Development of methods for time efficient scatter correction and improved attenuation correction in time-of-flight PET/MR

In der vorliegenden Dissertation wurden zwei fortdauernde Probleme der Bildrekonstruktion in der time-of-flight (TOF) PET bearbeitet: Beschleunigung der TOF-Streukorrektur sowie Verbesserung der emissionsbasierten Schwächungskorrektur. Aufgrund der fehlenden Möglichkeit, die Photonenabschwächung direkt zu messen, ist eine Verbesserung der Schwächungskorrektur durch eine gemeinsame Rekonstruktion der Aktivitäts- und Schwächungskoeffizienten-Verteilung mittels der MLAA-Methode von besonderer Bedeutung für die PET/MRT, während eine Beschleunigung der TOF-Streukorrektur gleichermaßen auch für TOF-fähige PET/CT-Systeme relevant ist. Für das Erreichen dieser Ziele wurde in einem ersten Schritt die hochauflösende PET-Bildrekonstruktion THOR, die bereits zuvor in unserer Gruppe entwickelt wurde, angepasst, um die TOF-Information nutzen zu können, welche von allen modernen PET-Systemen zur Verfügung gestellt wird. Die Nutzung der TOF-Information in der Bildrekonstruktion führt zu reduziertem Bildrauschen und zu einer verbesserten Konvergenzgeschwindigkeit. Basierend auf diesen Anpassungen werden in der vorliegenden Arbeit neue Entwicklungen für eine Verbesserung der TOF-Streukorrektur und der MLAA-Rekonstruktion beschrieben. Es werden sodann Ergebnisse vorgestellt, welche mit den neuen Algorithmen am Philips Ingenuity PET/MRT-Gerät erzielt wurden, das gemeinsam vom Helmholtz-Zentrum Dresden-Rossendorf (HZDR) und dem Universitätsklinikum betrieben wird. Eine wesentliche Voraussetzung für eine quantitative TOF-Bildrekonstruktionen ist eine Streukorrektur, welche die TOF-Information mit einbezieht. Die derzeit übliche Referenzmethode hierfür ist eine TOF-Erweiterung des single scatter simulation Ansatzes (TOF-SSS). Diese Methode wurde im Rahmen der TOF-Erweiterung von THOR implementiert. Der größte Nachteil der TOF-SSS ist eine 3–7-fach erhöhte Rechenzeit für die Berechnung der Streuschätzung im Vergleich zur non-TOF-SSS, wodurch die Bildrekonstruktionsdauer deutlich erhöht wird. Um dieses Problem zu beheben, wurde eine neue, schnellere TOF-Streukorrektur (ISA) entwickelt und implementiert. Es konnte gezeigt werden, dass dieser neue Algorithmus eine brauchbare Alternative zur TOF-SSS darstellt, welche die Rechenzeit auf ein Fünftel reduziert, wobei mithilfe von ISA und TOF-SSS rekonstruierte Schnittbilder quantitativ ausgezeichnet übereinstimmen. Die Gesamtrekonstruktionszeit konnte mithilfe ISA bei Ganzkörperuntersuchungen insgesamt um den Faktor Zwei reduziert werden. Dies kann als maßgeblicher Fortschritt betrachtet werden, speziell im Hinblick auf die Nutzung fortgeschrittener Bildrekonstruktionsverfahren im klinischen Umfeld. Das zweite große Thema dieser Arbeit ist ein Beitrag zur verbesserten Schwächungskorrektur in der PET/MRT mittels MLAA-Rekonstruktion. Hierfür ist zunächst eine genaue Kenntnis der tatsächlichen Zeitauflösung in der betrachten PET-Aufnahme zwingend notwendig. Da die vom Hersteller zur Verfügung gestellten Zahlen nicht immer verlässlich sind und zudem die Zählratenabhängigkeit nicht berücksichtigen, wurde ein neuer Algorithmus entwickelt und implementiert, um die Zeitauflösung in Abhängigkeit von der Zählrate zu bestimmen. Dieser Algorithmus (MLRES) basiert auf dem maximum likelihood Prinzip und erlaubt es, die funktionale Abhängigkeit der Zeitauflösung des Philips Ingenuity PET/MRT von der Zählrate zu bestimmen. In der vorliegenden Arbeit konnte insbesondere gezeigt werden, dass sich die Zeitauflösung des Ingenuity PET/MRT im klinisch relevanten Zählratenbereich um mehr als 250 ps gegenüber der vom Hersteller genannten Auflösung von 550 ps verschlechtern kann, welche tatsächlich nur bei extrem niedrigen Zählraten erreicht wird. Basierend auf den oben beschrieben Entwicklungen konnte MLAA in THOR integriert werden. Die MLAA-Implementierung erlaubt die Generierung realistischer patientenspezifischer Schwächungsbilder. Es konnte insbesondere gezeigt werden, dass auch Knochen und Hohlräume korrekt identifiziert werden, was mittels MRT-basierter Schwächungskorrektur sehr schwierig oder sogar unmöglich ist. Zudem konnten wir bestätigen, dass es mit MLAA möglich ist, metallbedingte Artefakte zu reduzieren, die ansonsten in den MRT-basierten Schwächungsbildern immer zu finden sind. Eine detaillierte Analyse der Ergebnisse zeigte