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.

Validation of the GATE Monte Carlo simulation platform for modelling a CsI(Tl) scintillation camera dedicated to small-animal imaging.

Monte Carlo simulations are increasingly used in scintigraphic imaging to model imaging systems and to develop and assess tomographic reconstruction algorithms and correction methods for improved image quantitation. GATE (GEANT4 application for tomographic emission) is a new Monte Carlo simulation platform based on GEANT4 dedicated to nuclear imaging applications. This paper describes the GATE simulation of a prototype of scintillation camera dedicated to small-animal imaging and consisting of a CsI(Tl) crystal array coupled to a position-sensitive photomultiplier tube. The relevance of GATE to model the camera prototype was assessed by comparing simulated 99mTc point spread functions, energy spectra, sensitivities, scatter fractions and image of a capillary phantom with the corresponding experimental measurements. Results showed an excellent agreement between simulated and experimental data: experimental spatial resolutions were predicted with an error less than 100 microns. The difference between experimental and simulated system sensitivities for different source-to-collimator distances was within 2%. Simulated and experimental scatter fractions in a [98-182 keV] energy window differed by less than 2% for sources located in water. Simulated and experimental energy spectra agreed very well between 40 and 180 keV. These results demonstrate the ability and flexibility of GATE for simulating original detector designs. The main weakness of GATE concerns the long computation time it requires: this issue is currently under investigation by the GEANT4 and the GATE collaborations.

An improved analytical detector response function model for multilayer small-diameter PET scanners.

The optimization of spatial resolution is a critical consideration in the design of small-diameter positron emission tomography (PET) scanners for animal imaging, and is often addressed with Monte Carlo simulations. As a faster and simpler solution, we have developed a new analytical model of the PET detector response function, and implemented the model for a small single-slice, multilayer PET scanner. The accuracy of the model has been assessed by comparison with both Monte Carlo simulations and experimental measurements published in the literature. Results from the analytical model agreed well with the Monte Carlo method, being noise free and two to three orders of magnitude faster. The only major discrepancy was a slight underestimation of the width of the point spread function by the analytical method as inter-crystal scatter is neglected. We observed good agreement between the predictions of the model and experimental measurements. For two large-diameter scanners additional discrepancies were seen due to photon acollinearity, which is not considered in the model. We have shown that the simple and fast analytical detector response function model can provide accurate estimates of spatial resolution for small-diameter PET scanners, and could be a useful tool for several applications, complementing or cross-validating other simulation methods.

Comparison of fluorine-18 and bromine-76 imaging in positron emission tomography.

State of the art positron emission tomography (PET) systems allow for scatter and attenuation correction. However, the size of the structure being studied and the region of interest (ROI) chosen also influence the accuracy of measurements of radioactive concentration. Furthermore, the limited spatial resolution of PET tomographs, which depends, among other factors, on the range of positrons in matter, can also contribute to a loss in quantitation accuracy. In this paper we address the influence of positron range, structure size and ROI size on the quantitation of radioactive concentration using PET. ECAT EXACT HR+ (HR+) and ECAT 953B/31 (ECAT 953B) PET systems were used in phantom acquisitions performed with two radioisotopes with different positron ranges. The 3D Hoffman phantom was scanned on both scanners with both radioisotopes, to visually analyse the image quality. A resolution phantom having six spheres of different diameters in a Plexiglas cylinder was used to calculate the values of the contrast recovery coefficient or hot spot recovery coefficient and of the spill-over or cold spot recovery coefficient under different imaging conditions used in clinical routine at our institution. Activity ratios were varied between 2 and 30 or between 0.4 and 200 by filling the spheres with fluorine-18 or bromine-76 respectively and the cylinder with 11C. Dynamic scans were performed on each scanner. Data were reconstructed using the same parameters as are used in clinical protocols. The variations in sphere and cylinder activities with time were fitted using the function M(t)=k1. A(t)+k2.B(t), where M(t) is the radioactivity concentration measured in an ROI placed on each sphere and A(t) and B(t) represent the true radioactivity concentrations present at time t in the spheres and in the cylinder respectively. k1 and k2 are factors representing the contrast recovery coefficient and the spill-over from surrounding activity on measurements respectively. The visual analysis of images obtained using a 3D Hoffman phantom showed that image resolution and image contrast between different regions are radioisotope dependent and clearly better when using 18F. Linear profiles taken on these images confirmed the visual assessment. For a given scanner, the k1 values obtained with 18F were systematically higher than those measured using 76Br in the same machine (especially for the smaller spheres) when using the same ROI. For a sphere of a particular diameter, the use of a wider ROI resulted in lower quantitative accuracy when using the same isotope and the same camera. Lower quantitative accuracy was found for smaller spheres for all ROI sizes used in image analysis. For the same scanner and for a similar imaging situation (same sphere and same ROI), it was found that k1 and k2 values depend on the radioisotope used. For the same isotope and tomograph, the k1 values obtained decreased with the size of the structures imaged, as well as with the increase in ROI size. The use of a tomograph with better spatial resolution (HR+, rather than ECAT 953B) greatly increased the k1 values for 18F while only a mild improvement in these values was observed for 76Br. The use of 76Br led to k2 values that were slightly higher than those measured using 18F. These differences may have been due to the difference in the range of the positrons emitted by the radioisotopes used in this study. The measurements performed in this study show that the comparison of studies obtained on the same camera depends on the radioisotope used and may require the adaptation of ROI size between examinations. Marked differences are visible if the positron ranges of such radioisotopes are very different. Therefore, when employing commercially available tomographs and imaging protocols used in clinical routine, the effects of differences in positron range on image quality and quantitation are noticeable and correction for these effects may be of importance. (ABSTRACT TRUNCATED)

