J. Donnard

High Spatial Resolution in

The autoradiography is an imaging technique able to locate in 2 dimensions molecules labeled with beta radioactive emitters. The usual technique in use is based on solid device, mostly films or phosphor screens. We report on tests performed with PIM (parallel ionization multiplier) on microscope slides demonstrating that MPGD (micro pattern gas detector) incorporating micromegas micro-meshes have to be considered for a next generation of beta-imager. The main advantages come from the high spatial resolution measured (25 mum FWHM in two dimensions) and from the event by event reconstruction position measurement resulting in ldquoon-linerdquo imaging availability in which no ldquoafter developmentrdquo have to be performed.

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3γ imaging is a new nuclear medical imaging technique which has been suggested by Subatech laboratory, this technique involves locating three-dimensional position of the decay of an innovative radioisotope (β+, γ) emitter, the 44SC. In order to demonstrate experimentally the feasibility of 3γ imaging, a first prototype, XEMISI (XEnon Medical Imaging System) was developed. The last published results of XEMISI, including the energy resolution obtained in the LXe and the very low electronic noise are very promising. The next step is to construct new prototype XEMIS2, which is a full liquid xenon cylindrical camera, dedicated to small animal imaging. To assess the performances of this camera for 3-gamma imaging, we developed a simulation of such a system using GATE (Geant4 Application for Emission Tomography). The simulation of XEMISI shows good data-simulation agreement. Based on this study, we are confident in the simulation of XEMIS2 and we expect a spatial resolution of ~500 J.lm and an energy resolution better than 5% depending on the electric field. Moreover, the full efficiency of the 3-gamma imaging technique has been estimated to ~5 % by simulating a rat phantom.

-Imaging With a PIM Device

The Beta autoradiography is widely used in pharmacology or in biological fields to study the response of an organism to a certain kind of molecule. The image of the distribution is processed by studying the concentration of the radioactivity into different organs. We report on the development of an integrated apparatus based on a PIM device (Parallel Ionization Multiplier) able to process the image of 10 microscope slides at the same time over an area of 18*18 cm2. Thanks to a vacuum pump and a regulation gas circuit, 5 minutes is sufficient to begin an acquisition. All the electronics and the gas distribution are included in the structure leading to a transportable device. Special software has been developed to process data in real time with image visualization. Biological samples can be labelled with β emitters of low energy like 3H/14C or Auger electrons of 125I/99mTc. The measured spatial resolution is 30 μm in 3H and the trigger and the charge rate are constant over more than 6 days of acquisition showing good stability of the device. Moreover, collaboration with doctors and biologists of INSERM (National Institute for Medical Research in France) has started in order to demonstrate that MPGDs can be easily proposed outside a physics laboratory.

Simulation of liquid xenon time projection chambers and 3γ camera with GATE

In the context of medical imaging system, we develop an innovative technique, called 3 gamma. It consists in a direct 3D reconstruction of each decays of 44Sc radionuclide with a resolution below the centimeter. This breakthrough in instrumentation technique is only possible by the use of a new detection medium (liquid xenon) and a new detection structure compared with conventional imaging technique. Thanks to an ultra-low noise front-end electronics (below 100 electrons ENC) operating at liquid xenon temperature and a fast UV sensitive PMT, high spatial resolution and high energy resolution are achievable in 3D. This is particularly important for Compton imaging since all interactions in the medium have to be identified to derive the incoming gamma ray direction. A prototype (XEMIS1) is now in test at Subatech and shows promising results. We achieve an energy resolution of 8.9 % (FWHM) at 1.2 MeV with an electric field of 1kV/cm. All the cryogenic system is fully operational with a high purification rate and shows a very good stability. I will review all these aspects and introduce the next step of the project, XEMIS2, a larger prototype dedicated to the 3 gamma imaging of small animals.

Advancements of labelled radio-pharmaceutics imaging with the PIM-MPGD

We report on the electronic dedicated to the acquisition of the ionization current signal induced by the interactions of γ-rays inside a Compton telescope with Liquid Xenon. In order to achieve sub-millimeter resolution we successfully adapted an existing ASIC originally designed for semi-conductor detector with low input capacitance and low dark current called IDeF-x-Lxe, and fabricated in a standard 0.35 μm CMOS technology. With a copper pad area of 0.25 in2 of our first prototype, and a Micromegas micromesh located 50 μm above the anode used as Frisch grid, the noise measurement shows an ENC of ~100 erms- thanks to a meticulous setup. Each part of the coupling between the detector and the electronic has been specially tailored to cope with temperature constraints and with noise requirements.

XEMIS: A new Compton camera with liquid xenon

The β autoradiography is commonly used in preclinical research. This technique is able to locate, in 2 dimensions with a high spatial resolution, molecule labeled with β emitters. We report on the development of an integrated and transportable device base on the PIM (Parallel Ionization Multiplier) structure able to perform the image of 10 microscope slides in the same time. Thin slices of organs can be labeled with β emitters of low energy like ³H / ¹⁴C or Auger electrons emitters such as ¹²⁵I / ^99mTc. The measured spatial resolution over the whole area of 18×18 cm² is 30μm Full Width at Half Maximum in 2D with a ³H labeling. The sensitivity threshold is estimated to be close to 0.005 cps/min/mm². The use in proportional counter mode is suitable to perform on-line imaging and monitoring. New modalities such as large area imaging of entire rat slices or imaging of β emitters of high energy like ¹³¹I or ¹⁸F are now under development. In case of labeling with β emitters of high energy, the spatial resolution is estimated to be around 200 μm. First results obtained with these new configurations will be presented.