THE EFFECT OF NON-IDEAL DETECTORS ON ENERGY WEIGHTED SPECTRA USED IN X-RAY MEDICAL IMAGING

1. THE EFFECT OF NON-IDEAL DETECTORS ON ENERGY WEIGHTED SPECTRA USED IN X-RAY MEDICAL IMAGING George D. Patatoukas1, Panagiotis F. Liaparinos1, Anastasios D. Gaitanis2, Ioannis S. Kandarakis2, George S. Panayiotakis1 1. Department of Medical Physics, Medical School, University of Patras, 265 00 Patras, Greece 2.Depatrment of Medical Instrumentation Technology, Technological Educational Institution of Athens, Agiou Spyridonos street, Aigaleo, 122 10 Athens, Greece

2. AIM • The present study investigates the effect that the energy weighting technique has on the quality of signal to noise ratio (SNR) in x-ray medical imaging under the assumption that the detector considered is non-ideal. A theoretical evaluation of the SNR under these conditions is carried out.

3. INTRODUCTION • Previous studies ignored scintillator-induced noise. • SNR evaluation under mammographic conditions. • Energy-sensitive pixel detectors, could define each photon not only spatially but also in terms of its energy.

4. METHOD • An algorithm was produced to study the variation of the weighting factor in terms of anode material, of energy and in terms of tumour or microcalcification thickness. • Different anode materials (Molybdenum and Wolfram) were used for a variety of different energies from 25 to 40 kVp. • Various possible thicknesses were considered for both microcalcifications and tumours. • The phantom designed was 1-dimensional.

5. METHOD (II) Φ (E) Tumor / microcalcification Region (varying thickness) Breast tissue of thickness 4.5 cm Φ’(E) Φ’’(E) Gd2O2S:Tb (scintillator) S2 S1 Figure 1. Typical x ray imaging situation using a phantom with two different regions (breast and microcalcification, or, breast and tumour).

6. METHOD (III) • The energy weighting factor is defined in the following way: • Attenuation coefficient values were calculated according to the following formulae for the cross section τ(E) and mass attenuation coefficient μ (E):

7. METHOD (IV) • SCINTILLATOR CHARACTERISTICS (Gd2O2S:Tb) • Emission: Forbidden 4f 4f transition • Highest intensity line: 545 nm (green) • High Z material (64) • X-ray to light conversion efficiency ηc=0.19 • Thickness: 32 mgcm-2 • Density: 7.3 gcm-3 • SIGNAL AND NOISE DEFINITION

8. METHOD (V) • SNR • SNR WEIGHTED • SNR RATIO

9. RESULTS • SNR ratio variation with microcalcification thickness at 30kVp using two different anode materials Mo and W. The SNR enhancement is clearly larger when using Molybdenum.

10. RESULTS(II) • SNR ratio variation with tumour thickness at 30kVp using two different anode materials Molybdenum and Wolfram. The SNR enhancement is again larger when using Molybdenum, but overall is less that when microcalcification is present.

11. RESULTS (III) • Weighting factor variation with energy for Mo at 36 kVp for microcalcification and tumour lesions both with size 0.2 cm

12. RESULTS (IV) • Variation of SNR ratio with tube voltage for microcalcification and for tumour using different anode materials.

13. DISCUSSION • Attenuation coefficients for microcalcification and tumor in the low-energy regions (up to 30 keV).

14. DISCUSSION (II) • Larger enhancement is achieved when using Molybdenum than Wolfram spectra, due to the strong variations of the Mo spectra.

15. CONCLUSION • SNR enhancement is achievable. • Larger enhancement when Molybdenum is used as an anode material. • Better SNR ratio values when microcalcifications are present.

16. FUTURE PROSPECTIVES • Consideration of other anode materials (e.g. Rhodium ). • Consideration of other scintillation detectors (e.g. CsI:Na). • Improvement of simulation geometry (3-D from 1-D). • Develop algorithm to calculate energy weighting on images.

17. ACKNOWLEDGEMENTS • This work was financially supported by the research programme EPEAEK “Archimedes”

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