AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
PET: Physics Principles and Equipment Design
Paul Kinahan, PhD, FIEEE
Director of PET/CT PhysicsImaging Research LaboratoryDepartment of RadiologyUniversity of Washington, Seattle, WA
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Disclosures• Research Contract, GE Healthcare• Advisory Board, Aposense Inc.
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Objectives1. Review physics of positron emission and detection of
annihilation photons2. Understand quantitative corrections that are needed3. Learn context of PET imaging in clinical practice
Outlinedecay and scintillation
• Coincidence event detection• Quantitative corrections• The PET imaging equation• CT-based attenuation correction• Image reconstruction• Patient imaging process• Scanner calibration, SUVs• Resolution versus noise tradeoffs
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET/CT Imaging is a powerful tool for detection, diagnosis, and staging of cancer
PET Image of Function
Function+Anatomy CT Image of Anatomy
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Diagnostic Accuracy of PET/CT exceeds CT or PET only
Weber et al. Nature Reviews Clinical Oncology 2008
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
• A nucleus is made of neutrons (N) and protons (Z) with mass number A = N+Z• Element X (i.e. the chemistry) is determined by the # protons Z• Symbolically • Black dots on plot are stable combinations of N and Z• N increases faster than Z for stable atoms• Combinations of N and Z away from the line of stability decay to the line
ZA X
Nuclear Stability and Decay
ISOTOPE LINE Z=50
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Decay modes when there are not enough neutrons• Positron emission: a proton converts to a neutron and emits
a positron (to conserve charge)
• For example: The positron then combines with a free electron and annihilates, producing 2 annihilation photons of 511 keV each
• Electron capture: an orbital electron (typically from inner K or L shells) combines with a proton to form a neutron
and also typically generating a characteristic x-ray
p n v
ZA X Z1
A X vor
918F 8
18O v
e 2 (E mc2 )
p e n v
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Positron Emission
Radioactive decay• start with neutron-deficient
isotope• decays to stable form by
converting a proton to a neutron and ejects a 'positron' to conserve electric charge
• positron annihilates with an electron, 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
+
e-
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Energy Spectrum of Positrons in Isotopes
Levin PMB 1999
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Calculated spatial resolution
20 cm system diameter and 2 mm wide detectors (pre-clinical scanner)
80 cm system diameter with 4 mm detectors (clinical scanner)
Resolution is typically at least 2x worse than this in practice• detector multiplexing• sinogram sampling• smoothing applied during/after image reconstruction to suppress noise• Formula by Moses:
d = detector width, D = ring diameter, R = positron range, b = 'block factor'
Levin PMB 1999
FWHM 1.25 d / 2 2 0.0022D 2 R2 b2
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Useful Positron Emitting Isotopes
Isotope Half life (min)
Most probable energy (keV)
FWHM of positron range* in water (mm)
109.7 203 0.102
1.3 1384 0.169
20.3 326 0.111
10.0 432 0.142
2.0 696 0.149
918F
3782Rb
611C
713N
815O
* very long tails> 90% of all clinical studies
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Scintillation Detectors
high energy511 keV photon
optical photons (~ 1eV)
scintillator(e.g. BGO Dense yet transparent)
current pulse for each UV photon
detected
photomultipliertubes (PMTs)gain of ~ 106
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Scintillators tried in PET
Used in commercial scanners
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET Detector Block
Reflective lightsealing tape
Two dual photocathode PMTs
gamma raysscintillation light
signal out to processing
• PET scanners are assembled in block modules
• Each block uses a limited number of PMTs to encode an array of scintillation crystals
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Block matrix6 x 8 crystals (axial by transaxial)Each crystal:
6.3 mm axial4.7 mm transaxial
Scanner constructionAxial:
4 blocks axially = 24 rings15.7 cm axial extent
Transaxial:70 blocks around = 560 crystals88 cm BGO ring diameter70 cm patient port
13,440 individual crystals
Block formation for a current PET scanner
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Line of response collimation by coincidence timing
t < 10 ns?
