A novel optical calorimetry approach is proposed for the dosimetry of therapeutic radiation,
based on the optical technique of Digital Holographic Interferometry (DHI). This detector
determines the radiation absorbed dose to water by measurement of the refractive index variations
arising from radiation induced temperature increases. The output consists of a time
series of high resolution, two dimensional images of the spatial distribution of the projected
dose map across the water sample. This absorbed dose to water is measured directly, independently
of radiation type, dose rate and energy, and without perturbation of the beam.
These are key features which make DHI a promising technique for radiation dosimetry.
A prototype DHI detector was developed, with the aim of providing proof-of-principle of the
approach. The detector consists of an optical laser interferometer based on a lensless Fourier
transform digital holography (LFTDH) system, and the associated mathematical reconstruction
of the absorbed dose. The conceptual basis was introduced, and a full framework was
established for the measurement and analysis of the results. Methods were developed for
mathematical correction of the distortions introduced by heat di usion within the system.
Pilot studies of the dosimetry of a high dose rate Ir-192 brachytherapy source and a small
eld proton beam were conducted in order to investigate the dosimetric potential of the technique.
Results were validated against independent models of the expected radiation dose
distributions.
Initial measurements of absorbed dose demonstrated the ability of the DHI detector to resolve
the minuscule temperature changes produced by radiation in water to within experimental
uncertainty. Spatial resolution of approximately 0.03 mm/pixel was achieved, and the dose
distribution around the brachytherapy source was accurately measured for short irradiation
times, to within the experimental uncertainty. The experimental noise for the prototype
detector was relatively large and combined with the occurrence of heat di usion, means that
the method is predominantly suitable for high dose rate applications.
The initial proof-of-principle results con rm that DHI dosimetry is a promising technique,
with a range of potential bene ts. Further development of the technique is warranted, to
improve on the limitations of the current prototype. A comprehensive analysis of the system
was conducted to determine key requirements for future development of the DHI detector
to be a useful contribution to the dosimetric toolbox of a range of current and emerging applications.
The sources of measurement uncertainty are considered, and methods suggested
to mitigate these. Improvement of the signal-to-noise ratio, and further development of the
heat transport corrections for high dose gradient regions are key areas of focus highlighted
for future development.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/10465 |
| Date | January 2015 |
| Creators | Cavan, Alicia Emily |
| Publisher | University of Canterbury. Physics and Astronomy |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Alicia Emily Cavan, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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