This document provides an overview of nuclear medicine imaging techniques including SPECT and PET. It discusses the basics of gamma cameras and how they form images using collimators. SPECT imaging is described including data acquisition in projections, reconstruction using filtered back projection, and corrections for attenuation and scatter. PET imaging concepts such as coincidence detection, time-of-flight, and the need for corrections for randoms, scatter, and attenuation are covered. The document compares the relative sensitivities, resolutions, and data corrections between SPECT and PET.

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. 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
4

5. 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

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 t
c o l l i m a t o r
d e t e c t o r
P M T
p r e - a m p
amplify & sum
position
analysis
Pulse Height
Analysis
c o m p u t e r
d i s p l a 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
Image of a point source is the whole
detector.
detector
sources
images
Image of a point source is a point.
w/o collimator with collimator
image
collimator

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
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR
• Most often used: parallel-hole collimator
• For thyroid and heart: pin-hole collimator
• For brain and heart: converging collimator

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)

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

14. Photopeak
All the energy of a g photon (E0) is
deposited in the detector
e.g. E0 = 140 keV for Tc-99m
14
p.e p.e
c.s
or

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

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

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

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

20. Filtered Back Projection
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps
• Attenuates streaks by filtering the projections

21. Iterative Reconstruction
• Quantitatively more accurate
 Can model various corrections
• Collimator
• Scatter
• System geometry
• Detector resolution
• Slow
• Being used increasingly in SPECT

22. Assume
Some Image (I)
Calculate
Projections (P’)
Calculation Includes
Attenuation
Scatter
Blur with depth
Compare to
Measured Projection (P)
Form New
Image (I’)
Use P’ & P
to form corrections
Is I-I’< *
Done

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

24. Attenuation correction:
Chang Method
• 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
I(x) = I0e-mx

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

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

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. Where was the event?
Coincidence
?
PET Imaging Concepts

29. Where was the event?

30. Annihilation Detection
Coincidence
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.

31. Time-of-Flight PET
Coincidence
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.

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)

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
Septa

35. Detector Needs
• High Stopping Power
 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

42. Scatter-to-True Ratio
• .2 - .5 brain
• .4 -2 body
• Scatter (and Trues) are proportional to
administered activity

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

44. Random-to-True Ratio
• .1 – 2 brain
• .1 -1 body
• 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).

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

48. PET: CT Based Attenuation Correction
© Nuclear medicine physics : a handbook for students and
teachers, International Atomic Energy Agency, 2014

49. SPECT vs PET
SPECT PET
(Step-and-shoot acquisition) (Simultaneous acquisition)
2015 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
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

51. 51
• October 7, 2015 -- 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
http://www.auntminnie.com/index.aspx?sec=s
up&sub=mol&pag=dis&ItemID=112051

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, https://rpop.iaea.org/
• SPECT Single Photon Emission Computed Tomography, David
S. Graff PhD, http://www.slideshare.net/david.s.graff/spect-
presentation
• 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, 032502 (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,
http://www.slideshare.net/david.s.graff/pet-ct-presentation
• 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