What are PET basics?
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The basic principle of PET
1. Positron-emitting tracer is injected into the
body
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2. Emitted positrons (+) travel 1 – 3 mm
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4
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4. Annihilation emits energy in the form of
two 511keV energy gamma rays at ~180
degrees
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3
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5. Gamma rays are detected by opposing
detectors
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5
2
3. Positrons collide with electrons (-)
causing an “annihilation”
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6. Energy discrimination (an “energy
window”) is used to ensure that each
gamma is ~511 keV
7. Timing discrimination (a “coincidence time
window”) is used to ensure that each
gamma ray comes from the same
annihilation, hence ensuring accurate
localization of the tracer
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Coincidence
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Trues
E energy window
 One annihilation
 Detection within coincidence window
 Energy within energy window
E energy window
 trues = const * activity
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Randoms
 Two annihilations
 Detection within coincidence
window
 Energy within energy window
 Randoms = const * activity *
activity
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Correction of randoms
 Randoms are related to the single rate of each detector
 Randoms are related to the length of the coincidence window
randoms  sngls1* sngls 2 * t 1 2
 Randoms can be calculated when the singles for each detector are measured,
and the coincidence window for each detector pair is known
 Randoms can be measured and corrected in real time for each LOR, using a
delayed coincidence window with exactly the same length as the “direct”
coincidence window
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Reduction of randoms
Relevant parameters:
12 ns
random coincidences
 Coincidence window
6 ns
4.5 ns (pico 3D)
coincidence window
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Scatter
 One annihilation
 Detection within coincidence
window
 Energy loss due to scatter
 But energy still within energy
window
 Scatter fraction is object
dependent!
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PET event energy spectra
PET events are distributed across a range of energy, not only in the 511 keV
range. An energy window is employed to reject scatter.
ENERGY
ENERGY
WINDOW
WINDOW
511 keV PHOTONS
SCATTER
Counts
BGO
LSO
350425
– 650
– 650
0
100
200
300
400
500
600
700
Energy (keV)
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Correction of scatter
Emission
Transmission
Scatter
Corrected
 Scatter is related to mu map
 Scatter is patient dependent
 Scatter needs to be measured for each patient
 Scatter can be estimated by phantoms (but a cylindrical phantom may be a good
approximation for the brain; everywhere else it is a very poor estimation)
 Scatter can be precisely modeled for each patient using the mu map: Watson method
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Correction of attenuation
 Patient absorbs some of the 511 keV photons
p(l , ) 
  

exp      ( x, y ) (l  x cos  y sin  )dxdy 
  

 

  f ( x, y) (l  x cos  y sin  )dxdy
 
 Attenuation is patient dependent
 mu map has to be measured for each patient
 mu map can be measured with external sources
 137Cs for estimated mu map
 68Ge for precise definition of mu map
 X-ray for high statistics and precise mu map
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Noise Equivalent Countrate (NEC)
 Main sources of statistical error in a PET system are randoms and scatter
 Comparison to a system that is resistant to randoms and scatter
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t
NEC 
t  s  (2)r
 NEC describes the effective number of counts measured by the PET scanner
as a function of the activity in the FOV
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Noise Equivalent Count Rate [per sec]
NEC – clinical performance
INJECTED DOSE RANGE
185 – 740 MBq
5 – 20 mCi
1 hour uptake
90
80
Biograph HI-REZ PICO
Biograph
70
60
50
40
2D
30
20
10
2
0.1
4
6 0.2 8
10 0.3 12
14 0.4 16
Specific Activity kBq/cc [uCi/cc]
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0.5
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*Ring difference and energy window unspecified; for Biograph HI-REZ all measurements are clinical
Source: Carney, et Al., “Regionally dependent count rate performance analysis
of patient data acquired with a PET/CT scanner,” abstract 364, SNM 2003.
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Sensitivity
A measure of the number of coincidence events a scanner is able to detect, assuming no
dead time. Four to five times improvement with 3D acquisition techniques.
2D acquisition mode
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3D acquisition mode
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Septa employed
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Low efficiency
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Higher dose required
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Lengthy scan times
Fewer counts per dose (low count rate)
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Low scatter
No septa
High efficiency
Lower dose required
Short scan times
Higher counts per dose (high count rate)
High scatter
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PET•CT Protocol
The typical protocol begins with a
CT topogram to identify the scan
range.
This is followed by a spiral CT
exam of the body part of interest.
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PET•CT Protocol
The patient is then automatically
positioned for the start of the PET
exam.
The PET exam is a series of bed
positions during which the
radioactive emissions are collected.
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PET•CT scan protocol
Survey
Spiral CT: seconds
CT
CT
Recon
CT
PET
scatter correction
attenuation correction
FUSION
WB PET: 10-20 min
CT
PET
Recon
PET
Fused PET•CT
PET
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Block detector components
Detector module
 169 crystal elements per detector
block
 4 photomultiplier tubes
(PMTs)/detector block
PMT
Channeled
scintillation light
Detector block
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Attenuation artifacts
Conventional CT: 50 cm FOV
Emission only PET
Attenuation correction PET
Note: arms not fully imaged, hardening
at edges of field of view
Note: arms fully imaged
Note: artifacts in liver and possible
lesion distortion
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Reduced image quality
Reduced accuracy
Increased artifacts
Potential diagnostic impact
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ACPlus™ Attenuation Correction
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Conventional attenuation scan
~120 sec scan time
106 counts
Conventional CT attenuation scan
~10 sec scan time
1012 counts
Siemens ACPlus
~10 sec scan time
1012 counts
FULL FOV
TRUNCATED FOV
FULL FOV (NOT TRUNCATED)
Extended 70 cm transverse FOV
Super fast attenuation scanning
Exceptionally high statistics
Unmatched attenuation image quality
Highest accuracy attenuation correction
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Standard PET: filtered backprojection
DETECTOR ELECTRONICS
GANTRY CROSS SECTION
COINCIDENCE TIMING WINDOW (4.5 nsec)
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Standard PET: filtered backprojection
DETECTOR ELECTRONICS
GANTRY CROSS SECTION
COINCIDENCE TIMING WINDOW (4.5 nsec)
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Time of flight
DETECTOR ELECTRONICS
CONVENTIONAL
TOF
COINCIDENCE
TIMING WINDOW
(4.5 nsec)
T, TIME DIFFERENCE
OF DETECTION
Source: Conti, et al., IEEE 2004
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Complex schematic of a PET•CT
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