THE EFFECT OF THE LSO/YSO CONCENTRATIONS RATIO ON THE IMAGING CHARACTERISTICS UNDER MAMMOGRAPHIC CONDITIONS Anastasios C. Konstantinidis 1, Panayiotis F. Liaparinos 1, George D. Patatoukas 1, Ioannis G. Valais 1,2, Dimitrios N. Nikolopoulos 2, George S. Panayiotakis 1 and Ioannis S. Kandarakis 2 1 Department of Medical Physics, Medical School, University of Patras, P.O. BOX Patras, Greece. 2 Department of Medical Instruments Technology, Technological Educational Institution of Athens, Ag. Spyridonos, Aigaleo, P.O. BOX Athens, Greece

AIM To investigate the influence of the LSO/YSO concentrations ratio on: – Signal to noise ratio - SNR –Detection quantum efficiency - DQE To investigate the effect of the anode material and the x-ray energy on the SNR and DQE using the aforementioned detectors under mammographic conditions.

INTRODUCTION INTRODUCTION Scintillators or phosphor screens are used as x-ray to light converters in radiation detectors of a large variety of medical imaging applications: conventional and digital X-ray Radiography, Mammography, X-ray Computed Tomography (CT), Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) Most radiation detectors consist of a scintillator coupled to an optical detector: (photographic emulsion film, photocathode, photodiode, CCD etc.)

INTRODUCTION II In the present study, performances of LSO:Ce, YSO:Ce and LYSO:Ce, varying from 90/10 up to 50/50 (fractions of LSO/YSO), were examined under Mammographic conditions (20-40 kVp). SNR and DQE were studied for the following luminescent materials: LSO, YSO, 90/10 LYSO, 80/20 LYSO, 70/30 LYSO, 60/40 LYSO and 50/50 LYSO. Phosphor coating weight was chosen to be equal to 30, 40 and 50 mg/cm 2, typical for mammographic conditions.

THEORY Name:Cerium doped Lutetium Oxyorthosilicate –Lu 2 SiO 5 :Ce (LSO:Ce) –high density (7.4 g/cm3) –high effective atomic number (Z eff =66) –fast response (40ns) –relatively high light yield (26000 ph/MeV) –it is non-hydroscopic !!Large crystals show inhomogeanity in light production and decay time and their energy resolution is poorer than expected

THEORY II Lu 2 Y 2 SiO 5 :Ce (LYSO:Ce) Cerium doped Lutetium Yttrium Oxyorthosilicate Product of LSO:Ce mixture with YSO:Ce –YSO:Ce =Cerium doped Yttrium Oxyorthosilicate LYSO:Ce has better performance and advantages compared with LSO:Ce. Yttrium is a low cost material and it exhibits high intrinsic efficiency due to its K-absorption edge at keV.

THEORY III Input signal to noise ratio –For photon fluence –For energy fluence

THEORY IV Absorbed signal to noise ratio –For photon fluence –where Φ abs (Ε)=Φ 0 (Ε)η q (E). –where Ψ abs (Ε)=Ψ 0 (Ε)η ε (E) –For energy fluence

THEORY V Detection quantum efficiency That describes the degradation of the SNR from the input to absorption within the scintillator mass. That describes the degradation of the SNR from the input to absorption within the scintillator mass.

RESULTS Molybdenum, Rhodium and Tungsten anode x- ray spectra, filtered by inherent and additional 30 mm of Lucite, for 28kVp tube voltage.

RESULTS II Variation of input SNR 2 with x-ray tube voltage for Mo anode with constant air Kerma in the range from 20 up to 40 kVp.

RESULTS III Variation of DQE abs with x-ray tube voltage for Rh anode, filtered by an additional 30 mm of Lucite, for all scintillators of same coating thickness (40 mg/cm 2 ) using Φ abs (E). DQE abs were found to decrease with increasing tube voltage, because of the behaviour of μ tot,t/ρ.

RESULTS IV Variation of DQE abs with x-ray tube voltage for Rh anode, filtered by an additional 30 mm of Lucite, for all scintillators of same coating thickness (40 mg/cm 2 ) using Ψ abs (E).

RESULTS V Variation of DQE abs with x- ray tube voltage for Mo, Rh and W anodes using Φ abs (E). Mo anode spectra exhibited the highest DQE abs, while the W anode spectra exhibited the lowest values of DQE abs. It was found that Rh anode produced highest amount of x-ray photons than the Mo anode.

RESULTS VI Variation of DQE abs with x-ray tube voltage for Mo, Rh and W anodes using Ψ abs (E). Mo anode spectra exhibited the highest DQE abs, especially in medium energies, due to the lower mean energy than Rh anode spectra.

CONCLUSIONS I When the absorbed x-ray photon fluence (Φ abs (E)) was used, YSO exhibited highest SNR 2 and DQE of absorbed x-rays in the energy range from 24 up to 40 kVp while LSO had the lowest values. When the absorbed x-ray energy fluence (Ψ abs (E)) was used, LSO had superior SNR 2 and DQE of absorbed x-rays to other materials, while YSO was found with the lowest values.

CONCLUSIONS II In the first case calculations were made considering that the produced K characteristic x-ray radiation totally escapes from the scintillator, while in the second case the K characteristic is totally absorbed by the scintillator It seems that the first point of view is an overestimation and the second is an underestimation. It seems that the first point of view is an overestimation and the second is an underestimation. The reality lies somewhere in between. In both cases of Φ abs (E) and Ψ abs (E), it was observed that the Mo anode spectra exhibited the highest DQE abs, while the W anode spectra exhibited the lowest values.

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