PET/CT Technology Updates:
Quality Assurance and Applications
Osama Mawlawi, Ph.D.
Department of Imaging Physics
MD Anderson Cancer Center
Disclosures
• GE SRA
• SIEMENS SRA
Introduction
• PET basic principles
• PET/CT
– Artifacts and solutions
•
•
•
•
New scanner features (all scanners)
Design developments
Future developments
Quality Assurance/Control
Learning Objectives
•
•
•
•
To learn the basic physics principles of PET &
PET/CT imaging
To understand the advantages and drawbacks
of using CT for attenuation correction of PET
images.
Become familiar with design specifics of
commercially available PET/CT scanners.
Learn quality control and assurance
techniques for PET/CT imaging
Annihilation
Detector ring
511keV
Photon
LOR
electron
positron
Nucleus
511keV
Photon
E=mC2
Electron rest mass = 9.11x10-31 kg
C=3.0x108 m/s
1 joule =1 kg m2/s2
1 e = 1.602x10-19 coulomb
1 eV = 1.602x10-19 J
Thus: E =1.02 MeV
O. Mawlawi MDACC
O. Mawlawi MDACC
Sample Sinograms
O. Mawlawi MDACC
O. Mawlawi MDACC
SUV = (Measured AC /Injected dose)/ Patient weight
Quantification: Power of PET
Measured Data
Random subtract
Normalize
Dead time
Correct Geometry
Calculate/subtract scatter
Correct Attenuation
FBP or IR reconstruction
calibration
Ideal measured data
O. Mawlawi MDACC
Effect of Attenuation
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
thickness (cm)
• The half value layer of water @ 511keV is 7.2 cm
• Both photons must exit the body to make a coincidence event
P1 = e−μL1
P2 = e−μL2
P1 * P2 = e−μL
Attenuation is dependant
on the path length and not
the depth of the source of
activity
Four ways to get an
attenuation map
1) Measured (MAC)
2) Calculated (CAC)
3) Segmented (SAC)
4) CT based (CTAC)
Nuclear Medicine: Diagnosis and therapy. Harbert J, Eckelman W., Neumann R.
O. Mawlawi MDACC
Well counter calibration
• Transformation of counts per
second to activity concentration
O. Mawlawi MDACC
SUV = (Measured AC /Injected dose)/ Patient weight
Standardized Uptake Value (SUV)
Decay corrected activity concentration (uCi/ml)
SUV =
Injected dose (mCi) / Patient weight (g)
1 mCi
SUV=1
1 mCi
SUV=1
1 Kg
10 Kg
Standardized Uptake Value (SUV)
Decay corrected activity concentration (uCi/ml)
SUV =
Injected dose (mCi) / Patient weight (g)
2 mCi
SUV=1
1 mCi
SUV=1
1 Kg
1 Kg
Dedicated PET Imaging
• Emission (2D mode)
• Transmission (scans are interleaved)
• 5 to 6 bed positions
• 8 min per position
• 5 EM, 3 Tx
• Total scan duration 50-60 min
15.5 cm
Tx rod sources
Emission
Transmission
Final
Disadvantages of dedicated PET
imaging techniques
• Transmission
– Noise due to low gamma ray flux from rod source
– Transmission is contaminated by emission data
• Scan duration
– Time consuming (emission & transmission )
– Increased patient movement (image blurring)
• Efficiency
– Decreased patient throughput
– Difficulty in correlating images to other diagnostic
modalities accurately
Hybrid Imaging
GE Discovery ST
Siemens/CTI Biograph
Phillips Gemini
Short duration, low noise CT-based attenuation correction
Improved patient throughput
•
•
•
•
•
•
•
Replace Tx scan by CT scan
3-5 min Tx scan duration
5-7 FOVs  15-35 min of Tx scanning
CT scan for whole body is about 1min
About 30 min of time saving per patient
Dedicated PET scan duration of 50 min
