1. Nuclear Medicine Physics
Gamma Camera, Scintillation Camera
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia

2. Medical College of Georgia And Sameer Tipnis, Ph.D.
A note of thanks to
Z. J. Cao, Ph.D.
Medical College of Georgia
And
Sameer Tipnis, Ph.D.
G. Donald Frey, Ph.D.
Medical University of South Carolina
for
Sharing nuclear medicine presentation content

3. How to obtain a NM image?
Administer radiopharmaceutical (a radionuclide labeled to a pharmaceutical)
The radiopharmaceutical is concentrated in the desired locations.
Nucleus of the radionuclide decays to emit g photons
Detect the g photons using a “gamma camera” (scintillation camera, Anger camera)

4. Basic principle
- rays directed towards a scintillation crystal - NaI(Tl)
Multiple PMTs detect light flashes
Signal ( E) is converted to electrical pulses
Pulses fed to energy discrimination and positioning circuits
Image of radionuclide distribution formed and displayed
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

5. Nuclear medicine is emission imaging.
g photons are emitted from inside of patient.
g energy: 70 to 511 keV
Relatively poor image quality due to limited photon number (severe image noise) and poor spatial resolution
Image noise caused by low count density (105 – 106 lower than x-ray imaging)
CT is transmission imaging

6. BUT: Nuclear medicine is molecular imaging
Interaction of the radiopharmaceutical with cells or molecules  molecular imaging
Bound directly to a target molecule (111In-monoclonal antibody)
Accumulated by molecular or cellular activities (18F-FDG, 99mTc-sestamibi, 131I)
Molecular or cellular activities (e.g. perfusion for heart, brain, kidney, lungs and metabolism of cancers)  earlier diagnosis
Antibodies are proteins that are generated by the immune system, specifically the white blood cells. They circulate in the blood and attach to foreign proteins called antigens in order to destroy or neutralize them. Monoclonal antibodies are laboratory produced antibodies cloned from specific antibodies.
FDG: glucose metabolism
Sestamibi: imaging of heart muscle
I-131: thyroid imaging/therapy

7. Major components of gamma camerai e n c o l m r
NaI(Tl) crystal P M T -
amplify & sum
position analysis
Pulse Height Analysis u d s y X Y Z

8. Gamma Camera Components

9. Major components of gamma camera
Collimator
to establish position relationship between g photon source and detector (projection imaging)
Scintillation detector (NaI(Tl))
to convert g photons to blue light photons
Photomultiplier tube (PMT)
to convert blue photons to electrons and to increase the number of electrons
Electronics
Pulse Height Analysis: estimates energy deposited in each detection (enables scatter rejection)
Position Analysis: center of luminescent intensity
Display
display distribution of radioactivity in patient

10. Why collimator? – image formation
w/o collimator with collimator
detector
sources
images
image
collimator
Collimator sorta like grid
Image of a point source is the whole detector.
Image of a point source is a point.

11. Why collimator? – image formation
to establish geometric relationship between the source and image
The collimator has a major affect on gamma camera count rate and spatial resolution
parallel-hole collimator

12. Parallel-hole collimator
A collimator with small diameter holes ‘d’ or long holes provides good resolution but few counts and hence noisy image
Design principle: to optimize the
trade-off between counts and
resolution
Thickness of lead between collimator holes (septal thickness) ‘t’ must make septal penetration less than 5%

13. Different parallel-hole collimators
low-energy all purpose (LEAP) collimator (Eg < 150 keV)  better efficiency but worse resolution
low-energy high resolution (LEHR) collimator (Eg < 150 keV)  better resolution but worse efficiency
medium-energy all purpose (MEAP) collimator (150 keV < Eg < 300 keV)
high-energy all purpose (HEAP) collimator for I-131 (Eg = 361 keV)

14. Collimators Most often used: parallel-hole collimator
For thyroid: pin-hole collimator
For brain and heart: converging collimator
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

15. Pinhole collimator single hole admitting photons
low efficiency and small FOV but potentially excellent resolution
Decreasing source-to-
detector distance leads to
larger image,
better resolution
higher count rate.

16. Detection of g photons in detector
An incident g photon may be stopped (absorbed) by or penetrate the detector
more penetration with higher photon energy
g photons recorded as counts (electrical pulses)
Counts represent concentration and distribution of radioactivity in the patient
A: absorption B A
B: penetration
p.e
c.s
A: absorption
p.e

17. Detection of g photons in detector
The pulse height is determined by the energy deposited by a g photon in the detector.
A penetrating g photon deposits less energy so the electrical pulse is smaller.
A photon scattered in the patient loses energy so the pulse is smaller when it is detected.
Scatter in detector make it impossible to know the entry point of the g photon.

