title: Optimizing Compton camera performance

authors: Uche, Chibueze Zimuzo

subjects: Nuclear medicine

abstract: Amore 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.

text: Optimizing Compton camera performance Amore 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.

year: 2011

title: Exploring usability evaluation of localised software in Malaysia

authors: Yeo, Alvin

abstract: This thesis postulates the theory that software usability evaluation should be adapted to the culture in which it is carried out. An experiment has been conducted to explore the effectiveness of usability assessment tools (UATs). These UATs are studied in the context of usability evaluation phase of the global software development life-cycle. In particular, the study investigates whether imported UATs were appropriate in the usability evaluation of a localised spreadsheet in Malaysia. The experiment reveals that there is some support for the thesis and recommends a further large-scale formal study.

text: Exploring usability evaluation of localised software in Malaysia This thesis postulates the theory that software usability evaluation should be adapted to the culture in which it is carried out. An experiment has been conducted to explore the effectiveness of usability assessment tools (UATs). These UATs are studied in the context of usability evaluation phase of the global software development life-cycle. In particular, the study investigates whether imported UATs were appropriate in the usability evaluation of a localised spreadsheet in Malaysia. The experiment reveals that there is some support for the thesis and recommends a further large-scale formal study.

year: 2002

title: Case roles / relations and discourse relations: A Maori language-based perspective

authors: Whaanga, Hēmi

subjects: case grammar

abstract: The overall aim of this research project is to explore case roles/ relations and discourse relations (referred to in this thesis as 'intra-propositional relations' and 'interpropositional relations' respectively) from the perspective of the Maori language. 
The thesis begins with an outline of the scope of the research and the approaches and methods used (Chapter 1). This is followed by a critical review of selected literature on case roles/ relations (intra-propositional relations) (see Chapter 2) and discourse relations (inter-propositional relations) (see Chapter 3) where it is noted that some of the relational theories and models appear to lack descriptive and/or explanatory adequacy. 
In Chapter 4, two models (an intra-propositional relational model and an interpropositional relational model) are developed on the basis of the critical review in Chapters 2 and 3, and these models are applied to a written corpus of Maori language texts, the primary aim being to track the ways in which intra-propositional and interpropositional relations are signalled in Maori. 
In Chapter 5, l supplement the corpus-based findings reported in Chapter 4 by reexamining the findings of two earlier studies from the perspective of the models introduced in Chapter 4. One of these earlier studies, reported in the early 1980s, is concerned with the distribution of prepositions in Maori in relation to case roles/ relations (intra-propositional relations); the other, produced much more recently (in 2001) is concerned with the signalling in Maori of a selection of what are referred to in that Work as 'semantico-pragmatic relations' (inter-propositional relations). 
Chapter 6 provides an overview of the research and a summary of the findings, together with an indication of its limitations, possible areas of application, and suggestions for future research.

text: Case roles / relations and discourse relations: A Maori language-based perspective The overall aim of this research project is to explore case roles/ relations and discourse relations (referred to in this thesis as 'intra-propositional relations' and 'interpropositional relations' respectively) from the perspective of the Maori language. 
The thesis begins with an outline of the scope of the research and the approaches and methods used (Chapter 1). This is followed by a critical review of selected literature on case roles/ relations (intra-propositional relations) (see Chapter 2) and discourse relations (inter-propositional relations) (see Chapter 3) where it is noted that some of the relational theories and models appear to lack descriptive and/or explanatory adequacy. 
In Chapter 4, two models (an intra-propositional relational model and an interpropositional relational model) are developed on the basis of the critical review in Chapters 2 and 3, and these models are applied to a written corpus of Maori language texts, the primary aim being to track the ways in which intra-propositional and interpropositional relations are signalled in Maori. 
In Chapter 5, l supplement the corpus-based findings reported in Chapter 4 by reexamining the findings of two earlier studies from the perspective of the models introduced in Chapter 4. One of these earlier studies, reported in the early 1980s, is concerned with the distribution of prepositions in Maori in relation to case roles/ relations (intra-propositional relations); the other, produced much more recently (in 2001) is concerned with the signalling in Maori of a selection of what are referred to in that Work as 'semantico-pragmatic relations' (inter-propositional relations). 
Chapter 6 provides an overview of the research and a summary of the findings, together with an indication of its limitations, possible areas of application, and suggestions for future research.

year: 2006