Reconstrucción de imagen en tomografía por emisión de positrones

Las imágenes de tomografía de emisión se generan mediante un algoritmo de reconstrucción, a partir de un conjunto de proyecciones adquiridas del objeto o paciente bajo examen. El procedimiento clásico de reconstrucción de imagen es la retroproyección filtrada (FBP). Este método es rápido y sencillo, pero no utiliza información estadística. Es un buen método para aplicaciones en las que el número de cuentas es alto (como tomografía de rayos X o CT), pero es peor cuando hay un bajo número de cuentas, como en imágenes de medicina nuclear. Los métodos iterativos de reconstrucción de imagen se han propuesto como alternativas a FBP. Estas técnicas tienen un coste computacional más alto que FBP pero producen imágenes de mejor contraste y relación señal-ruido. Los métodos iterativos eliminan los artefactos de líneas presentes en las imágenes FBP, reduciendo los falsos positivos y los falsos negativos cuando las lesiones están en la proximidad de órganos calientes. Este artículo presenta una visión de conjunto de los principios de la reconstrucción de imágenes para tomografía por emisión de positrones (PET); se introducen brevemente las bases matemáticas del método FBP para seguidamente presentar los métodos estadísticos de reconstrucción iterativa, principalmente los basados en la estimación de la máxima verosimilitud. También se comenta la técnica de subconjuntos ordenados para acelerar su cómputo, así como el uso de probabilidades a priori bayesianas, lo que permite la incorporación de información a priori (tal como restricciones de suavidad o información topológica parcialmente especificada) y así mejorar la calidad de la imagen. Finalmente se muestran ejemplos con fantomas y con estudios de pacientes para comparar los métodos presentadosIn emission tomography images are obtained from a reconstruction process using a set of measured projections of the object or the patient examined. The classical method for image reconstruction is filtered backprojection (FBP). This method is fast and simple, but it does not use any statistical information about the measurements. It is a good method for applications when the number of measured counts is high (e.g., in X-ray computed tomography, CT) but insufficient when applied for low-count projection data in nuclear molecular imaging. Iterative image reconstruction methods have been proposed as alternatives to FBP. Statistical iterative image reconstruction techniques show higher computational cost than FBP but are shown to produce images of better contrast and signalto- noise ratio. The elimination of streak artifacts present in FBP images with the use of iterative methods minimizes false-positive as well as false-negative results, when lesions are situated in the vicinity of hot organs. This paper presents an overview of the principles of image reconstruction for positron emission tomography (PET) and introduces briefly the mathematical background of the FBP method before entering into details of statistical iterative image reconstruction methods,mainly based on maximum likelihood estimation. A discussion is included about the ordered subsets technique for the acceleration of these methods and the Bayesian prior approach, which allows the incorporation of a priori information (such as smoothness constraints or partial specified topological information) and therefore further improves image quality. Some examples are included from phantom and patient studies to compare some of the methods presentedPublicad

Analysis and interpretation of dynamic FDG PET oncological studies using data reduction techniques

<p>Abstract</p> <p>Background</p> <p>Dynamic positron emission tomography studies produce a large amount of image data, from which clinically useful parametric information can be extracted using tracer kinetic methods. Data reduction methods can facilitate the initial interpretation and visual analysis of these large image sequences and at the same time can preserve important information and allow for basic feature characterization.</p> <p>Methods</p> <p>We have applied principal component analysis to provide high-contrast parametric image sets of lower dimensions than the original data set separating structures based on their kinetic characteristics. Our method has the potential to constitute an alternative quantification method, independent of any kinetic model, and is particularly useful when the retrieval of the arterial input function is complicated. In independent component analysis images, structures that have different kinetic characteristics are assigned opposite values, and are readily discriminated. Furthermore, novel similarity mapping techniques are proposed, which can summarize in a single image the temporal properties of the entire image sequence according to a reference region.</p> <p>Results</p> <p>Using our new cubed sum coefficient similarity measure, we have shown that structures with similar time activity curves can be identified, thus facilitating the detection of lesions that are not easily discriminated using the conventional method employing standardized uptake values.</p

Super-Resolution in Respiratory Synchronized Positron Emission Tomography

Respiratory motion is a major source of reduced quality in positron emission tomography (PET). In order to minimize its effects, the use of respiratory synchronized acquisitions, leading to gated frames, has been suggested. Such frames, however, are of low signal-to-noise ratio (SNR) as they contain reduced statistics. Super-resolution (SR) techniques make use of the motion in a sequence of images in order to improve their quality. They aim at enhancing a low-resolution image belonging to a sequence of images representing different views of the same scene. In this work, a maximum a posteriori (MAP) super-resolution algorithm has been implemented and applied to respiratory gated PET images for motion compensation. An edge preserving Huber regularization term was used to ensure convergence. Motion fields were recovered using a B-spline based elastic registration algorithm. The performance of the SR algorithm was evaluated through the use of both simulated and clinical datasets by assessing image SNR, as well as the contrast, position and extent of the different lesions. Results were compared to summing the registered synchronized frames on both simulated and clinical datasets. The super-resolution image had higher SNR (by a factor of over 4 on average) and lesion contrast (by a factor of 2) than the single respiratory synchronized frame using the same reconstruction matrix size. In comparison to the motion corrected or the motion free images a similar SNR was obtained, while improvements of up to 20% in the recovered lesion size and contrast were measured. Finally, the recovered lesion locations on the SR images were systematically closer to the true simulated lesion positions. These observations concerning the SNR, lesion contrast and size were confirmed on two clinical datasets included in the study. In conclusion, the use of SR techniques applied to respiratory motion synchronized images lead to motion compensation combined with improved image SNR and contrast, without any increase in the overall acquisition times