Robustness of anatomically guided pixel-by-pixel algorithms for partial volume effect correction in positron emission tomography.

Several algorithms have been proposed to improve positron emission tomography quantification by combining anatomical and functional information in a pixel-by-pixel correction scheme. The precision of these methods when applied to real data depends on the precision of the manifold correction steps, such as full-width half-maximum modeling, magnetic resonance imaging-positron emission tomography registration, tissue segmentation, or background activity estimation. A good understanding of the influence of these parameters thus is critical to the effective use of the algorithms. In the current article, the authors present a monodimensional model that allows a simple theoretical and experimental evaluation of correction imprecision. The authors then assess correction robustness in three dimensions with computer simulations, and evaluate the validity of regional SD as a correction performance criterion.

Absolute quantitation of iodine-123 epidepride kinetics using single-photon emission tomography: comparison with carbon-11 epidepride and positron emission tomography.

Epidepride labelled with iodine-123 is a suitable probe for the in vivo imaging of striatal and extrastriatal dopamine D2 receptors using single-photon emission tomography (SPET). Recently, this molecule has also been labelled with carbon-11. The goal of this work was to develop a method allowing the in vivo quantification of radioactivity uptake in baboon brain using SPET and to validate it using positron emission tomography (PET). SPET studies were performed in Papio anubis baboons using 123I-epidepride. Emission and transmission measurements were acquired on a dual-headed system with variable head angulation and low-energy ultra-high resolution (LEUHR) collimation. The imaging protocol consisted of one transmission measurement (24 min, heads at 90 degrees), obtained with two sliding line sources of gadolinium-153 prior to injection of 0.21-0.46 GBq of 123I-epidepride, and 12 emission measurements starting 5 min post injection. For scatter correction (SC) we used a dual-window method adapted to 123I. Collimator blurring correction (CBC) was done by deconvolution in Fourier space and attenuation correction (AT) was applied on a preliminary (CBC) filtered back-projection reconstruction using 12 iterations of a preconditioned, regularized minimal residual algorithm. For each reconstruction, a calibration factor was derived from a uniform cylinder filled with a 123I solution of a known radioactivity concentration. Calibration and baboon images were systematically built with the same reconstruction parameters. Uncorrected (UNC) and (AT), (SC + AT) and (SC + CBC + AT) corrected images were compared. PET acquisitions using 0.11-0.44 GBq of 11C-epidepride were performed on the same baboons and used as a reference. The radioactive concentrations expressed in percent of the injected dose per 100 ml (% ID/100 ml) obtained after (SC + CBC + AT) in SPET are in good agreement with those obtained with PET and 11C-epidepride. A method for the in vivo absolute quantitation of 123I-epidepride uptake using SPET has been developed which can be directly applied to other 123I-labelled molecules used in the study of the dopamine system. Further work will consist in using PET to model the radioligand-receptor interactions and to derive a simplified model applicable in SPET.