Browsing by Author "Fulton, RR"

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    Attenuation correction for freely moving small animal brain PET studies based on a virtual scanner geometry
    (IOP Publishing, 2014-09-05) Angelis, GI; Kyme, AZ; Ryder, WJ; Fulton, RR; Meikle, SR
    Attenuation correction in positron emission tomography brain imaging of freely moving animals is a very challenging problem since the torso of the animal is often within the field of view and introduces a non negligible attenuating factor that can degrade the quantitative accuracy of the reconstructed images. In the context of unrestrained small animal imaging, estimation of the attenuation correction factors without the need for a transmission scan is highly desirable. An attractive approach that avoids the need for a transmission scan involves the generation of the hull of the animal’s head based on the reconstructed motion corrected emission images. However, this approach ignores the attenuation introduced by the animal’s torso. In this work, we propose a virtual scanner geometry which moves in synchrony with the animal’s head and discriminates between those events that traversed only the animal’s head (and therefore can be accurately compensated for attenuation) and those that might have also traversed the animal’s torso. For each recorded pose of the animal’s head a new virtual scanner geometry is defined and therefore a new system matrix must be calculated leading to a time-varying system matrix. This new approach was evaluated on phantom data acquired on the microPET Focus 220 scanner using a custom-made phantom and step-wise motion. Results showed that when the animal’s torso is within the FOV and not appropriately accounted for during attenuation correction it can lead to bias of up to 10% . Attenuation correction was more accurate when the virtual scanner was employed leading to improved quantitative estimates (bias < 2%), without the need to account for the attenuation introduced by the extraneous compartment. Although the proposed method requires increased computational resources, it can provide a reliable approach towards quantitatively accurate attenuation correction for freely moving animal studies. © 2014 Institute of Physics
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    Impact of extraneous mispositioned events on motion-corrected brain SPECT images of freely moving animals
    (American Association of Physicists in Medicine, 2014-08-18) Angelis, GI; Ryder, WJ; Bashar, R; Fulton, RR; Meikle, SR
    Purpose: Single photon emission computed tomography (SPECT) brain imaging of freely moving small animals would allow a wide range of important neurological processes and behaviors to be studied, which are normally inhibited by anesthetic drugs or precluded due to the animal being restrained. While rigid body motion of the head can be tracked and accounted for in the reconstruction, activity in the torso may confound brain measurements, especially since motion of the torso is more complex (i.e., nonrigid) and not well correlated with that of the head. The authors investigated the impact of mispositioned events and attenuation due to the torso on the accuracy of motion corrected brain images of freely moving mice. Methods: Monte Carlo simulations of a realistic voxelized mouse phantom and a dual compartment phantom were performed. Each phantom comprised a target and an extraneous compartment which were able to move independently of each other. Motion correction was performed based on the known motion of the target compartment only. Two SPECT camera geometries were investigated: a rotating single head detector and a stationary full ring detector. The effects of motion, detector geometry, and energy of the emitted photons (hence, attenuation) on bias and noise in reconstructed brain regions were evaluated. Results: The authors observed two main sources of bias: (a) motion-related inconsistencies in the projection data and (b) the mismatch between attenuation and emission. Both effects are caused by the assumption that the orientation of the torso is difficult to track and model, and therefore cannot be conveniently corrected for. The motion induced bias in some regions was up to 12% when no attenuation effects were considered, while it reached 40% when also combined with attenuation related inconsistencies. The detector geometry (i.e., rotating vs full ring) has a big impact on the accuracy of the reconstructed images, with the full ring detector being more advantageous. Conclusions: Motion-induced inconsistencies in the projection data and attenuation/emission mismatch are the two main causes of bias in reconstructed brain images when there is complex motion. It appears that these two factors have a synergistic effect on the qualitative and quantitative accuracy of the reconstructed images. © 2014 American Association of Physicists in Medicine.
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    An investigation of the challenges in reconstructing PET images of a freely moving animal
    (Springer Nature, 2013-10-10) Akhta, M; Kyme, AZ; Zhou, V; Fulton, RR; Meikle, SR
    Imaging the brain of a freely moving small animal using positron emission tomography (PET) while simultaneously observing its behaviour is an important goal for neuroscience. While we have successfully demonstrated the use of line-of-response (LOR) rebinning to correct the head motion of confined animals, a large proportion of events may need to be discarded because they either ‘miss’ the detector array after transformation or fall out of the acceptance range of a sinogram. The proportion of events that would have been measured had motion not occurred, so-called ‘lost events’, is expected to be even larger for freely moving animals. Moreover, the data acquisition in the case of a freely moving animal is further complicated by a complex attenuation field. The aims of this study were (a) to characterise the severity of the ‘lost events’ problem for the freely moving animal scenario, and (b) to investigate the relative impact of attenuation correction errors on quantitative accuracy of reconstructed images. A phantom study was performed to simulate the uncorrelated motion of a target and non-target source volume. A small animal PET scanner was used to acquire list-mode data for different sets of phantom positions. The list-mode data were processed using the standard LOR rebinning approach, and multiple frame variants of this designed to reduce discarded events. We found that LOR rebinning caused up to 86 % ‘lost events’, and artifacts that we attribute to incomplete projections, when applied to a freely moving target. This fraction was reduced by up to 18 % using the variant approaches, resulting in slightly reduced image artifacts. The effect of the non-target compartment on attenuation correction of the target volume was surprisingly small. However, for certain poses where the target and non-target volumes are aligned transaxially in the field-of-view, the attenuation problem becomes more complex and sophisticated correction methods will be required. We conclude that there are limitations with the LOR rebinning approach and simplified attenuation correction for freely moving animals requiring the development and validation of more sophisticated approaches. © 2020 Springer Nature Switzerland AG