PET/CT scan duration of 20 min
Combines functional
and
Anatomical data
CT based attenuation correction
Attenuation
Correction
Inherent
Image
Registration
Converting CT Numbers to
Attenuation Values
Types of Artifacts
PET/CT imaging artifacts are due to :
• Contrast media
• Truncation
• Respiratory motion
Converting CT Numbers to
Attenuation Values
•
•
For CT values < 0 ,
materials are assumed to
have an energy
dependence similar to
water
For CT values > 0,
material is assumed to
have an energy
dependence similar to a
mixture of bone and
water
The green line shows the
effect of using water
scaling for all materials
Attenuation at 511keV
•
0.200
0.150
error
Bone/Water
0.100
Water/air
0.050
0.000
-1000
-500
0
500
CT number measured at 140kVp
1000
Transaxial
contrast bias
Coronal
Normal CT
Contrast enhanced CT
Contrast: Optiray 320, 100cc, 3cc/sec
CT attenuation map scaling
0.200
Linear attenuation at 511keV 1/cm
0.180
140kVp non
contrast
120kVp non
contrast
100kVp non
contrast
140kVp BaSO4
0.160
0.140
0.120
0.100
120kVp BaSO4
0.080
100kVp BaSO4
0.060
140kVp I
0.040
120kVp I
0.020
100kVp I
0.000
-1000
-500
0
500
CT Hounsfield values
1000
1500
Contrast Artifact- Intravenous
Contrast
injected
CT
PET CTAC
FUSED IMAGE
With contrast
Without contrast
61 year male with history of esophageal cancer
Truncation
CT and PET Fields of View
X-ray focal spot
PET Detector
PET 70cm image FOV
CT 50cm Fully sampled FOV
CT Detector
Wide View
Activity concentration recovered to within 5% of original value
CT
PET
FUSED
Attenuation map
Truncated
Truncated
corrected
54 yrs. male with history of metastatic melanoma of
the skin. SUV changed from 3.25 to 6.05
Breathing Artifacts
Mismatch between PET and CT
1 breathing cycle
CT captures the anatomy at one location of the breathing cycle
Mismatch between PET and CT
1 breathing cycle
PET averages the anatomy over the full breathing cycle
Results in image blurring and mismatch between PET and CT
PET/CT
low SUV
Photopenic
Breathing Artifact
Mismatch of lesion location between helical CT and PET
Breathing Artifact
Mismatch of lesion location between helical CT and PET
Mismatch between PET and CT – Cardiac Application
Courtesy of J. Brunetti, Holly name
Average CT – 4D CT
1
2
Courtesy of Tinsu Pan, MDACC
Clinical Studies
Mismatch 1:
CT diaphragm
position lower
than PET
+57%
Mismatch 2:
CT diaphragm
position higher
than PET
Courtesy of Tinsu Pan, MDACC
CT
PET
Fused
RACT
PET
Fused
(A)
(B)
(C)
(D)
Courtesy of Tinsu Pan, MDACC
No AC
Impact on treatment planning
Old GTV
New GTV
Previous GTV was outlined based on CT and clinical PET without motion correction.
New GTV was redefined based on the correct information from PET with ACT.
Courtesy of Tinsu Pan, MDACC
New PET Scanner Features
Gating to Reduce Motion Effects
• Image blurring
• Underestimation of activity concentration
Gated PET (Used to image repetitively moving objects: cardiac,
respiratory)
Trigger
Trigger
1
8
2
1
7
3
4
6
8
7
2
6
3
5
4
5
time
Bin 1
• Prospective fixed forward time binning
• Ability to reject cycles (cardiac) that don’t match
• Single 15 cm FOV Gated PET
• User defined number of bins and bin duration
• As number of bins increase, the duration and motion per bin decreases.
However images will be noisy unless acquired for longer durations.
