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Biomedical Imaging I

Biomedical Imaging I. Class 5 – Radionuclide Imaging (PET, SPECT), Part 3: Attenuation and Scatter Corrections 10/12/05. Recommended Reading. K. Miles, P. Dawson, and M. Blomley (Eds.), Functional Computed Tomography (Isis Medical Media, Oxford, 1997).

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Biomedical Imaging I

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  1. Biomedical Imaging I Class 5 – Radionuclide Imaging (PET, SPECT), Part 3: Attenuation and Scatter Corrections 10/12/05

  2. Recommended Reading • K. Miles, P. Dawson, and M. Blomley (Eds.), Functional Computed Tomography (Isis Medical Media, Oxford, 1997). • R. J. English, SPECT: Single Photon Emission Computed Tomography: A Primer (Society of Nuclear Medicine, Reston, VA, 1995). • M. Reivich and A. Alavi (Eds.), Positron Emission Tomography (A. R. Liss, NY, 1985).

  3. Characteristics of SPECT & PET images • Low spatial resolution • Dose limitations → long acquisition time • Imperfect tissue selectivity • Scatter effects • Low SNR • Dose limitations → poor counting statistics • Attenuation • High CNR • Essentially no signal except from the administered radionuclide • Post-processing operations improve SNR at the expense of spatial resolution • Spatial low pass (“long pass”) filtering

  4. Interplay of spatial resolution and CNR Ideal case: – No “bleeding” of signal from one location into the adjacent ones – Difference between signal arising from adjacent positions determines the contrast, and hence the CNR Reality: – Lateral spreading of image information results in lowering of high signal levels and raising of low ones – The apparent signal difference between adjacent sites is lower than in the ideal case, and so the CNR is lower

  5. Attenuation Correction

  6. Projection Operator a s θ x f(x,y) -a y  x 

  7. Projection Operator — CT vs. ECT x–ray CT: spatial distribution of attenuation coefficient ECT: spatial distribution of radioactivity

  8. 99mTc γ-ray photons e--e+ annihilation γ-ray photons X-Ray Energy & Contrast 20 keV 200 keV 2000 keV (= 2MeV)

  9. Other approaches to attenuation correction • Use co-registered anatomical image (e.g., MRI, x-ray CT) to generate an estimate of the tissue µ at each location, and insert that into Eq. (2) (Slide 7) • Use known-strength γ-emitting standards (e.g., 153Gd (Webb, §2.9.2, p. 79) or 68Ge (§ 2.11.4.1, p. 95)) in conjunction with image data collection, to estimateµ at eachtissue location • Iterative image reconstruction algorithms • In “odd-numbered” iterations, treat µ(u,v) as known and fixed, and solve for ρ(x,y) • In “even-numbered” iterations, treat ρ(x,y) as known and fixed, and solve for µ(u,v) • Even more elaborate mathematical techniques, e.g., • E. Y. Sidky and X. Pan, “Image reconstruction with a half–detector in single–photon emission computed tomography with nonuniform attenuation,” Optical Engineering42(9), 2506-2513 (2003). • I. Laurette et al., “A three–dimensional ray–driven attenuation, scatter and geometric response correction technique for SPECT in inhomogeneous media,” Physics in Medicine and Biology45, 3459-3480 (2000). • T. Kauppinen et al., “Improvement of brain perfusion SPET using iterative reconstruction with scatter and non–uniform attenuation correction,” European J. Nuclear Medicine27(9), 1380-1386 (2000).

  10. Scatter Correction

  11. Origins of scatter artifacts • Accidental coincidences (PET) • Compton scatter events (SPECT and PET) • Can redirect photons within measurement plane, or can redirect photons arising from outside measurement plane into it (assuming ring-shaped detector array) • Spatially and directionally distributed source • Higher photon energy than in x-ray CT means that detector collimation is less effective • More serious problem for area detectors (for whole-body imaging, for example) than for ring-shaped detector arrays

  12. Inherent Dimensionality

  13. Scatter correction techniques • Compton scattering (SPECT & PET) • Energy “sub-window” method (Webb, §2.9.2, p. 78) • Extrapolation of scattered “source” strength, from image regions outside the patient’s body, to image regions within it (§2.11.4.2, p. 97) • Accidental coincidences (PET) • True but impractical: if the radius r of the detector ring is varied, the rate of true coincidences is proportional to 1/r, while the accidental coincidence rate is proportional to 1/r2 • Accidental coincidence rate for a pair of detector is proportional to the product of the overall count rates for each • By increasing the delay between the timing pulses sent to each detector in a pair, can selectively detect only accidental coincidences

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