Image reconstruction algorithm for optically stimulated luminescence 2D dosimetry using laser-scanned Al₂O₃:C and Al₂O₃:C,Mg films

The objective of this work was to develop an image reconstruction algorithm for 2D dosimetry using Al₂O₃:C and Al₂O₃:C,Mg optically stimulated luminescence (OSL) films imaged using a laser scanning system. The algorithm takes into account parameters associated with detector properties and the readout system. Pieces of Al₂O₃:C films (∼8 mm  ×  8 mm  ×  125 µ m) were irradiated and used to simulate dose distributions with extreme dose gradients (zero and non-zero dose regions). The OSLD film pieces were scanned using a custom-built laser-scanning OSL reader and the data obtained were used to develop and demonstrate a dose reconstruction algorithm. The algorithm includes corrections for: (a) galvo hysteresis, (b) photomultiplier tube (PMT) linearity, (c) phosphorescence, (d) 'pixel bleeding' caused by the 35 ms luminescence lifetime of F-centers in Al₂O₃, (e) geometrical distortion inherent to Galvo scanning system, and (f) position dependence of the light collection efficiency. The algorithm was also applied to 6.0 cm  ×  6.0 cm  ×  125 μm or 10.0 cm  ×  10.0 cm  ×  125 µ m Al₂O₃:C and Al₂O₃:C,Mg films exposed to megavoltage x-rays (6 MV) and ¹²C beams (430 MeV u⁻¹). The results obtained using pieces of irradiated films show the ability of the image reconstruction algorithm to correct for pixel bleeding even in the presence of extremely sharp dose gradients. Corrections for geometric distortion and position dependence of light collection efficiency were shown to minimize characteristic limitations of this system design. We also exemplify the application of the algorithm to more clinically relevant 6 MV x-ray beam and a ¹²C pencil beam, demonstrating the potential for small field dosimetry. The image reconstruction algorithm described here provides the foundation for laser-scanned OSL applied to 2D dosimetry. (paper)