110,416 research outputs found
"Un problema fondamentale di Platone". Sul significato dell'idea del bene secondo Heidegger
No full textOptimizing Compton camera performance
No full textAmore realistic simulation approach is used to study the behavior of the Compton camera in
this thesis than previous studies to date. The Compton camera differs from gamma cameras
in that the collimator is replaced by a detector known as the ‘scatterer’. Gamma rays may be
Compton scattered in the scatterer and subsequently detected by an ‘absorber’ which is the
equivalent of the detector in a gamma camera. By measuring the energies and the positions
of the points on the scatterer and the absorber where the incident and scattered gamma rays
interacted with the detectors, an image of the source can be reconstructed. Because there is
no collimator present, the potential sensitivity of the Compton camera is much higher than
the gamma camera, resulting in reduced acquisition times.
Most of the work described in this thesis was done with the GEANT4 Monte Carlo
simulation software. GEANT4 has been proven to be very robust and efficient in modelling
physics problems of radiation transport and interactions with matter in complex geometries.
Four major studies are carried out to estimate and optimize the performance of this
novel equipment. The first study takes a look at the scatterer’s imaging parameters with
the aim of prescribing an optimal scatterer material and geometry. In the second study,
the contribution of the absorber to the overall Compton camera performance is evaluated,
considering detector material, interaction type and geometry. The third study explores the
limitations imposed by the detector energy threshold and dead time on the Compton camera
performance, using a simplified model of the general electronic architecture. An evaluation
of Compton camera for scintimammography was performed in the fourth study. For
this study, three dual-head Compton camera models (Si/CZT, Si/LaBr₃:Ce and Si/NaI(Tl)
Compton cameras) were simulated, and the effect of scintillation photons’ interactions with
the photomultipliers was implemented.
The results show that silicon of about 1 cm thickness would be adequate as the Compton
camera scatterer. Analyses suggest however, that the choice of silicon is not completely
flawless. Doppler broadening for this detector material contributes as much as 7.3 mm
and 2.4 mm to full-width-at-half-maximum (FWHM) image resolution at 140.5 keV and
511 keV respectively. On the other hand, detector spatial resolution which accounts for
the least image degradation at 140.5 keV is found to be the dominant degrading factor
at 511 keV, suggesting that the absorber parameters play major roles in image resolution
at higher diagnostic energies. Findings further suggest that cadmium zinc telluride (CZT)
would be themost suitable detector as the absorber since thematerial demonstrated the highest efficiency and least positioning error due to multiple interactions as well as good spatial
resolution. The inclusion of the energy threshold and detector dead time at 140.5 keV, reduced
the Compton camera detection efficiency by 48% and 17% respectively, but improved
the image resolution from 10.7 mm to 9.5 mm at the source-to-scatterer distance of 5 cm.
At 511 keV, the inclusion of these parameters reduced the efficiency by 6% and 13% respectively,
but made no significant difference on the camera resolution. For a challenging
detection case in scintimammography, 5 mm breast tumours of tumour/background uptakes
of 10:1 and 6:1 at 511 keV were used. The best signal-to-noise ratio (SNR) was attained
for the Si/CZT Compton camera model, with the SNR values of 12.2 and 5.3.
It is therefore envisioned that with an optimal camera geometry, improved reconstruction
technique and adequate filter algorithm, the combination of Si and CZT as the scatterer
and the absorber of the Compton camera would make a very promising imaging system for
nuclear medicine studies at higher gamma ray energies where the collimated SPECT systems
perform very poorly due to increased septal penetration. It is equally evident from the
studies that with improved technology, new detectors such as LaBr₃:Ce could replace the
traditional NaI(Tl) detector as imaging detectors
Fuji MX 700 Digital Camera
No full textDigital camera. Marked (to front): Fujinon lens f = 7.6mm 1:3:2 1.5 mega pixels Digital Camera MX700 Instructions and settings to rear. Silver metal housing. Marked: Fujifilm Digital camera MX700 on base.. Date: 1990 - 1999 (early) - from the The Betty Smithers Design Collection at Staffordshire University.
