Nuclear Medicine: Planar Imaging and the Gamma Camera

1. Nuclear Medicine: Planar Imaging and the Gamma Camera Katrina Cockburn Nuclear Medicine Physicist

2. Methods of Analysis • Once tracer has traced – need some method of analysing distribution Imaging Gamma Camera, PET Camera Compartmental Analysis Sample Counter

3. Radiation Detectors • Converts incident photon into electronic signal • Most commonly used detectors are scintillation • Photon interacts with crystal to convert incident photon into light photons • PMT changes light into electrical signal • Electrical signal recorded and analysed

4. Imaging Equipment • The Gamma Camera • Basic principle hasn’t changed since 1956!

5. Scintillation Imaging • Administration of Isotope

6. Scintillation Imaging • Localisation and Uptake

7. Scintillation Imaging • Localisation and Uptake

8. Scintillation Imaging • Localisation and Uptake

9. Scintillation Imaging • Localisation and Uptake

10. Scintillation Imaging • Localisation and Uptake

11. Scintillation Imaging • Localisation and Uptake

12. Scintillation Imaging • Localisation and Uptake Enhanced contrast between Organ of Interest and rest of body

13. Scintillation Imaging • Imaging distribution Gamma-rays emitted by radiopharmaceutical Collimator ‘selects’ only those rays travelling at right angles to face of camera Scintillation events in crystal recorded

14. Early Scintillation Study

15. Components of a Modern Gamma Camera • The components of a modern gamma camera Lead Shield Electronics PMTs Lightguide Crystal Collimator

16. The Collimator • The collimator consists of: • a lead plate • array of holes • It selects the direction of the photons incident on the crystal • It defines the geometrical field of view of the camera

17. The Collimator Detector Detector Patient Patient • In the absence of collimation: • no positional relationship between source – destination • In the presence of collimation: • all γ-rays are excluded except for those travelling parallel to the holes axis – true image formation

18. Types of Collimators • Several types of collimator: • Parallel-Hole • Converging • Diverging • Pin-Hole

19. Energy Ranges of Collimators

20. The Scintillation Crystal • First step of image formation • Photon detected by its interaction in the crystal • γ-rays converted into scintillations

21. Can be thought of as “partial ionisation” Electrons excited and gain energy As electrons fall back to ground state, photons emitted Use of doping (eg NaI:Tl) creates smaller gaps Scintillation

22. Scintillation Crystal Properties • High stopping efficiency • Stopping should be without scatter • High conversion of γ-ray energy into visible light • Wavelength of light should match response of PMTs • Crystal should be transparent to emitted light • Crystal should be mechanically robust • Thickness of scintillator should be short

23. Properties of NaI(Tl) Scintillator • The crystal – NaI(Tl) • emits light at 415 nm • high attenuation coefficient • intrinsic efficiency: 90% at 140 keV • conversion efficiency: 10-15% • energy resolution: 15-20 keV at 150 keV

24. Disadvantages of NaI(Tl) crystal • NaI(Tl) crystal suffers from the following drawbacks: • Expensive (~£50,000 +) • Fragile • sensitive against mechanical stresses • sensitive against temperature changes • Hygroscopic • encapsulated in aluminium case

25. Lightguide and Optical Coupling • Lightguide acts as optical coupler • Quartz doped plexiglass (transparent plastic) • The lightguide should: • be as thin as possible • match the refractive index of the scintillation crystal • Silicone grease to couple lightguide, crystal and PMT • No air bubbles trapped in the grease

26. The Photomultiplier Tube • A PMT is an evacuated glass envelope • It consists of: • a photocathode • an anode • ~ 10 dynodes

27. The Photomultiplier Tube • Photocathode of PMT emits 1 photoelectron per ~ 5 – 10 photons • Photoelectronaccelerated towards first dynode • Dynode emits 3 – 4 secondary e- per photoelectron • Secondary e- accelerated towards next dynode • Multiplication factor ~ 106 • Output of each PMT proportional to the number of light photons

28. PMT Properties • The photocathode should • be matched to blue light • have high quantum efficiency • High stability voltage supply: ~1kV

29. Positional and Energy Co-ordinates • PMT signals processed • spatial information –X and Y signals • energy information – Z signal • Z signal – the sum of the outputs of all PMTs • proportional to the total light output of the crystal • Light output proportional to the energy of incident gamma • Pulse height analyser accepts or rejects the pulse

30. Pulse Height Analysis • Z-signal goes to PHA • PHA checks the energy of the γ-ray • If Z-signal acceptable • γ-ray is detected • position determined by X and Y signals • 20% window stillincludes 30% ofscattered photons

31. Determining the Position of Events

32. Image Acquisition Techniques • Static - (Bones, Lungs) • Dynamic - (Renography) • Gated - (Cardiac) • Tomography • SPECT • PET • List Mode - (Cardiac)

33. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 1 Camera Computer Memory Image Display

34. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 1 1 Camera Computer Memory Image Display

35. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 1 1 1 Camera Computer Memory Image Display

36. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 1 1 1 1 Camera Computer Memory Image Display

37. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 2 1 1 1 Camera Computer Memory Image Display

38. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 2 1 1 1 1 Camera Computer Memory Image Display

39. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 3 1 1 1 1 Camera Computer Memory Image Display

40. Static Imaging • Camera FOV divided into regular matrix of pixels • Each pixel stores number of gamma rays detected at corresponding location on detector • Typical Matrix Sizes: 2562, 1282, 642 3 1 1 1 1 Camera Computer Memory Image Display

41. Dynamic Imaging • Series of sequential static frames • E.g. 90 frames each of 20s duration • Image rapidly changing distribution of activity within the patient • Used in Renography

42. Dynamic Imaging Analysis Split Renal Function Curves showing changing renal activity over time ROIs

43. Gated Imaging • Several frames acquired covering the cardiac cycle • Acquired over many cycles