Christian Morel

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.

GATE: a Geant4-based simulation platform for PET and SPECT integrating movement and time management

GATE, the Geant4 application for tomographic emission, is a simulation platform developed for PET and SPECT. It combines a powerful simulation core, the Geant4 toolkit, with newly developed software components dedicated to nuclear medicine. In particular, it models the passing of time during real acquisitions, allowing it to handle dynamic systems such as decaying source distributions or moving detectors. We present several series of results that illustrate the possibilities of this new platform. The simulation of decaying sources is illustrated on a dual-isotope acquisition with multiple time-frames. Count rate curves taking into account random coincidences and dead-time are shown for a dual-crystal setup and for a small-animal PET scanner configuration. Simulated resolution curves and reconstructed images are shown for rotating PET scanners. Lastly, we present first comparisons of simulated point-spread functions and spectra with experimental results obtained from a small-animal gamma camera prototype.

GATE, a Geant4-based simulation platform for PET integrating movement and time management

GATE, the Geant4 Application for Tomographic Emission, is a simulation platform developed for PET and SPECT. It combines a powerful simulation core (the Geant4 toolkit) and a large range of developments dedicated to nuclear medicine. In particular, it models the passing of time during real acquisitions, allowing to handle dynamic systems such as decaying source distributions or moving detectors. We present several series of results that illustrate the possibilities of this new platform. The simulation of decaying sources is illustrated on a dual-isotope acquisition with multiple time-frames. Count rate curves taking into account random coincidences and dead-time are shown for a dual-crystal set-up and for a small-animal PET scanner configuration. Simulated resolution curves and reconstructed images are shown for rotating PET scanners. Lastly, we present comparisons of simulated point-spread functions and spectra with experimental results obtained from a small-animal gamma camera prototype.

A data acquisition system for medical imaging

A data acquisition system for medical imaging applications is presented. Developed at CPPM, it provides high performance generic data acquisition and processing capabilities. The DAQ system is based on the PICMG xTCA standard and is composed of 1 up to 10 cards in a single rack, each one with 2 Altera Stratix IV FPGAs and a Fast Mezzanine Connector (FMC). Several mezzanines have been produced, each one with different functionalities. Some examples are: a mezzanine capable of receiving 36 optical fibres with up to 180 Gbps sustained data rates or a mezzanine with 12 × 5 Gbps input links, 12 × 5 Gbps output links and an SFP+ connector for control purposes. Several rack sizes are also available, thus making the system scalable from a one card desktop system useful for development purpose up to a full featured rack mounted DAQ for high end applications. Depending on the application, boards may exchange data at speeds of up to 25.6 Gbps bidirectional sustained rates in a double star topology through backplane connections. Also, front panel optical fibres can be used when higher rates are required by the application. The system may be controlled by a standard Ethernet connection, thus providing easy integration with control computers and avoiding the need for drivers. Two control systems are foreseen. A Socket connection provides easy interaction with automation software regardless of the operating system used for the control PC. Moreover a web server may run on the Envision cards and provide an easy intuitive user interface. The system and its different components will be introduced. Some preliminary measurements with high speed signal links will be presented as well as the signal conditioning used to allow these rates.

The FAVOR algorithm for 3D PET data and its implementation using a network of transputers

The recent appearance of septa-retractable positron emission tomography (PET) scanners has created a demand for fast three-dimensional (3D) reconstruction. While algorithms based on filtered backprojection have been developed to reconstruct PET data acquired in 3D (volume mode), they are computationally intensive owing to the large numbers of coincidence lines and image voxels that must be processed. Such linear algorithm are well suited to parallel implementation on a network of individual processors. The authors present an algorithm for fast volume reconstruction (FAVOR) of PET data, and describe its parallel implementation on a network of 44 T800 transputers. A reconstruction time of 1200 s has been achieved with this configuration of 44 T800 transputers. A reconstruction time of 1200 s has been achieved with this configuration of 44 T800 transputers comparable to that required by a single i860 processor. The prospects exist for 3D reconstruction times of around 120 s with the new generation of T9000 transputers.

