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PET/CT Technology Updates:Quality Assurance and Applications
Osama Mawlawi, Ph.D.
Department of Imaging PhysicsMD 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
O. Mawlawi MDACC
Nucleuspositron
electron
511keVPhoton
Detector ring
511keVPhoton
LOR
Annihilation
E=mC2
Electron rest mass = 9.11x10-31 kgC=3.0x108 m/s
1 joule =1 kg m2/s2
1 e = 1.602x10-19 coulomb1 eV = 1.602x10-19 JThus: E =1.02 MeV
O. Mawlawi MDACC
O. Mawlawi MDACC
Sample Sinograms
O. Mawlawi MDACC
SUV = (Measured AC /Injected dose)/ Patient weight
O. Mawlawi MDACC
Quantification: Power of PET
Ideal measured data
Measured Data
Random subtract
Normalize
Correct Geometry
Calculate/subtract scatter
Correct Attenuation
Dead time
FBP or IR reconstructioncalibration
Effect of Attenuation
00.10.20.30.40.50.60.70.80.9
1
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
O. Mawlawi MDACC
Attenuation is dependanton the path length and not the depth of the source of activity
Four ways to get anattenuation map1) Measured (MAC)2) Calculated (CAC)3) Segmented (SAC)4) CT based (CTAC)
11
LP e−= μ 22
LP e−= μ
1 2* LP P e−= μ
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
SUV = (Measured AC /Injected dose)/ Patient weight
Standardized Uptake Value (SUV)
SUV =Decay corrected activity concentration (uCi/ml)
Injected dose (mCi) / Patient weight (g)
1 Kg 10 Kg
1 mCi
SUV=1 1 mCi
SUV=1
Standardized Uptake Value (SUV)
SUV =Decay corrected activity concentration (uCi/ml)
Injected dose (mCi) / Patient weight (g)
1 Kg 1 Kg
1 mCi
SUV=1 2 mCi
SUV=1
15.5 cm
Tx rod sources
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
Emission
Transmission
Final
• 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
Disadvantages of dedicated PET imaging techniques
GE Discovery ST
Hybrid Imaging
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 functionaland
Anatomical data
CT based attenuation correction
AttenuationCorrection
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
0.000
0.050
0.100
0.150
0.200
-1000 -500 0 500 1000CT number measured at 140kVp
Atte
nuat
ion
at 5
11ke
VWater/air
Bone/Water
error
Normal CT Contrast enhanced CT
Coronal
Transaxial
contrast bias
Contrast: Optiray 320, 100cc, 3cc/sec
CT attenuation map scaling
0.000
0.020
0.040
0.060
0.080
0.100
0.120
0.140
0.160
0.180
0.200
-1000 -500 0 500 1000 1500
CT Hounsfield values
Line
ar a
ttenu
atio
n at
511
keV
1/c
m
140kVp noncontrast120kVp noncontrast100kVp noncontrast140kVp BaSO4
120kVp BaSO4
100kVp BaSO4
140kVp I
120kVp I
100kVp I
Contrast Artifact- Intravenous Contrast injected
CT PET CTAC FUSED IMAGE
With contrast Without contrast
61 year male with history of esophageal cancer
Truncation
PET Detector
PET 70cm image FOV
X-ray focal spot
CT Detector
CT 50cm Fully sampled FOV
CT and PET Fields of View
Wide View
Activity concentration recovered to within 5% of original value
54 yrs. male with history of metastatic melanoma of the skin. SUV changed from 3.25 to 6.05
CT PET FUSED Attenuation map
Truncated
Truncated corrected
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 SUVPhotopenic
Breathing ArtifactMismatch of lesion location between helical CT and PET
Breathing ArtifactMismatch of lesion location between helical CT and PET
Courtesy of J. Brunetti, Holly name
Mismatch between PET and CT – Cardiac Application
2
1
Average CT – 4D CT
Courtesy of Tinsu Pan, MDACC
Clinical Studies
Mismatch 1:
CT diaphragm position lower
than PET
Mismatch 2:
CT diaphragm position higher
than PET
+57%
Courtesy of Tinsu Pan, MDACC
CT PET Fused RACT PET Fused No AC(A) (B)
(C) (D)
Courtesy of Tinsu Pan, MDACC
Impact on treatment planning
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.
