Browsing by Author "Lehnert, W"

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    Analytical positron range modelling in heterogeneous media for PET Monte Carlo simulation
    (IOP Publishing Ltd, 2011-06-07) Lehnert, W; Grégoire, MC; Reilhac, A; Meikle, SR
    Monte Carlo simulation codes that model positron interactions along their tortuous path are expected to be accurate but are usually slow. A simpler and potentially faster approach is to model positron range from analytical annihilation density distributions. The aims of this paper were to efficiently implement and validate such a method, with the addition of medium heterogeneity representing a further challenge. The analytical positron range model was evaluated by comparing annihilation density distributions with those produced by the Monte Carlo simulator GATE and by quantitatively analysing the final reconstructed images of Monte Carlo simulated data. In addition, the influence of positronium formation on positron range and hence on the performance of Monte Carlo simulation was investigated. The results demonstrate that 1D annihilation density distributions for different isotope–media combinations can be fitted with Gaussian functions and hence be described by simple look-up-tables of fitting coefficients. Together with the method developed for simulating positron range in heterogeneous media, this allows for efficient modelling of positron range in Monte Carlo simulation. The level of agreement of the analytical model with GATE depends somewhat on the simulated scanner and the particular research task, but appears to be suitable for lower energy positron emitters, such as 18F or 11C. No reliable conclusion about the influence of positronium formation on positron range and simulation accuracy could be drawn. © 2011 IOP Publishing LTD
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    Attenuation correction for the large non-human primate brain imaging using microPET
    (IOP Publishing Ltd, 2010-04-21) Naidoo-Variawa, S; Lehnert, W; Kassiou, M; Banati, RB; Meikle, SR
    Assessment of the biodistribution and pharmacokinetics of radiopharmaceuticals in vivo is often performed on animal models of human disease prior to their use in humans. The baboon brain is physiologically and neuro-anatomically similar to the human brain and is therefore a suitable model for evaluating novel CNS radioligands. We previously demonstrated the feasibility of performing baboon brain imaging on a dedicated small animal PET scanner provided that the data are accurately corrected for degrading physical effects such as photon attenuation in the body. In this study, we investigated factors affecting the accuracy and reliability of alternative attenuation correction strategies when imaging the brain of a large non-human primate (papio hamadryas) using the microPET Focus 220 animal scanner. For measured attenuation correction, the best bias versus noise performance was achieved using a (57)Co transmission point source with a 4% energy window. The optimal energy window for a (68)Ge transmission source operating in singles acquisition mode was 20%, independent of the source strength, providing bias-noise performance almost as good as for (57)Co. For both transmission sources, doubling the acquisition time had minimal impact on the bias-noise trade-off for corrected emission images, despite observable improvements in reconstructed attenuation values. In a [(18)F]FDG brain scan of a female baboon, both measured attenuation correction strategies achieved good results and similar SNR, while segmented attenuation correction (based on uncorrected emission images) resulted in appreciable regional bias in deep grey matter structures and the skull. We conclude that measured attenuation correction using a single pass (57)Co (4% energy window) or (68)Ge (20% window) transmission scan achieves an excellent trade-off between bias and propagation of noise when imaging the large non-human primate brain with a microPET scanner. © 2010, IOP Publishing LTD.
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    Characterisation of partial volume effect and region-based correction in small animal positron emission tomography (PET) of the rat brain
    (Elsevier Inc, 2012-05-01) Lehnert, W; Grégoire, MC; Reilhac, A; Meikle, SR
    Accurate quantification of PET imaging data is required for a useful interpretation of the measured radioactive tracer concentrations. The partial volume effect (PVE) describes signal dilution and mixing due to spatial resolution and sampling limitations, which introduces bias in quantitative results. In the present study we investigated the magnitude of PVE for volumes of interest (VOIs) in the rat brain and the effect of positron range. In simulated 11C-raclopride studies we examined the influence of PVE on time activity curves in striatal and cerebellar VOIs and binding potential estimation. The performance of partial volume correction (PVC) was studied using the region-based geometric transfer matrix (GTM) method including the question of whether a spatially variant point spread function (PSF) is necessary for PVC of a rat brain close to the centre of the field of view. Furthermore, we determined the effect of spillover from activity outside the brain. The results confirmed that PVE is significant in rat brain PET and showed that positron range is an important factor that needs to be included in the PSF. There was considerable bias in time activity curves for the simulated 11C-raclopride studies and significant underestimation of binding potential even for very small centred VOIs. Good activity recovery was achieved with the GTM PVC using a spatially invariant simulated PSF when no activity was present outside the brain. PVC using a simple Gaussian fit point spread function was not sufficiently accurate. Spillover from regions outside the brain had a significant impact on measured activity concentrations and reduced the accuracy of PVC with the GTM method using rat brain regions alone, except for the smallest VOI size but at the cost of increased noise. Voxel-based partial volume correction methods which inherently compensate for spillover from outside the brain might be a more suitable choice. © 2012 Elsevier Inc.