allerdings verbleibende Probleme bezüglich der globalen Skalierung und des lokalen Übersprechens zwischen Aktivitäts- und Schwächungsschätzung auf. Daher werden zusätzliche Entwicklungen erforderlich sein, um auch diese Defizite zu beheben.The present work addresses two persistent issues of image reconstruction for time-of-flight (TOF) PET: acceleration of TOF scatter correction and improvement of emission-based attenuation correction. Due to the missing capability to measure photon attenuation directly, improving attenuation correction by joint reconstruction of the activity and attenuation coefficient distribution using the MLAA technique is of special relevance for PET/MR while accelerating TOF scatter correction is of equal importance for TOF-capable PET/CT systems as well. To achieve the stated goals, in a first step the high-resolution PET image reconstruction THOR, previously developed in our group, was adapted to take advantage of the TOF information delivered by state-of-the-art PET systems. TOF-aware image reconstruction reduces image noise and improves convergence rate both of which is highly desirable. Based on these adaptations, this thesis describes new developments for improvement of TOF scatter correction and MLAA reconstruction and reports results obtained with the new algorithms on the Philips Ingenuity PET/MR jointly operated by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the University Hospital. A crucial requirement for quantitative TOF image reconstruction is TOF-aware scatter correction. The currently accepted reference method — the TOF extension of the single scatter simulation approach (TOF-SSS) — was implemented as part of the TOF-related modifications of THOR. The major drawback of TOF-SSS is a 3–7 fold increase in computation time required for the scatter estimation, compared to regular SSS, which in turn does lead to a considerable image reconstruction slowdown. This problem was addressed by development and implementation of a novel accelerated TOF scatter correction algorithm called ISA. This new algorithm proved to be a viable alternative to TOF-SSS and speeds up scatter correction by a factor of up to five in comparison to TOF-SSS. Images reconstructed using ISA are in excellent quantitative agreement with those obtained when using TOF-SSS while overall reconstruction time is reduced by a factor of two in whole-body investigations. This can be considered a major achievement especially with regard to the use of advanced image reconstruction in a clinical context. The second major topic of this thesis is contribution to improved attenuation correction in PET/MR by utilization of MLAA reconstruction. First of all, knowledge of the actual time resolution operational in the considered PET scan is mandatory for a viable MLAA implementation. Since vendor-provided figures regarding the time resolution are not necessarily reliable and do not cover count-rate dependent effects at all, a new algorithm was developed and implemented to determine the time resolution as a function of count rate. This algorithm (MLRES) is based on the maximum likelihood principle and allows to determine the functional dependency of the time resolution of the Philips Ingenuity PET/MR on the given count rate and to integrate this information into THOR. Notably, the present work proves that the time resolution of the Ingenuity PET/MR can degrade by more than 250 ps for the clinically relevant range of count rates in comparison to the vendor-provided figure of 550 ps which is only realized in the limit of extremely low count rates. Based on the previously described developments, MLAA could be integrated into THOR. The performed list-mode MLAA implementation is capable of deriving realistic, patient-specific attenuation maps. Especially, correct identification of osseous structures and air cavities could be demonstrated which is very difficult or even impossible with MR-based approaches to attenuation correction. Moreover, we have confirmed that MLAA is capable of reducing metal-induced artifacts which are otherwise present in MR-based attenuation maps. However, the detailed analysis of the obtained MLAA results revealed remaining problems regarding stability of global scaling as well as local cross-talk between activity and attenuation estimates. Therefore, further work beyond the scope of the present work will be necessary to address these remaining issues

