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Inorganic scintillators are employed in most of the current medical imaging techniques [1], thanks to their good detection efficiency in the energy range of radiology and nuclear medicine (from 20 keV of mammography to 511 keV annihilation radiation used in PET).
Different imaging modalities pose different requirements to be fulfilled by the radiation detectors and not all these requirements are met by the crystals developed up to date. This is a reason why the R&D on the scintillators is always in progress.
As for the scintillators to be used in PET detectors, the requirements are:
• A high efficiency for -ray detection, associated with a high density and atomic number of the detector elements;
• A fast light signal that allows to achieve a good time resolution and to sustain high counting rates;
• A high light yield in order to achieve a good energy and time resolution;
• A good linearity of the light yield as a function of the energy to preserve the intrinsic energy resolution of the scintillator.
LSO:Ce doped was introduced in the 90’s as a high resolution and fast scintillator [2][3] and in 2001 Siemens /CTI introduced the first LSO:Ce based high resolution research tomograph (HRRT) for brain studies [4]. LSO is particularly appealing for PET thanks to its high efficiency (high density and atomic number) and to the ionic dopant Ce3+, which significantly improves the light output and the time response.
Recently developed LSO:Ce scintillator, co-doped with Ca, has been produced by the University of Tennessee group guided by Prof. C. Melcher. These crystals, tested with PMTs, showed a higher light output, faster light pulse, improved energy resolution and reduced afterglow, as compared to the standard LSO:Ce crystals.
The development and the optimization of the scintillators proceed in parallel with the research on new photodetectors. Even though the PMTs still represent the gold standard photodetectors, the recently available SiPMs are valid candidates to replace PMTs in the next generation PET scanners thanks to their compactness, high spatial resolution performances, low bias operating voltage and, most important for combined PET/MRI systems, insensitivity to static and RF fields.
In this work we present the performance of Ca co-doped LSO:Ce scintillator samples coupled to SiPMs from different manufacturers. The spectroscopic capabilities of the crystals and the timing performances have been assessed by using a Na-22 source. As an example, in figure 1 we show the energy spectrum of the Na-22source obtained with an LSO:Ce(Ca) sample of 4x4x5 mm^3 coupled to a MPPC Hamamatsu 3x3 mm^2, in time coincidence with another detector. The energy resolution is 8.5% (FWHM) at 511 keV.
Figure 1 Energy spectrum of the Na22source obtained with an LSO:Ca sample of 4x4x5 mm^3 coupled to a MPPC Hamamatsu 3x3 mm^2, in time coincidence with another detector. The energy resolution is 8.5% (FWHM) at 511 keV
References
[1] C. W. E van Ejik, “Inorganic scintillators in medical Imaging”, Phys. Med. Biol. 47 (2002), R85-106
[2] C. L. Melcher , US patent No 4958080, 1990.
[3] C. L. Melcher and J. S. Schweitzer, “Cerium doped lutetium oxyorthosilicate: a fast, efficient new scintillator”, IEEE Trans. Nucl. Sci. 39 (1992), 502-505
[4] Wienhard et al.”The ECAT HRRT:performance and first clinical application of the new high resolution research tomograph”, 2000 IEEE NSS-MIC Conference Record CDROM 17 (2001), 2-6
for pdf file, see the web site:
http://www.pi.infn.it/~bisogni/vienna_2010/ABSTRACT_vienna conference 2010_bisogni.pdf
