Fabricated Germanium-doped optical fibres in computed tomography radiation dosimetry

Newly fabricated germanium (Ge)-doped optical fibres as a potential dosimeter in computed tomography (CT) dosimetry are being investigated due to the several limitations of the current dosimeters and limited study of the optical fibres in the field of diagnostic imaging. As such, the responses of these fibres at radiation quality for CT applications (RQT beam quality) are being compared with gold standard thermoluminescence dosimeter (TLD-100 chip) and optically stimulated luminescence dosimeter (OSLD nanoDot). Four fibre types are studied with different Ge-dopant concentrations in flat fibre (FF) and cylindrical fibre (CF) shapes. These are 2.3 mol% Ge-FF, 6 mol% Ge-FF, 6 mol% Ge-CF, and 2.3 mol% Ge-CF. The first objective is to examine the basic characteristics in terms of dose linearity, RQT beam quality, dose reproducibility, dose sensitivity, minimum detectable dose, and fading. The thermoluminescence (TL) signals of all fibres are discovered to be linear for doses ranging from 2 to 40 milligray (mGy) for all RQT beam quality of 100 kilovoltage (kV) (RQT 8), 120 kV (RQT 9), and 150 kV (RQT 10). However, in the RQT beam quality study, the signal response decreased as the energy increased from 100 kV to 150 kV, exhibiting clear photoelectric dependence. The optical fibres also demonstrated dose reproducibility of better than 4% of the mean, surpassing the performance of the TLD-100 chip. In terms of dose sensitivity, all fibres demonstrated superior sensitivity, with 6 mol% and 2.3 mol% Ge-FF indicating 87 and 84 times more sensitivity than that of the TLD-100 chip. The optical fibres are also able to detect dose levels down to the least delivered dose of 2 mGy used in this study with superior lower dose detection competency of 2.3 mol% Ge-CF to identify doses down to 0.29 mGy. Finally, a higher fading rate of optical fibres ten days post-irradiation is observed due to the shallow electron trappings typical in low-dose radiation. Per the results, the TL response is dependent on the beam quality and fading. Thus, beam quality and fading correction factors are included in the absorbed dose measurement formula. The advanced characteristics in terms of the kinetic parameters of optical fibres have also been investigated prior to establishing absorbed dose methodology with uncertainty analysis. The response for absorbed and entrance skin dose measurements of the radiosensitive organs on CIRS ATOM® dosimetry verification paediatric phantom during head and lung CT scans revealed that the majority of the percentage difference of investigated dosimeters is less than 3% when compared against TLD-100 chips. The dosimetry verification in a real clinical setting for entrance skin dose measurement of the thyroids in paediatric patients undergoing computed tomography pulmonary angiogram (CTPA) examinations is also achieved with a percentage difference of less than 5%. The expanded uncertainties are 5.92%, 3.67%, 2.38%, 1.27%. 2.97% and 4.54% for 6 mol% Ge-FF, 2.3 mol% Ge-FF, 2.3 mol% Ge-CF, 6 mol% Ge-CF, TLD-100 chip and OSLD nanoDot, respectively, after multiplication by a coverage factor k = 2. This uncertainty provided a level of confidence of approximately 95%. In conclusion, all four fabricated Ge-doped optical fibres are feasible to be potential novel dosimeters in CT dosimetry RQT beam quality.

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