3,463 research outputs found
Evaluation of spacial resolution of a PET scanner through the simulation and experimental measurement of the Recovery coefficient
Purpose: In order to measure spatial resolution of a PET tomograph in clinical conditions, this study describes and
validates a method based on the recovery coefficient, a factor required to compensate underestimation in measured
radioactivity concentration for small structures.
Methods: In a PET image, the recovery factors of radioactive spheres were measured and their comparison with
simulated recovery coefficients yielded the tomographic spatial resolution. Following this methodology, resolution was
determined in different surrounding media and several conditions for reconstruction, including clinical conditions for brain
PET studies. All spatial resolution values were compared with those obtained using classical methods with point and
line sources.
Results: In each considered condition, spatial resolution of the PET image estimated using the recovery coefficient
showed good agreement with classical methods measurements, validating the procedure.
Conclusion: Measurement of the recovery coefficient provides an assessment of tomographic spatial resolution,
particularly in clinical studies conditions
Approaches Toward Combining Positron Emission Tomography with Magnetic Resonance Imaging
Positron emission tomography (PET) and magnetic resonance imaging (MRI) provide complementary information, and there has been a great deal of research effort to combine these two modalities. A major engineering hurdle is that photomultiplier tubes (PMT), used in conventional PET detectors, are sensitive to magnetic field. This thesis explores the design considerations of different ways of combining small animal PMT-based PET systems with MRI through experimentation, modelling and Monte Carlo simulation. A proof-of-principle hybrid PET and field-cycled MRI system was built and the first multimodality images are shown. A Siemens Inveon PET was exposed to magnetic fields of different strengths and the performance is characterized as a function of field magnitude. The results of this experiment established external magnetic field limits and design studies are shown for wide range of approaches to combining the PET system with various configurations of field-cycled MRI and superconducting MRI systems. A sophisticated Monte Carlo PET simulation workflow based on the GATE toolkit was developed to model the Siemens Inveon PET. Simulated PET data were converted to the raw Siemens list-mode format and were processed and reconstructed using the same processing chain as the data measured on the actual scanner. A general GATE add-on was developed to rapidly generate attenuation correction sinograms using the precise detector geometry and attenuation coefficients built into the emission simulation. Emission simulations and the attenuation correction add-on were validated against measured data. Simulations were performed to study the impact of radiofrequency coil components on PET image quality and to test the suitability of various MR-compatible materials for a dual-modality animal bed
Development Of A Scintillation Detector And The Influence On Clinical Imaging
The detector is the functional unit within a Positron Emission Tomography (PET) scanner, serving to convert the energy of radiation emitted from a patient into positional information, and as such contributes significantly to the performance of the scanner. While modern whole-body scanners use detectors composed of very many (i.e., 20000-30000) small pixels, typically ~4x4x20mm3 in size, several groups are actively investigating the performance of continuous crystals coupled to position sensitive photodetectors as an alternative detector design with a number of potential advantages, including improved spatial resolution and position sampling. This work in particular focuses on thick (≥14mm) continuous crystals in order to maintain the sensitivity of modern scanners. Excellent spatial resolution in continuous detectors that are thick, however, has proven difficult to achieve using simple positioning algorithms, leading to research in the field to improve performance. This thesis aims to investigate the effect of modifications to the scintillation light spread within the bulk of the scintillator to improve performance, focusing on the use of laser induced optical barriers (LIOBs) etched within thick continuous crystals, and furthermore aims to translate the effect on detector performance to scanner quantitation in patient studies.
The conventional continuous detector is first investigated by analyzing the various components of the detector as well as its limitations. It is seen that the performance of the detector is affected by a number of variables that either cannot be improved or may be improved only at the expense of greater complexity or computing time; these include the photodetector, the positioning algorithm, and Compton scatter in the detector. The performance of the detectors, however, is fundamentally determined by the light spread within the detector, and limited by the depth-dependence of the light spread and poor performance in the entrance region, motivating efforts to modify this aspect of the detector.
