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Imaging

Imaging. PET. Course Layout. Talk Layout. SPECT (Short introduction) PET – Physical principles and Structure PET corrections PET image reconstruction PET Typical applications in Brain Science. Principle of radionuclide imaging. Introduce radioactive substance into body

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Imaging

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  1. Imaging PET

  2. Course Layout

  3. Talk Layout • SPECT (Short introduction) • PET – Physical principles and Structure • PET corrections • PET image reconstruction • PET Typical applications in Brain Science

  4. Principle of radionuclide imaging • Introduce radioactive substance into body • Allow for distribution and uptake/metabolism of compound Functional Imaging! • Detect regional variations of radioactivity as indication of presence or absence of specific physiologic function • Detection by “gamma camera” or detector array • (Image reconstruction)

  5. Single photon emission CT (SPECT) • Single photon counting: • Windowing (reduces scatter, background) • Counting statistics limited by patient radiation dose • ~ 30 min examination w/ camera • First SPECT 1963 (Mark IV) used array of detectors • Rotation, Translation • High count rates • Many components • Mostly single-slice • Rotating camera: • Multiple slices • Multi-camera systems

  6. Brain: Perfusion (stroke, epilepsy, schizophrenia, dementia [Alzheimer]) Tumors Heart: Coronary artery disease Myocardial infarcts Respiratory Liver Kidney SPECT applications

  7. PET

  8. Positron emission

  9. Scintillation Detection

  10. Detector Assemblies 1-to-1 Coupling Excellent livetime characteristics, but expensive, and limited in size to smallest available PMT (~1cm2). Anger Camera Light from scintillator is distributed among several PMT’s; measured distribution determines location. Poor livetime, but can have good resolution with enough light output--NaI(Tl). Block Detector Individual crystals “pipe” light to detectors. More complex, but required with low light output

  11. Block Detector

  12. PET evolution

  13. PET

  14. DET 2 DET 1 Pulse Processing Pulse Processing AND Coincidence Detection

  15. Coincidences

  16. 5a 5b 4 1 2 3 Coincidence Events 1. Detected True Coincidence Event 2. True Event Lost to Sensitivity or Deadtime 3. True Event Lost to Photon Attenuation 4. Scattered Coincidence Event 5a,b. Random Coincidence Event

  17. Accidental (random) coincidences: • Two unrelated annihilation photons reach two opposing detectors within the time window of the coincidence resolving time  (~10-20 ns) g2 detector j D d g1 detector i t: Pulse legth (2t = resolving time) f : Fraction of detectors involved f ~ 1 Ci,Cj: Individual (single) count rates

  18. Attenuation Correction

  19. Scatter Elimination

  20. Filtered Back Projection

  21. Filtered Back Projection

  22. Filtered Back Projection

  23. Filtered backprojection • Filter the measured projection data at different projection • angles with a special function. • Backproject the filtered projection data to form the • reconstructed image. • Filtering can be implemented in 2 ways, in the spatial domain, the filter operation is • equivalent to to convolving the measured projection data using a special convolving • function h(t) • More efficient multiplication will be in the spatial frequency domain. • FFT the measured projection data into the frequency domain: • p(,)=FT {p(t, ) • Multiply the the fourier transform projections with the special function. • Inverse Fourier transform the product p’(,).

  24. 2D Vs. 3D

  25. Randoms

  26. Scatters

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