M.C. Maas

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.

Monolithic scintillator PET detectors with intrinsic depth-of-interaction correction

We developed positron emission tomography (PET) detectors based on monolithic scintillation crystals and position-sensitive light sensors. Intrinsic depth-of-interaction (DOI) correction is achieved by deriving the entry points of annihilation photons on the front surface of the crystal from the light sensor signals. Here we characterize the next generation of these detectors, consisting of a 20 mm thick rectangular or trapezoidal LYSO:Ce crystal read out on the front and the back (double-sided readout, DSR) by Hamamatsu S8550SPL avalanche photodiode (APD) arrays optimized for DSR. The full width at half maximum (FWHM) of the detector point-spread function (PSF) obtained with a rectangular crystal at normal incidence equals approximately 1.05 mm at the detector centre, after correction for the approximately 0.9 mm diameter test beam of annihilation photons. Resolution losses of several tenths of a mm occur near the crystal edges. Furthermore, trapezoidal crystals perform almost equally well as rectangular ones, while improving system sensitivity. Due to the highly accurate DOI correction of all detectors, the spatial resolution remains essentially constant for angles of incidence of up to at least 30 degrees . Energy resolutions of approximately 11% FWHM are measured, with a fraction of events of up to 75% in the full-energy peak. The coincidence timing resolution is estimated to be 2.8 ns FWHM. The good spatial, energy and timing resolutions, together with the excellent DOI correction and high detection efficiency of our detectors, are expected to facilitate high and uniform PET system resolution.

Optical simulation of monolithic scintillator detectors using GATE/GEANT4

Much research is being conducted on position-sensitive scintillation detectors for medical imaging, particularly for emission tomography. Monte Carlo simulations play an essential role in many of these research activities. As the scintillation process, the transport of scintillation photons through the crystal(s), and the conversion of these photons into electronic signals each have a major influence on the detector performance; all of these processes may need to be incorporated in the model to obtain accurate results. In this work the optical and scintillation models of the GEANT4 simulation toolkit are validated by comparing simulations and measurements on monolithic scintillator detectors for high-resolution positron emission tomography (PET). We have furthermore made the GEANT4 optical models available within the user-friendly GATE simulation platform (as of version 3.0). It is shown how the necessary optical input parameters can be determined with sufficient accuracy. The results show that the optical physics models of GATE/GEANT4 enable accurate prediction of the spatial and energy resolution of monolithic scintillator PET detectors.

Experimental characterization of monolithic-crystal small animal PET detectors read out by APD arrays

Minimizing dead space is one way to increase the detection efficiency of small-animal PET scanners. By using monolithic scintillator crystals (e.g., 20 mm/spl times/10 mm/spl times/10 mm LSO), loss of efficiency due to inter-crystal reflective material is minimized. Readout of such crystals can be performed by means of one or more avalanche photo-diode (APD) arrays optically coupled to the crystal. The entry point of a gamma photon on the crystal surface can be estimated from the measured distribution of the scintillation light over the APD array(s). By estimating the entry point, correction for the depth-of-interaction (DOI) is automatically provided. We are studying the feasibility of such detector modules. To this end, a 64-channel test setup has been developed. Experiments to determine the effect on the spatial resolution of crystal surface finish and detector geometry have been carried out. The first results of these experiments are presented and compared to simulation results. The crystal surface finish has only a small influence on the spatial resolution. The spatial resolution of 20 mm/spl times/10 mm/spl times/10 mm detectors is significantly better when read out on the front side than when read out on the back side. With a 20 mm/spl times/10 mm/spl times/20 mm crystal coupled to two APD arrays, a very small resolution degradation of only /spl sim/0.2 mm is observed for an incidence angle of 30/spl deg/ compared to normal incidence.

Using Cramer-Rao theory combined with Monte Carlo simulations for the optimization of monolithic scintillator PET detectors

We are investigating the possibility of using monolithic scintillator blocks as detectors for small animal positron emission tomography (PET). These detectors consist of several cm/sup 3/ of scintillating material, read out by one or more avalanche photo-diode (APD) arrays. The entry point of an incoming gamma photon is estimated from the distribution of the scintillation light over the APD pixels. To optimize the detector design, the influence of different design parameters is investigated using Geant4 simulations. To make it possible to study the influence of individual design parameters on the intrinsic spatial resolution of the detector, the use of a performance measure is proposed that is independent of the algorithm used to estimate the entry point, namely the Cramer-Rao lower bound on the estimation of the coordinates of a point source of light inside the crystal. To illustrate the use of this method, the influence of optical transport inside the detector is investigated for different detector designs, surface finishes and APD pixel sizes. A comparison with resolutions obtained from simulations involving beams of 511 keV annihilation photons indicates that this approach gives valid results.

