Class notes on the fundamental technology of Anger (Gamma) camera. Given at Lehigh Valley Hospital Nuclear Medicin Technologist training program, Allentown, PA

1. Gamma Cameras
David S. Graff PhD

2. Contact Information
• David S. Graff PhD
• David.S.Graff@gMail.com
• http://GraffNucMed.blogspot.com

3. Review of
How a gamma
Scintillation
camera works
cameras
Artifacts Collimators

4. Review of
How a gamma
Scintillation
camera works
cameras
Artifacts Collimators

5. A Gamma Camera is a giant
souped-up scintillation camera

7. A Gamma Camera is a giant
souped-up scintillation camera
Photomultiplier Tube
Scintillator
PMT

8. Some photons pass through the scintillator
some drop all their energy
some scatter out and deposit some energy

9. Deposited energy is converted to visible light by
scintilator
Amount of light proportional to deposited energy

10. Visible light is converted into electrons at surface
of photomultiplier tube
Number of electrons is proportional to amount of
light

11. Electrons pulled to positively charged plate
Knock more electrons off plate
These electrons pulled towards next plate with more positive voltage
Large number of electrons land on final plate, generate pulse
Size of pulse proportional to number of electrons

12. Photon deposits energy in scintillator
Size of pulse from PMT proportional to
deposited energy.

13. A gamma camera is simply a single
large scintillator crystal covered
with photomultiplier tubes.

18. A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the
total light

19. Review of
How a gamma
Scintillation
camera works
cameras
Artifacts Collimators

23. Anger circuit:
Sum of all outputs measures total light
= energy of incoming photon: Z

24. Anger circuit:
Weigted sum of all outputs measures
position: X+, X-
X-

25. Anger circuit:
Weigted sum of all outputs measures
position: X+, X-
X+

26. A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the
total light
The total light collected by all PMTs yields the energy
of the incoming photon.
The position of the photon is the weighted sum of
the signal from the different PMTs.
Weights must be precisely tuned.

27. Review of
How a gamma
Scintillation
camera works
cameras
Artifacts Collimators

28. What if the gain on a
PMT is too high?

34. Poor positioning can
lead to non-linearities

37. A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the
total light
The total light collected by all PMTs yields the energy
of the incoming photon.
The position of the photon is the weighted sum of
the signal from the different PMTs.
Weights of each PMT must be precisely tuned.
Miscorrections in position formula can lead to PMTs
visible in flood image
Technologist responsible for examining daily flood for
echos of PMTs

38. Review of
How a gamma
Scintillation
camera works
cameras
Artifacts Collimators

39. Collimators
For a camera to work, we need to be able to reconstruct
where the gamma ray came from.
Gamma rays can’t be focussed

40. Pinhole Collimator

41. A pinhole collimator only lets in
photons that pass through the small
hole. The rest are thrown away.

42. Pinhole collimators can magnify a
small object by moving close.

43. Parallel collimator

45. Collimators only pass rays
in a small set of angles

46. How collimators work
Most photons are
blocked by collimator Only photons
Radioactive tracer
parallel to
only goes to
collimator pass
certain tissues
through
Detector sees
Photons emitted blurred projection
in every direction of tracer in patient

47. Collimators are defined by
the following characteristics:
Collimator length
l
Septal Thickness Hole Diameter
a d

48. Acceptance angle is d/l

49. Bigger d, wider angle

50. Shorter l, wider angle

51. A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the
total light
The total light collected by all PMTs yields the energy
of the incoming photon.
The position of the photon is the weighted sum of
the signal from the different PMTs.
Weights of each PMT must be precisely tuned.
Miscorrections in position formula can lead to PMTs
visible in flood image
Technologist responsible for examining daily flood for
echos of PMTs
Collimators absorb nearly all photons.
Pinhole collimators can magnify, useful for small
objects.
Parallel collimators are most common, only let in
photons coming from single direction

52. Scatter can reduce
contrast

53. Idealized Gamma-Ray Spectrum
in NaI theoretical
Counts
per
Energy
Interval
Actual
Energy Eo

54. Too wide a window adds scattered
photons and reduces contrast