Reconstruction Algorithms for Novel Joint Imaging Techniques in PET

Positron emission tomography (PET) is an important functional in vivo imaging modality with many clinical applications. Its enormously wide range of applications has made both research and industry combine it with other imaging modalities such as X-ray computed tomography (CT) or magnetic resonance imaging (MRI). The general purpose of this work is to study two cases in PET where the goal is to perform image reconstruction jointly on two data types. The first case is the Beta-Gamma image reconstruction. Positron emitting isotopes, such as 11C, 13N, and 18F, can be used to label molecules, and tracers, such as 11CO2, are delivered to plants to study their biological processes, particularly metabolism and photosynthesis, which may contribute to the development of plants that have higher yield of crops and biomass. Measurements and resulting images from PET scanners are not quantitative in young plant structures or in plant leaves due to low positron annihilation in thin objects. To address this problem we have designed, assembled, modeled, and tested a nuclear imaging system (Simultaneous Beta-Gamma Imager). The imager can simultaneously detect positrons (β+) and coincidence-gamma rays (γ). The imaging system employs two planar detectors; one is a regular gamma detector which has a LYSO crystal array, and the other is a phoswich detector which has an additional BC-404 plastic scintillator for beta detection. A forward model for positrons is proposed along with a joint image reconstruction formulation to utilize the beta and coincidence-gamma measurements for estimating radioactivity distribution in plant leaves. The joint reconstruction algorithm first reconstructs the beta and gamma images independently to estimate the thickness component of the beta forward model, and then jointly estimates the radioactivity distribution in the object. We have validated the physics model and the reconstruction framework through a phantom imaging study and imaging a tomato leaf that has absorbed 11CO2. The results demonstrate that the simultaneously acquired beta and coincidence-gamma data, combined with our proposed joint reconstruction algorithm, improved the quantitative accuracy of estimating radioactivity distribution in thin objects such as leaves. We used the Structural Similarity (SSIM) index for comparing the leaf images from the Simultaneous Beta-Gamma Imager with the ground truth image. The jointly reconstructed images yield SSIM indices of 0.69 and 0.63, whereas the separately reconstructed beta alone and gamma alone images had indices of 0.33 and 0.52, respectively. The second case is the virtual-pinhole PET technology, which has shown that higher resolution and contrast recovery can be gained by adding a high resolution PET insert with smaller crystals to a conventional PET scanner. Such enhancements are obtained when the insert is placed in proximity of the region of interest (ROI) and in coincidence with the conventional PET scanner. Intuitively, the insert may be positioned within the scanner\u27s axial field-of-view (FOV) and radially closer to the ROI than the scanner\u27s ring. One of the complicating factors of this design is the insert\u27s blocking the scanner\u27s lines-of-response (LORs). Such data may be compensated through attenuation and scatter correction in image reconstruction. However, a potential solution is to place the insert outside of the scanner\u27s axial FOV and to move the body to be in proximity of the insert. We call this imaging strategy the surveillance mode. As the main focus of this work, we have developed an image reconstruction framework for the surveillance mode imaging. The preliminary results show improvement in spatial resolution and contrast recovery. Any improvement in contrast recovery should result in enhancement in tumor detectability, which will be of high clinical significance

