Tae Yong Song

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.

Optimization of pinhole collimator for small animal SPECT using Monte Carlo simulation

The aim of this study is to design an optimized pinhole collimator using Monte Carlo simulation for the development of an ultra high-resolution SPECT using a position sensitive photo-multiplier tube. Simulations using Monte Carlo N-Particle Transport code, version 4c were performed to model the pinhole SPECT system. The simulation geometries consist of a cone-shaped pinhole collimator with tungsten aperture and a NaI(Tl) scintillation crystal measuring 6 mm in thickness and 120 mm in diameter. Spatial resolution, sensitivity, edge penetration, and scatter fraction were simulated by changing the pinhole diameter and channel height. The optimal ranges of pinhole diameter and channel height were determined from tradeoff curves of resolution and sensitivity and from penetration and scatter fraction. Tradeoff curves allowed us to determine the optimal range of pinhole diameter to be from 1 mm to 1.5 mm for the system configured in this study. The penetration and scatter fraction curve indicated that the channeled aperture was preferable over knife-edge. The optimal range of channel height was from 0.3 to 0.6 mm. The results demonstrate that the pinhole collimator designed in this study could be utilized to perform ultra high-resolution small animal imaging.

Evaluation of maximum-likelihood position estimation with Poisson and Gaussian noise models in a small gamma camera

It has been reported that maximum-likelihood position-estimation (MLPE) algorithms offer advantages of improved spatial resolution and linearity over conventional Anger algorithm in gamma cameras. While the fluctuation of photon measurements is more accurately described by Poisson than Gaussian distribution model, the likelihood function of a scintillation event assumed to be Gaussian could be more easily implemented and might provide more consistent outcomes than Poisson based MLPE. The purpose of this study is to evaluate the performances of the noise models, Poisson and Gaussian, in MLPE for the localization of photons in a small gamma camera (SGC) using NaI(Tl) plate and PSPMT. The SGC consisted of a single NaI(Tl) crystal, 10 cm diameter and 6 mm thick, optically coupled to a PSPMT (Hamamatsu R3292-07). The PSPMT was read out using a resistive charge divider, which multiplexes 28(X) by 28(Y) cross wire anodes into four channels. Poisson and Gaussian based MLPE methods have been implemented using experimentally measured detector response functions (DRF). The intrinsic resolutions estimated by Anger logic, Poisson and Gaussian based MLPE were all 3.1 mm. Integral uniformities were 19.9%, 12.0% and 9.8%, and linearities were 1.0 mm, 0.5 mm and 0.05 mm, for Anger logic, Poisson and Gaussian based MLPE, respectively. MLPEs considerably improved linearity and uniformity compared to Anger logic. Gaussian based MLPE, which is easy to implement, allowed to obtain better linearity and uniformity performances than the Poisson based MLPE.

A Compact SPECT/CT System for Small Animal Imaging

A dual-modality compact SPECT/CT system for small animal imaging was developed. The SPECT system consisted of a pinhole collimator and continuous NaI(Tl) scintillation crystal coupled to a PSPMT. The CT system consisted of a microfocus X-ray tube and a CMOS flat-panel detector. The SPECT system was mounted perpendicular to the X-ray system. Individual projections of the SPECT and the CT were acquired by rotating the animal on a vertical axis in front of the detectors. The SPECT and CT images were reconstructed using OSEM and Feldkamps cone-beam algorithms, respectively. Mouse and rat SPECT images demonstrated detailed activity distribution at the expected structures. A CT image obtained with 40 kVp and 0.5 mA presented high-resolution anatomic details. Fused SPECT/CT images demonstrated good agreement between the CT images and the corresponding uptake of the radiotracer. The SPECT/CT system developed in this study provides high-quality dual-modality images and could be useful to obtain functional images with high resolution morphology information

Performance amelioration for small animal SPECT using optimized pinhole collimator and image correction technique

The aim of this study was to improve the performance of pinhole single photon emission computed tomography (SPECT) fabricated with a position sensitive photomultiplier tube using an optimized pinhole collimator and image correction technique. The center-of-rotation was aligned to prevent mechanical shift using a leveling-laser system. Based on the previous results of pinhole aperture simulation and optimization study, a pinhole collimator for the small animal SPECT was fabricated in the range of optimal pinhole diameter and channel height. Previously reported image correction technique of position mapping, energy calibration and flood correction procedures developed for array type scintillator was revised and applied to plate type crystal. Phantom and small animal studies were performed to investigate the system performance. Image corrections, center-of-rotation alignment, and collimator optimization were necessary to obtain high resolution and high quality SPECT images. The SPECT system ameliorated by this study could be utilized in small animal and molecular imaging studies required to provide high spatial resolution with moderate sensitivity.