Bin 8
4D-Gated PET
Phantom Study
Coronal
Sagital
Mip
Axial
Sphere (volume)
1 (16.5 cc)
Gated
31624
Static
33546
Error (%)
-6.1
2 (8.4 cc)
31316
21872
30.2
3 (4 cc)
4 (2 cc)
5 (1 cc)
6 (0.5 cc)
29919
22857
19087
11897
20607
13643
8264
5143
31.1
40.3
56.7
56.8
Impact of Whole-body Respiratory Gated PET/CT
Static wholebody
Single respiratory phase
(1 of 7, so noisier)
1 cc lesion on CT
• The max SUV of the lesion goes from 2 in the static image to 6 in the
respiratory-gated image sequence
Courtesy of P Kinahan, UW
New CT Application… Advantage
4D CT
Respiratory tracking with Varian RPM optical monitor
CT images acquired over complete respiratory cycle
“Image acquired”
signal to RPM
system
X-ray on
First couch position
Second couch position
Third couch position
Respiratory motion defined retrospective gating
4D-PET/CT
4D PET/CT
Motion tracking with 4D CT
Acquire image that accurately
represents respiratory motion
Draw treatment volume on Cine
movie
Image Courtesy of Tinsu Pan
Advantage 4D CT provides a method for accurately
defining tumor motion for GTV determination
Gated PET (Used to image repetitively moving objects: cardiac, respiratory)
Trigger
Trigger
1
8
2
7
3
4
6
5
1
8
7
2
6
3
4
5
time
Bin 1
Bin 8
Methods to improve SNR of gated PET
•
•
•
•
•
Deformable registration (image based)
Motion incorporation during reconstruction
Increasing scan duration
DIBH
End expiration period gating
PET data acquisition of different duration per FOV
Static
3 min
Static
3 min
Static
5 min
Static
6 min
LIST Mode
X2
Y1
• Time ticks are fixed at 1msec intervals
• The number of events between time ticks depends
on the amount of activity in the field of view
Y3
Y4
• The more activity, the more the events between
time ticks.
• Very flexible, data can be rebinned as static,
dynamic, or gated.
• Requires large amount of memory
X1
X4
X3
X1
Y1
X2
Y2
TIME
X3
Y3
X4
1msec
X8
Y8
X9
Y4
X5
1msec
Y9
X10
Y10
X11
Y11
Y5
X6
Y6
Y2
TIME
X7
Y7
LIST data format
Tick marks
1 msec
Section of a list file (141MB) using a 25MBq (0.7mCi) Ge-68 rod source acquired for
a duration of 15 seconds.
Advantages of List mode acquisition:
• Ability to post process the data using different prescriptions.
(static, dynamic, gated)
• Duration of the scan can be changed (compare different scan
durations)
• Chop off data segments based on user definition.
List mode allows rebinning of acquired data into different durations
New PET Scanner Designs
SIEMENS
GE
PHILIPS
Advantages of the extended Axial Field-of-View
Sensitivity
FOV
• increased sensitivity = shorter imaging per bed (or more counts)
• larger axial FOV = fewer bed positions for same axial coverage
• reduction in imaging time (or dose) of ~2 for comparable quality
Courtesy of D Townsend
Transverse resolution recovery
Courtesy of D Townsend
Transverse resolution recovery
Courtesy of D Townsend
BrainPET for MAGNETOM Trio,
The first results
* Works
in Progress. The information about this product is preliminary. The product is under development and is not commercially
available in the U.S., and its future availability cannot be ensured.
First ever in-vivo images simultaneously acquired by MR and PET (17.11.06)
MR
MR-PET
Courtesy of Townsend and Nahmias University of Tennessee, USA and
76 Pichler, Claussen University of Tuebingen, Germany Siemens Medical Solutions
Schlemmer,
PET
Innovation is in our genes.
Molecular Imaging
Simultaneous
PET/MR scanner
design
Schwaiger M 2005
Separated PET/MR scanner design
Schwaiger M 2005
VUE Point – Iterative Reconstruction
Conventional
Iterative Reconstruction
Randoms
Correction
Deadtime/
Normalization
Correction
Radial
Repositioning
Quadrant
Scatter
Correction
Attenuation
Correction
VUE Point
Deadtime/
Normalization
Correction
Radial
Repositioning
Iterative Reconstruction w/ Distance Driven Projectors
Quadrant
Randoms
Attenuation
Scatter
Correction
Correction
Correction
VUE Point HD (mid-2007 update)
Iterative Reconstruction w/ Distance Driven Projectors
Deadtime/
Normalization
Correction
Detector
Geometry
Modeling
Randoms
Correction
Volume Scatter
Correction
Attenuation
Correction
GE Healthcare, Molecular Imaging
Page 80
Average CT to match with PET
Average CT
Helical CT
Helical CT
w/o thorax
Helical CT w/o thorax plus
average CT of thorax
Pan et al, J. Nuc Med, 2005
Motion Freeze 4D PET/CT
Time-of-Flight Acquisition:
Principles
∆t
∆x = c
2
C = 3*108 m/s
Back projection
along the LOR
Time of Flight and SNR, examples…
∆t
∆x = c
2
SNRTOF
Time
Resolution
(ns)
∆x
D
≅
⋅ SNRconv
∆x
(cm)
SNR
improvement
(20 cm object)
SNR
improvement
(40 cm object)
0.1
1.5
3.7
5.2
0.3
4.5
2.1
3.0
0.5
7.5
1.6
2.3
1.2
18.0
1.1
1.5
Colon cancer
CT
119 kg (BMI = 46.5)
non-TOF
30 min scan
TOF
MIP
Improvement in lesion detectability with TOF
Courtesy of J. karp
Average-weight patient study
Tongue CA,
lung mets
67 kg
BMI = 29.0
CT
20 min scan
non-TOF
non-TOF
TOF
TOF
Improvement in lesion contrast with TOF
Courtesy of J. karp
non-Hodgkin’s lymphoma 136 kg (45 BMI)
nonTOF 30 min
TOF
30 min
TOF tumor contrast (SUV) higher than non-TOF by 1.5
TOF
10 min
Courtesy of J. karp
TOF tumor contrast superior to non-TOF for 10 min as well as 30 min scan
Philips Gemini Big Bore PET/CT
 85cm large bore for Radiation Therapy planning.
