DROITE Workshop

24/10/2012 and 25/10/2012

in Valpré Lyon, accès / directions

Programme

Participants

Registration is free but mandatory / Inscription gratuite mais obligatoire

14h00 : Welcome, Valpré in Lyon

Wednesday 24 October

14h30 : Alignement et reconstruction par approche inverse pour la tomographie électronique,

Viet Dung (IFPEN)

Dans le domaine du raffinage, les supports de catalyseurs ont besoin d’être qualifiés/quantifiés à une échelle nanométrique; ceci nécessite des techniques d’imagerie 3D de haute précision afin de pouvoir remonter à diverses caractéristiques physico-chimiques. Une des techniques d'acquisition utilisées est la tomographie électronique (ou nanotomographie) et plus précisément, en ce qui concerne notre équipe, la Microscopie Electronique en Transmission (MET). Cette technique permet de reconstruire des volumes 3D à partir de séries de projections sous différents angles ceci à l'échelle nanométrique. Un problème majeur dans ce contexte est le mauvais alignement des projections. Les techniques d'alignement actuelles emploient habituellement des marqueurs de référence tels que des nanoparticules d'or. Lorsque l'utilisation de marqueurs n'est pas possible, l'alignement de projections adjacentes est obtenu par corrélation entre ces projections. Cependant, cette méthode donne parfois de mauvais résultats. Dans cette présentation, nous proposons une nouvelle méthode robuste de reconstruction nanotomographique, alternant alignements des projections et reconstruction. Elle est en deux étapes. La première étape consiste en un processus d'alignement initial s’appuyant sur la minimisation d'une fonction de coût basée sur des statistiques robustes, qui mesure la similarité entre une projection et les projections précédentes de la série. Elle vise à réduire les forts déplacements, résultant de l'acquisition entre les projections successives. Dans la seconde étape, les projections pré-recalées sont employées pour initialiser un processus itératif et alterné d'alignement et reconstruction, minimisant alternativement une fonction coût basée sur la reconstruction du volume et une autre fonction de coût basée sur l'alignement d’une projection avec sa version simulée obtenue à partir du volume reconstruit. A la fin de ce processus, on a une reconstruction correcte du volume, les projections étant correctement alignées. Nous avons testé avec succès notre méthode pour les projections réelles d’un support de catalyseur de zéolite.

15h15 : Mass and line geometry conservation in 2D dynamic ROI reconstruction,

Laurent Desbat (TIMC-IMAG, Grenoble)

We consider the mass preservation in dynamic tomography. We present an analytic compensation of deformations preserving the mass and the acquisition line geometry.We show 2D ROI reconstructions of truncated dynamic data.

16h15 : Pause...

16h30 : Nonlinear phase retrieval in line-phase tomography,

Valentina Davidoiu (CREATIS/INSA de Lyon)

Due to the rapid development of the third-generation X-rays synchrotron sources, phase-contrast X-rays imaging has attracted much interest recently. The phase contrast with hard X-ray can be obtained with propagation based or interferometry based techniques. In propagation based techniques, the hard X-ray beam is observed when it has propagated some distances after the sample in free space. The phase contrast image formation can be explained in the framework of the Fresnel diffraction theory and it is a highly nonlinear process. Yet, until recently, the best known algorithms were based on the linearization of the forward problem under restrictive assumptions. The spatial resolution of the linear methods can be refined by other method that takes into account the nonlinearity of the inverse problem. Here, we present a convergence comparison of a nonlinear approach for the phase retrieval problem involving regularizations with sparsity constraints. This approach uses alternatively a solution of the nonlinear problem based on the Fréchet derivative and a solution of the linear problem in wavelet coordinate with an iterative thresholding.

20h00 : Dinner, à / at Valpré (et nuit si réservée / and overnight, if registered)

Thursday 25 October

09h00 : Co-registration between a C-arm x-ray imaging system and positron tracking system,

Benjamin Spencer, (Carleton Univ. Ottawa, now at UJF Grenoble 1)

A technique has been developed at Carleton University to enable real time 3D tracking of objects using implanted positron emission sources termed PeTrack. PeTrack is useful for real time tracking of objects such as abdominal or lung tumours to allow for the accurate delivery of radiation therapy during patient respiration, as well as for the tracking and localization of surgical tools or devices used during interventional procedures. Co-registration between the two systems allows the display of objects tracked in 3D on to a 3D reconstructed image. This presentation outlines the methods utilized to acquire accurate 3D images from an x-ray C-arm: image intensifier distortion correction, x-ray scanner geometric calibration, and analytic short-scan image reconstruction. The method is then evaluated by comparing the generated reconstructed images to those obtained by the on-board Siemens C-arm reconstruction system. Additionally, a method of PeTrack geometric calibration was developed and evaluated. Lastly, the x-ray and PeTrack systems were co-registered and evaluated presenting an error of 1.6 +/- 0.2 mm between the location of sources identified on an image and their projected locations on to the image using the co-registration parameter. Sources of systematic error are discussed and subject to further evaluation.

