In-vivo dark-field and phase-contrast x-ray imaging.

Novel radiography approaches based on the wave nature of x-rays when propagating through matter have a great potential for improved future x-ray diagnostics in the clinics. Here, we present a significant milestone in this imaging method: in-vivo multi-contrast x-ray imaging of a mouse using a compact scanner. Of particular interest is the enhanced contrast in regions related to the respiratory system, indicating a possible application in diagnosis of lung diseases (e.g. emphysema).

Nonlinear least-squares modeling of 3D interaction position in a monolithic scintillator block.

This paper presents a study of possible models to describe the relation between the scintillation light point-of-origin and the measured photo detector pixel signals in monolithic scintillation crystals. From these models the X, Y and depth of interaction (DOI) coordinates can be estimated simultaneously by nonlinear least-square fitting. The method depends only on the information embedded in the signals of individual events, and therefore does not need any prior position training or calibration. Three possible distributions of the light sources were evaluated: an exact solid-angle-based distribution, an approximate solid-angle distribution and an extended approximate solid-angle-based distribution which includes internal reflection at side and bottom surfaces. The performance of the general model using these three distributions was studied using Monte Carlo simulated data of a 20 x 20 x 10 mm lutetium oxyorthosilicate (Lu2SiO5 or LSO) block read out by 2 Hamamatsu S8550 avalanche photo diode arrays. The approximate solid-angle-based model had the best compromise between resolution and simplicity. This model was also evaluated using experimental data by positioning a narrow 1.2 mm full width at half maximum (FWHM) beam of 511 keV photons at known positions on the 20 x 20 x 10 mm LSO block. An average intrinsic resolution in the X-direction of 1.4 mm FWHM was obtained for positions covering the complete block. The intrinsic DOI resolution was estimated at 2.6 mm FWHM.

GATE: a simulation toolkit for PET and SPECT.

Monte Carlo simulation is an essential tool in emission tomography that can assist in the design of new medical imaging devices, the optimization of acquisition protocols and the development or assessment of image reconstruction algorithms and correction techniques. GATE, the Geant4 Application for Tomographic Emission, encapsulates the Geant4 libraries to achieve a modular, versatile, scripted simulation toolkit adapted to the field of nuclear medicine. In particular, GATE allows the description of time-dependent phenomena such as source or detector movement, and source decay kinetics. This feature makes it possible to simulate time curves under realistic acquisition conditions and to test dynamic reconstruction algorithms. This paper gives a detailed description of the design and development of GATE by the OpenGATE collaboration, whose continuing objective is to improve, document and validate GATE by simulating commercially available imaging systems for PET and SPECT. Large effort is also invested in the ability and the flexibility to model novel detection systems or systems still under design. A public release of GATE licensed under the GNU Lesser General Public License can be downloaded at http:/www-lphe.epfl.ch/GATE/. Two benchmarks developed for PET and SPECT to test the installation of GATE and to serve as a tutorial for the users are presented. Extensive validation of the GATE simulation platform has been started, comparing simulations and measurements on commercially available acquisition systems. References to those results are listed. The future prospects towards the gridification of GATE and its extension to other domains such as dosimetry are also discussed.

A fully 3D small PET scanner.

A fully 3D small PET scanner based on a novel detection principle for gamma rays is described. It uses BaF2 scintillator and photosensitive wire chambers. Extensive tests with technical prototypes have shown that such a system will have a detection efficiency for gamma rays comparable with what can be obtained with the more traditional approach, and a spatial resolution determined by the size of the crystals. The expected performances of the scanner, based on our measurements and on simulations, are given.

Progress in the development of a PET scanner based on BaF2 scintillator and photosensitive wire chambers.

The work presented is part of a design study for a Positron Emission Tomograph (PET) scanner based on the use of BaF2 scintillator and photosensitive wire chambers. The detection efficiency for gamma radiation of 511 keV is found close to 100% for a sufficiently large crystal. For a matrix of small and elongated crystals as one would use in a PET scanner (5 x 5 x 50 mm3) we obtained 6 photoelectrons per 511 keV deposited. The following variants and alternatives were also studied: operation of the wire chamber at atmospheric pressure; double readout where the crystals are read on one side with a photomultiplier to give time and energy resolution, and on the other side with a wire chamber to localise the event; and Csl photocathodes. Encouraging results have been obtained for each of these, but particularly the Csl photocathodes look very promising.