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

Abstract

The objective of this work was to develop an image reconstruction algorithm for 2D dosimetry using AlO:C and AlO: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 AlO:C films (~8 mm × 8 mm × 125 µm) 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 AlO, (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 µm AlO:C and AlO:C,Mg films exposed to megavoltage x-rays (6 MV) and C beams (430 MeV u). 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 C 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.

Characterization of Al2O3optically stimulated luminescence films for 2D dosimetry using a 6 MV photon beam

“…pixel bleeding due to the long F-center luminescence lifetime) and for system properties (e.g. galvo dist ortion, light collection efficiency) (Ahmed et al 2016). The algorithm uses two parameters in the corrections, related to the relative contributions to the total signal from F-center (parameter R 1 ) and from phosphorescence (parameter R 2 ).…”

“…The OSL system was described by Ahmed et al (2016). The OSL films are scanned with a focused 532 nm Nd:YAG laser using galvo mirrors, the OSL signal being detected by a photomultiplier tube (PMT).…”

“…The complete image reconstruction was performed as described in Ahmed et al (2016). One of the major steps in the image reconstruction is the pixel bleeding correction algorithm, which corrects for image blurring caused by the slow luminescence from F-center and, if existent, by slower phosphorescence emission.…”

“…Based on the pixel-bleeding correction algorithm mentioned above, a laser-scanning 2D OSL reader was built for proof-of-principle demonstration (Ahmed et al 2014), and an image reconstruction algorithm for Al 2 O 3 :C and Al 2 O 3 :C,Mg films was developed and demonstrated (Ahmed et al 2016). The system works particularly well using Al 2 O 3 :C,Mg, a material originally developed for optical data storage and as fluorescence nuclear track detectors (Akselrod and Akselrod 2006), and which has a greater ratio of fast luminescence (335 nm emission associated with F + -center, <7 ns lifetime) to slow F-center luminescence compared to regular Al 2 O 3 :C. The dosimetric properties of this new 2D OSL dosimetry technique have not yet been demonstrated.…”

New optically stimulated luminescence dosimetry film optimized for energy dependence guided by Monte Carlo simulations

“…However, several problems need to be addressed i.e. corrections related to geometrical distortion, detector non-linearity, signal fading, quantum efficiency (QE) as well as pixel bleeding' (Ahmed et al 2016). Therefore, it is worthwhile to search for new material, that can overcome the limitations of these detectors.…”