detector A
detector B
record coincident event for this line
of response (LOR)
scannerFOV
+ + e-
annihilation
• In SPECT this is achieved though use of a collimator• In CT the known source-detector geometry is used
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET Imaging Equation• With enough coincident events for each line of
response, we can approximate measures as line-integral data of the radioisotope concentration
g(l,) f (x(s), y(s))ds
The integral is along a lineL(l, ) (x, y) x cos ysin l With rotated coordinates (l,s)x(s) l cos ssiny(s) l sin s cos
s
f (x, y)
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Sinograms• We can represent the projection data g(l,), as a 2-D image, which is
called a sinogram• Each row is a projection at a fixed angle , with an intensity of g(l,• A point in the object projects to a sine wave in the sinogram• Useful for understanding scanner problems
l
object sinogram
scanner FOV
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
More complex sinogram example
l
x
y
l
s
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
“2D” versus “Fully-3D” PET imaging
2D Emission Scan Fully-3D Emission Scan
detectedphotons absorbed
by septa
detecteddetected
• lower sensitivity, simpler to reconstruct
septa
• lower sensitivity, simpler to reconstruct
• higher sensitivity, harder to reconstruct
end shield
scintillator
tracer accumulation
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Quantitative errors in measurement
Lost (attenuated)event
Scattered coincidenceevent
Random coincidenceevent
incorrectly determined LORs
Compton scatter
no LOR
• Corrections have to be applied for these effects
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
QuestionWhat physical property has the biggest impact on PET image quality?
1. Type of scintillator material2. Attenuation3. Scatter4. Random coincidences5. Amount of injected FDG6. Scanner calibration and QA/QC
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Answer: Attenuation
Scans performed using the same scanner and protocols
Thin not Thin
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Effects of Attenuation: Patient Study
PET: without attenuation correction
PET: with attenuation correction (accurate)
CT image (accurate)
Enhanced skin uptake
reduced mediastinal
uptake
Non-uniform liver
'hot' lungs
Attenuation, and errors in attenuation correction, can dominate image quality
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Attenuation in PET Imaging• A key issue is that there are 2 photons along the line of response (LOR)
so that total attenuation is always the same (unlike case for SPECT)
annihilation location
detector A
detector B
attenuation distance from annihilation location to detector B
NA N0 exp (x( s ), y( s );E)d sS0
R
NB N0 exp (x( s ), y( s );E)d s
R
S0
photons detected from a single
annihilation location at s0
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Attenuation in PET Imaging• Total number of annihilation photons arriving in
coincidence is the product of the attenuation factors
• If we now allow for a distributed source of positrons
even better, we have attenuation as a simple multiplication
NC N0 exp (x( s ), y( s );E)d sS0
R
exp (x( s ), y( s );E)d sR
S0
N0 exp (x( s ), y( s );E)d s
R
R
(l,) K A(x(s), y(s))exp (x( s ), y( s );E)d sR
R
dsR
R
(l,) K A(x(s), y(s))dsR
R
exp (x(s), y(s);E)dsR
R
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Types of transmission imaging
1. PET transmission source (68Ge/68Ga): Coincident annihilation photons (mono-energetic @ 511 keV), 265 day half life
2. Single photon source (137Cs): Single -rays (mono energetic @ 662 keV), 20 yr half-life
3. X-ray CT scan: X-rays with a distribution of energies from ~30 to 130 keV (effective energy of ~70 keV)
E
I0(E) X-ray source spectra
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Comparing X-ray, -camera (SPECT) and PET
SPECT:
X-ray CT:
PET:L1L2
L1
Attenuation only, but with complicated energy weighting
Uncoupled mono-energetic attenuation and emission
Coupled mono-energetic attenuation and emission
I0(E)
(x,y,E)
I I 0 (E)e (x .y ,E )dL
0
L
0
dE
I I 0 (x, y)
dL
e
(x .y ,511keV )dL
I0 (x, y)
I I0 (x, y)0
L
e (x .y ,EO )dL
0
L
dL
E
I0(E)
I0 (x, y)
L1 L2 Lconstant attenuation length:
L1variable attenuation length:
emission (sinogram) dataattenuation factors
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET Transmission imaging
• Using 3-point coincidences, we can reject TX scatter• (x,y) is measured at needed value of 511 keV• Near-side detectors, however, suffer from deadtime due to high countrates• Subject to bias from emission photons from patient