 Time of Flight
 Possible loss of Res/Sens due to large bore
88
FUTURE
Mixed acquisition protocol
Static
Gated
Static
Partial Volume Effects (PVE)
• Scanner resolution: 4-7mm
• Image resolution: 7-12mm
• Pixel size: 4-6mm
• Objects that are smaller than the
resolution of the scanner will
experience PVE – underestimate the
activity concentration
Standardized Uptake Value (SUV)
Decay corrected activity concentration (uCi/ml)
SUV =
Injected dose (mCi) / Patient weight (g)
SUV1
>
SUV2
Effect of Partial voluming
13
All spheres contain
the same activity
concentration
17
22
10
Profile (10 mm)
28
Recovery (%)
37
100
22
28
37
17
50
13
0
Standard
8 x 8 detector
10
Sphere diameter
HI-REZ
13 x 13 detector
Recovery coefficients
Courtesy of Siemens
Phase Gating vs. Amplitude Gating
1
8
2
1
4
7
2
7
3
8
4
5
8
Amplitude varying
1
7
2
6
3
6
5
time
1
SUM
2
3
3
6
4
5
4
5
6
7
8
1
time
D. Mohammad et al, Med. Phys. 34(7):3067-76, 2007
Phase Gating vs. Amplitude Gating
1
8
2
1
4
7
2
7
3
8
6
3
6
4
5
5
time
Frequency varying
1
SUM
8
2
7
3
4
6
5
1
2
3
4
5
6
7
8
time
Chang et al. Young inv. Award, Sun. 12:45-1:15
D. Mohammad et al, Med. Phys. 34(7):3067-76, 2007
Standardizing the
Standardized Uptake Value (SUV)
Decay corrected activity concentration (uCi/ml)
SUV =
Injected dose (mCi) / Patient weight (g)
Quantitative PET Performance
3 Categories that “might” affect SUV measurements
•
Patient Compliance
–
–
•
Fasting
Blood Glucose levels
15% (75%)
Scan Conditions
–
–
–
–
•
15% (30%)
Scan time post injection
Patient anxiety/comfort during uptake (room temperature, etc.)
30% (60%)
Patient motion during acquisition
15% (50%)
PET/CT vs Dedicated PET
Intrinsic System Parameters and Capability
–
–
–
–
Calibration
QA – Maintenance of operating parameters
Performance characteristics – Partial Volume Effects
Image processing algorithms
30%
15%
Up to 80%
Boellaard et al 2009, J Nucl Med
Quantitative PET Performance
• RSNA - QIBA
• ACR
• SNM • AAPM - TG145
• NIH – NCI, QIN
• ACRIN
Measureable Parameters of Tumor
Growth and Malignancy
Gene expression
?
Receptors
18-F L-DOPA
Angiogenesis
18-FLT
18 FMISO
64-Cu ATSM
DNA Synthesis/Hypoxia
Protein & membrane Catabolism
PET/CT 2008
Energy Metabolism
Vascularity
Blood Flow
Blood Brain Barrier
Cellular Transport
C-11 methionine
C-11, F-18 choline
C-11, F-18 acetate
F-18 DG
Quality Control Schedule
• Daily:
– Check singles, coincidences, timing, energy
– Sinograms
• Weekly:
– Update gains
• Quarterly
– Normalization and well counter calibration
• Annually
– ACR or NEMA tests, TG126.