Reconstruction Algorithms for Novel Joint Imaging Techniques in PET

Positron emission tomography (PET) is an important functional in vivo imaging modality with many clinical applications. Its enormously wide range of applications has made both research and industry combine it with other imaging modalities such as X-ray computed tomography (CT) or magnetic resonance imaging (MRI). The general purpose of this work is to study two cases in PET where the goal is to perform image reconstruction jointly on two data types. The first case is the Beta-Gamma image reconstruction. Positron emitting isotopes, such as 11C, 13N, and 18F, can be used to label molecules, and tracers, such as 11CO2, are delivered to plants to study their biological processes, particularly metabolism and photosynthesis, which may contribute to the development of plants that have higher yield of crops and biomass. Measurements and resulting images from PET scanners are not quantitative in young plant structures or in plant leaves due to low positron annihilation in thin objects. To address this problem we have designed, assembled, modeled, and tested a nuclear imaging system (Simultaneous Beta-Gamma Imager). The imager can simultaneously detect positrons (β+) and coincidence-gamma rays (γ). The imaging system employs two planar detectors; one is a regular gamma detector which has a LYSO crystal array, and the other is a phoswich detector which has an additional BC-404 plastic scintillator for beta detection. A forward model for positrons is proposed along with a joint image reconstruction formulation to utilize the beta and coincidence-gamma measurements for estimating radioactivity distribution in plant leaves. The joint reconstruction algorithm first reconstructs the beta and gamma images independently to estimate the thickness component of the beta forward model, and then jointly estimates the radioactivity distribution in the object. We have validated the physics model and the reconstruction framework through a phantom imaging study and imaging a tomato leaf that has absorbed 11CO2. The results demonstrate that the simultaneously acquired beta and coincidence-gamma data, combined with our proposed joint reconstruction algorithm, improved the quantitative accuracy of estimating radioactivity distribution in thin objects such as leaves. We used the Structural Similarity (SSIM) index for comparing the leaf images from the Simultaneous Beta-Gamma Imager with the ground truth image. The jointly reconstructed images yield SSIM indices of 0.69 and 0.63, whereas the separately reconstructed beta alone and gamma alone images had indices of 0.33 and 0.52, respectively. The second case is the virtual-pinhole PET technology, which has shown that higher resolution and contrast recovery can be gained by adding a high resolution PET insert with smaller crystals to a conventional PET scanner. Such enhancements are obtained when the insert is placed in proximity of the region of interest (ROI) and in coincidence with the conventional PET scanner. Intuitively, the insert may be positioned within the scanner\u27s axial field-of-view (FOV) and radially closer to the ROI than the scanner\u27s ring. One of the complicating factors of this design is the insert\u27s blocking the scanner\u27s lines-of-response (LORs). Such data may be compensated through attenuation and scatter correction in image reconstruction. However, a potential solution is to place the insert outside of the scanner\u27s axial FOV and to move the body to be in proximity of the insert. We call this imaging strategy the surveillance mode. As the main focus of this work, we have developed an image reconstruction framework for the surveillance mode imaging. The preliminary results show improvement in spatial resolution and contrast recovery. Any improvement in contrast recovery should result in enhancement in tumor detectability, which will be of high clinical significance

A Four-Dimensional Image Reconstruction Framework for PET under Arbitrary Geometries

Positron Emission Tomography (PET) is a functional imaging modality with applications ranging from the treatment of cancer, studying neurological diseases and disease models. Virtual-Pinhole PET technology improves the image quality in terms of resolution and contrast recovery. The technology calls for having a detector with smaller crystals placed near a region of interest in a conventional whole-body PET scanner. The improvement is from the higher spatial sampling of the imaging area near the detector. A prototype half-ring PET insert built to study head-and-neck cancer imaging was extended to breast cancer imaging. We have built a prototype half-ring PET insert for head-and-neck cancer imaging applications. In the first half of this work, we extend the use of the insert to breast imaging and show that such a system provides high resolution images of breast and axillary lymph nodes while maintaining the full imaging field of view capability of a clinical PET scanner. We are focused on designing unconventional PET geometries for specific applications. A general purpose 4D PET reconstruction framework was created to estimate the radionuclide uptake in the subject. Quantitative estimation in PET requires precise modeling of PET physics. Data acquired in a PET scanner is well modeled as a Poisson counting process. Reconstruction given the forward model is implemented using MAP-OSEM. The framework is capable of reconstructing PET data under arbitrary position of the detector elements and different crystal sizes. A novel symmetry finding algorithm is created to reduce the system matrix size, without loss of resolution. The framework motivates investigation into different PET system geometries for different applications, as well as optimizing the design of PET systems. A generalized normalization procedure was developed to model unknown components. The programs are parallelized using OpenMP and MPI to run on small workstations as well as super-computing clusters. The performance of our reconstruction framework is presented through four novel and unconventional PET systems, each designed specifically for a different geometry. The Virtual-Pinhole half-ring system is a half-ring insert integrated into a Siemens Biograph-40, for head and neck imaging. The Flat-panel system is a modular insert system integrated into the Biograph-40, designed for breast cancer imaging. The MicroInsert II is the second generation full ring insert device, integrated into the MicroPET scanner to improve the resolution and contrast recovery of the MicroPET scanner. The Plant PET system is a PET system designed to image plants vertically, and integrated into a plant growth chamber. The improvement in speed/memory from symmetry finding is as high as a factor of 50 in some cases. Further improvements to the framework and state of the field are also discussed

