by the application of resolution modeling. For smaller chambers and simulated lesions, contrast generally increases with resolution modeling. Edges of lesions also appear sharper with resolution modeling. No significant differences were seen between the various levels of resolution modeling. However, Gibbs artifacts are amplified at the boundaries of high contrast regions, which can significantly affect absolute quantification measurements. Convergence with resolution modeling is also notably slower, requiring more iterations with OSEM to reach a stable mean activity concentration. Additionally, reconstructions require far more computing time with resolution modeling due to the increase in number of sampling rays. Thus while the edge enhancement and contrast improvements may benefit lesion detection, the artifacts, slower convergence, and increased reconstruction time limit the utility of resolution modeling for both absolute quantification and clinical imaging studies. </p><p>Finally, a clinical pilot study was initiated to measure the average uptake of Tc-99m Sestamibi in healthy breast tissue. Subjects were consented from those undergoing diagnostic parathyroid studies at Duke. Each subject was injected with 25mCi of Sestamibi as part of their pre-surgical parathyroid SPECT imaging studies and scanned with the dedicated breast SPECT-CT system before their diagnostic parathyroid SPECT scan. Based on phantom studies of CT reconstructed attenuation coefficient accuracy, a slice-by-slice segmentation algorithm was developed to separate breast CT data into adipose and glandular tissue. SPECT data were scatter, attenuation, and decay corrected to the time of injection. Segmented CT images were used to measure average radiotracer concentration in the whole breast, as well as adipose and glandular tissue. With 8 subjects scanned, the average measured whole breast activity concentration was found to be 0.10µCi/mL. No significant differences were seen between adipose and glandular tissue uptake. </p><p>In conclusion, the application of various characterization and correct methods for quantitative SPECT imaging were investigated. Changes in detected scatter distribution appear to have minimal impact on quantification, and characterization of low-energy tailing for a modified scatter subtraction method yields inferior overall quantification results. Comparable quantification accuracy is seen with SPECT and CT-based attenuation maps, assuming the SPECT-based volume is fairly accurate. In general, resolution recovery within OSEM yields higher contrast, but quantification accuracy appears more susceptible to measurement location. Finally, scatter, attenuation, and resolution recovery methods, along with a breast segmentation algorithm, were implemented in a clinical imaging study for quantifying Tc-99m Sestamibi uptake. While the average whole breast uptake was measured to be 0. 10µCi/mL, no significant differences were seen between adipose and glandular tissue or when implementing resolution recovery. Thus, for future clinical imaging, it's recommended that the application of the investigated correction methods should be limited to the traditional DEW method and CT-based attenuation maps for quantification studies.</p> / Dissertation

This thesis reports on algorithmic design and software development completed for the Medipix All Resolution System (MARS) multi-energy CT scanner. Two areas of research are presented - the speed and usability improvements made to the post-reconstruction material decomposition software; and the development of two algorithms designed for the implementation of a novel voxel system into the MARS image reconstruction chain. The MARS MD software package is the primary material analysis tool used by members of the MARS group. The photon-processing ability of the MARS scanner is what makes material decomposition possible. MARS MD loads reconstructed images created after a scan and creates a new set of images, one for every individual material within the object. The software is capable of discriminating at least six different materials, plus air, within the object. A significant speed improvement to this program was attained by moving the code base from GNU Octave to MATLAB and applying well known optimisation routines, while the creation of a graphical user interface made the software more accessible and easy to use. The changes made to MARS MD represented a significant contribution to the productivity of the entire MARS group. A drawback of the MARS image reconstruction chain is the time required to generate images of a scanned object. Compared to commercially available CT systems, the MARS system takes several orders of magnitude longer to do essentially the same job. With up to eight energy bins worth of data to consider during reconstruction, compared to a single energy bin in most com- mercial scanners, it is not surprising that there is a shortfall. A major performance limitation of the reconstruction process lies in the calculation of the small distances travelled by every detected photon within individual portions of the reconstruction volume. This thesis investigates a novel volume geometry that was developed by Prof. Phil Butler and Dr. Peter Renaud, and is designed to partially mitigate this time constraint. By treating the volume as a cylinder instead of a traditional cubic structure, the number of individual path length calculations can be drastically reduced. Two sets of algorithms are prototyped, coded in MATLAB, C++ and CUDA, and finally compared in terms of speed and visual accuracy.

No description available.

No description available.

This study investigated the hypothesis that severity of depressive symptoms following acute ischemic stroke is associated with degree of tissue infarction and severity of white matter changes (WMCs). It employed a novel quantitative region-based approach considering both infarction and WMCs. Of 54 ischemic stroke patients recruited, 50 (72.3 ± 12.8 years, 52.0% male) had useable CT scans. The typical patient was recruited within 3 weeks of their stroke (19.7 ± 31.0 days), exhibited minor cognitive impairment (MMSE score 25.8 ± 4.6), and had mild to moderate stroke severity (NIHSS score 6.5 ± 5.4). 28.0% of patients screened positive for clinical depression with a CES-D score ≥16. While neither degree of infarction nor severity of WMCs (ARWMC score) in the 12 brain regions correlated with depressive symptoms (CES-D score), stroke severity was a significant predictor of depressive symptoms. This stressor, related to physical disability, was a predominant predictor over lesion characteristics.

