The axial field of view (AFOV) of the current generation of clinical whole-body PET scanners range from 15–22 cm, which limits sensitivity and renders applications such as whole-body dynamic imaging, or imaging of very low activities in whole-body cellular tracking studies, almost impossible. Generally, extending the AFOV significantly increases the sensitivity and count-rate performance. However, extending the AFOV while maintaining detector thickness has significant cost implications. In addition, random coincidences, detector dead time, and object attenuation may reduce scanner performance as the AFOV increases. In this paper, we use Monte Carlo simulations to find the optimal scanner geometry (i.e. AFOV, detector thickness and acceptance angle) based on count-rate performance for a range of scintillator volumes ranging from 10 to 90 l with detector thickness varying from 5 to 20 mm. We compare the results to the performance of a scanner based on the current Siemens Biograph mCT geometry and electronics. Our simulation models were developed based on individual components of the Siemens Biograph mCT and were validated against experimental data using the NEMA NU-2 2007 count-rate protocol. In the study, noise-equivalent count rate (NECR) was computed as a function of maximum ring difference (i.e. acceptance angle) and activity concentration using a 27 cm diameter, 200 cm uniformly filled cylindrical phantom for each scanner configuration. To reduce the effect of random coincidences, we implemented a variable coincidence time window based on the length of the lines of response, which increased NECR performance up to 10% compared to using a static coincidence time window for scanners with large maximum ring difference values. For a given scintillator volume, the optimal configuration results in modest count-rate performance gains of up to 16% compared to the shortest AFOV scanner with the thickest detectors. However, the longest AFOV of approximately 2 m with 20 mm thick detectors resulted in performance gains of 25–31 times higher NECR relative to the current Siemens Biograph mCT scanner configuration.
The ability to acquire a representation of the spatial environment and the ability to localize within it are essential for successful navigation in a-priori unknown environments. The hippocampal formation is believed to play a key role in spatial learning and localization in animals in general and rodents in particular. This paper briefly reviews the relevant neurobiological and cognitive data, and their relation to computational models of spatial learning and localization used in contemporary mobile robots. It proposes a hippocampal model of spatial learning and localization, and characterizes it using a Kalman filter based tool for information fusion from multiple uncertain sources. The resulting model not only explains neurobiological and behavioral data from rodent experiments, but also allows a robot to learn a place-based metric representation of space and to localize itself in a stochastically optimal manner. The paper presents an algorithmic implementation of the model and results of several experiments that demonstrate its capabilities. These include the ability to disambiguate perceptually similar places, scale well with increasing errors, and the automatic acquisition of spatial information at multiple resolutions. keywords: spatial learning, robot localization, hippocampal model, Kalman filter, probabilistic localization.
Abstract.The UV-LED mission demonstrates the precise control of the potential of electrically isolated test masses that is essential for the operation of space accelerometers and drag-free sensors. Accelerometers and drag-free sensors were and remain at the core of geodesy, aeronomy and precision navigation missions as well as gravitational science experiments and gravitational wave observatories. Charge management using photoelectrons generated by the 254nm UV line of Hg was first demonstrated on Gravity Probe B and is presently part of the LISA Pathfinder technology demonstration. The UV-LED mission and prior ground testing demonstrates that AlGaN UVLEDs operating at 255 nm are superior to Mercury vapor lamps because of their smaller size, lower power draw, higher dynamic range, and higher control authority. We show flight data from a small satellite mission on a Saudi Satellite that demonstrates AC charge control (UV-LEDs and bias are AC modulated with adjustable relative phase) between a spherical test mass and its housing. The result of the mission is to bring the UV-LED device Technology Readiness Level (TRL) to TRL-9 and the charge management system to TRL-7. We demonstrate the ability to control the test mass potential on an 89 mm diameter spherical test mass over a 20 mm gap in a drag-free system configuration. The test mass potential was measured with an ultra-high impedance contact probe. Finally, the key electrical and optical characteristics of the UV-LEDs showed less than 7.5% change in performance after 12 months in orbit.
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