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Anastasios C. Konstantinidis 1 , Panayiotis F. Liaparinos 1 , George D. Patatoukas 1 , Ioannis

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

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Anastasios C. Konstantinidis 1 , Panayiotis F. Liaparinos 1 , George D. Patatoukas 1 , Ioannis

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  1. THE EFFECT OF THE LSO/YSO CONCENTRATIONS RATIO ON THE IMAGING CHARACTERISTICS UNDER MAMMOGRAPHIC CONDITIONS Anastasios C. Konstantinidis1, Panayiotis F. Liaparinos1, George D. Patatoukas1, Ioannis G. Valais1,2, Dimitrios N. Nikolopoulos2, George S. Panayiotakis1 and Ioannis S. Kandarakis2 1 Department of Medical Physics, Medical School, University of Patras, P.O. BOX 26500 Patras, Greece. 2 Department of Medical Instruments Technology, Technological Educational Institution of Athens, Ag. Spyridonos, Aigaleo, P.O. BOX 122 10 Athens, Greece

  2. 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.

  3. 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.)

  4. 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/cm2, typical for mammographic conditions.

  5. THEORY • Name:Cerium doped Lutetium Oxyorthosilicate • Lu2SiO5:Ce (LSO:Ce) • high density (7.4 g/cm3) • high effective atomic number (Zeff=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

  6. THEORY II • Lu2Y2SiO5:Ce (LYSO:Ce) • Cerium doped Lutetium Yttrium Oxyorthosilicate • Product of LSO:Cemixture 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 17.04 keV.

  7. THEORY III • Input signal to noise ratio • For photon fluence • For energy fluence

  8. THEORY IV • Absorbed signal to noise ratio • For photon fluence • where Φabs(Ε)=Φ0(Ε)ηq(E). • where Ψabs(Ε)=Ψ0(Ε)ηε(E) • For energy fluence

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

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

  11. RESULTS II • Variation of input SNR2 with x-ray tube voltage for Mo anode with constant air Kerma in the range from 20 up to 40 kVp.

  12. RESULTS III • Variation of DQEabs 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/cm2) using Φabs(E) . • DQEabs were found to decrease with increasing tube voltage, because of the behaviour of μtot,t/ρ .

  13. RESULTS IV • Variation of DQEabs 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/cm2)using Ψabs(E).

  14. RESULTS V • Variation of DQEabs with x-ray tube voltage for Mo, Rh and W anodesusing Φabs(E). • Mo anode spectra exhibited the highest DQEabs, while the W anode spectra exhibited the lowest values of DQEabs. It was found that Rh anode produced highest amount of x-ray photons than the Mo anode.

  15. RESULTS VI • Variation of DQEabs with x-ray tube voltage for Mo, Rh and W anodesusing Ψabs(E). • Mo anode spectra exhibited the highest DQEabs, especially in medium energies, due to the lower mean energy than Rh anode spectra.

  16. CONCLUSIONS I • When the absorbed x-rayphoton fluence (Φabs(E)) was used, YSO exhibited highest SNR2 and DQE of absorbed x-rays in the energy range from 24 up to 40 kVpwhile LSO had the lowest values. • When the absorbed x-ray energy fluence (Ψabs(E)) was used, LSO had superior SNR2 and DQE of absorbed x-rays to other materials, while YSO was found with the lowest values.

  17. 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. • The reality lies somewhere in between. • In both cases of Φabs(E) and Ψabs(E), it was observed that the Mo anode spectra exhibited the highest DQEabs, while the W anode spectra exhibited the lowest values.

