PET/CT Technology Updates: Quality Assurance and Applications · PET/CT Technology Updates: Quality...

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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

Annihilation

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

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

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)

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

SUV=1 1 mCi

1 Kg 1 Kg

SUV=1 2 mCi

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

• 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

-1000 -500 0 500 1000CT number measured at 140kVp

VWater/air

Bone/Water

Normal CT Contrast enhanced CT

Coronal

Transaxial

contrast bias

Contrast: Optiray 320, 100cc, 3cc/sec

CT attenuation map scaling

-1000 -500 0 500 1000 1500

CT Hounsfield values

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

Courtesy of J. Brunetti, Holly name

Mismatch between PET and CT – Cardiac Application

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

CT PET Fused RACT PET Fused No AC(A) (B)

(C) (D)

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

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)

Trigger1

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)

Trigger1

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

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

Courtesy of D Townsend

Transverse resolution recovery

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

Separated PET/MR scanner design

GE Healthcare, Molecular Imaging

VUE Point – Iterative Reconstruction

Deadtime/ Normalization

Correction

Randoms Correction

Radial Repositioning

Quadrant Scatter

Correction

Attenuation Correction

Iterative Reconstruction

Correction

Randoms Correction

Radial Repositioning

Quadrant Scatter

Correction

Iterative Reconstruction w/ Distance Driven Projectors

Correction

Randoms Correction

Volume Scatter 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

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

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

FUTURE

Static

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

Standard 8 x 8 detector

HI-REZ 13 x 13 detector

All spheres contain the same activity concentration Profile (10 mm)

Sphere diameter

37282217

Recovery coefficients

Effect of Partial voluming

Courtesy of Siemens

Phase Gating vs. Amplitude Gating

SUMAmplitude varying

D. Mohammad et al, Med. Phys. 34(7):3067-76, 2007

Phase Gating vs. Amplitude Gating

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)

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%

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.

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:

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:

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:

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:

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

PET/CT Technology Updates: Quality Assurance and Applications · PET/CT Technology Updates: Quality...

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