Comparative evaluation of two commercial PET scanners, ECAT EXACT HR+ and Biograph 2, using GATE

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Abstract

Geant4 application for tomographic emission (GATE) is a generic Monte Carlo simulation platform based on a general-purpose code GEANT4 and designed to simulate positron emission tomography (PET) and single photon emission tomography systems. Monte Carlo simulations are used in nuclear medicine to model imaging systems and develop and assess tomographic reconstruction algorithms and correction methods for improved image quantification. The purpose of this study is to validate two GATE models of the commercial available PET scanner HR+ and the PET/CT Biograph 2. The geometry of the system components has been described in GATE, including detector ring, crystal blocks, PMTs etc. The energy and spatial resolution of the scanners as given by the manufacturers have been taken into account. The GATE simulated results are compared directly to experimental data obtained using a number of NEMA NU-2-2001 performance protocols, including spatial resolution, sensitivity and scatter fraction. All the respective phantoms are precisely modeled. Furthermore, an approximate dead-time model both at the level of single and coincidence events was developed so that the simulated count rate curve can satisfactorily match the experimental count rate performance curve for each scanner In addition a software tool was developed to build the sinograms from the simulated data and import them into the software for tomographic image reconstruction where the reconstruction algorithm of FBP3DRP was applied. An agreement of less than 0.8 mm was obtained between the spatial resolution of the simulated system and the experimental results. Also the simulated scatter fraction for the NEMA NU 2-2001 scatter phantom matched the experimental results to within 3% of measured values. Finally the ratio of the simulated sensitivities with sources radially offset 0 and 10 cm from the central axis of each of the two scanners reaches an agreement of less than 1% between the simulated and experimental values. This simulation code will be used in a second phase in order to study scatter phenomena and motion artifacts. The simulation results will be used to optimize image reconstruction algorithms, with emphasis on dynamic PET studies.

Introduction

Monte Carlo simulations are used in nuclear medicine to model positron emission tomography (PET) or single photon emission tomography (SPECT) imaging systems in order to develop and assess tomographic reconstruction algorithms and correction methods for improved image quantification [1]. For these purposes several simulation codes have been used in the past. Geant4 application for tomographic emission (GATE) is a generic Monte Carlo simulation platform based on a general-purpose code GEANT4 [2] and designed to answer the specific needs of PET/SPECT applications [3], [4]. GATE includes specific modules necessary to perform realistic simulations, including modules managing time and time-dependent processes (detector and source movements, radioactive decay, and dynamic acquisitions), complex source distributions and easy description of scanner geometry. The ability to synchronize all time-dependent components allows a coherent description of the acquisition process and is one of the most innovative features of this package. GATE is an open source software and its development and validation is carried out by members of the OpenGATE collaboration.

The purpose of this study is to validate a Monte Carlo model for the simulation of the commercial available SIEMENS PET scanners of ECAT EXACT HR+ and the PET/CT Biograph 2 using GATE simulation package (version 2.2.0). Both scanners were chosen because of their wide area of applications throughout the last years. Their comparative evaluation on a simulation level is going to contribute to our better understanding of the influence of each one of their basic operational parameters on the efficiency and sensitivity of these systems. Furthermore, the comparative presentation of both the simulation and experimental data can trigger a thorough analysis in order to optimize the simulation parameter values at GATE and achieve tolerable and satisfying agreement between simulation and experimental output. This simulation code will be used in a second phase in order to study scatter phenomena and motion artifacts. The simulation results will be used to optimize image reconstruction algorithms, with emphasis on dynamic PET studies.

Section snippets

Description of geometry

The scanner's physical and technical specifications were obtained from Siemens medical and CTI innovations data sheets. Direct measurements at the place of the installation were performed as well.

The ECAT EXACT HR+ PET scanner (Fig. 1) is located at Hammersmith Imanet Hospital at London. It consists of 4 block rings of 72 detector blocks each. Each block is constructed of an 8×8 BGO crystal array. The dimensions of each crystal element are 4.05 mm×4.39 mm×30 mm. The detector ring diameter is 82.4 

Results

The National Electrical Manufacturers Association (NEMA) performance protocols describe a series of explicitly defined experiments that have a significant recognition at the field of performance measurements for both PET and SPECT systems [8], [9]. Therefore we had designed our simulation experiments according to these protocols. We have used ROOT data analysis framework [10] as an output data format. The simulated values produced for each scanner about the performance parameters of spatial

Discussion and conclusion

The spatial resolution values, obtained using the simulated GATE model, are within 9% of the experimental values. Furthermore we observe that the results of the simulation are consistently indicating improved spatial resolution in comparison to the respective experimental measurements. These discrepancies are observed due to the absence of modeling within GATE the light shielding both within and between the detector blocks, the inherent limitations of the resolution of the photomultiplier

Acknowledgments

The authors would like to thank all the GATE users and developer community for their valuable suggestions and insight they have provided us during this validation study.

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