The feasibility and potential of LIOBs to fine-tune this light spread and improve these limitations is then studied using both experiments and simulations. The behavior of the LIOBs in response to optical light is investigated, and the opacity of the etchings is shown to be dependent on the parameters of the etching procedure. Thick crystals were also etched with LIOBs in their entrance region in a grid pattern in order to improve the resolution in the entrance region. Measurements show an overall improvement in spatial resolution: the resolution in the etched region of the crystals is slightly improved (e.g., ~0.8mm for a 25mm thick crystal), though in the unetched region, it is slightly degraded (e.g., ~0.4mm for a 25mm thick crystal). While the depth-dependence of the response of the crystal is decreased, the depth-of-interaction (DOI) performance is degraded as well. Simulation studies informed by these measurements show that the properties of the LIOBs strongly affect the performance of the crystal, and ultimately further illustrate that trade-offs in spatial resolution, position sampling, and DOI resolution are inherent in varying the light spread using LIOBs in this manner; these may be used as a guide for future experiments.
System Monte Carlo simulations were used to investigate the added benefit of improved detector spatial resolution and position sampling to the imaging performance of a whole-body scanner. These simulations compared the performance of scanners composed of conventional pixelated detectors to that of scanners using continuous crystals. Results showed that the improved performance (relative to that of 4-mm pixelated detectors) of continuous crystals with a 2-mm resolution, pertinent to both the etched 14mm thick crystal studied as well as potential designs with the etched 25mm thick crystal, increased the mean contrast recovery coefficient (CRC) of images by ~22% for 5.5mm spheres.
Last, a set of experiments aimed to test the correspondence between quantification in phantom and patient images using a lesion embedding methodology, so that any improvements determined using phantom studies may be understood clinically. The results show that the average CRC values for lesions embedded in the lung and liver agree well with those for lesions embedded in the phantom for all lesion sizes. In addition, the relative changes in CRC resulting from application of post-filters on the subject and phantom images are consistent within measurement uncertainty. This study shows that the improvements in CRC resulting from improved spatial resolution, measured using phantom studies in the simulations, are representative of improvements in quantitative accuracy in patient studies.
While unmodified thick continuous detectors hold promise for both improved image quality and quantitation in whole-body imaging, excellent performance requires intensive hardware and computational solutions. Laser induced optical barriers offer the ability to modify the light spread within the scintillator to improve the intrinsic performance of the detector: while measurements with crystals etched with relatively transmissive etchings show a slight improvement in resolution, simulations show that the LIOBs may be fine-tuned to result in improved performance using relatively simple positioning algorithms. For systems in which DOI information is less important, and transverse resolution and sensitivity are paramount, etching thick detectors with this design, fine-tuned to the particular thickness of the crystal and application, is an interesting alternative to the standard detector design
Small animal PET imaging using GATE Monte Carlo simulations : Implementation of physiological and metabolic information
Tese de doutoramento, (Engenharia Biomédica e Biofísica), Universidade de Lisboa, Faculdade de Ciências, 2010O rato/ratinho de laboratório é o modelo animal de escolha para o estudo dos processos fundamentais associados a determinadas patologias, como o cancro. Esta escolha deve-se a uma gama de factores que incluem uma grande homologia genética com o Homem. Assim sendo o
rato/ratinho é amplamente utilizado em laboratórios por todo o Mundo para estudo dos processos
celulares básicos associados á doença e à terapia. A comunidade laboratorial tem, nos últimos
anos, desenvolvido um grande interesse pela imagiologia não-invasiva destes animais. De entre as
diversas tecnologias de imagem aplicadas aos estudosin vivo de pequenos animais, a Tomografia
por Emissão de Positrões (PET) permite obter informação sobre a distribuição espacial e temporal
de moléculas marcadas com átomo emissor de positrões, de forma não invasiva.
