THE EFFECT OF NON-IDEAL THE EFFECT OF NON-IDEAL DETECTORS ON ENERGY WEIGHTED DETECTORS ON ENERGY WEIGHTED

SPECTRA USED IN SPECTRA USED IN X-RAY MEDICAL IMAGINGX-RAY MEDICAL IMAGING

George D. PatatoukasGeorge D. Patatoukas11, Panagiotis F. , Panagiotis F. LiaparinosLiaparinos11, Anastasios D. Gaitanis, Anastasios D. Gaitanis22, Ioannis S. , Ioannis S.

KandarakisKandarakis2, George S. Panayiotakis, George S. Panayiotakis11

1. Department of Medical Physics, Medical School, University of Patras, 265 1. Department of Medical Physics, Medical School, University of Patras, 265 0000

Patras, GreecePatras, Greece2.Depatrment of Medical Instrumentation Technology, Technological 2.Depatrment of Medical Instrumentation Technology, Technological

Educational Institution of Athens, Agiou Spyridonos street, Aigaleo, 122 10 Educational Institution of Athens, Agiou Spyridonos street, Aigaleo, 122 10 Athens, GreeceAthens, Greece

AIMAIM

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.

INTRODUCTIONINTRODUCTION

Previous studies ignored scintillator-induced noise.Previous studies ignored scintillator-induced noise.

SNR evaluation under mammographic conditions.SNR evaluation under mammographic conditions.

Energy-sensitive pixel detectors, could define each photon Energy-sensitive pixel detectors, could define each photon not only spatially but also in terms of its energy.not only spatially but also in terms of its energy.

METHODMETHOD

An algorithm was produced to study the variation of the An algorithm was produced to study the variation of the weighting factor in terms of anode material, of energy and in weighting factor in terms of anode material, of energy and in terms of tumour or microcalcification thickness.terms of tumour or microcalcification thickness.

Different anode materials (Molybdenum and Wolfram) were Different anode materials (Molybdenum and Wolfram) were used for a variety of different energies from 25 to 40 used for a variety of different energies from 25 to 40 kVp.kVp.

Various possible thicknesses were considered for both Various possible thicknesses were considered for both microcalcifications and tumours.microcalcifications and tumours.

The phantom designed was 1-dimensional.The phantom designed was 1-dimensional.

METHOD (II)METHOD (II)

Tumor / microcalcificationRegion

(varying thickness)

Gd2O2S:Tb (scintillator)

Breast tissue of thickness

S1 S2

Φ’(E) Φ’’(E)

Φ (E)

Figure 1. Typical x ray imaging situation using a phantom with two different regions (breast and microcalcification, or, breast and tumour).

4.5 cm

METHOD (III)METHOD (III)

The energy weighting The energy weighting factor is defined in the factor is defined in the following way:following way:

Attenuation coefficient Attenuation coefficient values were calculated values were calculated according to the following according to the following formulae for the cross formulae for the cross section section ττ(E) and mass (E) and mass attenuation coefficient attenuation coefficient μ μ ((EE))::

ZEZE 56.015.24)( 32.4

A

EE 0)(

METHOD (IV)METHOD (IV)

SCINTILLATOR CHARACTERISTICS (GdSCINTILLATOR CHARACTERISTICS (Gd22OO22S:Tb)S:Tb)

Emission: Forbidden 4f 4f transitionEmission: Forbidden 4f 4f transition

Highest intensity line: 545 nm (green)Highest intensity line: 545 nm (green)

High Z material (64)High Z material (64)

X-ray to light conversion efficiency X-ray to light conversion efficiency ηηcc=0.19=0.19

Thickness: Thickness: 32 mgcm32 mgcm-2-2

Density: 7.3 gcmDensity: 7.3 gcm-3-3

SIGNAL AND NOISE DEFINITIONSIGNAL AND NOISE DEFINITION dEdTg

E

EteES

E

ct

T

'','0

'','2,1

E ct

TdEdTg

E

EteEN

2

'','0

'','2,1

METHOD (V)METHOD (V)

SNRSNR

SNR WEIGHTEDSNR WEIGHTED

SNR RATIOSNR RATIO

21

21

21

NN

SSSNR

2

12

21

221

][

][

WNN

WSSSNRweighted

SNR

SNRSNR weighted

ratio

RESULTSRESULTS

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

RESULTS(II)RESULTS(II)

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

RESULTS (III)RESULTS (III)

Weighting factor variation with energy for Mo at 36 kVp Weighting factor variation with energy for Mo at 36 kVp for microcalcification and tumour lesions both with size for microcalcification and tumour lesions both with size 0.2 cm0.2 cm

RESULTS (IV)RESULTS (IV)

Variation of SNR ratio with tube voltage for Variation of SNR ratio with tube voltage for microcalcification and for tumour using different anode microcalcification and for tumour using different anode materialsmaterials..

DISCUSSIONDISCUSSION

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

DISCUSSION (II)DISCUSSION (II)

Larger enhancement is achieved when using Larger enhancement is achieved when using Molybdenum than Wolfram spectra, due to the strong Molybdenum than Wolfram spectra, due to the strong variations of the Mo spectra.variations of the Mo spectra.

CONCLUSIONCONCLUSION

SNR enhancement is achievable.SNR enhancement is achievable.

Larger enhancement when Molybdenum is used as an Larger enhancement when Molybdenum is used as an anode material.anode material.

Better SNR ratio values when microcalcifications are Better SNR ratio values when microcalcifications are present.present.