Improving Quantification in Lung PET/CT for the Evaluation of Disease Progression and Treatment Effectiveness

Positron Emission Tomography (PET) allows imaging of functional processes in vivo by measuring the distribution of an administered radiotracer. Whilst one of its main uses is directed towards lung cancer, there is an increased interest in diffuse lung diseases, for which the incidences rise every year, mainly due to environmental reasons and population ageing. However, PET acquisitions in the lung are particularly challenging due to several effects, including the inevitable cardiac and respiratory motion and the loss of spatial resolution due to low density, causing increased positron range. This thesis will focus on Idiopathic Pulmonary Fibrosis (IPF), a disease whose aetiology is poorly understood while patient survival is limited to a few years only. Contrary to lung tumours, this diffuse lung disease modifies the lung architecture more globally. The changes result in small structures with varying densities. Previous work has developed data analysis techniques addressing some of the challenges of imaging patients with IPF. However, robust reconstruction techniques are still necessary to obtain quantitative measures for such data, where it should be beneficial to exploit recent advances in PET scanner hardware such as Time of Flight (TOF) and respiratory motion monitoring. Firstly, positron range in the lung will be discussed, evaluating its effect in density-varying media, such as fibrotic lung. Secondly, the general effect of using incorrect attenuation data in lung PET reconstructions will be assessed. The study will compare TOF and non-TOF reconstructions and quantify the local and global artefacts created by data inconsistencies and respiratory motion. Then, motion compensation will be addressed by proposing a method which takes into account the changes of density and activity in the lungs during the respiration, via the estimation of the volume changes using the deformation fields. The method is evaluated on late time frame PET acquisitions using ¹⁸F-FDG where the radiotracer distribution has stabilised. It is then used as the basis for a method for motion compensation of the early time frames (starting with the administration of the radiotracer), leading to a technique that could be used for motion compensation of kinetic measures. Preliminary results are provided for kinetic parameters extracted from short dynamic data using ¹⁸F-FDG

Incorporating accurate statistical modeling in PET: reconstruction for whole-body imaging

Tese de doutoramento em Biofísica, apresentada à Universidade de Lisboa através da Faculdade de Ciências, 2007The thesis is devoted to image reconstruction in 3D whole-body PET imaging. OSEM ( Ordered Subsets Expectation maximization ) is a statistical algorithm that assumes Poisson data. However, corrections for physical effects (attenuation, scattered and random coincidences) and detector efficiency remove the Poisson characteristics of these data. The Fourier Rebinning (FORE), that combines 3D imaging with fast 2D reconstructions, requires corrected data. Thus, if it will be used or whenever data are corrected prior to OSEM, the need to restore the Poisson-like characteristics is present. Restoring Poisson-like data, i.e., making the variance equal to the mean, was achieved through the use of weighted OSEM algorithms. One of them is the NECOSEM, relying on the NEC weighting transformation. The distinctive feature of this algorithm is the NEC multiplicative factor, defined as the ratio between the mean and the variance. With real clinical data this is critical, since there is only one value collected for each bin the data value itself. For simulated data, if we keep track of the values for these two statistical moments, the exact values for the NEC weights can be calculated. We have compared the performance of five different weighted algorithms (FORE+AWOSEM, FORE+NECOSEM, ANWOSEM3D, SPOSEM3D and NECOSEM3D) on the basis of tumor detectablity. The comparison was done for simulated and clinical data. In the former case an analytical simulator was used. This is the ideal situation, since all the weighting factors can be exactly determined. For comparing the performance of the algorithms, we used the Non-Prewhitening Matched Filter (NPWMF) numerical observer. With some knowledge obtained from the simulation study we proceeded to the reconstruction of clinical data. In that case, it was necessary to devise a strategy for estimating the NEC weighting factors. The comparison between reconstructed images was done by a physician largely familiar with whole-body PET imaging