Os traçadores utilizados para obter esta “imagem molecular” são administrados em baixas quantidades,
de tal forma que os processos biológicos que envolvem concentrações da ordem do nano
molar, ou mesmo inferiores, podem ser determinadas sem perturbar o processo em estudo. Muitas
combinações de diferentes moléculas com diferentes radionúclidos permitem traçar uma gama de
caminhos moleculares específicos (e.g. processos biológicos de receptores e síntese de transmissores
em caminhos de comunicação em células, processos metabólicos e expressão genética).
A imagem pode ser executada repetidamente antes e depois de intervenções permitindo o uso de
cada animal como o seu próprio controlo biológico.
A investigação já realizada em curso que aplicam a PET ao estudos de pequenos animais, tem permitido
compreender, entre outras coisas, a evolução de determinadas doenças e suas potenciais
terapias. Contudo, existem algumas dificuldades de implementação desta técnica já que a informação
obtida está condicionada pelos fenómenos físicos associados à interacção da radiação com
a matéria, pelos instrumentos envolvidos na obtenção da informação e pela própria fisiologia do
animal (por exemplo o seu movimento fisiológico). De facto, a fiabilidade da quantificação das imagens
obtidas experimentalmente, em sistemas PET dedicados aos pequenos animais, é afectada
ao mesmo tempo pelos limites de desempenho dos detectores (resolução espacial e em energia,
sensibilidade, etc.), os efeitos físicos como a atenuação e a dispersão, que perturbam a reconstrução
da imagem, e os efeitos fisiológicos (movimentos do animal). Na prática estes efeitos são
corrigidos com métodos de correcção específicos com a finalidade de extrair parâmetros quantitativos
fiáveis. Por outro lado, as características fisiológicas dos animais a estudar e a necessidade
da existência de animais disponíveis, são factores adicionais de complexidade.
Recentemente, tem sido dedicada alguma atenção aos efeitos resultantes dos movimentos fisiológicos,
nomeadamente do movimento respiratório, na qualidade das imagens obtidas no decurso de
um exame PET. Em particular, no caso do estudo dos tumores do pulmão (algo infelizmente muito
frequente em humanos), o movimento fisiológico dos pulmões é uma fonte de degradação das imagens
PET, podendo comprometer a sua resolução e o contraste entre regiões sãs e doentes deste
orgão. A precisão quantitativa na determinação da concentração de actividade e dos volumes funcionais
fica assim debilitada, sendo por vezes impedida a localização, detecção e quantificação do
radiotraçador captado nas lesões pulmonares. De modo a conseguir diminuir estes efeitos, existe
a necessidade de melhor compreender a influência deste movimento nos resultados PET.
Neste contexto, as simulações Monte Carlo são um instrumento útil e eficaz de ajuda à optimização
dos componentes dos detectores existentes, à concepção de novos detectores, ao desenvolviBaseados em modelos matemáticos dos processos físicos, químicos e, sempre que possível, biológicos,
os métodos de simulação Monte Carlo são, desde há muito, uma ferramenta privilegiada
para a obtenção de informação fiável da previsão do comportamento de sistemas complexos e por
maioria de razão, para uma sua melhor compreensão.
No contexto da Imagiologia Molecular, a plataforma de simulação Geant4 Application for Tomographic
Emission (GATE), validada para as técnicas de imagem de Medicina Nuclear, permite a
simulação por Monte Carlo dos processos de obtenção de imagem. Esta simulação pode mesmo
ser feita quando se pretende estudar a distribuição de emissores de positrões cuja localização
varia ao longo do tempo. Adicionalmente, estas plataformas permitem a utilização de modelos
computacionais para modelar a anatomia e a fisiologia dos organismos em estudo mediante a
utilização de uma sua representação digital realista denominada de fantôma. A grande vantagem
na utilização destes fantômas relaciona-se com o facto de conhecermos as suas características
geométricas (“anatómicas”) e de podermos controlar as suas características funcionais (“fisiológicas”).