This study investigated the hypothesis that severity of depressive symptoms following acute ischemic stroke is associated with degree of tissue infarction and severity of white matter changes (WMCs). It employed a novel quantitative region-based approach considering both infarction and WMCs. Of 54 ischemic stroke patients recruited, 50 (72.3 ± 12.8 years, 52.0% male) had useable CT scans. The typical patient was recruited within 3 weeks of their stroke (19.7 ± 31.0 days), exhibited minor cognitive impairment (MMSE score 25.8 ± 4.6), and had mild to moderate stroke severity (NIHSS score 6.5 ± 5.4). 28.0% of patients screened positive for clinical depression with a CES-D score ≥16. While neither degree of infarction nor severity of WMCs (ARWMC score) in the 12 brain regions correlated with depressive symptoms (CES-D score), stroke severity was a significant predictor of depressive symptoms. This stressor, related to physical disability, was a predominant predictor over lesion characteristics.

Es wurde der Einsatz der Mehrschicht-Spiral-CT (MS-CT) zur zerebralen Perfusionsbildgebung und zur zerviko-zerebralen Angiographie bei Schlaganfallpatienten evaluiert. Bei 52 Patienten mit klinischen Zeichen einer akuten Ischämie wurde im Mittel 3,4 Stunden nach Symptombeginn an einem MS-CT Gerät eine CT-Perfusion (CTP) durchgeführt. Parameterbilder der zerebralen Blutperfusion (CBP), des zerebralen Blutvolumens (CBV) und der mittleren Transitzeit (MTT) wurden generiert, Perfusionsstörungen ermittelt und mit bildgebenden sowie klinischen Verlaufskontrollen korreliert. Eine CT-Angiographie (CTA) wurde initial bei 12 Patienten angewandt, um die Eignung des Untersuchungsprotokolls zu prüfen. Darüber hinaus wurden bei 45 Patienten mit Zeichen einer akuten zerebrovaskulären Insuffizienz die Ergebnisse der CTA mit denen anderer bildgebender Modalitäten (Magnetresonanz-Angiographie, digitale Subtraktionsangiographie, Dopplerultraschall) verglichen. Perfusionsbilder konnten von 44 Patienten generiert werden, hiervon entwickelten 22 Patienten einen im Verlauf gesicherten Infarkt. An Hand der MTT-Bilder konnten ischämische Veränderungen mit einer Sensitivität von 95 % erfasst werden, die Spezifität war mit 100 % für die CBV-Bilder am höchsten. Patienten mit Infarkt zeigten seitenvergleichend eine signifikante Reduktion der CBP in ischämischen Arealen. Die Ausdehnung der CBV Reduktion ergab die beste Korrelation mit dem endgültigen partiellen Infarktvolumen. Mit der CTA konnte anfänglich bei 12 / 12 Patienten, später bei 43 / 45 (96 %) eine umfassende Darstellung des zerviko-zerebralen Gefäßsystems erreicht werden, wobei in 22 Infarktpatienten die zu Grunde liegende Gefäßpathologie erkannt werden konnte. Das evaluierte Protokoll zur Perfusionsbildgebung mittels Mehrschicht-Spiral-CT ist zur frühzeitigen Erkennung und Quantifizierung einer akuten zerebralen Ischämie geeignet und bietet zusammen mit der CTA, welche das gesamte zerviko-zerebrale Gefäßsystem verlässlich visualisieren kann, die Möglichkeit einer umfassenden Bildgebung mittels MS-CT bei Schlaganfallpatienten. / The application of multi-slice spiral computed tomography (MS-CT) in the diagnostic assessment of stroke patients using cerebral perfusion imaging and cervicocerebral angiography was evaluated. Fifty-two patients with clinically suspected acute ischemia underwent CT perfusion (CTP), performed 3.4 hours, on average, after the onset of symptoms, by using MS-CT. Perfusion images of the cerebral blood perfusion (CBP), cerebral blood volume (CBV) and mean transit time (MTT) were calculated. The amount and extension of perfusion disturbances were measured and correlated with the outcome. CT angiography (CTA) was initially performed on twelve patients to verify the suitability of the examination protocol. In addition, forty-five patients with signs of acute cerebrovascular insufficiency underwent CTA. CTA findings were compared with those of other imaging modalities (magnetic resonance imaging, digital subtraction angiography, doppler ultrasonography). Of 44 patients in whom perfusion maps could be generated, 22 developed infarction confirmed at follow-up. On MTT-maps ischemic changes could be detected with the highest sensitivity (95%). Specificity was highest (100%) for CBV-maps. Patients with infarction showed significant reduction of CBP in ischemic tissue compared to the contra lateral hemisphere. Extension of CBV reduction showed the best correlation with final infarct volume. Initially in twelve out of twelve patients, later in 43 out of 45 (96%), the cervicocranial vascular system could be comprehensively visualized using CTA. In 22 stroke patients the underlying vascular pathology could be detected. CT perfusion using multi-slice CT is a suitable tool for the early identification and quantification of acute cerebral ischemia. Multi-slice CT angiography permits reliable visualization of the cervicocranial vascular system. Together these tools offer comprehensive assessment of stroke patients by means of multi-slice CT.