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Structured scintillators for medical imaging

Final Report Summary - STRING (Structured scintillators for Medical Imaging)

Most of X-ray imagers use scintillator layer for conversion of the x-ray photons into visible light that can be detected by standard photo sensors (CCD, CMOS and flat panel). This technique is quite common in medical imaging. Radiation dose is a major concern in medical imaging and thus the detection efficiency of the scintillator layer has to be maximal. Therefore, it is important to make the scintillator layer thick enough to absorb all the X-ray photons. The layer thickness is thus larger than the pixel dimensions, which means that scintillation light originating in one point spreads over several pixels of the photo sensor lowering the spatial resolution and the contrast in resulting radiograms.

State-of-the-art positron emission tomographs require scintillators with a high density (at least 5.0 g/cm3), a high light yield (25 000 photons/MeV or more), a short decay time (preferably less than 20 ns), the absence of a build-up in the emission intensity (rise time < 1 ns) and a high energy resolution (smaller than 10 %). To completely absorb the high energy radiation, the crystals have a thickness of several mm, this requires transparent scintillators as otherwise many of the photons generated will not reach the photo detectors.

Currently, PET machines use single crystalline materials: in high end time-of-flight PET scanners almost 40 000 of them are needed. This contributes significantly to the costs of such scanners. For this reason, it is very worthwhile to look for possibilities to reduce the costs of the scintillators. This will reduce the price of PET machines and increase their proliferation, in this way enhancing the level of medical diagnostics at reduced costs. Cubic materials enable ceramic scintillators (no single crystal growth needed to obtain transparent materials), this results in a reduction in scintillator costs of about 30-40 %.

The STRING project aimed at providing ceramic scintillators. Decay times in the order of 20 ns can be achieved with activator ions emitting in the UV only and this does not comply with the photo detectors used. Therefore, in the project, we also looked for organic wavelength shifters, that are adapted to the emission of the scintillators on the one hand and the sensitivity of the photo detectors on the other hand. To use the light as effectively as possible, interference filters were used to prevent UV light, generated by the scintillator from entering the photo detector and wavelength converted light from entering the scintillator.

The final aim of the project was the creation of the so-called STRING stack: a ceramic scintillator with a wavelength shifter, sandwiched between two interference filters and its characterisation and assessment of its impact on the image quality in PET.

The STRING stack shows a slightly reduced light output, this is presumably mainly due to the suppression of the 4f-4f emission lines, as it mainly shows up in the measurements with the short gating time. In view of the rather low light yield, the project omitted testing the STRING stack under PET like conditions. They also omitted the manufacturing of an industrial prototype. Instead they modelled the time-dependent emission signal of the complete stack, as a function of the decay time of the scintillator and the wavelength shifter.

The project leader has applied for a patent describing the STRING concept. In addition, he has published a book on luminescence, in which scintillators are discussed. The project leader also plans to use results of the STRING project for teaching purposes at the universities where he is active.