Quantitative Yttrium-90 Bremsstrahlung SPECT/CT and PET/CT Study for 3D Dosimetry in Radiomicrosphere Therapy

Liver cancer ranks the third most common cause of cancer related mortality worldwide. Radiomicrosphere therapy (RMT), a form of radiation therapy, involves administration of Yttrium-90 (90Y) microspheres to the liver via the hepatic artery. 90Y microspheres bremsstrahlung SPECT/CT or PET/CT imaging could potentially identify an extrahepatic uptake. An early detection of such an uptake, thus, could initiate preventative measures early on. However, the quantitative accuracy of bremsstrahlung SPECT/CT images is limited by the wide and continuous energy spectrum of 90Y bremsstrahlung photons. 90Y PET/CT imaging is also possible but limited by the extremely small internal pair production decay. These limitation lead to inaccurate quantitation of microsphere biodistribution especially in small tumors. SPECT/CT and PET/CT acquisition of a Jasczak phantom with eight spherical inserts filled with 90Y3Cl solution were performed to measure the quantitative accuracy of the two imaging modalities. 90Y microsphere SPECT/CT data of 17 patients who underwent RMT for primary or metastatic liver cancer were acquired. Technetium-99m macroaggregated albumin (99mTc-MAA) SPECT/CT scans were also collected, but available for only twelve of the patients. SPECT/CT images from phantoms were used to determine the optimal iteration number for the iterative spatial resolution recovery algorithm. Methods for image based calculation of calibration factors for activity estimation from the patient and phantom 90Y bremsstrahlung SPECT/CT images were developed. Tumor areas were segmented using an active contour method. The 99mTc-MAA and 90Y microsphere SPECT/CT images were co-registered a priori for correlation analysis. Comparison of uptake on 99mTc-MAA and 90Y microsphere SPECT/CT images was assessed using tumor to healthy liver ratios. Furthermore, a three dimensional absorbed dose estimation algorithm was developed using the voxel S-value method. Absorbed doses within the tumor and healthy part of the liver were investigated for correlation with administered activity. Improvement in contrast to noise ratio and contrast recovery coefficients (QH) on patient and phantom 90Y bremsstrahlung SPECT/CT images as well as PET/CT images were achieved. Total activity estimations in liver and phantom gave mean percent errors of -4 ± 12% and -23 ± 41% for patient and phantom SPECT/CT studies. The pre and post-treatment images showed significant correlation (r = 0.9, p \u3c 0.05) with mean TLR of 9.2 ± 9.4 and 5.0 ± 2.2 on 99mTc-MAA and 90Y microspheres SPECT/CT respectively. The correlation between the administered activity and tumor absorbed dose was weak (r = 0.5, p \u3e 0.05), however, healthy liver absorbed dose increased with administered activity (r = 0.8, p \u3c 0.05)