Design of a high resolution small animal octagonal PET scanner: preliminary studies

[Abstract] AMI International Conference 2003, September 21 - 27, Madrid, Spain: "High Resolution Molecular Imaging: from Basic Science to Clinical Applications"We present a preliminary study on the design of a high resolution small animal octagonal positron emission tomography (PET) scanner, based on Monte Carlo simulations. The purpose of this study is to evaluate the impact of several critical design parameters on the reconstructed image quality, as well as the calculation of the system matrix for iterative image reconstruction based on statistical modelsPublicad

Isotretinoin and psychopathology: a review

Isotretinoin, a synthetic oral retinoid that is used against severe nodulocystic acne, has been associated with various psychiatric side effects such as depression, suicidality and psychotic symptoms. A great number of reports on its effects have been published since its introduction into the market. However, a causal relationship has not been established and the link between isotretinoin use and psychiatric events remains controversial. The present paper reviews the available evidence regarding the association of isotretinoin and psychiatric side effects. All published material reporting psychiatric side effects following isotretinoin treatment, including case reports, case series, reports from adverse drug event reporting systems, prospective surveys and retrospective case-control studies, are presented. In addition, the neurobiology of the retinoids and possible biological mechanisms that may lead to psychopathology are described

Efficient methodology for 3D statistical reconstruction of high resolution coplanar PET/CT scanner

Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008A fully 3D statistical image reconstruction algorithm has been developed for a high-resolution coplanar PETtCT scanner based on rotating planar PET detectors. The system matrix has been modeled with custom Monte Carlo techniques optimized for the specific scanner architecture. The system model includes positron range, non-colinearity of gamma rays and crystal interaction modelling with attenuation and Compton scattering effects. Only 0.21 % of the system matrix columns are modeled in detail, obtaining the rest of the values with axial and transaxial voxel-driven symmetries. The iterative algorithm is a fully 3D approach, regularized with the anatomical registered image using a novel version of the minimum cross entropy (MXE) scheme, and accelerated employing ordered subsets. The proposed method has been shown to produce images with superior quality than 3D hybrid (FORE+2D-OSEM) algorithms applied on synthetic GATE data, as well as on real small animal acquisitionsThis work has been partly funded by the CDTEAM project and CENIT programme (Spanish Ministry of Industry), EMIL (ED Network of Excellence), CIBER CB07/09/0031 and RETIC-RECAVA (Spanish Ministry of Health) and TEC2007-64731/TCM(Spanish Ministry of Education and Science

Modeling the acquisition front-end in high resolution gamma-ray imaging

Proceeding of: 2004 IEEE Nuclear Science Symposium Conference Record, Rome, Italy, 16-22 October 2004The availability of synthetic realistic data enables design optimization, algorithm evaluation and verification of any digital system where a significant amount of digital signal processing is performed. The evolution of positron emission tomography cameras towards continuous sampling of individual position-sensitive photomultiplier anodes with processing algorithms implemented on digital programmable logic devices creates a new framework where new approaches to the γ-event detection are possible. We have developed a system model of the acquisition chain, including multi-layer phoswich, photomultiplier, front-end analog electronics, data acquisition and data processing. This processing includes estimation algorithms for the most relevant event parameters: energy, layerof- interaction, time picking-off and event location. The selected simulation platform couples gently to digital hardware simulation tools, in such a way that implemented models may generate reallike stimuli for the digital system under development. The modeling of the whole front-end electronics enables deeper understanding and tuning of different system trade-offs and provides a rapid and soft transition between specification and hardware development

Modeling the acquisition front-end in high resolution gamma-ray imaging

The availability of synthetic realistic data eases design optimization, algorithm evaluation and verification of any digital system where a significant amount of digital signal processing is performed. The evolution of positron emission tomography cameras towards continuous sampling of individual position-sensitive photomultiplier anodes with processing algorithms implemented on digital programmable logic devices creates a new framework where new approaches to the γ-event detection are possible. We have developed a system model of the acquisition chain, including multi-layer phoswich, photomultiplier, front-end analog electronics, data acquisition and digital processing. This processing includes estimation algorithms for the most relevant event parameters: energy, layer-of-interaction, time picking-off and event location. The selected simulation platform couples gently to digital hardware simulation tools, in such a way that implemented models may generate real-like stimuli for the digital system under development. The modeling of the whole front-end electronics enables deeper understanding and tuning of different system trade-offs and provides a rapid and soft transition between specification and hardware developmentPublicad

Olanzapine-associated neuroleptic malignant syndrome: Is there an overlap with the serotonin syndrome?

BACKGROUND: The neuroleptic malignant syndrome is a rare but serious condition mainly associated with antipsychotic medication. There are controversies as to whether "classical" forms of neuroleptic malignant syndrome can occur in patients given atypical antipsychotics. The serotonin syndrome is caused by drug-induced excess of intrasynaptic 5-hydroxytryptamine. The possible relationship between neuroleptic malignant syndrome and serotonin syndrome is at present in the focus of scientific interest. METHODS: This retrospective phenomenological study aims to examine the seventeen reported olanzapine – induced neuroleptic malignant syndrome cases under the light of possible overlap between neuroleptic malignant syndrome and serotonin syndrome clinical features. RESULTS: The serotonin syndrome clinical features most often reported in cases initially diagnosed as neuroleptic malignant syndrome are: fever (82%), mental status changes (82%) and diaphoresis (47%). Three out of the ten classical serotonin syndrome clinical features were concurrently observed in eleven (65%) patients and four clinical features were observed in seven (41%) patients. CONCLUSION: The results of this study show that the clinical symptoms of olanzapine-induced neuroleptic malignant syndrome and serotonin syndrome are overlapping suggesting similarities in underlying pathophysiological mechanisms