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Corrections for physical effects in Positron Emission Mammography

Corrections for physical effects in Positron Emission Mammography. Nuno C. Ferreira Dep. Biofísica – IBILI Fac. Medicina – Univ. Coimbra PEM Workshop – Lisbon, 1-2 July 2002. Why correct data for physical effects?. To improve image quality

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Corrections for physical effects in Positron Emission Mammography

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  1. Corrections for physical effects in Positron Emission Mammography Nuno C. Ferreira Dep. Biofísica – IBILI Fac. Medicina – Univ. Coimbra PEM Workshop – Lisbon, 1-2 July 2002

  2. Why correct data for physical effects? • To improve image quality • Image should represent the true activity distribution • Corrections improve contrast and tumour detectability, remove artefacts • To preserve quantification • Useful in comparative studies (e.g. follow-up to therapy)

  3. Data corrections are necessary... assumed measured projection projection integral of the activity along the line or tube of response Detector efficiencyeffects Scatteredcoincidences component Attenuationof radiation Truecoincidencescomponent Random coincidencescomponent • ...because the measured projections are not the same as the projections assumed during image reconstruction Object(uniformcylinder)

  4. Correct data for what physical effects? Random coincidences Random Compton Scatter Compton True event Event lost dueto sensitivity Event lost due to Attenuation Attenuation Sensitivity variations(Normalization correction)

  5. Correct data for what physical effects? • Random coincidences • Compton Scatter • Attenuation • Sensitivity variations (Normalization correction) Random

  6. Randoms Correction Using Delayed Channel PROMPTcoincidences = TRUES + RANDOMS delay Detector 2 + delay DELAYEDcoincidences Estimate(TRUES) = (TRUES+RANDOMS) – Estimate(RANDOMS) = Estimate(RANDOMS) = – “TRUES” PROMPTS DELAYED time Detector 1 Detector 2

  7. Random coincidences correction S2 R12 R12 = 2T . S1 . S2 S1 Detector 1 Detector 2 Width of coincidence window Singles rate in detectors 1 and 2 Randomsrate • To improve the estimation of random coincidences: • Option 1: Averaging of estimations obtained with various delays • Option 2: Measurement of the single events:

  8. Random coincidences correction • Randoms: PEM vs. PET • PEM has less activity in the FOV  less randoms than PET • In PEM most activity is outside the FOV and there are no end-shields  larger % of randoms from outside the FOV • ClearPEM: LuAP is fast  very short coinc. window  less randoms Brain PETDetectors PEMDetectors Myocardium Torso End-shields Liver Bladder

  9. Activity outside the FOV Contributions of different sources to the total random coincidences from outside the FOV Brain(1%) Myocardium(9%) Liver(7%) Torso(82%) Bladder(1%) PEMDetectors Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923

  10. Correct data for what physical effects? • Random coincidences • Compton Scatter • Attenuation • Sensitivity variations (Normalization correction) Compton

  11. Effect of scatter • Main effect: • Reduces image contrast (target-to-background activity ratio) • Affects tumour detectability • Other effects: • Reduces quantification accuracy • Slight reduction of the total number of useful events detected • Slight degradation of spatial image resolution

  12. Scatter correction • PEM vs. PET • Relative to PET, scatter is less important in PEM... • ...but influence of activity from outside the FOV is more important • Activity is mostly outside the FOV (heart, chest) • No side shielding in PEM • Correction methods: • Proposed by other groups for PEM: • Monte-Carlo simulation [Holdsworth et al., IEEE TNS, February 2002] • Convolution-subtraction (Bergström) [Raylman et al., IEEE TNS, June 2001] • Proposed by our group for PET: • Hybrid energy-based correction [Ferreira et al., Phys Med Biol, May 2002] Advantage: takes scatter from outside the FOV into account

  13. Correct data for what physical effects? • Random coincidences • Compton Scatter • Attenuation • Sensitivity variations (Normalization correction) Event lost due to Attenuation

  14. Attenuation correction When a radiation beam penetrates a material with thickness x and linear attenuation coefficient m, it is attenuated by a factor: % of true (unscattered) coincidences detected: x=50 cm  1% e-mx x=15 cm  24% x= 5 cm  62% For tissue, m=0.096 cm-1 @ 511 keV • The amount of attenuation is mainly dictated by the dimensions of the human body. • Attenuation is more important in PET than in PEM • Attenuation needs to be measured in PET, but probably not in PEM • attenuation coefficient @ 511keV is ~ constant for the breast • use of consistency conditions may be sufficiently accurate

  15. Attenuation correction Effect of Attenuation Correction (PET) Source: Meikle, Bailey et al., J NucMed (1995); 36:1680-1688

  16. Correct data for what physical effects? • Includes various corrections, mainly dependent of the scanner: • geometrical effects • intrinsic efficiencies of individual detectors • interferences between individual detectors • deadtime • Random coincidences • Compton Scatter • Attenuation • Sensitivity variations (Normalization correction)

  17. Results from bibliography FDG uncorrected corrected Water (Cold) Normalized+ Random correction+ Scatter correction Raw image(no corrections) Normalized+ Random correction Normalized Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923 • Scatter phantom

  18. Results from bibliography (2) Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923 • Breastphantom (?) Raw image(no corrections) Normalized+ Random correction+ Scatter correction Normalized+ Random correction Normalized

  19. Summary • Specificities of PEM: • Large influence of activity outside the FOV • Little activity in the FOV • Most activity is outside the FOV • Side-shielding is not possible • Small FOV, object • Less attenuation • High sensitivity • Large solid angle • Less attenuation • High resolution • Indications for PEM camera design: • Randoms correction • Smallest possible coinc. window (LuAP is OK) • Capability to count singles/channel... • ...or count randoms with multiple delays • Scatter correction • Good energy information (~256 energy channels) • List-mode (with energy information) • Normalization correction • Information about single events (for deadtime correction) • Specificities of PEM Corrections: • Randoms correction • Less randoms than PET • Larger % of randoms from outside the FOV than PET • Same methods as in PET • Scatter correction • Less scatter than PET • More scatter from outside the FOV than PET • Hybrid energy-based methods • Attenuation correction • Less attenuation than PET • Attenuation in breast is more homogeneous than in PET • No transmission measurement (but attenuation correction seems feasible) • Normalization correction • Correction is scanner dependent, as in PET

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