Positron Emission Tomography
UniversitΓ€t Bonn
Presenter: Difei Wang
June,2018
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Contents
1
2
3
4
Positron emission
Detected events
Detectors and configuration
Data acquisition
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Positron emission
β’ Positron emission = π½+decay
π β π + π+ + π
β’ On atomic level
π β π + π+ + ππ΄πβ1
π΄π
2 Γ 511 keV
Positron range
Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences.
Springer London, 2004.
Collinearity β no physical collimation needed
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Positron emitting nuclide
Nuclide Maximum positron
energy (MeV)
Range in water (mm) Half lifetime
Deduced RMS
11 C 0.96 3.9 0.4 20.4 min
13 N 1.2 5.1 0.6 9.96 min
15 O 1.7 8.0 0.9 2.05 min
18 F 0.64 2.3 0.2 108 min
82 Rb 3.4 18 2.6 1.3 min
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Positron emission
D : distance between two
coincidence detectors
Source: Pat Zanzonico. Positron emission tomography: a review of basic principles, scanner design and performance,
and current systems. Sem Nucl Med 34:87-111, 2004.
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Spatial resolution
Spatial resolution = physical + instrumentation factors
β’ Physical factors
1. Positron range degrades spatial resolution. The range-related blurring is
reduced by the tortuous path and the spectral distribution of positron
energies.
2. Non-collinearity related blurring depends on the
distance between two coincidence detectors.
Ξπ Γ π·
whole body scan: ~2 mm
small animal scan: ~ 0.3 mm
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Coincidence event
True coincidence event
energy range : 250 ~ 650 keV Source: Pat Zanzonico. Positron emission tomography: a review of basic principles, scanner design and performance,
and current systems. Sem Nucl Med 34:87-111, 2004.
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Timing window
1. The position of the annihilation
2. Signal processing
3. Scintillation decay time
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Detected Events in PET
1. True
True count rate is linearly proportional to
the activity.
2. Random
Random count rate increases more rapidly
than the true count rate.
Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission
Tomography: Basic Sciences. Springer London, 2004.
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Detected Events in PET
3. Scatter
Scatter count rate is linearly proportional
to the activity. The scatter-to-true ratio is
independent of timing window.
Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences.
Springer London, 2004.
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Detector materials
Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences.
Springer London, 2004.
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Detector materials
Material NaI (Tl) BGO LSO GSO
Density (g/cm3) 3.7 7.1 7.4 6.7
Effective atomic number
Zeff
51 75 66 59
Attenuation coefficient π
(/cm) 0.34 0.95 0.88 0.70
Light output
(photons/ keV) 41,000 9,000 30,000 8,000
Scintillation decay time (ns) 230 300 40 60
Energy resolution
(% FWHM) 8 12 10 9
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Spatial resolution
β’ instrumentation factors
Depth of interaction effect (DOI effect):
The relatively thick detector elements lead
to a loss of resolution.
Center :
resolution: π πππ‘ = π/2
Away from the center :
πβ² = π cos π + π₯ sin π
resolution: π πππ‘β² β π πππ‘ Γ [cos π + π₯ π sin π]
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
x : 2 β 3 cm
d : 0.3 β 0.6 cm
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Block detectors
π = (ππππ΄+ ππππ΅) β (ππππΆ+ ππππ·)
ππππ΄ + ππππ΅ + ππππΆ + ππππ·
π = (ππππ΄+ ππππΆ) β (ππππ΅+ ππππ·)
ππππ΄ + ππππ΅ + ππππΆ + ππππ·
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
(left) D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences. Springer
London, 2004 (right)
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Modified block detectors
1.Quadrant sharing block design
Each PMT monitors corners of
four different blocks.
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
+ : reduce the number of PMTs
β : need more time to process the signal
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Modified block detectors
2. Phoswich
This approach makes use of the
difference in decay times of two
scintillators. The event can be
localized into upper or lower layer.
LSO
GSO
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
+ : reduce the DOI effect
β : worse performance
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Detector configurations
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
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Detector configurations
2D: Fan beam
3D: Cone beam
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
Useful field-of-view
Multi-coincidence fan beam detection: Each detector element is operated in
coincidence with multiple opposed detector elements.
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Data acquisition
π: polar angle
π: azimuthal angle
Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron
Emission Tomography: Basic Sciences. Springer London, 2004.
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Data acquisition
π ( π , π)
Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey.
Positron Emission Tomography: Basic Sciences. Springer London,
2004 (left)
S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear
Medicine. (4th ed). Philadelphia, PA, Saunders, 2012 (right)
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Data acquisition
Direct plane: Crystal ring
collects data from a single
slice.
Cross plane: Crystal ring
collects data from adjacent
rings
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
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Data acquisition
Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012
The axial sensitivity profile for 3-D acquisition reaches its
maximum at the center of the field-of-view.
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Attenuation correction
PET β only system: simultaneous emission/transmission scan
β’ Blank scan: without the patient, once a day
β’ Transmission scan: with the patient; all events
β’ Emission scan: with patient, coincidence events
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Conclusion
Positron emission
Possible events
Detectors
Data acquisition