title: Fast Monte Carlo Simulation of ⁹⁰Y Bremsstrahlung using a Kernel-based Photon Source

authors: Zakariah, Suhaili

subjects: Monte Carlo

abstract: In targeted radionuclide therapy using ⁹⁰Y, bremsstrahlung photons can be used for imaging and subsequently for absorbed dose estimation. Monte Carlo simulation is a reliable method to study bremsstrahlung imaging but it takes a long computation time as not all ⁹⁰Y disintegrations result in emission of a bremsstrahlung photon. Furthermore the electron transport simulation is particularly time consuming. This research proposes that bremsstrahlung photons produced from the decay of a ⁹⁰Y point source are replaced with a kernel-based photon source for faster Monte Carlo simulation.
This study is divided into three main parts. First, a ⁹⁰Y point source in a spherical water phantom is fully simulated using Monte Carlo simulation. The characteristics of the bremsstrahlung photons produced from the ⁹⁰Y decay are investigated. The full Monte Carlo simulation of ⁹⁰Y point source provides the relationships between the emission position, energy and emission angle of the bremsstrahlung photon. These are recorded as probability distribution functions (PDFs).
Then, a kernel-based photon source which comprises of an array of photon-emitting concentric spherical shells is developed. The energy spectrum and angular distributions of the emitted photons are defined for each shell using the PDFs obtained previously from the full ⁹⁰Y Monte Carlo simulation. The kernel-based photon source shows a very close approximation to the distribution of bremsstrahlung photons generated by the full Monte Carlo simulation of ⁹⁰Y point source if the shells are sufficiently closely spaced.
Finally, the accuracy and speed of the kernel-based photon source are evaluated. A simplified gamma camera model, which consists of a collimator and NaI(Tl) detector is simulated to obtain the point spread function (PSF) of the point source ‘image’. The PSF can be represented as a Gaussian function. Estimation of σ of the Gaussian function and thus the full-width half-maximum (FWHM) is used to compare the different kernel-based photon source models to represent a ‘real’ source generated by a full Monte Carlo ⁹⁰Y simulation. The results show that the FWHMs of the photon source with kernels are comparable to full Monte Carlo simulation of ⁹⁰Y point source. The FWHM determined for a photon point source with the spectrum of ⁹⁰Y bremsstrahlung underestimates the full Monte Carlo simulation of the true ⁹⁰Y point source. This demonstrates that it is important to account for the photons being emitted at a distance away from the source and all of the photons cannot be assumed to come from a single point. It is more computationally efficient to use this source model than the kernel-based photon sources as it has the shortest computation time, however it is quite inaccurate. Simplifying the kernel-based source by assuming the photon emission as isotropic reduces the accuracy of the model slightly, though the reduction in simulation time is sufficiently small that the more accurate anisotropic kernel should be preferred.
The kernel-based photon source proposed in this study can be used to accurately represent the bremsstrahlung photons produced from the beta decay of a ⁹⁰Y point source. This approach greatly improves the simulation speed by almost 30 times.

text: Fast Monte Carlo Simulation of ⁹⁰Y Bremsstrahlung using a Kernel-based Photon Source In targeted radionuclide therapy using ⁹⁰Y, bremsstrahlung photons can be used for imaging and subsequently for absorbed dose estimation. Monte Carlo simulation is a reliable method to study bremsstrahlung imaging but it takes a long computation time as not all ⁹⁰Y disintegrations result in emission of a bremsstrahlung photon. Furthermore the electron transport simulation is particularly time consuming. This research proposes that bremsstrahlung photons produced from the decay of a ⁹⁰Y point source are replaced with a kernel-based photon source for faster Monte Carlo simulation.
This study is divided into three main parts. First, a ⁹⁰Y point source in a spherical water phantom is fully simulated using Monte Carlo simulation. The characteristics of the bremsstrahlung photons produced from the ⁹⁰Y decay are investigated. The full Monte Carlo simulation of ⁹⁰Y point source provides the relationships between the emission position, energy and emission angle of the bremsstrahlung photon. These are recorded as probability distribution functions (PDFs).
Then, a kernel-based photon source which comprises of an array of photon-emitting concentric spherical shells is developed. The energy spectrum and angular distributions of the emitted photons are defined for each shell using the PDFs obtained previously from the full ⁹⁰Y Monte Carlo simulation. The kernel-based photon source shows a very close approximation to the distribution of bremsstrahlung photons generated by the full Monte Carlo simulation of ⁹⁰Y point source if the shells are sufficiently closely spaced.
Finally, the accuracy and speed of the kernel-based photon source are evaluated. A simplified gamma camera model, which consists of a collimator and NaI(Tl) detector is simulated to obtain the point spread function (PSF) of the point source ‘image’. The PSF can be represented as a Gaussian function. Estimation of σ of the Gaussian function and thus the full-width half-maximum (FWHM) is used to compare the different kernel-based photon source models to represent a ‘real’ source generated by a full Monte Carlo ⁹⁰Y simulation. The results show that the FWHMs of the photon source with kernels are comparable to full Monte Carlo simulation of ⁹⁰Y point source. The FWHM determined for a photon point source with the spectrum of ⁹⁰Y bremsstrahlung underestimates the full Monte Carlo simulation of the true ⁹⁰Y point source. This demonstrates that it is important to account for the photons being emitted at a distance away from the source and all of the photons cannot be assumed to come from a single point. It is more computationally efficient to use this source model than the kernel-based photon sources as it has the shortest computation time, however it is quite inaccurate. Simplifying the kernel-based source by assuming the photon emission as isotropic reduces the accuracy of the model slightly, though the reduction in simulation time is sufficiently small that the more accurate anisotropic kernel should be preferred.
The kernel-based photon source proposed in this study can be used to accurately represent the bremsstrahlung photons produced from the beta decay of a ⁹⁰Y point source. This approach greatly improves the simulation speed by almost 30 times.

