Transparent Barriers on Scintillators for Flat Panel Detectors (in Digital X-ray Imaging)
Nurcan Yanarcan Dogan, Argus Imaging
Flat panel-indirect conversion systems are a well-accepted technology for digital imaging in medical applications: they have a compact size, an immediate access to the digital image and a high image quality. A flat panel detector has three main parts; the scintillator, the photodiode transistor array and the readout electronics. When the scintillator layer is exposed to x-rays, the beam is absorbed and converted to visible light. This light is converted into an electric charge by the photodiodes, where each photodiode represents a pixel. The charge at each
pixel is read out by the electronics, converted into digital data and sent to an image processor.
The scintillator performance determines how efficiently and homogeneously the light is onverted and transported to the photodiodes. Thallium doped cesium iodide (CsI:Tl) is one of the most preferred scintillator materials for flat panel detectors due to its needle like structure acting as a light guide.
Although a CsI:Tl scintillator is advantageous in terms of image quality, its lifetime is very limited. Even very small amounts of moisture will decrease the scintillator efficiency and image quality. Therefore, to ensure a proper commercial product with a lifetime of many years, the CsI:Tl scintillators must be screened from any moisture by a proper barrier. The barrierrequirements are amongst the highest when comparing to other products requiring a moisture barrier.
In this project, industrial problem solving strategies were used for obtaining commercial scintillator products. First of all, the problem was defined by figuring out the failure mechanisms of the scintillators exposed to moist air and the project requirements were set. Second, potential solutions were defined by reviewing the literature and gathering information from similar industrial fields. Third, the best barrier combination consisting of an organic levelling layer deposited by chemical vapour deposition, an inorganic nanolaminate layer deposited by atomic layer deposition, and an organic protective layer deposited by chemical vapour deposition technique, was created as the solution. Finally, implementation and optimization studies were performed. In the implementation and the optimization steps, six sigma and design of experiment methods were used extensively.
The prototypes were tested successfully, both internally and by customers.
Argus Imaging started to use this barrier structure in the production line.
Nurcan conducted her PDEng graduation project at Argus Imaging in Heerlen, The Netherlands. The project was supervised by Ed Gillissen, project manager, and Erik Jacobs both at Argus Imaging and Ward Cottaar, director at the Design and Technology Instrumentation program from TU Eindhoven.