  18. REFERENCES • [1] Melcher, C. L., Schweitzer, J.S. (1992) ‘‘Cerium-doped Lutetium Oxyorthosilicate: A Fast, Efficient New Scintillator’’, IEEE Trans. on Nucl. Sci., Vol. 39, pp. 502. • [2] Ren, G., Qin, L., Lu, S., Li, H. (2004), ‘‘Scintillation characteristics of lutetium oxyorthosilicate (Lu2SiO5:Ce) crystals doped with cerium ions’’, Nucl. Instr. Meth. Phys. Res. A, Vol. 531, pp. 560-565. • [3]Valais, I., Kandarakis, I., Nikolopoulos, D., Konstantinidis, A., Sianoudis, I., Cavouras, D., Dimitropoulos, D., Nomicos, C., Panayiotakis, G. (2005) ‘‘Evaluation of light emission efficiency of LYSO: Ce scintillator under x-ray excitation for possible applications in medical imaging’’, Nucl. Instr. Meth. Phys. Res. A, (Article in Press). • [4]Swank, R.K. (1973), ‘‘Calculation of Modulation Transfer Functions of X-ray Fluorescent Screens’’, Appl. Opt., Vol. 12, pp.1865-1870. • [5]Swank, R.K. (1974), ‘‘Absorption and noise in x-ray phosphors’’, J. Appl. Phys., Vol. 45, pp. 4109-4203. • [6]Nishikawa, R.M., Yaffe, M.J. (1985), ‘‘Signal-to-noise properties of mammographic film-screen systems’’, Med. Phys., Vol. 12, pp. 32-39. • [7]Dick, C. E., Motz, J. W. (1981), ‘‘Image information transfer properties of x-ray fluorescent screens’’, Med. Phys., Vol. 8, pp. 337-346. • [8]Ginsburg, A., Dick, C. E. (1993), ‘‘Image information transfer properties of x-ray intensifying screens in the energy range from 17 to 320 keV’’, Med. Phys., Vol. 20, pp. 1013-1021. • [9]Cavouras, D., Kandarakis, I., Nikolopoulos, D., Kalatzis, I., Episkopakis, A., Linardatos, D., Roussou, M., Nirgianaki, E., Margetis, D., Valais, I., Kalivas, N., Sianoudis, I., Kourkoutas, K., Dimitropoulos, N., Louizi, A., Nomicos, C., Panayiotakis, G. (2005), ‘‘Light emission efficiency and imaging performance of Y3Al5O12: Ce (YAG: Ce) powder scintillator under diagnostic radiology conditions’’, Applied Physics B, Vol. 00, pp. 1-11. • [10]Konstantinidis, A., Episkopakis, A., Pyrovolakis, I., Valais, I., Patatoukas, G., Nikolopoulos, D., Panayiotakis, G., Kandarakis I. (2005), ‘‘Modelling the Signal to Noise Ratio in energy integrating radiation detectors’’, 1st International Conference on Experiments / Process / System Modelling / Simulation / Optimization (EpsMsO), Athens Greece, 6-9 July, CD-ROM Proceedings. • [11]Hubbell, J.H., Seltzer, S.M. (1995), Tables of X-ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients 1 KeV to 20 MeV for Elements Z=1 to 92 and 48 Additional Substances of Dosimetric Interest, NISTIR 5632, Gaithersburg. • [12]NIST: X-Ray Mass Attenuation Coefficients, http:// physics.nist.gov/PhysRefData/XrayMassCoef/cover.html • [13]Siemens - Vacuum Technology Division - Simulation of X-ray spectra, http:// www.healthcare.siemens.com/med/rv/spektrum/radIn.asp • [14]Boone, J.M. (2000), X-ray production, interaction, and detection in diagnostic imaging in: Handbook of Medical Imaging, Vol. 1, Physics and Psychophysics, edited by Beutel J., Kundel H.L., Van Metter R.L., SPIE, Bellingham. • [15]Boone, J.M., Seibert., J.A. (1997), ‘‘An accurate method for computer-generating tungsten anode x-ray spectra from 30 to 140 kV’’, Med. Phys., Vol. 24, pp. 1661-1670. • [16]Kandarakis, I., Cavouras, D., Sianoudis, I., Nikolopoulos, D., Episkopakis, A., Linardatos, D., Margetis, D., Nirgianaki, E., Roussou, M., Melissaropoulos, P., Kalivas, N., Kalatzis, I., Kourkoutas, K., Dimitropoulos, N., Louizi, A., Nomicos, K., Panayiotakis, G. (2005), ‘‘On the response of Y3Al5O12:Ce (YAG:CE) powder scintillating screens to medical imaging X-rays’’, Nucl. Instr. Meth. Phys. Meth. A, Vol. 538, pp. 615-630.

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