1. Physics of Nuclear Medicine, SPECT and PET
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia
Augusta University

2. Outline Radionuclides in Nuclear Medicine Radiation Dose
Gamma Camera Basics
SPECT (Single Photon Emission Computed Tomography)
PET (Positron Emission Tomography

3. Radionuclides used in nuclear medicine
Less than 20 radionuclides but hundreds of labeled compounds
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps, 4th edition, 2012

4. Effective dose of NM procedures

5. Dose Definition
Effective dose E (Sv): measure of absorbed dose to whole body, the product of equivalent dose and organ specific weighting factors
Whole body dose equivalent to the nonuniform dose delivered
Equivalent dose: averahe absorbed dose across an organ or tissue with radiation specific weighting factors (Sv)

6. How to obtain a NM image?
Administer radiopharmaceutical (a radionuclide labeled to a pharmaceutical)
The radiopharmaceutical concentrates in the desired locations
Nucleus of the radionuclide decays to emit photons (g , x-ray)
Detect the photons using a “gamma camera”

7. Gamma Camera Basics p a t i e n c o l m r d P M T - amplify & sum
position analysis
Pulse Height Analysis u s y X Y Z

8. Photomultiplier tube (PMT)
40 to 100 PM tubes (d = 5 cm) in a modern gamma camera
photocathode directly coupled to detector or connected using plastic light guides
ultrasensitive to magnetic fields

9. Why collimator? – image formation
w/o collimator with collimator
detector
sources
images
image
collimator
Image of a point source is the whole detector.
Image of a point source is a point.

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

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

12. Collimator Summary
Collimator must be matched to energy of radionuclide
Efficiency changes little with distance to source (patient)
Resolution falls off quickly with distance to source (patient)
Low energy: < 160 keV
Medium energy: < 250 keV
High energy

13. Energy spectrum of detector
Septal penetration & scatter: energy deposited in detector is between 0 and E0.
photopeak: all energy of g photons (E0) deposited in detector
energy window

14. 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

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

16. Modern Camera Design Most cameras use rectangular heads
Most cameras are designed to do SPECT imaging
The dual head is the most common design

17. Tomographic imaging (SPECT)
SPECT (Single Photon Emission Computed Tomography)
Tomographic imaging (SPECT)
Tomographic images can be produced by acquiring conventional gamma camera projection data at several angles around the patient
Similar to CT

18. © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps
Sinogram
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

19. © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

20. Filtered Back Projection
Attenuates streaks by filtering the projections
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps
1D projections convolved with a ramp filter (actually a ramp with some roll-off)
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

21. Iterative Reconstruction
Quantitatively more accurate
Can model various corrections
Collimator
Scatter
System geometry
Detector resolution
Slow
Being used increasingly in SPECT
Improve comments on limitations of filtered back projection.

22. Measured Projection (P)
Assume
Some Image (I)
Calculate
Projections (P’)
Calculation Includes
Attenuation
Scatter
Blur with depth
Compare to
Measured Projection (P)
Use P’ & P
to form corrections
Image estimate may be a uniform image
Assumed image undergoes forward projection to produce sinogram
Form New
Image (I’)
Is I-I’< *
Done

23. Data Collection Image matrix is collected Each image row makes a slice
64 x 64 or 128 x 128
Each image row makes a slice
Multiple slices can be added to reduce noise
Anything higher than 128 x 128?

24. Attenuation correction: Chang Method
I(x) = I0e-mx
Assume uniform attenuation
m = linear attenuation coefficient of soft tissue (0.15 per cm for Tc-99m)
X is tissue thickness along projection from emission data
Get SPECT images that are attenuated and not attenuated

25. Attenuation correction: Transmission measurements
X-ray source (SPECT/CT)
Non-diagnostic CT
Diagnostic CT

26. PET (Positron Emission Tomography)
Positron decay characteristics
Coincidence and angular correlation
Time of flight
PET detector/scanner design
Data corrections

27. Positron is an Anti-particle
When a particle and and antiparticle interact they annihilate
Both particles are destroyed
Two photons(Gamma-rays) are created
Two photons are emitted in ~opposite directions (± 0.25 degrees for F-18)
Gamma 1
Gamma 2 + -

28. PET Imaging Concepts
Where was the event? ?
Coincidence

29. Where was the event?

30. Annihilation Detection
In coincidence counting an event is ONLY registered
if a signal is received from two detectors within a
narrow window of time.
A few nanoseconds is usually used.
Coincidence

31. Time-of-Flight PET
In “Time-of-Flight” pet, use of a very small time window
(<100 picoseconds) can localize an annihilation event to
within a few cm along the line of coincidence.
Time-of-Flight PET can improve SNR.
Coincidence

32. PET Scanner
Ring (multiple rings) with lots of little detectors (up to 23,040)
Rings have axial coverage of up to 26cm.
Detectors must have good stopping power
Detector must be fast for accurate coincidence measurements
Lutecium silicate LSO (LYSO) is commonly used (&BGO)
Modern detector materials?
Philips Vereos Digital PET 23, 040 detectors