orbiting 68Ge/68Ga
source
PET scanner
scattered TX photon
near-side detectors
511 keV annihilation
photon
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Single-photon transmission imaging
• We cannot reject TX scatter with 3-point method, so additional scatter corrections needed• (x,y) is measured at 662 keV, so scaling or segmentation is needed (to impose correct
value), which can add bias• no deadtime for near-side detectors, so much higher source strengths can be used
orbiting 137Cs source
PET scanner
scattered TX photon
662 keV g-ray photonshielding for near-
side detectors
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
X-ray CT transmission imaging
• Scatter is suppressed with rejection grid, although beam-hardening occurs• (x,y,E) is measured as an (unknown) weighted average from ~30-130 keV, so some other method to get
(x,y,511keV), potentially introducing bias. Such as the hybrid method [Kinahan et al, Med Phys, 25:2046-2053, 1998], but which has bias problems.
• photon flux is very high, so very low noise, but much higher patient dose• greatly improved contrast
orbiting X-ray tube
X-ray detectors
30-130 keV X-ray photon
scatter-rejection grid
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET TX: 3min511 keVbest quantitationhighest noise
singles TX: 3min662 keV
X-ray CT TX: 20 s~30-120 keVworst quantitationlowest noise
Comparison of transmission imaging methods
• Due to the diagnostic superiority of PET/CT over PET, all attenuation correction is done use CT-based methods
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
CT-based Attenuation Correction• The mass-attenuation coefficient () is similar for all non-bone
materials since Compton scatter dominates for these materials• Bone has a higher photoelectric absorption cross-section due to
presence of calcium• Can use two different scaling factors: one for bone and one for
everything else
0.01
0.10
1.00
10.00
100.00
10 100 1000keV
Bone, Cortical
Muscle,SkeletalAir
bone
everything else
70 511
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
0.00
0.05
0.10
0.15
0.20
-1000 -500 0 500 1000 1500
CT Hounsfield Units
CT-based Attenuation Correction• Bi-linear scaling methods apply different scale factors for bone and non-
bone materials• Should be calibrated for every kVp and/or contrast agent
air-water mixture
water-bone mixture
air soft tissue dense bone
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET/CT Anatomy
All 3 (couch, CT and PET) must be in accurate alignment
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
• CT images are also used for calibration (attenuation correction) of the PET data
• Note that images are not really fused, but are displayed as fused or side-by-side with linked cursors
• Note also that the CT is used for attenuation correction, thus a significant potential for error
Data Flow and Processing
X-ray acquisition
Anatomical (CT) Reconstruction
PET Emission Acquisition
CT Image
Translate CT to PET Energy (511 keV)
Smooth to PET Resolution
Attenuation Correct PET Emission Data
Functional (PET) Reconstruction
PET Image
Display of PET and CT DICOM image stacks
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Respiratory Artifacts: Propagation of CT breathing artifacts via CT-based attenuation correction
Attenuation artifacts can dominate true tracer uptake values
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
• Examples of respiratory and gross motion artifacts• As a check, compare PET image without attenuation correction
Coronal section of attenuation-corrected FDG-PET image
PET image (now in hot metal color scale) overlaid on CT image
Coronal section of FDG-PET image without attenuation-correction
Motion Artifacts
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Commercial/Clinical PET/CT ScannerPET detector blocksthermal barrierrotating CT system
unit
hum
an
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
GE Discovery 690 Siemens mCT Philips Gemini TF
Some Current Scanners
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Some Current Scanners Specifications*
*May have errors or be obsolete in some categories
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Quantitative Corrections• Attenuation• Scattered coincidences• Random coincidences• Detector efficiency corrections• Deadtime corrections• Scanner calibration• Image reconstruction
In principle corrections for these effects can be performed accurately, but any errors will produce errors in the PET image.