10h00 : Design of a spatially-variable-focusing collimator and impact of the forward projection model in reconstruction for small-animal SPECT,

Didier Benoit, (QIM IMNC-IN2P3/CNRS, Orsay)

In SPECT imaging, the collimator is one of the most important components limiting the trade-off between spatial resolution and sensitivity. We recently proposed a spatially-variable-focusing cone-beam (SVFCB) collimator that can be used to improve the sensitivity compared to a parallel-beam collimator while reducing the truncation problem associated with fan-beam collimation. Using Monte Carlo simulations, we designed an SVFCB collimator in which the shortest focal length was within (SVFCB-In) the field of view (FOV). We studied the reconstruction feasibility in presence of the multiplexing induced by the collimation geometry. We also characterized the spatial resolution and sensitivity achievable with such a collimator compared to a parallel-hole collimator. For these two collimators, we studied the impact of the ray-tracing projector used in the forward projection and the improvement of image quality brought by modelling the point spread function (PSF) in the reconstruction process. Results show that spatial resolution and sensitivity are improved with the SVFCB-In collimator compared to parallel collimators. The projector model had a significant impact on the resulting image quality, with a spatial resolution decreasing from 1.7mm with a simple Siddon projector and no PSF model to 1.35 mm with a sophisticated projector including a PSF model for the parallel beam collimator. For the SVFCB-In collimator, a simple stationary and isotropic PSF is not an ideal model and introduces image distortion. Yet, using a sophisticated projector, the spatial resolution could also be decreased to 1.35 mm compared to 1.7 mm with the Siddon projector.

11h00 : Modeling a flexible Detector Response Function in small animal SPECT using Geant4,

Ziad El Bitar (IPHC, Strasbourg)

Clinical SPECT (Single Photon Emission Computed Tomography) systems equipped with pinhole collimators can achieve small animal imaging since they enable high magnification factor leading to high spatial resolution images. The utilization of Monte Carlo simulations to model the acquisition process and the propagation of the photons from their point of emission to their detection point and integrating the model into an iterative reconstruction algorithm improves the quality and the spatial resolution in the reconstructed images. However, pinhole SPECT systems are known to be very sensitive to geometrical misalignments. Geometrical misalignements are defined as the radial or axial shift of the collimator pinhole and/or twist and tilt of the detector heads and are introduced in the system each time the collimation device is changed (pinhole to parallel holes or vice versa). In this work we present a flexible detector response function design that takes into account the geometric misalignement and avoids a re-calculation of the whole system matrix for each exam.

12h00 : Lunch, at workshop site Valpre

14h00 : Metal artifact reduction in computed tomography

Katrien Van Slambrouck (Nuclear Medicine and Medical Imaging Research Center, K.U.Leuven)

Iterative reconstruction with a more sophisticated model can be used to reduce metal artefacts in computed tomography. However, it is usually very time consuming. Projection completion is more popular mainly because it is much faster. An important disadvantage of projection completion is the loss of information in the interpolated projections rays which may introduce new artefacts or loss of contrast for structures lying in these projection rays. In this work an accelerated iterative method is used. To reduce calculation time, the more sophisticated model is only applied to the small area in and around the metals. The remainder of the reconstruction volume is reconstructed using a less complex and less computationally demanding model. Both projection completion and the iterative method clearly reduce the streak and shadow artefacts. A better compromise between artefacts and contrast is observed for the iterative method.

14h45 : Resolution modeling in digital breast tomosynthesis

Koen Michielsen (Nuclear Medicine and Medical Imaging Research Center, K.U.Leuven)

Visualizing micro-calcifications adequately remains a challenge in digital breast tomosynthesis. We propose a maximum a posteriori algorithm which uses a plane by plane updating scheme for faster convergence. The scheme enables efficient implementation of an approximate model for position dependent resolution. An observer study shows an improvement in detection of micro-calcifications compared to the filtered backprojection method currently in use.

15h30 : Pause...

16h00 : Algorithme LM-MLEM de reconstruction d'images pour caméra Compton,

Xavier Lojacono (CREATIS, Lyon)

La caméra Compton est un dispositif dit à collimation électronique permettant de réaliser la reconstruction tomographique d'une source de rayonnement gamma. Les avantages de ce type de système résident dans la possibilité d'avoir une plus grande sensibilité par l'absence de collimateur mécanique, et dans la possibilité de reconstruire en 3D avec un dispositif fixe. Ces avantages en font un candidat prometteur pour application en médecine nucléaire et en hadronthérapie. La détection du rayonnement se fait en deux temps, d'abord par diffusion Compton des photons gamma, ensuite par absorption des photons diffusés. Les données recueillies à partir d'une détection permettent de localiser la source d'emission sur une surface de cône et l'ensemble de ces cônes permet de reconstruire la source. A l'heure actuelle, les algorithmes de reconstruction les plus en vogue sont itératifs et basés sur le maximum de vraisemblance. Nous présenterons ici le système de détection, la modélisation du problème de reconstruction et nous montrerons comment le modèle peut être implémenté dans l'algorithme de reconstruction MLEM.

17h00 : Registration of sliding objects using direction dependent B-splines decomposition,

Vivien Delmon (CLB-CREATIS, Lyon)

Abstract to follow.

This DROITE workshops are supported by a grant from << Région Rhône Alpes>> (ARC 6, T.I.C. et Usages Informatiques Innovants)

This workshop is supported by the labex CAMI, PRIMES and the ECCAMI project.