34. PET scanner
PET scanners lack conventional collimation so they have a high geometric efficiency
Some had septal rings to reduce cross talk from ring to ring
When rings in 2D
When rings out 3D
Septa
2D growing out of favor?
Septa

35. Detector Needs High Stopping Power Light Output Short Decay Time
Much higher gamma ray energy (511 keV)
Light Output
Not as important because each gamma ray leaves a lot of energy in the crystal
Short Decay Time
Very important because of high count rate
Limits activity given to patient

37. Events in PET Scanners

39. Trues

40. Trues Rtrue = AO g2 gACDe-mT gACD~ h/2D for ring(s)
Where Rtrue = true coincidence rate
Ao = Administered activity
g = intrinsic efficiency
gACD = geometric efficiency
e-mT = object attenuation
h = detector thickness
D = detector diameter

41. Scatter
A 511 keV photon can give up 340 keV in a single 180 degree scatter
Scatter most probable about 45 degrees from incident direction, leaving a scattered photon of 285keV

42. Scatter-to-True Ratio
brain
.4 -2 body
Scatter (and Trues) are proportional to administered activity
Random-to-True ratio higher for 3D since there are no septa to eliminate scatter

43. RRnd = CTW Rtrue Rtrue
CTW = timing window
Random

44. Random-to-True Ratio .1 – 2 brain .1 -1 body
Random-to-True Ratio high near high activity (Bladder)
Random-to-True Ratio high near high activity (Bladder)

45. Corrections
PET scanners use energy discrimination (pulse height analysis) system like the gamma camera to help eliminate scatter
Randoms are corrected for by measuring coincidence rates with a delay of time between 511 keV photon arrivals (so there are no trues).
Actually measures anticoincident 511 keV photons

46. Attenuation Correction
Like all radionuclide imaging there is a problem due to attenuation.
It is much less for PET than for Tc-99m imaging
Correction is important for quantifying the metabolic activity of lesions (SUVs)

47. Attenuation Correction
CT data reconstructed to make a attenuation map of the body
Attenuation map information is used in image reconstruction
Replace w/ CT data

48. PET: CT Based Attenuation Correction
Get PET images that are attenuated and not attenuated
See Darko’s 2015 head and neck lecture
© Nuclear medicine physics : a handbook for students and teachers, International Atomic Energy Agency, 2014

49. SPECT vs PET PET SPECT (Simultaneous acquisition)
(Step-and-shoot acquisition)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

50. SPECT & PET SPECT – 2 views from opposite sides
Res. ~ collimator res., which degrades rapidly with increasing distance from collimator face
PET – Simultaneous acquisition
Res. ~ detector width; is max in center of ring
SPECT sensitivity ~ 0.02%
Huge losses due to absorptive collimators
PET sensitivity- 2D ~ 0.2%; 3D ~ 2% or higher
High sensitivity due to ACD (electronic collimation)
Allows higher frequency filters / higher spatial resolution
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

51. October 7, Researchers at the University of California, Davis (UC Davis) have received a five-year, $15.5 million grant to develop what they are calling the world's first total-body PET scanner.
National Cancer Institute and will fund the Explorer project, led by Simon Cherry, PhD, distinguished professor of biomedical engineering and Ramsey Badawi, PhD, a professor of radiology.
The total-body PET scanner would image an entire body all at once, and it would acquire images much faster or at a much lower radiation dose by capturing almost all of the available signal from radiopharmaceuticals. … the design would line the entire inside of the PET camera bore with multiple rings of PET detectors.
… such a total-body PET design could reduce radiation dose by a factor of 40 or decrease scanning time from 20 minutes to 30 seconds

52. References
Physics in Nuclear Medicine: Simon Cherry, James Sorenson and Michael Phelps, 4th Edition, Elsevier, 2012
International Atomic Energy Agency, SPECT/CT TECHNOLOGY & FACILITY DESIGN,
SPECT Single Photon Emission Computed Tomography, David S. Graff PhD,
Quantitative capabilities of four state-of-the-art SPECT-CT cameras; Alain Seret, Daniel Nguyen and Claire Bernard, EJNMMI Research 2012, 2:45
Characterization of the count rate performance of modern gamma cameras, M. Silosky, V. Johnson, C. Beasley, and S. Cheenu Kappadath, Medical Physics 40, (2013)
Nuclear medicine physics : a handbook for students and teachers, International Atomic Energy Agency, 2014

53. References
Physics in Nuclear Medicine: Simon Cherry, James Sorenson and Michael Phelps, 4th Edition, Elsevier, 2012
Physics of PET-CT, David S. Graff PhD,
The Challenge of Detector Designs for PET, Thomas K. Lewellen, AJR:195, August 2010
Basics of PET Imaging; Physics, Chemistry, and Regulations, Gopal B. Saha, Springer, 2005