PET Daily QA Scan
Daily Quality Assurance
• Pre-calibrated Phantom
Daily QA
Sample Sinograms
Quarterly Scanner Calibration
• Well Counter Correction (WCC)
• Transformation of counts per
second to activity concentration
Pre-Normalization
Post-Normalization
Assess quantification accuracy using SUV measurement
Annual ACR Phantom Images
• Uses the ACR (Esser phantom)
Annual ACR phantom
images
Contrast
Uniformity
Resolution
Consider a 1 liter (~1 kg) tank of water that has 10 mCi (370
MBq) of 18F-FDG. Let’s assume that the FDG is uniformly
distributed in the water tank. 1) What is the measured SUVbw
in a region of interest in the water tank? 2) what is the unit of
SUVbw ? Hint: SUVbw = (measured AC/injected dose)/Pt. Wt.
0%
0%
0%
0%
1.
2.
3.
4.
10, uCi/cc
10, ml/g
1, unitless
1, g/ml
10
Consider a 1 liter (~1 kg) tank of water that has 10 mCi (370
MBq) of 18F-FDG. Let’s assume that the FDG is uniformly
distributed in the water tank. 1) What is the measured SUVbw in
a region of interest in the water tank? 2) what is the unit of
SUVbw ? Hint: SUVbw = (measured AC/injected dose)/Pt. Wt.
1.
2.
3.
4.
10, uCi/cc
10, ml/g
1, unitless
1, g/ml
Ref: Standards for PET image acquisition and quantitative data
analysis. Boellaard R. J Nucl Med. 2009 May;50 Suppl 1:11S-20S.
2009 Apr 20. Review
Attenuation in PET imaging is dependent on:
0%
0%
0%
0%
1.
2.
3.
4.
Annihilation photon path length
Depth of annihilation event in the source
Amount of radioactivity in the source
Number of detectors in the scanner
10
Attenuation in PET imaging is dependent on:
1.
2.
3.
4.
Annihilation photon path length
Depth of annihilation event in the source
Amount of radioactivity in the source
Number of detectors in the scanner
Ref: Physics in nuclear medicine. 3rd edition, Cherry S, Sorenson J,
Phelps M. Chapter 18, page 356.
All of the following are advantages of
PET/CT over dedicated PET except:
0%
0%
0%
0%
1.
2.
3.
4.
Shorter overall scan duration
Better image resolution
Low noise attenuation correction
Automatic fusion of anatomical and
functional images
10
All of the following are advantages of
PET/CT over dedicated PET except:
1.
2.
3.
4.
Shorter overall scan duration
Better image resolution
Low noise attenuation correction
Automatic fusion of anatomical and
functional images
Ref: Townsend DW, Carney JP, Yap JT, Hall NC. PET/CT today and
tomorrow. J Nucl Med 2004;45 (suppl 1):4S–14S.
The main advantage of TOF over non a TOF
scanner is:
0%
0%
0%
0%
1.
2.
3.
4.
Higher resolution
Higher contrast
Higher count rate performance
Lower scatter
10
The main advantage of TOF over non a TOF
scanner is:
1.
2.
3.
4.
Higher resolution
Higher contrast
Higher count rate performance
Lower scatter
Ref: Cherry SR. The 2006 Henry N. Wagner Lecture: of mice and men
(and positrons) - advances in PET imaging technology. J Nucl Med.
2006;47:1739.
A well counter calibration in PET imaging is
used to:
0%
0%
0%
0%
1. Correct for variations in image uniformity
2. Correct for variations in detector gains
3. Correct for differences in detector coincidence
timing
4. Transform detected count rate to activity
concentration
10
A well counter calibration in PET imaging is
used to:
1. Correct for variations in image uniformity
2. Correct for variations in detector gains
3. Correct for differences in detector coincidence
timing
4. Transform detected count rate to activity
concentration
Ref: Ref: Physics in nuclear medicine. 3rd edition, Cherry S, Sorenson J,
Phelps M. Chapter 18, page 357.
Thank You