Attenuation correction errors are the most significant in generalCalibration errors have typically received the least attention
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET Image Reconstruction• We have an inverse problem since the scanner
measures
• In other words, given g(l,), what is f(x,y)?
• There are two main classes of image reconstruction– Analytic: Using the Filtered-backprojection algorithm (FBP)
– Iterative: Based on statistical modeling of the data, which is Poisson distributed
• Iterative methods are most commonly used– Typically some variant of expectation maximization (EM)
used for maximum likelihood estimation
g(l,) f (x(s), y(s))ds
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Clinical Considerations
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Clinical Applications
IMV 2008 PET Imaging Market Summary Report
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET Scanning Process
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Imageinterpre‐tation
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Patient
preparation
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Scan
acquisition
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Image
recon-
struction
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Image
analysis
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
1. Scout scan (5-10 sec)
CT PET
4. Whole-body PET (15-30 min)
CT PET
Typical PET/CT Scan Protocol
3. Helical CT (30 sec)
CT PET
2. Selection of scan region
Scout scan image
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Typical Radiation Doses
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
What Do PET Scans Measure?• If everything goes well, the role of the PET scanner is to measure the
radioactivity per unit volume• Typically measured as kBq/ml or Ci/ml• Start with a simple example:
10 mCi = 370 MBq 70 kg water = 70 L inject
concentration = 370,000 kBq / 70,000 ml= 5.3 kBq/ml
suppose there is a very small object that takes up 5x the local concentration, so its concentration = 26.5 kBq/ml
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
What if there are different activities or distribution volumes?
• Injecting different amounts or changing the volume will change the concentration
10 mCi = 370 MBq inject
concentration = 5.3 kBq/ml
5 mCi = 185 MBq inject
concentration = 2.8 kBq/ml
10 mCi = 370 MBq inject
concentration = 10.6 kBq/ml
35 kg = 35 L
26.5 kBq/ml
13.3 kBq/ml
53.0 kBq/ml
The hot spot has different uptake values in kBq/ml even though it has the same relative uptake compared to background
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Standardized uptake values (SUVs)• Normalize by amounts injected per volume (i.e. weight) to get the same
relative distribution with SUV = 1.0 for a uniform distribution
10 mCi = 370 MBq inject
SUV = 5.3 kBq/ml / (370MBq/70 Kg)= 1.0 gm/ml
5 mCi = 185 MBq inject
SUV = 1.0 gm/ml
10 mCi = 370 MBq inject
SUV = 1.0 gm/ml
35 kg = 35 L
SUV = 5.0
SUV = 5.0
SUV = 5.0
The hot spot now has the same SUV uptake values independent in activity injected or volume of distribution (i.e. patient size)
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Sources of Error in SUV Values
It is important to minimize SUV errors for serial (e.g. response to Rx) or multi-center studies
Some potential sources of error are:• High blood glucose levels• Variations in dose uptake time• Uncalibrated clocks (including scanner) and cross calibration of scanner with dose
calibrator• Errors in radioactive dose assay• Variations in image reconstruction and other processing protocols and parameters• Variations in images analysis methods: E.g. how ROIs are drawn and whether max or
mean SUV values are reported
SUV PETROI
DINJ / V
PET = measured PET activity concentrationD' = decay-corrected injected doseV' = surrogate for volume of distribution
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
SUV calculation chain for PET
9.6 mCi
dose calibrator
pre- and post injection assays
decay correctednet activity
PET scanner scanner global
calibration factor
patient weight (& height)
scanner units Bq/ml SUVs
SUV scale factor s
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
• Modified NEMA NU-2 Image Quality Phantom (30 cm x 23 cm cross section)
• Sphere diameters:1.0, 1.3, 1.7, 2.2, 2.8, 3.7 cm• 4:1 target:background ratio and typical patient activity• RC = measured / true