Old GTV
New GTV
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)
time
7
3
4 5
6
8
3
45
6
7
Bin 8
82
Trigger1
Bin 1
21
Trigger
• 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.
4D-Gated PET
Phantom Study
AxialCoronal Sagital Mip
Sphere (volume) Gated Static Error (%)1 (16.5 cc) 31624 33546 -6.1
2 (8.4 cc) 31316 21872 30.23 (4 cc) 29919 20607 31.14 (2 cc) 22857 13643 40.35 (1 cc) 19087 8264 56.7
6 (0.5 cc) 11897 5143 56.8
Impact of Whole-body Respiratory Gated PET/CT
• The max SUV of the lesion goes from 2 in the static image to 6 in the respiratory-gated image sequence
Static wholebody Single respiratory phase(1 of 7, so noisier)
1 cc lesion on CT
Courtesy of P Kinahan, UW
New CT Application… Advantage 4D CT
Respiratory motion defined retrospective gating
X-ray on
First couch position Second couch position Third couch position
“Image acquired” signal to RPM system
Respiratory tracking with Varian RPM optical monitorCT images acquired over complete respiratory cycle
4D-PET/CT
4D PET/CT
Motion tracking with 4D CT
Advantage 4D CT provides a method for accurately defining tumor motion for GTV determination
Image Courtesy of Tinsu Pan
Acquire image that accurately represents respiratory motionDraw treatment volume on Cine movie
Gated PET (Used to image repetitively moving objects: cardiac, respiratory)
time
7
3
4 5
6
8
3
45
6
7
Bin 8
82
Trigger1
Bin 1
21
Trigger
Methods to improve SNR of gated PET
• Deformable registration (image based)• Motion incorporation during reconstruction• Increasing scan duration• DIBH• End expiration period gating
Static3 min
PET data acquisition of different duration per FOV
Static3 min
Static5 min
Static6 min
LIST Mode
• 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
• 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 Y1 X2 Y2 X3 Y3TIME X4 Y4 TIME
1msec 1msec
X5 Y5 X6 Y6
Y8 X9 Y9 X10 Y10 X11 Y11X8
X7 Y7
X1
Y1X2
Y2X3
Y3
X4
Y4
Section of a list file (141MB) using a 25MBq (0.7mCi) Ge-68 rod source acquired for a duration of 15 seconds.
LIST data format Tick marks
1 msec
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
SIEMENSGE
PHILIPS
Advantages of the extended Axial Field-of-View
• 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
Sensitivity
FOV
Courtesy of D Townsend
Transverse resolution recovery
Courtesy of D Townsend
Courtesy of D Townsend
Transverse resolution recovery
Innovation is in our genes.76 Siemens Medical Solutions Molecular Imaging
First ever in-vivo images simultaneously acquired by MR and PET (17.11.06)
BrainPET for MAGNETOM Trio, The first results
PETMR MR-PETCourtesy of Townsend and Nahmias University of Tennessee, USA and Schlemmer, Pichler, Claussen University of Tuebingen, Germany
* 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.
Simultaneous PET/MR scanner design
Schwaiger M 2005
Schwaiger M 2005
Separated PET/MR scanner design
GE Healthcare, Molecular Imaging
Page 80
VUE Point – Iterative Reconstruction
Deadtime/ Normalization
Correction
Randoms Correction
Radial Repositioning
Quadrant Scatter
Correction
Attenuation Correction
Iterative Reconstruction
Deadtime/ Normalization
Correction
Randoms Correction
Radial Repositioning
Quadrant Scatter
Correction
Attenuation Correction
Iterative Reconstruction w/ Distance Driven Projectors
Deadtime/ Normalization
Correction
Randoms Correction
Volume Scatter Correction
Attenuation Correction
Iterative Reconstruction w/ Distance Driven Projectors
Conventional
VUE Point
VUE Point HD (mid-2007 update)
Detector Geometry Modeling
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
2tx c∆
∆ =
C = 3*108 m/s
Back projection along the LOR
Time of Flight and SNR, examples…
convTOF SNRx
DSNR ⋅∆
≅ctx2∆
=∆
Time Resolution
(ns)
∆x
(cm)
SNRimprovement(20 cm object)
SNRimprovement(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
30 min scan
MIP
Colon cancer 119 kg (BMI = 46.5)
Improvement in lesion detectability with TOF
non-TOF TOF CT
Courtesy of J. karp
Average-weight patient studyTongue CA, lung mets
67 kgBMI = 29.0 20 min scan
CTTOFnon-TOF
Improvement in lesion contrast with TOF
