Damulira, Edrine
(2021)
Development of an led array for dosimetry in diagnostic radiology.
PhD thesis, Universiti Sains Malaysia.
Abstract
The first goal of this research is to explore the dosimetric response of surface
mount device (SMD) light-emitting diodes (LEDs) to diagnostic X-rays and
radiotherapy beams. The response to diagnostic X-rays was examined using five LED
strips colors based on variable diagnostic X-ray parameters, including kilovoltage peak
(kVp), tube current-time product (mAs), dose, and source to detector distance. The
response to radiotherapeutic beams was preliminarily investigated with a cold white
LED, while varying the irradiation angle, beam energy, source-surface distance, field
size, and absorbed dose. This work’s second objective is to amplify diagnostic X-ray
radiation-induced signals by increasing the number of LED chips and using an
amplifier board. Additionally, a detection capability comparison between the cold
white LED and a bpw43 photodiode is presented. Finally, this investigation aims at
designing and fabricating an LED array prototype (LAP) dosimetric system. The LAP
comprises a 20 × 20 cm2 array of photovoltaic cold white LED chips sandwiched
between two intensifying screens. The system was placed inside an air cavity shielded
from optical noise using black vinyl tape. The screens converted diagnostic X-ray
beams to fluorescent blue light. The LEDs herein were executed in detector mode;
thus, they converted the fluorescent light into radiation-induced currents. These analog
currents were quantified and converted into digital voltage signals using a digital
multimeter. LAP characterization was implemented with (i) beam qualities established
by the IEC 61267, i.e., RQR 7 (90 kVp) and RQR 8 (100 kVp), and (ii) low (25 mAs)
and high (80 mAs) beam quantities defined herein. The cold white LED demonstrated
a better dosimetric behavior. LED chip number increment produced higher
amplification coefficients than the amplifier board. Both the photodiode and LEDs
demonstrated similar signal precision, linearity to mAs (dose), and dose and energy
dependence. The minimum dose detected by the LAP was 0.1386 mGy, whereas the
maximum dose implemented here was ~ 13 mGy. Whereas the LAP absorbed dose
linearity was 99.18 %, mAs linearity was 98.64 %. The sensitivity of the system
fluctuated by ± 4.69 %, ± 6.8 %, and ± 7.7 % during energy, dose, and dose rate
variation, respectively. Two LAP data sets were 89.93 % repeatable. Thus, this study
proposed an ultrathin (5 mm), lightweight (130 g), and relatively low cost (US $255)
LED-based dosimetric prototype system. This prototype’s dosimetric mechanism was
simple, efficient, and accurate.
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