Nuclide Imaging: Planar Scintigraphy, SPECT, PET - PDF Document

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  1. Nuclide Imaging: Planar Scintigraphy, SPECT, PET Yao Wang Polytechnic University, Brooklyn, NY 11201 Based on J. L. Prince and J. M. Links, Medical Imaging Signals and Systems, and lecture notes by Prince. Figures are from the textbook except otherwise noted.

  2. Lecture Outline • Nuclide Imaging Overview • Review of Radioactive Decay • Planar Scintigraphy – Scintillation camera – Imaging equation • Single Photon Emission Computed Tomography (SPECT) • Positron Emission Tomography (PET) • Image Quality consideration – Resolution, noise, SNR, blurring EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 2

  3. What is Nuclear Medicine • • Also known as nuclide 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) • • • • From H. Graber, Lecture Note for BMI1, F05 EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 3

  4. Examples: PET vs. CT • X-ray projection and tomography: – X-ray transmitted through a body from a outside source to a detector (transmission imaging) – Measuring anatomic structure Nuclear medicine: – Gamma rays emitted from within a body (emission imaging) – Imaging of functional or metabolic contrasts (not anatomic) • Brain perfusion, function • Myocardial perfusion • Tumor detection (metastases) • From H. Graber, Lecture Note, F05 EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 4

  5. What is Radioactivity? EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 5

  6. Positron Decay and Electron Capture • Also known as Beta Plus decay – A proton changes to a neutron, a positron (positive electron), and a neutrino – Mass number A does not change, proton number Z reduces The positron may later annihilate a free electron, generate two gamma photons in opposite directions – These gamma rays are used for medical imaging (Positron Emission Tomography) • From: http://www.lbl.gov/abc/wallchart/chapters/03/2.html EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 6

  7. Gamma Decay (Isometric Transition) • A nucleus (which is unstable) changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons) (called gamma rays). The daughter and parent atoms are isomers. – The gamma photon is used in Single photon emission computed tomography (SPECT) Gamma rays have the same property as X-rays, but are generated different: – X-ray through energetic electron interactions – Gamma-ray through isometric transition in nucleus • From: http://www.lbl.gov/abc/wallchart/chapters/03/3.html EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 7

  8. Measurement of Radioactivity Bq=Bequerel Ci=Curie: (orig.: activity of 1 g of 226Ra) Naturally occurring radioisotopes discovered 1896 by Becquerel First artificial radioisotopes produced by the Curies 1934 (32P) The intensity of radiation incident on a detector at range r from a radioactive source is AE I π = 2 4 r A: radioactivity of the material; E: energy of each photon EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 8

  9. Radioactive Decay Law • • N(t): the number of radioactive atoms at a given time A(t): is proportional to N(t) dN = − = λN A dt decay : λ constant • From above, we can derive − λ t = N t N e ( ) 0 − λ − λ t t = = λ A t A e N e ( ) 0 0 • The number of photons generated (=number of disintegrations) during time T is ∫ ∫ = = ∆ T T • − λ − λ t T λ = − N A t dt N e dt N e ( ) 1 ( 0 ) 0 0 0 EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 9

  10. Common Radiotracers Thyroid function Kidney function Most commonly used Oxygen metabolism EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 10

  11. Overview of Imaging Modalities • Planar Scintigraphy – Use radiotracers that generate gammay decay, which generates one photon in random direction at a time – Capture photons in one direction only, similar to X-ray, but uses emitted gamma rays from patient – Use an Anger scintillation camera SPECT (single photon emission computed tomography) – Use radiotracers that generate gammay decay – Capture photons in multiple directions, similar to X-ray CT – Uses a rotating Anger camera to obtain projection data from multiple angles PET (Positron emission tomography) – Uses radiotracers that generate positron decay – Positron decay produces two photons in two opposite directions at a time – Use special coincidence detection circuitry to detect two photons in opposite directions simultaneously – Capture projections on multiple directions • • EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 11

  12. Planar Scintigraphy • Capture the emitted gamma photons (one at a time) in a single direction • Imaging principle: – By capturing the emitted gamma photons in one particular direction, determine the radioactivity distribution within the body – On the contrary, X-ray imaging tries to determine the attenuation coefficient to the x-ray EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 12

  13. Anger Scintillation Camera Compare the detected signal to a threshold Compute the location with highest activity Convert light to electrical currents Convert detected photons to lights Absorb scattered photons EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 13

  14. Collimators EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 14

  15. Scintillation Detector • Scintillation crystal: – Emit light photons after deposition of energy in the crystal by ionizing radiation – Commonly used crystals: NaI(Tl), BGO, CsF, BaF2 – Criteria: Stopping power, response time, efficiency, energy resolution • Detectors used for planar scintigraphy EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 15

  16. Photomultiplier Tubes • Each tube converts a light signal to an electrical signal and amplifies the signal EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 16

  17. Inside a Photomultiplier Tube 10^6-10^8 electrons reach anode for each electron liberated from the cathode Increasing in voltage, Repeatedly generates more electrons, 10-14 steps Dynode: positively charged For each electron reaching a dynode, 3-4 electrons are released For every 7-10 photons incident upon the photocathode, an electron is released Outputs a current pulse each time a gamma photon hits the scintillation crystal. This current pulse is then converted to a voltage pulse through a preamplifier circuit. EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 17