Podemos assim obter padrões a partir dos quais podemos avaliar e aumentar a qualidade
dos equipamentos e técnicas de imagem.
O objectivo do presente trabalho consiste na modelação e validação de uma plataforma de simulação
do sistema microPET® FOCUS 220, usado em estudos de PET para pequenos animais,
utilizando a plataforma de simulação GATE. A metodologia adoptada procurou reproduzir de uma
forma realista, o ambiente de radiação e factores instrumentais relacionados com o sistema de
imagem, assim como o formato digital dos dados produzidos pelo equipamento. Foram usados
modelos computacionais, obtidos por segmentação de imagem de exames reais, para a avaliação
da quantificação das imagens obtidas. Os resultados obtidos indicam que a plataforma produz
resultados reprodutíveis, adequados para a sua utilização de estudos de pequenos animais em
PET.
Este objectivo foi concretizado estudando os efeitos combinados do tamanho das lesões, do rácio
de concentração de actividade lesão-para-fundo e do movimento respiratório na recuperação
de sinal de lesões esféricas localizadas no pulmão em imagens PET de pequenos animais. Para
este efeito, foi implementada no código GATE uma representação digital em 4D de um ratinho de
corpo inteiro (o fantôma MOBY). O MOBY permitiu reproduzir uma condição fisiológica que representa
a respiração em condição de "stress", durante um exame típico de PET pequeno animal, e
a inclusão de uma lesão esférica no pulmão tendo em conta o movimento da mesma. Foram realizadas
um conjunto de simulações estáticas e dinâmicas usando 2-Deoxy-[18F]fluoro-D-glucose
(FDG) tendo em consideração diferentes tamanhos das lesões e diferentes captações deste radiofármaco.
O ruído da imagem e a resolução temporal foram determinadas usando imagens 3D
e 4D. O rácio sínal-para-ruído (SNR), o rácio contraste-para-ruído (CNR), a relação lesão-fundo
(target-to-background activity concentration ratio- TBR), a recuperação de contraste (CR) e a recuperação
de volume (VR) foram também avaliados em função do tamanho da lesão e da actividade
captada. Globalmente, os resultados obtidos demonstram que a perda de sinal depende tanto do
tamanho da lesão como da captação de actividade na lesão. Nas simulações estáticas, onde não
foi simulado movimento, os coeficientes de recuperação foram influenciados pelo efeito de volume parcial para os tamanhos mais reduzidos de lesão. Além disso, o aumento do contraste na lesão produz um aumento significativo no desvio padrão da média de sinal recuperado resultando numa
diminuição no CNR e no SNR. Também concluímos que o movimento respiratório diminui significativamente
a recuperação do sinal e que esta perda depende principalmente do tamanho da lesão.
A melhor resolução temporal e resolução espacial foram obtidas nas simulações estáticas, onde
não existia movimento envolvido.
Os resultados simulados mostram que o efeito de volume parcial é dominante nas lesões mais
pequenas devido à resolução espacial do sistema FOCUS, tanto nas imagens estáticas como
nas dinâmicas. Além disso, para concentrações baixas de radiofármaco existe uma dificuldade
inerente em quantificar a recuperação de sinal nas lesões comprometendo a análise quantitativa dos dados obtidos.Organ motion has become of great concern in medical imaging only recently. Respiratory motion
is one source of degradation of PET images. Respiratory motion may lead to image blurring, which
may result in reduced contrast and quantitative accuracy in terms of recovered activity concentration
and functional volumes. Consequently, the motion of lungs hinders the localization, detection, and
the quantification of tracer uptake in lung lesions. There is, therefore, a need to better understand
the effects of this motion on PET data outcome.
Medical imaging methods and devices are commonly evaluated through computer simulation. Computer
generated phantoms are used to model patient anatomy and physiology, as well as the imaging
process itself. A major advantage of using computer generated phantoms in simulation studies
is that the anatomy and physiological functions of the phantom are known, thus providing a gold
standard from which to evaluate and improve medical imaging devices and techniques.
