how it works: positron emission - university of...
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How it works: Positron Emission
Radioactive decay• unstable atomic nuclei due to too
many protons relative to thenumber of neutrons
• decays to stable form byconverting a proton to a neutron
• ejects a 'positron' to conserveelectric charge
• positron annihilates with anelectron, releasing two anti-colinear high-energy photons
npnp
n
pnp n
pn
pn p
p
pn
p n
pn
p
n
p n npnp
n
pnp n
pn
pn p
n
pn
p n
pn
p
n
p n
~2 mm
18F 18O
~180 deg
E = mc2
= 511 keV
β+
e-
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Types of Photons
• X-ray photons• gamma (γ) ray photons (Greek letters for
radiation from nuclear decay processes)• annihilation photons
all can have the same energy
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Molecular Imaging: Glu Metabolism
FDG-6-PO4 is ‘trapped’ and is agood marker for glucosemetabolic rates*
glucose
glucose 6-phosphate
pyruvate lactate
gylcolysis(anaerobic,inefficient)
TCA(oxidative,efficient)
HOCH2
H 18F
HOH HHO
H
OH
H
radioactivefluorine
O
[18F]fluorodeoxyglucose (FDG)
whatwesee
FDG
FDG 6-phosphate
X
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How it works: Scintillation
high energy511 keV photon
optical photons (~ 1eV)
scintillator(e.g. BGO Denseyet transparent)
currentpulse foreach UVphoton
detected
photomultipliertubes (PMTs)gain of ~ 106
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Scintillators used in PET Scanners
newtechnology
veryshort
somewhatlower thanLSO
very highmoreexpensive
GSO
newtechnology
veryshort
highhighmoreexpensive
LSO
workhorselonghighestlowestexpensive
BGO
Hygroscopic
longlowesthighestcheap(relatively)
NaI(Tl)
CommentsDecaytime (µs)determinesdeadtimeandrandoms
EffectiveDensitydeterminesscannersensitivity
Effective numberof scintillationphotons @ 511keV determinesenergy and spatialresolution
CostMaterial
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How it works: Timing coincidence
Δt < 10 ns?
detector A
detector B
recordpositrondecayevent
scannerFOV
β+ + e-
annihilation
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Typical PET Scanner Detector Ring
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Anatomy: PET gantry
Detector+ PMTassem-blies
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Typical PET Image
Lung cancer example: Very obvious!
Elevated uptake of FDG (related to metabolism)
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What is Attenuation?The single most important physical effect in PET imaging:• The number of detected photons is significantly reduced compared to
the number of positron decays in a spatially-dependent manner• For PET it is due to Compton scatter out of the detector ring• For CT it is a combination of Compton scatter and photoelectric
absorption
one 511 keV photonscattered out of scanner one 511 keV photon absorbed
scannerpatient
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PET image withoutattenuation correction
Simulation of the Effects of not PerformingAttenuation Correction of PET Emission Image
True PET image(simulation of abdomen)
profile
Enhanced 'skin'
Reduced interior(even neg.!)
Locallyincreasedcontrast
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Effects of Attenuation: Patient Study
PET: withoutattenuation correction
PET: with attenuationcorrection (accurate)
CT image (accurate)
Enhancedskin uptake
reducedmediastinal
uptake
Non-uniformliver
'hot' lungs
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Attenuation Correction• Transmission scanning with an external photon source is used
for attenuation correction of the emission scan• The fraction absorbed in a transmission scan, along the same
line of response (LOR) can be used to correct the emission scandata
• The transmission scan can also be used to form a 'transmission'or 'attenuation' image
θ
sy
x
same line of response(LOR) L(s,θ)
Emission scan (EM) Transmission (TX)
tracer uptake tissue density
photon sourcerotation
t
f (x, y)
FOVscanner
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PET Transmission imaging(annihilation photon imaging)
• Using 3-point coincidences, we can reject TX scatter• µ(x,y) is measured at needed value of 511 keV• near-side detectors, however, suffer from deadtime due to high
countrates, so we have to limit the source strength (particularly in 3D)
µ(x,y)
orbiting68Ge/68Ga
source
PETscanner
near-sidedetectors
511 keVannihilation
photon
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And, if you have PET/CT scanner: X-ray TX
• Photon flux is very high, so very low noise• Greatly improved contrast at lower photon energies.• Scatter and beam-hardening can introduce bias.• µ(x,y,E) is measured as an weighted average from ~30-120 keV, so µ
(x,y,511keV) must be calculated, potentially introducing bias
µ(x,y)
orbiting X-ray tube and
detectorassembly
X-raydetectors
30–130 keVX-ray photon
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X-ray and Annihilation Photon Transmission Imaging
Linear attenuation coeffcient at511 keV
Not a physical quantitySlowFastNoisyLow noise
PET Transmission (511 keV)X-ray (~30-120 keV)
Energy spectra
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Quantitative errors in measurement
Lost (attenuated)event
Scattered coincidenceevent
Random coincidenceevent
incorrectly determined LORs
Comptonscatter
no LOR
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3D versus 2D PET imaging
2D Emission Scan 3D Emission Scan
detected absorbedby septa
detecteddetected
fewer true, scattered, andrandom coincidences
more true, scattered, andrandom coincidences
septa
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Effect of random coincidencecorrections in 2D and 3D
2D Emission Scan 3D Emission Scan
FOV for randomcoincidences
FOV for truecoincidences
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Noise Equivalent Counts or NEC
•NEC ~ 'Effective' count rate•Prompt coincidences are what the scanner sees: P=T+S+R•but true coincidences are what we want: T=P-S-R, which addsnoise•so overall we have:
SNR2!NEC =
T
1+S/T+R/T
In this measuredexample, at 10 kBq/cc(about 6 mCi) thescanner's count rate forcoincidences will be~450 kcps, but theeffective count rate(aka NEC) will be only~75 kcps
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NEC comparisons• Major arguing point for some vendors• Determined partly by detector type, detector and scanner
geometry, acquisition mode, and front-end electronics• Important, but not sole factor for image quality
0
20
40
60
80
100
120
140
0 5 10 15 20 25 30 35 40
kcps
Range for whole-body scansPeak NEC rates
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Partial Volume Effect
• Apparent SUV drops with volume• Also effected by image smoothing
Final ImageFillable spheres
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PET Resolution Losses
• Simulation study withtypical imaging protocols
• Limits quantitation inoncology imaging,important for followingtherapy if size changes
true tracer uptake reconstructed values(scanner resolution + smoothing of noisy data)
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Time of Flight (TOF) PET/CT• Uses difference in photon detection times to
guess at tracer emission point• without timing info, emission point could be
anywhere along line• c = 3x1010cm/s, so Δt = 600 ps ~ Δd = 10 cm
in resolution
Δd = c Δt/2
timingresolutionuncertainty
best guess aboutlocation (d)
β+ + e-
annihilation
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Philips Gemini TF
PET scanner LYSO : 4 x 4 x 22 mm3
28,338 crystals, 420 PMTs 70-cm bore, 18-cm axial FOV
CT scanner Brilliance 16-slice
Installation at U.Penn Nov ’05Validation and research patient imaging
Nov ’05 – Apr ’06 50 patientsBeta testing and upgrade to production release software
May ’06 – Jun ’06 40 patients (to date)
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Heavy-weight patient study
13 mCi 2 hr post-inj3 min/bed
MIP
Colon cancer119 kgBMI = 46.5
non-TOF
Improvement in lesion detectability with TOF
TOF LDCT