Signal to Noise Ratio of APD-Based Monolithic Scintillator Detectors for High Resolution PET

Monolithic scintillator detectors, consisting of several cm3 of scintillating material coupled to one or more Hamamatsu S8550 avalanche photodiode (APD) arrays, are proposed as detectors for high resolution positron emission tomography (PET). In this work, the factors contributing to the variance on the signals are investigated, and their effects on the energy, time and spatial resolutions are analyzed. Good agreement was found between a model of the energy resolution and experiments with a 20 x 10 x 10 mm3 LYSO:Ce crystal coupled to a single channel large-area APD (LAAPD). With the same crystal coupled to an APD array, differences between model and experiment were observed at high APD gain. The measured energy resolution of ~11% FWHM was dominated by scintillation photon statistics, with less important roles for the APD excess noise factor and electronic noise. On the other hand, electronic noise was an important factor both for the time and the spatial resolutions. The time resolution was found to depend strongly on the APD bias voltage, and was best at the highest bias. A time resolution of 1.6 ns full width at half maximum (FWHM) was measured against a BaF2 -PMT detector. The best spatial resolution measured was 1.64 mm FWHM, without correction for the ~0.9 mm FWHM measurement beam. It is estimated that an intrinsic spatial resolution of 1.26 mm FWHM can be achieved at the center of the detector with an infinitely narrow test beam.

Initial Characterization of a Nonpixelated Scintillator Detector in a PET Prototype Demonstrator

Previous experimental results have demonstrated that a PET detector module based on a 20times10times10 mm LSO block read out by a Hamamatsu S8550 APD array is able to determine the impinging position of perpendicularly incident 511-keV photons with a resolution better than 2-mm FWHM. A prototype PET demonstrator using two of these detector heads was build and evaluated. The axial and transaxial resolution measured in two-dimensional (2-D) OSEM and 2-D FBP reconstructed images is better than 1.5- and 2-mm FWHM, respectively, for radial distances less than 15 mm. A Monte Carlo simulation showed that a four-ring PET scanner based on these monolithic scintillator blocks has a sensitivity which is about 2.5 times higher than the sensitivity of a similar system equipped with detector heads using a 4times8 matrix of 2times2times10 mm LSO pixels

Model of the point spread function of monolithic scintillator PET detectors for perpendicular incidence

PURPOSE Previously, we demonstrated the potential of positron emission tomography detectors consisting of monolithic scintillation crystals read out by arrays of solid-state light sensors. We reported detector spatial resolutions of 1.1-1.3 mm full width at half maximum (FWHM) with no degradation for angles of incidence up to 30 degrees, energy resolutions of approximately 11% FWHM, and timing resolutions of approximately 2 ns FWHM, using monolithic LYSO:Ce3+ crystals coupled to avalanche photodiode (APD) arrays. Here we develop, validate, and demonstrate a simple model of the detector point spread function (PSF) of such monolithic scintillator detectors. METHODS A PSF model was developed that essentially consists of two convolved components, one accounting for the spatial distribution of the energy deposited by annihilation photons within the crystal, and the other for the influences of statistical signal fluctuations and electronic noise. The model was validated through comparison with spatial resolution measurements on a detector consisting of an LYSO:Ce3+ crystal read out by two APD arrays. RESULTS The model is shown to describe the measured detector spatial response well at the noise levels found in the experiments. In addition, it is demonstrated how the model can be used to correct the measured spatial response for the influence of the finite diameter of the annihilation photon beam used in the experiments, thus obtaining an estimate of the intrinsic detector PSF. CONCLUSIONS Despite its simplicity, the proposed model is an accurate tool for analyzing the detector PSF of monolithic scintillator detectors and can be used to estimate the intrinsic detector PSF from the measured one.

Experimental characterization of novel small animal PET detector modules based on scintillation crystal blocks read out by APD arrays

Minimizing dead space is one way to increase the detection efficiency of small-animal PET scanners. By using monolithic blocks of scintillating material (e.g. 20 mmtimes10 mmtimes10 mm LSO), loss of efficiency due to inter-crystal reflective material is minimized. Readout of such blocks can be performed by means of one or more avalanche photo-diode (APD) arrays optically coupled to the block. The primary event position and depth of interaction (DOI) information are derived from the measured distribution of the scintillation light over the APD array(s). We are studying the feasibility of such detector modules, both by simulation and by experiment. A 64-channel setup for testing the above type of detector modules has been developed. Experiments to verify the effect of crystal surface finish, detector geometry and reconstruction algorithm parameters on the spatial resolution have been carried out. The first results of these experiments are presented in this paper, and compared to simulation results. This research is conducted in collaboration with the Crystal Clear Collaboration (CCC)

Performance of APD-based monolithic-crystal detectors for small animal PET

Recent years have seen an increased interest in high resolution, high sensitivity detectors for small animal positron emission tomography (PET). Detectors based on large (e.g. several cm/sup 3/ of LSO) monolithic scintillators optically coupled to one or more avalanche photodiode (APD) array(s) minimize loss of sensitivity due to dead space. In such detectors, the primary event position and depth of interaction (DOI) correction are derived from the measured distribution of scintillation light over the APD array(s). The performance of detectors consisting of a monolithic LYSO scintillator read out by two Hamamatsu S8550 APD arrays is investigated. Monte Carlo simulations indicate that the scanner sensitivity and uniformity are improved when trapezoidal crystals are used instead of rectangular ones. The performance of both types of detectors was studied. Experiments show that the influence of the angle of incidence on the spatial resolution of both detector types is small, ranging from /spl sim/2.0 mm FWHM at 0/spl deg/ to /spl sim/2.4 mm FWHM at 30/spl deg/. An energy resolution of 11.5% at 511 keV and a time resolution of 2.4 ns was measured. Monte Carlo simulations predict a sensitivity of /spl sim/21% for a point source at the center of the field of view for a four-ring scanner based on trapezoidal monolithic detectors.