1. Introduction to Gamma Camera
Unit III: Part B
Introduction to Gamma Camera

2. Objectives
Describe the major components of the Anger gamma camera and the development and concept of positioning logic, including the requirement for energy normalization
Discuss later improvements to the Anger camera’s design

3. Circa 1950s Rectilinear Scanner

4. Anger Camera—circa 1958

5. Hal Anger’s original plan using 7 PMTs
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 253.

6. Cross-sectional Schematic of Gamma Camera
PMTs
NaI (Tl) Crystal

8. What we want to get is…

9. But the schematic says we’ll get…

10. Because of its particulate, high energy, nature, we can only use High “Z” material to “absorb” gamma radiation (i.e. lead).

11. … adding a multi-hole collimator to the exterior-facing portion of the large scintillation crystal

12. Collimators…”discriminate based on direction of flight…” (Christian pg
Collimators allow only gamma photons traveling in a desired direction to be detected.
Collimators themselves cannot discriminate different photon energies from one another.

13. Position Logic Circuitry

14. Position Logic Circuitry with normalization of event energy
Sum circuit totals all output to create the Z pulse
The Z pulse is sent to the PHA and the division circuit to normalize X and Y output for positioning

15. Related positioning logic concept: edge packing
Picture from: accessed 11/11/12

16. NaI(Tl) Crystals Usually 3/8 to 5/8 inch thick
Hygroscopic so hermetically sealed
Surrounded by inward reflective cover

17. Thicker crystal : more sensitive crystal
Gamma photon
Less likely gamma photon will pass through undetected
But PMTs will see it more as a foggy diffuse light

18. Thinner crystal : better resolution
Gamma photons
But PMTs will see a gamma photon as a more distinct point of light
More gamma photons will pass through undetected

19. 3/8 inch crystal required to efficiently detect gamma photons up to 200 keV
Most modern gamma cameras have 38 x 50 cm rectangular FOVs

20. More resolute thicker crystals are possible for higher photon energies.
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 69.

21. Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 69.

22. Putting It All Together

23. Putting It All Together
Collimator –
Only allows direct path photon to interact with NaI(Tl) crystal.

24. Putting It All Together
Crystal –
Emits visible light when energy from gamma photon is deposited within in

25. Putting It All Together
Light Pipe—
Anger’s original design included a thick clear plastic sheet between crystal & PMT. The photoemissive material is thicker at the center of the photo anode resulting in making the PMTs shapes visible on a uniform image. This was an attempt to reduce this effect.
Newer cameras correct this digitally and no longer use light pipes.

26. Putting It All Together
PMTs –
Generate electric pulse in response to visible light from crystal. (Closest PMT to light generates greatest pulse)
Modern cameras have an array of PMTs
More recently produced systems include software for thresholding PMT responses for more accurate positioning

27. Putting It All Together
Position Logic Circuit –
Compares PMT pulses and assigns a location on an image matrix that corresponds with the event detected by the crystal.

28. Putting It All Together
Position Logic Circuit –
Sends location signals in terms of x and y values

29. Putting It All Together
DIVISION CIRCUITRY
Z Pulse
Sum of PMT pulses generated by a detection event (represents total energy deposited on the crystal).
The Z pulse is also used in the “Division Circuitry” to “normalize” the X & Y positioning signals.

30. Putting It All Together
Pulse Height Analyzer-
As with any scintillation detector, allows us to select pulses of a certain height and reject others.

31. -y +y +x -x
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 253 & 255.

32. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg

33. Multiformatter High quality CRT
Can direct image to multiple spots on film
2, 4, 8, 16, 30 images on one film
Works great, but NO MISTAKES!
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg

34. Variable Persistance Scope (P-Scope)
More commonly digitized now
Screen is “refreshed” based on time or counts
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg

35. Digital Images
Digitized means a discrete value is ascribed to each pulse height
(no longer an infinite variety of pulse heights—they are quantitatively categorized)
A MCA discriminates acceptable pulses
An accepted pulse is 1 count
A matrix is an electronic/digital array of square pixels used for count storage and display on a CRT, LCD, LED, or plasma monitor

36. Pixels ADC: Analog to Digital Converter Display
(For most modern gamma cameras, the ADC is at the end of each PMT)
Display
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 362.
Information Storage for Display
Kai Lee, Computers in Nuclear Medicine: A Practical Approach, 2nd Ed., (Reston, VA: SNM 2005) p. 83

37. Important Matrix Concepts
Using the above gray scale, we can see in the simple 3 X 3 matrix below that the maximum counts are in pixel nbr. 9, and the minimum are in pixel 5. 1 2 3 4 5 6 7 8 9

38. Modern Gamma Cameras
Most modern gamma camera systems come with two cameral linked to a moveable gantry and/or movable bed.
This is to facilitate:
Circular geometry
Angled planar views
SPECT
Head to toe (whole body) imaging
Most dual-head systems are variable angle
180 degrees (opposing planar and 360 degree SPECT)
90 degrees (180 degree Cardiac SPECT)

39. In Summary
With its positioning logic circuitry, the Anger camera led to the development of Nuclear Medicine
Modern cameras offer a number of improvements through digitization
Many more and better refined PMTs are used
Positioning and sum signals are digitized
Computer algorithms have replaced the hard-wired positioning circuit
X/Y positioning is no longer displayed by a CRT but as part of a computer image matrix

40. Nuclear medicine techs and physician view cowardly lion scan