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Title: Evaluation of physical characteristics of the Lu2SiO5:Ce3+ (LSO:Ce) scintillator in single crystal and in granular form for applications in X-ray medical imaging systems
Other Titles: Πειραματική και θεωρητική αξιολόγηση φυσικών χαρακτηριστικών φωσφόρων-σπινθηριστών ταχείας απόκρισης με ενεργοποιητή ιόντων δημητρίου (Ce3+) για εφαρμογή σε συστήματα ιατρικής απεικόνισης
Authors: Δαυίδ, Ευστράτιος
Issue Date: 2009-03-27T07:33:23Z
Keywords: X-ray detectors
Keywords (translated): Ανιχνευτές ακτινοβολίας Χ
Abstract: For all medical imaging systems using X-rays or γ-rays, radiation detector development in general and scintillator development in particular are in full progress. There is a strong interest in the introduction of new dense, high-atomic-number inorganic scintillation crystals with a high light yield and a fast response, especially for PET and SPECT. Powder scintillators are of interest for projection X-ray imaging. For PET, research is focused on CeP3+P doped scintillators, employing the 5d → 4f transitions. A high light yield is expected in the visible region. The time response in PET/CT applications will be in the 25–100 ns range. Improved energy resolution will also be a point of interest. For CT, time response requirements are at the microsecond level for decay time and the afterglow should be well below 10P−4Ps. For X-ray screens light spreading should be kept under control, e.g. by denser material like LSO:Ce, or columnar phosphors (CsI:TI) and by shorter luminescence emission wavelength which shows higher light attenuation of laterally directed photons. In this study we examine, both in powder and in crystal form, the detection efficiency of LuB2BSiOB5B:Ce, the absolute luminescence efficiency, the X-ray to luminescence efficiency, the spectral compatibility and the effective efficiency using various optical detectors. All these measurements were conducted in the X-ray energy range from 22 to 140 kVp used in medical X-ray imaging. In conclusion the X-ray quantum detection efficiency and the X-ray energy absorption efficiency of a LSO:Ce powder scintillator screen of 25 mg/cmP2 Pcoating thickness were found higher than currently employed materials (e.g. GdB2BOB2BS: Tb and CsI:Tl) for detection of X-rays used in mammographic applications. The absolute luminescence efficiency of this screen maintains high values, within the mammographic energy range and the intrinsic conversion efficiency was found close to that of CsI:Tl but lower than that of GdB2BOB2BS:Tb. In ragiographic energy range the screen of 172.5 mg/cmP2P exchibit the higher values of ALE and XLE. The emission spectrum of LSO:Ce screens showed excellent spectral compatibility with currently used digital detectors and taking also into account its very fast response it could be considered for applications in digital X-ray imaging systems. The LSO:Ce scintillator crystal shows higher absolute luminescence efficiency values (17,86 at 140kVp) than the corresponding of BGO crystal (3,40 at 140 kVp). LSO:Ce X-ray luminescence efficiency was found higher than the corresponding of BGO crystal in the whole range of energies used in our study. The higher value of DOG, 2430 gain units, showed at 140 kVp X-ray tube voltage for the LSO:Ce scintillator when the corresponding value at the same energy of the BGO is 1670 units. In the mammographic energy range the difference between the above measured values was smaller than the ones obtained in the radiographic energy region. This lead us to the assumption that LSO:Ce crystal can be efficiently used for X-ray energy imaging (under 100 kVp) as those used in CT applications. The intrinsic conversion efficiency was estimated to be significantly higher for LSO:Ce, which in addition is higher than more of the currently employed scintillators. The light emission spectrum of the LSO:Ce scintillator, peaking at about 420 nm, was found compatible to many currently employed optical photon detectors. Its very short scintillation decay time and its high detection efficiency, both in terms of QDE and EAE, they can be crucial for the applications of this scintillator in modern fast image producing X-ray computed tomography systems, especially those employed in combined PET/CT devices. 12/−−⋅⋅smRmWμ12/−−⋅⋅smRmWμ The comparison of the ALE and the XLE of the LSO:Ce single crystal scintillator with that of the efficient powder LSO:Ce scintillator shows that the LSO:Ce screens can be used: i) in modern scinti-mammography/ X-ray mammography systems in which one common fast scintillator in powder form may satisfy the requirements of both modalities and the strict requirements in spatial resolution and ii) in applications where single crystals have to be replaced by powder or ceramic scintillators in order to improve spatial resolution (eg. in ring type SPECT detectors and in combined SPECT/CT detectors). In addition, this comparison may be of interest since powder scintillators are of lower cost than the corresponding single crystals.
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