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19 March 2014Optimizing two radioluminescence based quality assurance devices for diagnostic radiology utilizing a simple model
The extrinsic (absolute) efficiency of a phosphor is expressed as the ratio of light energy emitted per unit area at the
phosphor surface to incident x-ray energy fluence. A model described in earlier work has shown that by knowing the
intrinsic efficiency, the particle size, the thickness and the light extinction factor ξ, it is possible to deduce the extrinsic
efficiency for an extended range of particle sizes and layer thicknesses for a given design. The model has been tested on
Gd2O2S:Tb and ZnS:Cu fluorescent layers utilized in two quality assurance devices, respectively, aimed for the
assessment of light field and radiation field congruence in diagnostic radiology. The first unit is an established device
based on both fluorescence and phosphorescence containing an x-ray sensitive phosphor (ZnS:Cu) screen comprising a
long afterglow. Uncertainty in field edge position is estimated to 0.8 mm (k=2). The second unit is under development
and based on a linear CCD sensor which is sensitized to x-rays by applying a Gd2O2S:Tb scintillator. The field profiles
and the corresponding edge location are then obtained and compared. Uncertainty in field edge location is estimated to
0.1 mm (k=2).
The properties of the radioluminescent layers are essential for the functionality of the devices and have been optimized
utilizing the previously developed and verified model. A theoretical description of the maximization of phosphorescence
is also briefly discussed as well as an interesting finding encountered during the development processes: focal spot
wandering. The oversimplistic physical assumptions made in the radioluminescence model have not been found to lead
the optimizing process astray. The obtained functionality is believed to be adequate within their respective limitations for
both devices.
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Jan Lindström, Markus Hulthén, Gudrun Alm Carlsson, Michael Sandborg, "Optimizing two radioluminescence based quality assurance devices for diagnostic radiology utilizing a simple model," Proc. SPIE 9033, Medical Imaging 2014: Physics of Medical Imaging, 90333R (19 March 2014); https://doi.org/10.1117/12.2042921