Characterization of two deep-diffusion avalanche photodiode array prototypes with different optical coatings

The aim of this paper was to characterize two deep-diffusion avalanche photodiode (APD) arrays for positron emission tomography (PET). Two different prototypes developed by radiation monitoring devices (RMD) were investigated. Quantum efficiency was determined as a function of wavelength in the visible-ultraviolet rays region. Capacitance was determined as a function of bias. Dark current and gain were measured at several fixed temperatures versus bias. Finally, bulk and surface contributions to the dark current were estimated.

Development of a motion correction system for respiratory-gated PET study

A respiratory motion during whole-body imaging has been recognized as a source of image quality degradation and reduces the quantitative accuracy of positron emission tomography (PET) study. The aim of this study is to evaluate respiratory gating system and to develop a respiratory motion correction system using trigger generating device built in-house and gated-PET data acquisition mode. We utilized a commercially available laser optical sensor to detect respiratory motion during PET scanning. Each respiratory cycle is divided into 4 bins defined from average peak interval and irregular peak within the breathing motion. The acquired data within the time bins correspond to different positions within the breathing cycle and stored for the post motion correction. Motion data of diaphragm and chest wall was calculated by CT image acquisition during the normal inspiration and expiration position. In the images of a phantom, the blurring artifact due to breathing motion was reduced by our correction method. This technique improves the quantitative specific activity of the tracer which is distorted because of the respiratory motion.

Development of a motion detecting system for respiratory-gated PET using laser optical displacement sensor

A respiratory motion during whole-body imaging has been recognized as a source of image degradation and induces distortions in positron emission tomography (PET) study. The aim of this study is to evaluate respiratory motion using lung simulator and to develop a respiratory motion detecting system using optical laser sensor. We utilized a commercially available laser optical sensor to detect respiratory motion during PET scanning. Each respiratory cycle is divided into 4 bins defined as average peak interval and irregular peak within the patients breathing motion. The acquired data within the time bins correspond to different positions within the breathing cycle and stored for the post motion correction. Artifact of breathing motion was evaluated with the phantom study. In the images of a human subject, the blurring artifact due to breathing motion was divided by our detecting system. This technique improves the quantitative specific activity of the tracer which is distorted by the respiratory motion.

A simulation study for SPECT multi-pinhole detector optimization

The aim of this study is to derive optimized parameters for a detector module employing scintillation crystal and multi-pinhole collimator which could be utilized in a variety of SPECT systems. A detector module was simulated to have 100 mm /spl times/ 100 mm active area with 7 mm thick CsI(Tl) crystal. Monte Carlo simulation studies were performed to determine the optimal number of pinholes using Geant4 application for tomographic emission (GATE). The number of pinholes was varied from 1 to 225, with a 2 mm pinhole diameter and 50 mm focal length. Perpendicular lead septa were employed, between pinholes, to prevent projections from overlapping. Hot and cold rod phantoms were used to evaluate the performance of the proposed system. SPECT images were reconstructed using the OSEM algorithm. Activity distribution was well visualized, and 12 to 6 mm diameter rods could be resolved in the reconstructed images. In this study we have designed a multi-pinhole imaging system providing good resolution and system sensitivity over a large FOV. The detector module, applicable to various SPECT systems, could provide improved performance.

Motion Correction of Respiratory-Gated PET/CT Images Using Polynomial Warping

A respiratory motion during whole-body imaging has been recognized as a source of image quality degradation and reduces the quantitative accuracy of positron emission tomography (PET) study. The aim of this study is to correct respiratory gated PET/CT image using region growing and nonlinear warping. We utilized a commercially available optical laser sensor to detect respiratory motion during PET/CT scanning. The acquired data within the time bins corresponded to different positions within the breathing cycle and stored for the post motion correction. Motion data of diaphragm and chest wall were calculated by gated-CT image acquisition during the normal inspiration and expiration. This technique improves the quantitative specific activity of the tracer which is distorted because of the respiratory motion.