Download - Innovation is in our genes. 1 Siemens Medical Solutions Molecular Imaging What are PET basics?
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What are PET basics?
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The basic principle of PET
1. Positron-emitting tracer is injected into the body
2. Emitted positrons (+) travel 1 – 3 mm
3. Positrons collide with electrons (-) causing an “annihilation”
4. Annihilation emits energy in the form of two 511keV energy gamma rays at ~180 degrees
5. Gamma rays are detected by opposing detectors
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
E energy window
One annihilation Detection within coincidence window Energy within 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 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
12*2*1 tsnglssnglsrandoms
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Reduction of randoms
Relevant parameters:
Coincidence window
coincidence window
ran
do
m c
oin
cid
ence
s
12 ns
6 ns
4.5 ns (pico 3D)
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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 WINDOW
350 – 650
ENERGYWINDOW
425 – 650
Co
un
ts
Energy (keV)
0 100 200 300 400 700
600500
SCATTER
511 keV PHOTONS
BGO
LSO
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Correction of scatter
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
Emission Transmission Scatter Corrected
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Correction of attenuation
Patient absorbs some of the 511 keV photons
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
NEC describes the effective number of counts measured by the PET scanner as a function of the activity in the FOV
rst
tNEC
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2
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NEC – clinical performance
*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.
INJECTED DOSE RANGE185 – 740 MBq5 – 20 mCi1 hour uptake
0.1 0.2 0.3 0.4 0.52 4 6 8 10 12 14 16 18 20
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Specific Activity kBq/cc [uCi/cc]
No
ise
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uiv
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per
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2D
Biograph HI-REZ PICO
Biograph
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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.
Septa employed Low efficiency Higher dose required Lengthy scan times Fewer counts per dose (low count rate) Low scatter
No septa High efficiency Lower dose required Short scan times Higher counts per dose (high count rate) High scatter
2D acquisition mode 3D acquisition mode
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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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scatter correction attenuation correction
PET Recon
Spiral CT: seconds CT
CT PET
Survey
WB PET: 10-20 min
PET
CT PET
CT Recon
Fused PET•CT
FUSION
PET•CT scan protocol
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Block detector components
169 crystal elements per detector block
4 photomultiplier tubes (PMTs)/detector block
Detector block
PMT
Detector module
Channeled scintillation light
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Attenuation artifacts
Conventional CT: 50 cm FOV
Note: arms not fully imaged, hardeningat edges of field of view
Emission only PET
Note: arms fully imaged
Attenuation correction PET
Note: artifacts in liver and possible lesion distortion
Reduced image quality Reduced accuracy Increased artifacts Potential diagnostic impact
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ACPlus™ Attenuation Correction
Extended 70 cm transverse FOV Super fast attenuation scanning Exceptionally high statistics Unmatched attenuation image quality Highest accuracy attenuation correction
Conventional attenuation scan
~120 sec scan time106 counts
FULL FOV
Conventional CT attenuation scan
~10 sec scan time1012 counts
TRUNCATED FOV
Siemens ACPlus~10 sec scan time1012 counts
FULL FOV (NOT TRUNCATED)
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Standard PET: filtered backprojection
COINCIDENCE TIMING WINDOW (4.5 nsec)
DETECTOR ELECTRONICSGANTRY CROSS SECTION
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Standard PET: filtered backprojection
COINCIDENCE TIMING WINDOW (4.5 nsec)
GANTRY CROSS SECTIONDETECTOR ELECTRONICS
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Time of flight
DETECTOR ELECTRONICS
COINCIDENCE TIMING WINDOW (4.5 nsec) T, TIME DIFFERENCE OF DETECTION
CONVENTIONAL TOF
Source: Conti, et al., IEEE 2004
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Complex schematic of a PET•CT