year: 2011

title: Application of optical and infrared remote sensing to the retrieval of selected land and sea features

authors: Jelenak, Aleksandar

abstract: This thesis discusses two main areas of remote sensing research and thus is divided into two parts. The first part deals with novel methods for sea surface temperature (SST) retrieval using measurements of the Advanced Very High Resolution Radiometer (AVHRR) on-board the National Oceanic and Atmospheric Administration's (NOAA) series of satellites.

The basis for this work is the Dynamic Water Vapour method developed by Steyn-Ross et al. [1993], Smith [1993]. Two new methods are proposed in this thesis: The Dynamic Water Vapour or Atmospheric Temperature (DWVT) and the Simultaneous Water Vapour and Atmospheric Temperature (SimWVT).

The second part of the thesis is a feasibility study on the potential applicability of airborne multispectral remote sensing technology to pasture biomass prediction in New Zealand. This was a pilot project, done in collaboration with AgResearch Ltd. and Dexcel Ltd. Charles Sturt University’s four-band airborne multispectral imaging system was used to acquire images of 65 test pasture plots.

Biomass of the test plots is predicted using a selection of vegetation indices, and multispectral reflectances. The results confirm the widely-held view that vegetation indices are not applicable to New Zealand pastures. Noticeably better results are achieved using the multispectral test plot reflectance in linear multiple regression-type equations.

text: Application of optical and infrared remote sensing to the retrieval of selected land and sea features This thesis discusses two main areas of remote sensing research and thus is divided into two parts. The first part deals with novel methods for sea surface temperature (SST) retrieval using measurements of the Advanced Very High Resolution Radiometer (AVHRR) on-board the National Oceanic and Atmospheric Administration's (NOAA) series of satellites.

The basis for this work is the Dynamic Water Vapour method developed by Steyn-Ross et al. [1993], Smith [1993]. Two new methods are proposed in this thesis: The Dynamic Water Vapour or Atmospheric Temperature (DWVT) and the Simultaneous Water Vapour and Atmospheric Temperature (SimWVT).

The second part of the thesis is a feasibility study on the potential applicability of airborne multispectral remote sensing technology to pasture biomass prediction in New Zealand. This was a pilot project, done in collaboration with AgResearch Ltd. and Dexcel Ltd. Charles Sturt University’s four-band airborne multispectral imaging system was used to acquire images of 65 test pasture plots.

Biomass of the test plots is predicted using a selection of vegetation indices, and multispectral reflectances. The results confirm the widely-held view that vegetation indices are not applicable to New Zealand pastures. Noticeably better results are achieved using the multispectral test plot reflectance in linear multiple regression-type equations.