First K-Edge Imaging With a Micro-CT Based on the XPAD3 Hybrid Pixel Detector

We investigate the capability to perform K-edge imaging with the newly developed micro-CT PIXSCAN based on the XPAD3 hybrid pixel detector. The XPAD3 detector surface of 8 cm ×11 cm makes it possible to perform whole body mouse imaging. We present a proof of principle of K-edge imaging of mouse-size phantoms filled with Silver and Iodine solutions. Results are compared with standard X-ray absorption tomography for several solution densities.

Normalisation of Histogrammed List Mode Data

Many PET scanners nowadays have the possibility to record event-by-event information, known as list mode data. This has the advantage of keeping the data in the highest possible resolution (both temporal and spatial). In most cases, list mode data are then binned into sinogram format before reconstruction. In this paper, we discuss at which stage normalisation factors should be introduced. It is shown that noise is greatly reduced by performing the normalisation after the binning. We illustrate this with acquired and simulated data for the quad HiDAC camera.

In Vivo Measurement of Glucose Utilization in Rats using a β-Microprobe: Direct Comparison with Autoradiography

A new β-microprobe (βP) has been used to locally measure the time–concentration curve of a radiolabeled substance. The βP, analogous to positron emission tomography methodology, is useful for in vivo animal studies because it can acquire time–concentration curves with high temporal and spatial resolution. Using [18F]fluoro-2-deoxy-d-glucose and βP, we evaluated the reliability of the biologic parameters and we compared this method with the [14C]2-deoxy-d-glucose autoradiography. βP time–concentration curves in three regions of the brain were obtained from 24 rats. Four kinetic parameters (K1-k4) were estimated from 60-minute experimental periods using a three-compartment model. Best fits were obtained when the vascular fraction (Fv) was estimated simultaneously with the four kinetic parameters (K1-k4). The mean estimated Fv values were about 5.5% for the frontal cortex regions and 8.0% for the cerebellum. Correlation coefficients higher than 0.830 were observed between regional cerebral metabolic rates for glucose (rCMRglc) values obtained by βP and autoradiography. In addition, the βP-derived input function was similar to that obtained by manual sampling. Our findings show that reliable rCMRglc values can be obtained using βP.

Design study for the ClearPET/XPAD small animal PET/CT scanner

We present a small animal PET/CT scanner design based on the ClearPET high resolution PET scanner and the XPAD3 hybrid pixel X-ray detector developed at CPPM. The design study of the configuration and functionality of the combined system is discussed. Using the GATE Monte Carlo simulation platform, the complete system is being investigated for achieving a fully integrated high performance PET/CT scanner. For the new design, we rearrange the LSO/LuYAP phoswich detector modules of the ClearPET demonstrator on its rotating gantry, eliminating the existing axial gaps between the rings of ClearPET modules and increasing its sensitivity by 71%. The functionality for simultaneous scans has been verified experimentally using CT and PET components in one setup. These measurements showed that an additional 500 mum thick brass shielding of the ClearPET detector modules is sufficient to prevent saturation of the PET detectors by low energy scattered X-ray photons.

Measured and simulated specifications of Lausanne ClearPET scanner demonstrator

This paper presents the measurements obtained with the partial ring Lausanne ClearPET scanner demonstrator and compares them against GATE Monte Carlo simulations. Radial and tangential spatial resolutions were measured up to 4 cm from the scanner axis. FWHM spatial resolutions range from 1.3 mm on axis to 2.6 mm at 4 cm from the axis. Excellent agreement is observed between the measured and simulated spatial resolutions. Coincidence time resolution of the LSO/LuYAP detector modules amounts to 4.8 ns FWHM. Absolute sensitivity measured for two detector modules facing each other amounts to 0.068%, compared to 0.070% simulated for 7.1 g/cc LuYAP with 70% lutetium atomic fraction. Simulation of a 4-ring ClearPET scanner with 20 LSO/LuYAP detector modules per ring gives an absolute sensitivity of 4.42%. DOI effect is shown on a reconstructed image of a Mini-Derenzo phantom. Images of a [ 18F]FDG rat brain scan are presented