18. Scintillation process in detector
Most detectors are ~3/8” of NaI (Tl). Tl (activator) facilitates scintillation at room temperature
As a g photon creates ionizations in the detector
Ionizations free e- from the atom to create ion-e- pairs
The ion-e- pairs excite Tl atoms.
Tl atoms return to ground state by emitting blue light (~ 3 ev)
p.e
c.s

19. Scintillation process in detector
The detector converts g photons to a number of blue photons.
The number of blue photons is proportional to the energy deposited by g photon
e.g. 140 keV  5000 and 70 keV  2500 blue photons
The number of blue photons determines the number of electrons liberated in the photocathodes of PMTs and in turn, the electrical pulse height.
Electrical pulse height is proportional to g photon energy deposited in the crystal

20. Desirable Scintillator Properties
High , Z  high absorption efficiency
Improves detector sensitivity
High light output (conversion efficiency)
Improves energy discrimination, spatial resolution
Light output proportional to energy deposited
Improves linearity
Transparent to light emissions
Improves sensitivity
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

21. How good (bad) is NaI (Tl) detector?
Good stopping power for low-energy g photons by photoelectric process
at 69 keV, penetration  0% for a thickness (t) of 3/8”
at 140 keV, penetration = 7.7% for t of 3//8”
at 247 keV, penetration = 48.5% for t of 3/8”
Slow scintillation decay
(230 ns) which limits count
rate (avoid pulse pile-up)

22. How good (bad) is NaI (Tl) detector?
relatively dense, high Z (~ 55)
good conversion efficiency: ~ 26 eV/blue photon
good transparency for blue photons
blue photons matched with PMTs photocathode sensitivity
Compton scatter dominates at Eg > 250 keV  poor spatial resolution
fragile and hygroscopic (can absorb water, turn yellow)
Hermetically sealed

23. Photomultiplier tube Create and amplify electric pulses
photocathode (CsSb): to convert blue light to e-
dynodes: each to increase electrons 3 – 6 times
anode: to collect e-
gain in e- number:
610  6 × 107
 very efficient

24. Photomultiplier tube (PMT)
40 to 100 PM tubes (d = 5 cm) in a modern gamma camera
photocathod directly coupled to detector or connected using plastic light guides
anode connected
to electronics in
the tube base
ultrasensitive to
magnetic field

25. Each PMT provides a weighted X+, X-, Y+ and Y- signal
Weighting factors for
19 tube camera Y+ 16 17 18 15 6 7 19 X- 14 5 1 2 8 X+ 13 4 3 9 12 11 10 Y-
Each PMT provides a weighted X+, X-, Y+ and Y- signal

26. Energy Signal Z = x+ + x- + y+ - y-
The outputs from all the PMT’s are summed to estimate energy deposited

27. Event Location X+ = x+ - x- Z Y+ = y+ - y- Z
The X, Y outputs from all the PMT’s are summed to estimate the center of scintillation

28. Pulse height analyzer
selects Z pulses of certain voltage amplitudes  to discriminate against unwanted (scattered)  photons 3
V2 (154 keV)
V1 (126 keV) 1 2

29. Absorbed energy spectrum of detector
Penetration/scatter: energy deposited in detector is between 0 and E0.
photopeak: all energy of g photons (E0) deposited in detector
energy window

30. Photopeak
All the energy of a g photon (E0) is deposited in the detector
e.g. E0 = 140 keV for Tc-99m
p.e
c.s or

31. Image Formation (Photopeak)
USEFUL FOR IMAGING
counted
Stops > 99.95% of  s
stopped
Typical efficiency of a LEHR collimator ~ 0.02 %
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

32. Penetration/scatter spectrum
Some of the energy of a g photon (E0) is deposited in the detector
c.s
NOT USEFUL FOR IMAGING
p.e
30 keV x-ray
p.e
x-ray

33. Scatter Major source of image degradation in NM
Increases image noise and reduces lesion contrast
Windowing the photopeak allows suppression of scatter events (but not complete elimination)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

34. Scatter in patient
scatter
Photopeak

35. Image degradation Septal penetration Counted
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

36. Image degradation Simultaneous detections detected detected
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

37. Image degradation Scatter detected
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

38. System spatial resolution
system resolution Rsys
intrinsic (detector) resolution Rint
collimator resolution Rcol
Rint typically 2.9mm to 4.5mm
Rcol typically 7.4mm to 13.2mm
Rsys typically 1cm

39. Collimator Resolution
Spatial resolution degrades with increasing pt – collimator distance.
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

40. Effect of  coll-to-pt distance
Type
Spat. Res.
Efficiency
FOV
Parallel hole  
Converging 
Diverging
Pinhole
Increasing collimator to pt distance ALWAYS degrades spatial resolution
Parallel hole collimator has very favorable properties
This is the main collimator used in NM
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

41. Gamma camera energy resolution
energy spread due to fluctuation of the blue photon number in the detector, and fluctuation of electric signal in subsequent electronics
Energy resolution determines the width of the energy window.
Typical system energy resolution: 9 – 11%
Typical clinical energy window: 20%, 140±10% keV, 126 – 154 keV
better energy resolution  smaller energy window  acquiring most of the photopeak counts but fewer scatter counts

42. Data acquisition collimator: match the radioisotope
energy window: match the radioisotope
pixel size: 1/3 ~ 1/2 of spatial resolution
usually, 64×64, 128×128 or 256×256
2 bytes in pixel depth
count rate < 20000/sec
patient close to the detector

43. Effect of matrix size
64× ×128

44. Planar NM Imaging
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

45. Quality control of gamma camera
uniformity: daily, 256×256, > 4M counts
resolution: weekly, 512×512, > 4M counts
acquisition of new uniformity maps and possible energy map: quarterly, > 30M counts

46. Uniformity
a collimator defect a bad PMT shift of energy peak

47. Bar phantom
made of lead stripes with different orientations and spacing in 4 quadrants
to measure extrinsic and intrinsic linearity and spatial resolution
extrinsic: place a Co-57 sheet source with the bar phantom on the top of the collimator
intrinsic: take collimator off and place a Tc-99m point source 5 × detector size away
detector
bar
pt source
g ray