Resolution Effects
Recovery Coeffcient (RC) with 2D FBP
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4Diameter (cm)
Mean RC for ROI
Max RC for ROI
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Variations in resolution loss vs. size and smoothingM
ean
Max
FBP OSEM
Incr smoothing
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Resolution versus noise
10 mm smoothing4 mm smoothing 7 mm smoothing
RC for 1 cm spheres
0.85
0.92
0.52
0.80
0.40
0.72SNM Chest phantom: True RC is 1.0
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
QuestionWhat is the goal of a combined PET/CT scanner?1. Accurate attenuation correction2. Accurate image alignment3. Revitalize nuclear medicine4. Job security for physicists
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
References• Cherry SR, Sorenson JA, Phelps ME. Physics in
Nuclear Medicine. Saunders: Philadelphia, PA, 2003• Positron Emission Tomography: Basic Science and
Clinical Practice. Peter E Valk, Dale L Bailey, David W Townsend and Michael N Maisey Eds., London, Springer Verlag, 2003
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Extra slides
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
• An atomic nucleus is made of neutrons (N) and protons (Z) with mass number A = N+Z
• Element X (i.e. the chemistry) is determined by the # protons Z
• Symbolically • Black dots on plot are stable
combinations of N and Z• N increases faster than Z for
stable atoms• Combinations of N and Z away
from the line of stability decay to the line
ZA X
Nuclear Stability and Decay
ISOTOPE LINE Z=50
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
PET Image Reconstruction• From the sinograms, or line integral data collected as
large numbers of coincident events, we can solve the inverse problem
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Size-Dependent Resolution Losses
• Hot sphere diameters of 10, 13, 17, 22, 28, and 37-mm
• Target/background ratio 4:1
• Max and mean activity concentrations measured via 10-mm diameter ROIs
similar to abdominal x-section
Modified NEMA NU-2 IQ Phantom
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Not meant as a "Consumer's Report" evaluation, but rather to facilitate multi-center comparisons
SNM Validation Phantom Study• Sample images of the IDENTICAL object from 12 different PET and PET/CT
scanners
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
‘Coffee Break’ Repeat PET/CT scans with Repositioning
GE DSTE-16 PET/CT Scanner Siemens Biograph HI-REZ-16 PET/CT Scanner
20%
30%
40%
50%
60%
70%
80%
90%
100%
5 10 15 20 25 30 35Sphere Diameter (mm
Max
Mean
20%
30%
40%
50%
60%
70%
80%
90%
100%
5 10 15 20 25 30 35Sphere Diameter (mm)
Max
Mean
SUVs from 20 3D-OSEM scans with 7-mm smoothing
• Intra-scanner short-term variability is 3% - 4%
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
SNM Phantom: Key results of SUV measurements
Variations are introduced by the scanner type,acquisition protocol, calibration differences,processing (e.g. image reconstruction methodor smoothing) and ROI definition method.
averaged coefficients of variation
mean SUV: 8.6%, max SUV: 11.1%
Plots of recovery coefficient (RC) = measured in ROI/true
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
2 cm sphere
5 cm sphere
33 cm
profile
Resolution Effects
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Image Reconstruction: Modeling Detector BlurringInter-crystal scattering Parallax error
true LOR
variable depth of
interactionassigned line of
response (LOR)
Shape of detector blurring point spread function (PSF) • Radially variant• Asymmetric in transaxial direction• Two-fold symmetric about FOV center
crystal thickness
true event crystal
assigned event crystal due to scattering
scintillation(Compton scatter)
light collection
annihilation photon
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Spatially‐Variant Image Resolution
standard OSEM
OSEM with detector blurring
modeled
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
Typical PET Scanner Detector Ring
AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation
Paul Kinahan
X-ray and Annihilation Photon Transmission Imaging for Attenuation Correction
0
25,000
50,000
75,000
100,000
0 100 200 300 400 500 600
keV
I
X-ray (~30-120 keV) PET Transmission (511 keV)Low noise Noisy
Fast SlowPotential for bias when
scaled to 511 keVQuantitatively accurate
for 511 keV
Transform?