J.M. Létang

X-ray tomography applied to the characterization of cellular materials. Related finite element modeling problems

The analyses of several materials exhibiting a cellular structure have been carried out using X-ray tomography. This new technique allows the three dimensional and non destructive visualisation of the studied materials at the scale of their cellular microstructure. Qualitative examples are given for metal foams, bread and cellular concrete. The similarity between these materials is striking. It has been measured by quantitative 3D image processing. The different Finite Element Methods available today to produce meshes from these images are presented and discussed in the final part of this paper.

On the Application of X‐ray Microtomography in the Field of Materials Science

The principle of the tomography technique and the different possible set-ups, which can be used to obtain medium-(10 lm) and high(1 lm) resolution, three-dimensional, non-destructive images, are shown in this paper. Illustrations are made of the applications of the technique in the field of materials science. Examples are given for medium-resolution images of metallic foams and model metal matrix composites that are reinforced with spherical particles. High-resolution examples are shown for aluminium alloys. For low-absorbent materials we show that the phase contrast obtained using synchrotron radiation can provide a valuable solution. The quantitative use of these images, coupled with in-situ tensile tests or used for the simple analysis of the initial microstructure of several structural materials, is also described.

Monitoring the Bragg peak location of 73MeV∕u carbon ions by means of prompt γ-ray measurements

By means of a time-of-flight technique, we measured the longitudinal profile of prompt γ-rays emitted by 73MeV∕u C13 ions irradiating a polymethyl methacrylate target. This technique allowed us to minimize the shielding against neutrons and scattered γ-rays, and to correlate prompt gamma emission to the ion path. This correlation, together with a high counting rate, paves the way toward real-time monitoring of the longitudinal dose profile during ion therapy treatments. Moreover, the time correlation between the prompt gamma detection and the transverse position of the incident ions measured by a beam monitor can provide real-time three dimensional control of the irradiation.

Design Guidelines for a Double Scattering Compton Camera for Prompt-

In hadrontherapy in order to fully take advantage of the assets of the ion irradiation, the position of the Bragg peak has to be monitored accurately. Here, we investigate a monitoring method relying on the detection in real time of the prompt γ emitted quasi instantaneously during the nuclear fragmentation processes. Our detection system combines a beam hodoscope and a double scattering Compton camera. The prompt-γ emission points are reconstructed by intersecting the ion trajectories given by the hodoscope and the Compton cones reconstructed with the camera. We propose here to study in terms of point spread function and efficiency the theoretical feasibility of the emission points reconstruction with our set-up in the case of a photon point source in air. First we analyze the nature of all the interactions which are likely to produce an energy deposit in the three detectors of the camera. It is underlined that upper energy thresholds in both scatter detectors are required in order to select mainly Compton events (one Compton interaction in each scatter detector and one interaction in the absorber detector). Then, we study the influence of various parameters such as the photon energy and the inter-detector distances on the Compton camera response. These studies are carried out by means of Geant4 simulations. We use a source with a spectrum corresponding to the prompt-γ spectrum emitted during the carbon ion irradiation of a water phantom. In the current configuration, the spatial resolution of the Compton camera is about 6 mm (Full Width at Half Maximum) and the detection efficiency 10-5. Finally, provided the detection efficiency is increased, the clinical applicability of our system is considered.

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PURPOSE Proton CT (pCT) has the potential to accurately measure the electron density map of tissues at low doses but the spatial resolution is prohibitive if the curved paths of protons in matter is not accounted for. The authors propose to account for an estimate of the most likely path of protons in a filtered backprojection (FBP) reconstruction algorithm. METHODS The energy loss of protons is first binned in several proton radiographs at different distances to the proton source to exploit the depth-dependency of the estimate of the most likely path. This process is named the distance-driven binning. A voxel-specific backprojection is then used to select the adequate radiograph in the distance-driven binning in order to propagate in the pCT image the best achievable spatial resolution in proton radiographs. The improvement in spatial resolution is demonstrated using Monte Carlo simulations of resolution phantoms. RESULTS The spatial resolution in the distance-driven binning depended on the distance of the objects from the source and was optimal in the binned radiograph corresponding to that distance. The spatial resolution in the reconstructed pCT images decreased with the depth in the scanned object but it was always better than previous FBP algorithms assuming straight line paths. In a water cylinder with 20 cm diameter, the observed range of spatial resolutions was 0.7 - 1.6 mm compared to 1.0 - 2.4 mm at best with a straight line path assumption. The improvement was strongly enhanced in shorter 200° scans. CONCLUSIONS Improved spatial resolution was obtained in pCT images with filtered backprojection reconstruction using most likely path estimates of protons. The improvement in spatial resolution combined with the practicality of FBP algorithms compared to iterative reconstruction algorithms makes this new algorithm a candidate of choice for clinical pCT.