non-TOF TOF
Courtesy of J. karp
non-Hodgkin’s lymphoma 136 kg (45 BMI)
TOF tumor contrast (SUV) higher than non-TOF by 1.5
nonTOF 30 min TOF 30 min TOF 10 min
TOF tumor contrast superior to non-TOF for 10 min as well as 30 min scanCourtesy of J. karp
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
Static
Static
Gated
Mixed acquisition protocol
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)
SUV =Decay corrected activity concentration (uCi/ml)
Injected dose (mCi) / Patient weight (g)
SUV1
SUV2>
10
37 28
22
1713
Standard 8 x 8 detector
HI-REZ 13 x 13 detector
All spheres contain the same activity concentration Profile (10 mm)
Sphere diameter
Rec
over
y (%
)
100
50
0 10
37282217
13
Recovery coefficients
Effect of Partial voluming
Courtesy of Siemens
Phase Gating vs. Amplitude Gating
time
7
3
4 5
6
8
3
45
6
7
82
1
21
time
7
3
45 6
83
45
6
7
8
21 2
1
1
SUMAmplitude varying
D. Mohammad et al, Med. Phys. 34(7):3067-76, 2007
Phase Gating vs. Amplitude Gating
time
7
3
4 5
6
8
3
45
6
7
82
1
21
SUM
time
7
34
56
8
3
45
6
7
82
12
1
Frequency varying
D. Mohammad et al, Med. Phys. 34(7):3067-76, 2007Chang et al. Young inv. Award, Sun. 12:45-1:15
Standardizing the Standardized Uptake Value (SUV)
SUV =Decay corrected activity concentration (uCi/ml)
Injected dose (mCi) / Patient weight (g)
Quantitative PET Performance
3 Categories that “might” affect SUV measurements• Patient Compliance
– Fasting– Blood Glucose levels
• Scan Conditions– Scan time post injection– Patient anxiety/comfort during uptake (room temperature, etc.)– Patient motion during acquisition– 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%
Up to 80%
15%
30% (60%)
15% (75%)
15% (30%)
15% (50%)
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
DNA Synthesis/Hypoxia
Protein & membrane Catabolism
Energy Metabolism
VascularityBlood Flow
Blood Brain BarrierCellular Transport
Receptors
PET/CT 2008
?
F-18 DG
C-11 methionineC-11, F-18 cholineC-11, F-18 acetate
18-FLT18 FMISO64-Cu ATSM
18-F L-DOPAAngiogenesis
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
Sample Sinograms
Daily QA
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 SUVbwin a region of interest in the water tank? 2) what is the unit of SUVbw ? Hint: SUVbw = (measured AC/injected dose)/Pt. Wt.
10
0%
0%
0%
0% 1. 10, uCi/cc2. 10, ml/g3. 1, unitless4. 1, g/ml
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. 10, uCi/cc2. 10, ml/g3. 1, unitless4. 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:
10
0%
0%
0%
0% 1. Annihilation photon path length2. Depth of annihilation event in the source3. Amount of radioactivity in the source4. Number of detectors in the scanner
Attenuation in PET imaging is dependent on:
1. Annihilation photon path length2. Depth of annihilation event in the source3. Amount of radioactivity in the source4. 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:
10
0%
0%
0%
0% 1. Shorter overall scan duration2. Better image resolution3. Low noise attenuation correction4. Automatic fusion of anatomical and
functional images
All of the following are advantages of PET/CT over dedicated PET except:
1. Shorter overall scan duration2. Better image resolution3. Low noise attenuation correction4. 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:
10
0%
0%
0%
0% 1. Higher resolution2. Higher contrast3. Higher count rate performance4. Lower scatter
The main advantage of TOF over non a TOF scanner is:
1. Higher resolution2. Higher contrast3. Higher count rate performance4. 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:
10
0%
0%
0%
0% 1. Correct for variations in image uniformity2. Correct for variations in detector gains3. Correct for differences in detector coincidence
timing4. Transform detected count rate to activity
concentration
A well counter calibration in PET imaging is used to:
1. Correct for variations in image uniformity2. Correct for variations in detector gains3. Correct for differences in detector coincidence
timing4. 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
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