FUTURE PROSPECTIVESFUTURE PROSPECTIVES

Consideration of other anode materials (e.g. Rhodium ).Consideration of other anode materials (e.g. Rhodium ).

Consideration of other scintillation detectors (e.g. Consideration of other scintillation detectors (e.g. CsI:Na).CsI:Na).

Improvement of simulation geometry (3-D from 1-D).Improvement of simulation geometry (3-D from 1-D).

Develop algorithm to calculate energy weighting on Develop algorithm to calculate energy weighting on images.images.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

This work was financially supported by the research This work was financially supported by the research programme EPEAEK “Archimedesprogramme EPEAEK “Archimedes””

REFERENCESREFERENCES

[1] Cahn, R.N, Cederstrőm, B., Danielsson, M., Hall, A., Lundqvist, M., [1] Cahn, R.N, Cederstrőm, B., Danielsson, M., Hall, A., Lundqvist, M., Nygren, D. (1999), ‘Detective quantum efficiency dependence on x ray Nygren, D. (1999), ‘Detective quantum efficiency dependence on x ray energy weighting in mammography’, Medical Physics, Vol. 26, pp.2680-energy weighting in mammography’, Medical Physics, Vol. 26, pp.2680-2683.2683.[2] Griesh, J., Niederlőhner, D., Anton, G. (2004), ‘The influence of energy [2] Griesh, J., Niederlőhner, D., Anton, G. (2004), ‘The influence of energy weighting on X-ray imaging quality’, Nuclearweighting on X-ray imaging quality’, Nuclear Instruments and Methods in Instruments and Methods in Physics Research Section A, Vol. 531, pp. 68-74.Physics Research Section A, Vol. 531, pp. 68-74.[3] Van Eijk, C.W.E (2002), ‘Inorganic scintillators in medical imaging’, [3] Van Eijk, C.W.E (2002), ‘Inorganic scintillators in medical imaging’, Phys. Med. Biol., Vol. 47, pp. R85-R106. Phys. Med. Biol., Vol. 47, pp. R85-R106. [4] http://www.med.siemens.com/med/rv/spektrum/mamIn.asp[4] http://www.med.siemens.com/med/rv/spektrum/mamIn.asp[5] Boone, J. M., Seibert, J.A. (1997), ‘An accurate method for computer-[5] Boone, J. M., Seibert, J.A. (1997), ‘An accurate method for computer-generating tungsten anode x-ray spectra from 30 to 140 kV’, Medical generating tungsten anode x-ray spectra from 30 to 140 kV’, Medical Physics, Vol. 24, Issue 11, pp. 1667-1670.Physics, Vol. 24, Issue 11, pp. 1667-1670.[6] Boone, J. M., Fewell, T.R, Jennings, R.J. (1997), ‘Molybdenum, [6] Boone, J. M., Fewell, T.R, Jennings, R.J. (1997), ‘Molybdenum, rhodium, and tungsten anode spectral models using interpolating rhodium, and tungsten anode spectral models using interpolating polynomials with application to mammography’, Medical Physics, Vol. 24, polynomials with application to mammography’, Medical Physics, Vol. 24, Issue 12, pp. 1863-1874.Issue 12, pp. 1863-1874.

REFERENCES (II)REFERENCES (II)

[7] Ludwig, G. W. (1971), ‘X-ray efficiency of powder phosphors’, J. [7] Ludwig, G. W. (1971), ‘X-ray efficiency of powder phosphors’, J. Electrochem. Soc., Vol.Electrochem. Soc., Vol. 118, pp.118, pp. 1152–1159.1152–1159.[8] Swank, R. K. (1973), ‘Calculation of modulation transfer functions of [8] Swank, R. K. (1973), ‘Calculation of modulation transfer functions of x-ray fluorescent screens’, Appl. Opt., Volx-ray fluorescent screens’, Appl. Opt., Vol. . 1212, , pp.pp. 1865–701865–70[9] Beutel, J., Apple, B. A., Shaw., R. (1993) , ‘The role of screen [9] Beutel, J., Apple, B. A., Shaw., R. (1993) , ‘The role of screen parameters and print-through in the performance of film/screen systems’, parameters and print-through in the performance of film/screen systems’, Phys. Med. Biol.,Phys. Med. Biol., Vol. 38, pp.Vol. 38, pp. 1181–2061181–206 . .[10] Kandarakis, I., Cavouras, D., Panayiotakis, G.S., Nomicos, C.D.[10] Kandarakis, I., Cavouras, D., Panayiotakis, G.S., Nomicos, C.D.(1997), ‘Evaluating x-ray detectors for radiographic applications: A (1997), ‘Evaluating x-ray detectors for radiographic applications: A comparison of with and screens’, Phys. Med. Biol., Vol. 42, pp. 1351-comparison of with and screens’, Phys. Med. Biol., Vol. 42, pp. 1351-1373.1373.[11] Kandarakis, I., Cavouras, D., Nomicos, C.D., Panayiotakis, G.S.[11] Kandarakis, I., Cavouras, D., Nomicos, C.D., Panayiotakis, G.S.(2001), ‘X-ray luminescence of phosphor using X-ray beams for medical (2001), ‘X-ray luminescence of phosphor using X-ray beams for medical applications ’, Nuclearapplications ’, Nuclear Instruments and Methods in Physics Research Instruments and Methods in Physics Research Section B, Vol.179, pp. 215-224.Section B, Vol.179, pp. 215-224.