  18. Positioning Logic Each incident photon causes responses at all PMTs, but the amplitude of the response is proportional to its distance to the location where the photon originates. Positioning logic is used to estimate this location. EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 18

  19. Pulse Height Calculation EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 19

  20. Pulse Height Analysis EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 20

  21. Acquisition Modes EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 21

  22. List Mode EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 22

  23. Single Frame Mode The value in each pixel indicates the number of events happened in that location over the entire scan time EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 23

  24. Dynamic Frame Mode EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 24

  25. Multiple Gated Acquisition EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 25

  26. Imaging Geometry and Assumption (x,y) z EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 26

  27. Imaging Equation EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 27

  28. Planar Source EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 28

  29. Examples • Example 1: Imaging of a slab • Example 2: Imaging of a two-layer slab • Go through on the board EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 29

  30. SPECT • Instrumentation • Imaging Principle EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 30

  31. SPECT Instrumentation • Similar to CT, uses a rotating Anger camera to detect photons traversing paths with different directions • Recent advances uses multiple Anger cameras (multiple heads), reducing scanning time (below 30 minutes) • Anger cameras in SPECT must have significantly better performances than for planar scintigraphy to avoid reconstruction artifacts EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 31

  32. A typical SPECT system Fig. 9.1 A dual head system EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 32

  33. Imaging Equation: θ=0 (z,l) R Replace x by l EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 33

  34. General Case: Imaging Geometry s l R EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 34

  35. General Case: Imaging Equation EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 35

  36. Approximation Under this assumption, A can be reconstructed using the filtered backprojection approach The reconstructed signal needs to be corrected! EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 36

  37. Correction for Attenuation Factor • Use co-registered anatomical image (e.g., MRI, x-ray CT) to generate an estimate of the tissue µ at each location • 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 estimate µ at each tissue location • Iterative image reconstruction algorithms – In “odd-numbered” iterations, treat µ(x,y) as known and fixed, and solve for A(x,y) – In “even-numbered” iterations, treat A(x,y) as known and fixed, and solve for µ(x,y) From Graber, Lecture Slides for BMI1,F05 • EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 37

  38. Example • Imaging of a rectangular region, with the following structure. Derive detector readings in 4 positions (A,B,C,D) B A C Α1, µ1 Α2,µ2 D Do you expect the reading at B and D be the same? What about at A and C? EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 38

  39. SPECT applications • Brain: – Perfusion (stroke, epilepsy, schizophrenia, dementia [Alzheimer]) – Tumors Heart: – Coronary artery disease – Myocardial infarcts Respiratory Liver Kidney • • • • •From Graber, Lecture Slides for BMI1,F05 •See Webb Sec. 2.10 EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 39

  40. PET Principle EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 40

  41. Annihilation Coincidence Detection • Detect two events in opposite directions occurring “simultaneously” • Time window is 2-20 ns, typically 12 ns • No detector collimation is required – Higher sensitivity EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 41

  42. Detected PET Events EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 42

  43. Coincidence Timing EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 43

  44. PET Detector Block BGO is chosen because of the higher energy (511KeV) of the photons EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 44

  45. Multiple Ring Detector EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 45

  46. PET Detector Configuration EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 46

  47. A Typical PET Scanner EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 47

  48. Combined PET/CT Systems • CT: provides high resolution anatomical information • PET: Low resolution functional imaging • Traditional approach: – Obtain CT and PET images separately – Registration of CT and PET images, to help interpretation of PET images • Combined PET/CT: Performing PET and CT measurements within the same system without moving the patient relative to the table – Make the registration problem easier – But measurement are still taken separately with quite long time lag EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 48

  49. Imaging Equation     ' R ∫ s ( ) s 0 + = − µ N N x s y s E ds exp ( ( ), ' ( ' )); ) 0   0 −  s   ' 0 ∫ − ( ) s 0 − = µ N N x s y s E ds exp ( ( ), ' ( ' )); ) 0   R −    ' R ∫ s ( ) s 0 = µ N N x s y s E ds exp ( ( ), ' ( ' )); ) c 0   0 −  s     ' 0 ∫ − • µ x s y s E ds exp ( ( ), ' ( ' )); )   R −  R ∫ −   ' = µ N x s y s E ds exp ( ( ), ' ( ' )); ) 0   R     R ∫ − R ∫ − R ∫ − R ∫ −         ϕ θ = − µ = • − µ l K A x s y s x ), ' s y s ds ds  K A x s y s ds x ), ' s y s ds ( , ) ( ( ), ( )) exp ( ( ( ' )) ' ( ( ), ( )) exp ( ( ( ' )) '    R R R R separated! be can µ(x,y) A x y ( , and ) EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 49

  50. Attenuation Correction • One can apply filtered backprojection algorithm to reconstruct A(x,y) from the corrected sinogram EL5823 Nuclear Imaging Yao Wang, Polytechnic U., Brooklyn 50