More Accurate Pinhole Camera Calibration with Imperfect Planar Target
No full textThis paper presents a novel approach to camera calibration that improves final accuracy with respect to standard methods using precision planar targets, even if now inaccurate, unmeasured, roughly planar targets can be used. The work builds on a recent trend in camera calibration, namely concurrent optimization of scene structure together with the intrinsic camera parameters. A novel formulation is presented that allows maximum likelihood estimation in the case of inaccurate targets, as it extends the camera extrinsic parameters into a tight parametrization of the whole scene structure. It furthermore observes the special characteristics of multi-view perspective projection of planar targets. Its natural extensions to stereo camera calibration and hand-eye calibration are also presented. Experiments demonstrate improvements in the parametrization of the camera model as well as in eventual stereo reconstruction
Automated camera ranking and selection using video content and scene context
No full textPhDWhen observing a scene with multiple cameras, an important problem to solve is to automatically
identify “what camera feed should be shown and when?” The answer to this question is of interest
for a number of applications and scenarios ranging from sports to surveillance. In this thesis we
present a framework for the ranking of each video frame and camera across time and the camera
network, respectively. This ranking is then used for automated video production. In the first stage
information from each camera view and from the objects in it is extracted and represented in a way
that allows for object- and frame-ranking. First objects are detected and ranked within and across
camera views. This ranking takes into account both visible and contextual information related to
the object. Then content ranking is performed based on the objects in the view and camera-network
level information. We propose two novel techniques for content ranking namely: Routing Based
Ranking (RBR) and Multivariate Gaussian based Ranking (MVG). In RBR we use a rule based
framework where weighted fusion of object and frame level information takes place while in MVG
the rank is estimated as a multivariate Gaussian distribution. Through experimental and subjective
validation we demonstrate that the proposed content ranking strategies allows the identification of
the best-camera at each time.
The second part of the thesis focuses on the automatic generation of N-to-1 videos based on the
ranked content. We demonstrate that in such production settings it is undesirable to have frequent
inter-camera switching. Thus motivating the need for a compromise, between selecting the best
camera most of the time and minimising the frequent inter-camera switching, we demonstrate that
state-of-the-art techniques for this task are inadequate and fail in dynamic scenes. We propose three
novel methods for automated camera selection. The first method (¡go f ) performs a joint optimization
of a cost function that depends on both the view quality and inter-camera switching so that a
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Abstract ii
pleasing best-view video sequence can be composed. The other two methods (¡dbn and ¡util) include
the selection decision into the ranking-strategy. In ¡dbn we model the best-camera selection
as a state sequence via Directed Acyclic Graphs (DAG) designed as a Dynamic Bayesian Network
(DBN), which encodes the contextual knowledge about the camera network and employs the past
information to minimize the inter camera switches. In comparison ¡util utilizes the past as well
as the future information in a Partially Observable Markov Decision Process (POMDP) where the
camera-selection at a certain time is influenced by the past information and its repercussions in
the future. The performance of the proposed approach is demonstrated on multiple real and synthetic
multi-camera setups. We compare the proposed architectures with various baseline methods
with encouraging results. The performance of the proposed approaches is also validated through
extensive subjective testing
Camera Self-Calibration for the ARToolkit
No full textCamera calibration is an essential and important part of an Augmented Reality (AR) system. The use of a planebased calibration technique can give a good accuracy, which can be important for AR applications. The calibration technique used in the current ARToolKit requires user intervention, which is prone to error and involves a lengthy calibration time. The camera has to be recalibrated every time the focal length changes which is cumbersome and less suitable for applications where a more automated and easier approach is needed. This paper investigates the use of camera self-calibration for the ARToolKit, which has the advantage of simplicity of implementation. In order to improve its accuracy, a distortion model is also investigated. In this context several interesting results are presented
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