year: 2001

title: Challenges Faced – Implications for Policy: The Everyday Lives of Eastern European Women in New Zealand

authors: Ember, Adrienna

subjects: everyday life

abstract: This study explored the everyday lives, aspirations, and coping strategies of seven Eastern European immigrant women in New Zealand who came from Bulgaria, the former Eastern Germany, Hungary, Poland, Romania, Serbia, and Slovakia. The answers to the research questions aimed to contribute to the deconstruction of the invisibility and marginalisation of these women in New Zealand.
Guided by the philosophies of Kaupapa Māori Research; the theory of Human Wellbeing; and the theory of Positive Psychology, and using the methods of narrative inquiry, three interviews were conducted with each participant. The focus of the first interviews was to learn about the everyday lives of research participants in their countries of origin. The second interview explored the reasons of relocation to New Zealand, the settlement process, and how women re-established their lives in their new locations. In the final interviews women talked about their current everyday lives and their plans for the future.
Interviews were recorded and summarised in interview summary reports which research participants modified in collaboration with the researcher until a final version was achieved. In addition, women participated in two focus group sessions. The first session was devoted to establish connections among the participants and allow themes to emerge from their conversations. The second group session aimed to explore areas women struggled with in their lives the most: relationships and employment opportunities.
The study yielded contributions to the research topic, to the acculturation literature, and to the design of research with migrants. Firstly, it revealed both negative and positive aspects of the participants’ everyday lives and highlighted some under-researched cultural differences from the mainstream population.
Women highlighted as an important cultural difference their strong preference for straightforward communication which was often experienced as offensive and blatant by local New Zealanders. Research participants critiqued the necessity of networking to obtain jobs in New Zealand which is a clear obstacle for a newly arrived migrant.
This study also highlighted how the acculturation process is more individual and complex than conventional models have sought to explain. While traditional acculturation literature suggests that migrants go through some common patterns during their settlement that ideally leads to their assimilation to the host culture, in this research not all migrant women intended to assimilate. Indeed, those with a stronger wish to become part of the host culture reported more disappointments than those who embraced their cultural otherness as a positive aspect and did not mind reminding different from the dominant Pakeha culture.
Finally, for migrant studies a less common research design was applied by using the Kaupapa Māori framework. This philosophy proposes a research process that empowers immigrants to voice their needs and strengthen them as agents of their lives beyond the research process. While two strength based theories were used for data interpretation (the 3-D model of the Human Wellbeing Theory and the PERMA model of Positive Psychology), the data analysis revealed that both models showed some limitations by failing to incorporate the dimension of spiritual and physical wellbeing within their domains.

text: Challenges Faced – Implications for Policy: The Everyday Lives of Eastern European Women in New Zealand This study explored the everyday lives, aspirations, and coping strategies of seven Eastern European immigrant women in New Zealand who came from Bulgaria, the former Eastern Germany, Hungary, Poland, Romania, Serbia, and Slovakia. The answers to the research questions aimed to contribute to the deconstruction of the invisibility and marginalisation of these women in New Zealand.
Guided by the philosophies of Kaupapa Māori Research; the theory of Human Wellbeing; and the theory of Positive Psychology, and using the methods of narrative inquiry, three interviews were conducted with each participant. The focus of the first interviews was to learn about the everyday lives of research participants in their countries of origin. The second interview explored the reasons of relocation to New Zealand, the settlement process, and how women re-established their lives in their new locations. In the final interviews women talked about their current everyday lives and their plans for the future.
Interviews were recorded and summarised in interview summary reports which research participants modified in collaboration with the researcher until a final version was achieved. In addition, women participated in two focus group sessions. The first session was devoted to establish connections among the participants and allow themes to emerge from their conversations. The second group session aimed to explore areas women struggled with in their lives the most: relationships and employment opportunities.
The study yielded contributions to the research topic, to the acculturation literature, and to the design of research with migrants. Firstly, it revealed both negative and positive aspects of the participants’ everyday lives and highlighted some under-researched cultural differences from the mainstream population.
Women highlighted as an important cultural difference their strong preference for straightforward communication which was often experienced as offensive and blatant by local New Zealanders. Research participants critiqued the necessity of networking to obtain jobs in New Zealand which is a clear obstacle for a newly arrived migrant.
This study also highlighted how the acculturation process is more individual and complex than conventional models have sought to explain. While traditional acculturation literature suggests that migrants go through some common patterns during their settlement that ideally leads to their assimilation to the host culture, in this research not all migrant women intended to assimilate. Indeed, those with a stronger wish to become part of the host culture reported more disappointments than those who embraced their cultural otherness as a positive aspect and did not mind reminding different from the dominant Pakeha culture.
Finally, for migrant studies a less common research design was applied by using the Kaupapa Māori framework. This philosophy proposes a research process that empowers immigrants to voice their needs and strengthen them as agents of their lives beyond the research process. While two strength based theories were used for data interpretation (the 3-D model of the Human Wellbeing Theory and the PERMA model of Positive Psychology), the data analysis revealed that both models showed some limitations by failing to incorporate the dimension of spiritual and physical wellbeing within their domains.

year: 2016