Steven Jude Duclos

Development of the HiLightTM scintillator for computed tomography medical imaging

Abstract The image quality of computed tomography (CT) medical scanners is extremely sensitive to afterglow, radiation damage and optical non-uniformities of scintillators used in X-ray detectors. This represents a tough challenge in the design of scintillator materials with optimum properties. Discussion will center on the development and properties of the first commercialized transparent ceramic scintillator, the (Y,Gd)2O3:Eu-based HiLightTM scintillator used in GE Medical Systems CT products. The flexibility of the ceramic scintillator platform has enabled it to be engineered to satisfy the changing needs of CT imaging, which is demonstrated by its successful incorporation into over 8000 CT systems worldwide since 1988. The ceramic process makes possible uniform co-doping at ppm levels to control electronic defects responsible for afterglow, reducing it to levels below detectibility in CT images. Annealing of the material in controlled oxygen atmospheres, combined with rapid oxygen diffusion along grain boundaries in the ceramic, reduces radiation damage to negligible values. Transient thermoluminescence of these materials will be discussed as a diagnostic of electronic trap levels responsible for both afterglow and radiation damage. Finally, with the increased scan speed requirements of modern CT systems, energy transfer between the Eu activator and other rare-earth ions can be used to speed the radiative decay of the scintillator, ensuring the materials continued viability in future CT systems.

Luminescence of Pr

The optical properties of Pr3+ activated LuCl3 and LuBr3 under intra-ionic 4f2rarr 4f1 5d1 and band gap (lambdaex = 193 nm; X-ray) excitations have been evaluated. The luminescence of Pr 3+ in these hosts is found to be strongly dependent on the excitation energy: for energies less than the absorption edge of the host, the luminescence is dominated by the 4f1 5d1 rarr 4f2 interconfigurational optical transitions. When luminescence is excited by photons with energies corresponding to the absorption edge or the band gap of the host, the emission spectrum is dominated by the intraconfigurational 4f2rarr 4f2 optical transitions, which emanate from the Pr3+ 3PJ states. A model to account for the dependence of the emission behavior of Pr3+ ion on the excitation energy is presented.

^{3+}

The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of several factors controlling the energy transfer from the host matrix to activator, two of which are investigated in the present paper. The first one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the row LuCl 3 -LuBr 3 -LuI 3 . The STE structure and the efficiency of STE to cerium energy transfer are verified by cluster ab initio calculations. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4 f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI 3 , and inefficient in LuCl 3 . In order to justify this channel we perform calculations of density of states using a periodic plane-wave density functional approach. The performed estimations theoretically justify the high LuI 3 :Ce 3+ scintillator yield.

Doped LuCl

The optical properties of Pr3+ activated LuCl3 under intra-ionic 4f2 rarr 4f1 5d1 and band gap (lambdaex = 193 nm; X-ray) excitations has been evaluated. The luminescence of LuCl3 :Pr3+ is found to be strongly dependent on the excitation energy: for energies less than the absorption edge of the host, the luminescence of LuCl3: Pr3+ is dominated by the 4f1 5d1 rarr 4f2 interconfigurational optical transitions. When luminescence is excited by photons with energies corresponding to the absorption edge or the band gap of the host, the emission spectrum is dominated by the intraconfigurational 4f2 rarr 4f2 optical transitions, which emanate from the Pr3+ 3PJ states. The luminescence efficiency of the inter- and intraconfigurational optical transitions is high at room temperature. A model to account for the dependence of the emission behavior of Pr3+ ion on the excitation energy is presented.