Fast Monte Carlo Simulation of ⁹⁰Y Bremsstrahlung using a Kernel-based Photon Source
Zakariah, S. (2011). Fast Monte Carlo Simulation of ⁹⁰Y Bremsstrahlung using a Kernel-based Photon Source (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/5549
Permanent Research Commons link: http://hdl.handle.net/10289/5549
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.
University of Waikato
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