Towards 3D Dosimetry with Optical Fluorescence Tomography Using a Radiofluorogenic Material: Setup and Data Processing

In this project, a system for 3D readout of the absorbed dose in a radiofluorogenic and radiochromic solid-state polymer dosimeter has been developed. Optical fluorescence tomography is used to readout the 3D dosimeter. A sheet of laser light is used to excite fluorescence in the dosimeter, and a camera then acquires 2D images of the resulting fluorescent emission. 3D information is obtained by moving the dosimeter sample through the light sheet. The main features are that the readout only takes 1.5 minute per cm of sample length to complete, and the spatial resolution is 0.1 x 0.1 x 1 mm, limited by the optics in the system. In this project, a system for 3D readout of the absorbed dose in a solid-state polymer dosimeter has been developed. Optical fluorescence tomography is used to readout the radiofluorogenic dosimeter. A sheet of laser light is used to excite fluorescence in the dosimeter, and a camera then acquires 2D images of the resulting fluorescent emission. 3D information is obtained by moving the dosimeter sample through the light sheet. The main features are that the readout only takes 1.5 minute per cm of sample length to complete, and the spatial resolution is 0.1 x 0.1 x 1 mm, limited by the optics in the system. The measured data are fluorescence images excited by the a sheet of laser light. The calculation of the absorbed dose 3D distribution from the fluorescence images is complicated by the fact that the dosimeter material is both radiofluorogenic and significantly radiochromic: The fluorescence response is linear with the absorbed dose and the intensity of the excitation light, but the excitation light is absorbed by the sample in a locally dose-dependent manner. An analytical solution to this problem has been derived for calculating the 3D distribution of the absorbed dose from the 3D readout fluorescence images. Two interesting special cases of this solution have been highlighted. The impact of uncertainty in the dosimeter material parameters and boundary conditions was illustrated. An alternative algorithmic solution to the dose reconstruction was made. It works by fitting dose images represented by Fourier components to the observed data, and then optimizing the Fourier components to improve the fit. This solution was demonstrated to converge uniformly towards the true dose solution, if the given material parameters and boundary conditions for the dosimeter are correct. This solution method is relatively computationally intensive, and it could benefit from better optimization. A complete list of design recommendations and considerations for the 3D setup has been compiled; A diode laser at 520 nm is suitable for excitation of the pararosaniline dye used in the dosimeter. A cylindrical lens is used to generate a light sheet. The vertical intensity line profile of the light sheet must be characterized for each readout series in order to perform the data analysis. This can be done with a fluorescence reference in the field of view of the camera. The refractive index matching fluid should match the dosimeter at the fluorescence emission wavelengths, but be slightly higher, to prevent total internal reflection of the emitted light. A list of material properties required for the dosimeter to function ideally has been established. The decadic attenuation coefficient at the wavelength of the excitation laser should be at most 0.007 cm−1. The decadic radiochromic response at the excitation wavelength should not exceed 0.0036 cm−1Gy−1. The ratio of the fluorescence before irradiation and the radiofluorogenic response should not exceed 2 Gy. The radiochromic and radiofluorogenic response should be stable over time and homogeneous within a dosimeter sample. Some experiments with a radiofluogenic dosimeter material have been made. The dosimeter used in this project did not fulfil these requirements mentioned above and further work on the polymer material is required to meet the requirements. Some dosimeter readouts with the 3D setup were made, and it was demonstrated that the dose reconstruction algorithm can work with data from the 3D readout setup.