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    NEMA NU 4-2008 validation and applications of the PET-SORTEO Monte Carlo simulations platform for the geometry of the Inveon PET preclinical scanner
    (IOP Publishing, 2013-09-10) Boisson, F; Wimberley, CA; Lehnert, W; Zahra, D; Pham, TQ; Perkins, G; Hamze, H; Grégoire, MC; Reilhac, A
    Monte Carlo-based simulation of positron emission tomography (PET) data plays a key role in the design and optimization of data correction and processing methods. Our first aim was to adapt and configure the PET-SORTEO Monte Carlo simulation program for the geometry of the widely distributed Inveon PET preclinical scanner manufactured by Siemens Preclinical Solutions. The validation was carried out against actual measurements performed on the Inveon PET scanner at the Australian Nuclear Science and Technology Organisation in Australia and at the Brain and Mind Research Institute and by strictly following the NEMA NU 4-2008 standard. The comparison of simulated and experimental performance measurements included spatial resolution, sensitivity, scatter fraction and count rates, image quality and Derenzo phantom studies. Results showed that PET-SORTEO reliably reproduces the performances of this Inveon preclinical system. In addition, imaging studies showed that the PET-SORTEO simulation program provides raw data for the Inveon scanner that can be fully corrected and reconstructed using the same programs as for the actual data. All correction techniques (attenuation, scatter, randoms, dead-time, and normalization) can be applied on the simulated data leading to fully quantitative reconstructed images. In the second part of the study, we demonstrated its ability to generate fast and realistic biological studies. PET-SORTEO is a workable and reliable tool that can be used, in a classical way, to validate and/or optimize a single PET data processing step such as a reconstruction method. However, we demonstrated that by combining a realistic simulated biological study ([11C]Raclopride here) involving different condition groups, simulation allows one also to assess and optimize the data correction, reconstruction and data processing line flow as a whole, specifically for each biological study, which is our ultimate intent. © 2017 IOP Publishing
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    Scatter correction for large non-human primate brain imaging using microPET
    (Institute of Physics, 2011-04-07) Naidoo-Variawa, S; Lehnert, W; Banati, RB; Meikle, SR
    The baboon is well suited to pre-clinical evaluation of novel radioligands for positron emission tomography (PET). We have previously demonstrated the feasibility of using a high resolution animal PET scanner for this application in the baboon brain. However, the non-homogenous distribution of tissue density within the head may give rise to photon scattering effects that reduce contrast and compromise quantitative accuracy. In this study, we investigated the magnitude and distribution of scatter contributing to the final reconstructed image and its variability throughout the baboon brain using phantoms and Monte Carlo simulated data. The scatter fraction is measured up to 36% at the centre of the brain for a wide energy window (350–650 keV) and 19% for a narrow (450–650 keV) window. We observed less than 3% variation in the scatter fraction throughout the brain and found that scattered events arising from radioactivity outside the field of view contribute less than 1% of measured coincidences. In a contrast phantom, scatter and attenuation correction improved contrast recovery compared with attenuation correction on its own and reduced bias to less than 10% at the expense of the reduced signal-to-noise ratio. We conclude that scatter correction is a necessary step for ensuring high quality measurements of the radiotracer distribution in the baboon brain with a microPET scanner, while it is not necessary to model out of field of view scatter or a spatially variant scatter function. © 2011, Institute of Physics