고양자효율 다양극 광전자증배관을 이용한 비정시간 측정 가능 PET

학위논문 (박사)-- 서울대학교 대학원 : 의과대학 의과학과, 2019. 2. 이재성.In vivo imaging of physiological activities in molecular level has proven to be useful for diagnosing diseases in early stage. Positron emission tomography (PET) is a widely used molecular imaging technique that provides three-dimensional images of functional changes with high sensitivity by estimating the distribution of injected radiotracers. The advent of time-of-flight (TOF) PET scanners has dramatically improved the quality of generated images, which led to enhanced diagnostic power and/or reduced scan time or patient radiation dose. Herein, a prototype TOF PET scanner based on advanced high-quantum-efficiency multianode photomultiplier tubes (PMTs) is presented. Superior time performance of the scanner was demonstrated and improvement of reconstructed images was confirmed from various phantom studies. Finally, our system was verified to be capable of serving as a demonstration system that provides experimental evidences of the benefits of excellent time performance and to be useful for validating the feasibility of new PET applications that have been traditionally challenging.생체 내의 생리적 현상을 분자 수준으로 촬영하는 것이 가능해짐에 따라 질병의 조기 진단이 가능해지고 있다. 양전자 방출 단층촬영기법(PET)은 이와 같은 분자 영상 기법 중 하나로, 체내에 주사한 방사성 의약품의 체내 분포를 추정함으로써 생리적 기능의 변화를 높은 민감도로 촬영하여 3차원 영상으로 제공한다. 비정 시간 측정 가능 PET 장치는 영상의 품질을 향상시킴으로써 질병을 진단할 확률을 높이거나 환자의 촬영 시간 또는 방사선 피폭 정도를 감소시키는 효과를 나타내는 것으로 알려져 있다. 이에 본 연구에서는 고양자효율 다양극 광전자증배관을 이용하여 고성능의 비정 시간 측정 PET 프로토타입을 개발하였다. 다양한 팬텀 촬영을 수행한 결과 이 프로토타입은 우수한 시간 성능과 향상된 영상을 제공함을 확인하였다. 본 시스템을 활용하여 우수한 시간 성능이 갖는 장점을 실험적으로 입증할 수 있음과 기술적인 한계로 적용이 불가능했던 새로운 PET 응용 분야의 실현 가능성 평가가 가능함을 확인하였다.Contents Abstract i Contents iii List of Figures v List of Tables vii General Introduction 1 Chapter 1. Development of TOF PET detectors 3 1.1. Background 3 1.2. Materials and Methods 4 1.2.1. Photomultiplier Tube 4 1.2.2. Detector Design 6 1.2.3. Front-end Electronics 7 1.2.4. Data Acquisition 9 1.2.5. Experimental Setup 10 1.2.6. Analysis 12 1.2.7. Timing Resolution Optimization 13 1.3. Results 14 1.3.1. Optimal Setup 14 1.3.2. Detector Performance 16 1.3.3. Verification of the Optimized Timing Resolution 19 1.4. Discussion 21 Chapter 2. Proof-of-concept prototype TOF PET system 23 2.1. Background 23 2.2. Materials and Methods 24 2.2.1. Prototype TOF PET Scanner 24 2.2.2. System Performance Measurement 26 2.2.3. Image Quality Measurement 27 2.2.4. Robustness to Errors in Data Correction 29 2.2.5. Partial Ring Geometry and Limited Angle Tomography 30 2.2.6. Joint Estimation of Activity and Attenuation 32 2.2.7. Comparison to Conventional 600-ps TOF PET 34 2.3. Results 35 2.3.1. System Performance Measurement 35 2.3.2. Image Quality Measurement 40 2.3.3. Robustness to Errors in Data Correction 44 2.3.4. Partial Ring Geometry and Limited Angle Tomography 47 2.3.5. Joint Estimation of Activity and Attenuation 49 2.4. Discussion 51 General Discussion 55 Reference 57 국문 초록 64Docto

Regularized Image Reconstruction Algorithms for Dual-Isotope Myocardial Perfusion SPECT (MPS) Imaging Using a Cross-Tracer Prior

In simultaneous dual-isotope myocardial perfusion SPECT (MPS) imaging, data are simultaneously acquired to determine the distributions of two radioactive isotopes. The goal of this work was to develop penalized maximum likelihood (PML) algorithms for a novel cross-tracer prior that exploits the fact that the two images reconstructed from simultaneous dual-isotope MPS projection data are perfectly registered in space. We first formulated the simultaneous dual-isotope MPS reconstruction problem as a joint estimation problem. A cross-tracer prior that couples voxel values on both images was then proposed. We developed an iterative algorithm to reconstruct the MPS images that converges to the maximum a posteriori solution for this prior based on separable surrogate functions. To accelerate the convergence, we developed a fast algorithm for the cross-tracer prior based on the complete data OS-EM (COSEM) framework. The proposed algorithm was compared qualitatively and quantitatively to a single-tracer version of the prior that did not include the cross-tracer term. Quantitative evaluations included comparisons of mean and standard deviation images as well as assessment of image fidelity using the mean square error. We also evaluated the cross tracer prior using a three-class observer study with respect to the three-class MPS diagnostic task, i.e., classifying patients as having either no defect, reversible defect, or fixed defects. For this study, a comparison with conventional ordered subsets-expectation maximization (OS-EM) reconstruction with postfiltering was performed. The comparisons to the single-tracer prior demonstrated similar resolution for areas of the image with large intensity changes and reduced noise in uniform regions. The cross-tracer prior was also superior to the single-tracer version in terms of restoring image fidelity. Results of the three-class observer study showed that the proposed cross-tracer prior and the convergent algorithms improved the ima- - ge quality of dual-isotope MPS images compared to OS-EM.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85873/1/Fessler3.pd