Imaging During Ion Beam Therapy: A Monte Carlo Simulation Study

Prompt-gamma profile was measured at WPE-Essen using 160 MeV protons impinging a movable PMMA target. A single collimated detector was used with time-of-flight (TOF) to reduce the background due to neutrons. The target entrance rise and the Bragg peak falloff retrieval precision was determined as a function of incident proton number by a fitting procedure using independent data sets. Assuming improved sensitivity of this camera design by using a greater number of detectors, retrieval precisions of 1 to 2 mm (rms) are expected for a clinical pencil beam. TOF improves the contrast-to-noise ratio and the performance of the method significantly.

Filtered backprojection proton CT reconstruction along most likely paths

Monte Carlo simulations are nowadays essential tools for a wide range of research topics in the field of radiotherapy. They also play an important role in the effort to develop a real-time monitoring system for quality assurance in proton and carbon ion therapy, by means of prompt-gamma detection. The internal theoretical nuclear models of Monte Carlo simulation toolkits are of decisive importance for the accurate description of neutral or charged particle emission, produced by nuclear interactions between beam particles and target nuclei. We assess the performance of Geant4 nuclear models in the context of prompt-gamma emission, comparing them with experimental data from proton and carbon ion beams. As has been shown in the past and further indicated in our study, the prompt-gamma yields are consistently overestimated by Geant4 by a factor of about 100% to 200% over an energy range from 80 to 310 MeV/u for the case of (12)C, and to a lesser extent for 160 MeV protons. Furthermore, we focus on the quantum molecular dynamics (QMD) modeling of ion-ion collisions, in order to optimize its description of light nuclei, which are abundant in the human body and mainly anticipated in hadrontherapy applications. The optimization has been performed by benchmarking QMD free parameters with well established nuclear properties. In addition, we study the effect of this optimization on charged particle emission. With the usage of the proposed parameter values, discrepancies reduce to less than 70%, with the highest values being attributed to the nucleon-ion induced prompt-gammas. This conclusion, also confirmed by the disagreement we observe in the case of proton beams, indicates the need for further investigation on nuclear models which describe proton and neutron induced nuclear reactions.

Real-time proton beam range monitoring by means of prompt-gamma detection with a collimated camera

In hadron therapy in order to fully take advantage of the assets of the ion irradiation, the position of the Bragg peak has to be monitored accurately. Here, we propose a monitoring method relying on the detection in real time of the prompt γ emitted quasi instantaneously during the nuclear fragmentation processes. Our detection system combines a beam hodoscope and a double scattering Compton camera. The prompt γ emission points are reconstructed by intersecting the ion trajectories given by the hodoscope and the Compton cones reconstructed with the camera. We studied the influence of various parameters such as the photon energy and the inter-detector distances on the Compton camera response to a photon point source. This study was carried out by means of Geant4 simulations. In the current configuration, for a photon source with a typical prompt γ spectrum, the spatial resolution of the Compton camera is about 5.6 mm and the detection efficiency 10-5.

Assessment and improvements of Geant4 hadronic models in the context of prompt-gamma hadrontherapy monitoring

Proton computed tomography (CT) has been described as a solution for imaging the proton stopping power of patient tissues, therefore reducing the uncertainty of the conversion of x-ray CT images to relative stopping power (RSP) maps and its associated margins. This study aimed to investigate this assertion under the assumption of ideal detection systems. We have developed a Monte Carlo framework to assess proton CT performances for the main steps of a proton therapy treatment planning, i.e. proton or x-ray CT imaging, conversion to RSP maps based on the calibration of a tissue phantom, and proton dose simulations. Irradiations of a computational phantom with pencil beams were simulated on various anatomical sites and the proton range was assessed on the reference, the proton CT-based and the x-ray CT-based material maps. Errors on the tissues RSP reconstructed from proton CT were found to be significantly smaller and less dependent on the tissue distribution. The imaging dose was also found to be much more uniform and conformal to the primary beam. The mean absolute deviation for range calculations based on x-ray CT varies from 0.18 to 2.01 mm depending on the localization, while it is smaller than 0.1 mm for proton CT. Under the assumption of a perfect detection system, proton range predictions based on proton CT are therefore both more accurate and more uniform than those based on x-ray CT.

Design study of a Compton camera for prompt γ imaging during ion beam therapy

The authors present a scheme for motion detection exploiting temporal integration and local contextual information. A multiscale temporal decomposition is supplied to the original sequence. Change detection is performed using a likelihood test at each temporal scale. The decision process is formalized within a statistical regularization framework and takes advantage of a tracking module. Motion detection is achieved by minimizing an energy function. This function involves three terms, expressing (1) adequacy between temporal variations at different scales and motion labels, (2) local spatial regularization, and (3) coherence between temporal prediction of change area locations and motion labels. Experimental results on real scenes are reported.<<ETX>>