In this thesis, are presented the results of a research studied the combined effects of lesion size,
lesion-to-background activity concentration ratio and respiratory motion on signal recovery of spherical
lesions in small animal PET images using Monte Carlo simulation. Moreover, background
activity is unavoidable and it causes significant noise and contrast loss in PET images. For these
purposes, has been used the Geant4 Application for Tomographic Emission (GATE) Monte Carlo
platform to model the microPET®FOCUS 220 system. Additionaly, was implemented the digital 4D
Mouse Whole-Body (MOBY) phantom into GATE. A physiological “stress breathing” condition was
created for MOBY in order to reproduce the respiratory mouse motion during a typical PET examination.
A spherical lung lesion was implemented within this phantom and its motion also modelled.
Over a complete respiratory cycle of 0.37 s was retrieved a set of 10 temporal frames (including the
lesion movement) generated in addition to a non-gated data set. Sets of static (non-gated data) and
dynamic (gated data) 2-Deoxy-[18F]fluoro-D-glucose (FDG) simulations were performed considering
different lesion sizes and different activity uptakes. Image noise and temporal resolution were
determined on 3D and 4D images. Signal-to-Noise Ratio (SNR), Contrast-to-Noise Ratio (CNR),
Target-to-Background activity concentration Ratio (TBR), Contrast Recovery (CR) and Volume Recovery
(VR) were also evaluated as a function of lesion size and activity uptake.
Globally, the results obtained show that signal loss depends both on lesion size and lesion activity
uptake. In the non-gated data, where was no motion included (perfect motion correction), the recovery
coefficients were influenced by the partial volume effect for the smallest lesion size. Moreover,
the increased lesion contrast produces a significant increase on the standard deviation of the mean
signal recover. This led to a decrease in CNR and SNR. In addition, respiratory motion significantly
deteriorates signal recovery and this loss depends mainly of the lesion size. Best temporal resolution
(volume recovery) and spatial resolution was given by the non-gated data, where no motion is
involved.
The simulated results show that the partial volume effect is dominant for small objects due to limited
FOCUS system resolution in both 3D and 4D PET images. In addition, lower activity concentrations
significantly deteriorates the lesion signal recovery compromising quantitative analysis.Fundação para a Ciência e a Tecnologia (FCT) under grant nº SFRH/BD/22723/200
Efficient methodologies for system matrix modelling in iterative image reconstruction for rotating high-resolution PET
A fully 3D iterative image reconstruction algorithm has been developed for high-resolution PET cameras composed of pixelated scintillator crystal arrays and rotating planar detectors, based on the ordered subsets approach. The associated system matrix is precalculated with Monte Carlo methods that incorporate physical effects not included in analytical models, such as positron range effects and interaction of the incident gammas with the scintillator material. Custom Monte Carlo methodologies have been developed and optimized for modelling of system matrices for fast iterative image reconstruction adapted to specific scanner geometries, without redundant calculations. According to the methodology proposed here, only one-eighth of the voxels within two central transaxial slices need to be modelled in detail. The rest of the system matrix elements can be obtained with the aid of axial symmetries and redundancies, as well as in-plane symmetries within transaxial slices. Sparse matrix techniques for the non-zero system matrix elements are employed, allowing for fast execution of the image reconstruction process. This 3D image reconstruction scheme has been compared in terms of image quality to a 2D fast implementation of the OSEM algorithm combined with Fourier rebinning approaches. This work confirms the superiority of fully 3D OSEM in terms of spatial resolution, contrast recovery and noise reduction as compared to conventional 2D approaches based on rebinning schemes. At the same time it demonstrates that fully 3D methodologies can be efficiently applied to the image reconstruction problem for high-resolution rotational PET cameras by applying accurate pre-calculated system models and taking advantage of the system's symmetries
- …