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Magnus Dahlbom Ph D PET/CT QC/QA Magnus Dahlbom, Ph.D. Division of Nuclear Medicine Ahmanson Biochemical Imaging Clinic David Geffen School of Medicine at UCLA Los Angeles Quality Control in PET Verify the operational integrity of the system Detectors Acquisition Electronics Maintain consistent and high image quality Minimize chances for artifacts Catch potential problems early Maintain quantitative accuracy Eliminate unnecessary repeat scans Data Flow in a PET System PET Detectors Most modern PET system use a detector technology referred to as Block Detectors. A large number of scintillation crystals are coupled to a smaller number of PMTs. In the block detector, a matrix of cuts are made into a solid block of scintillator material t d fi th dt t l t to define the detector elements. The depth of the cuts are adjusted to direct the light to the PMTs. The light produced in each crystal, will produce a unique combination of signals in the PMTs, which will allow the detector to be identified.
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PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Apr 18, 2018

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Page 1: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Magnus Dahlbom Ph D

PET/CT QC/QA

Magnus Dahlbom, Ph.D.Division of Nuclear MedicineAhmanson Biochemical Imaging ClinicDavid Geffen School of Medicine at UCLALos Angeles

Quality Control in PET

Verify the operational integrity of the systemDetectorsAcquisition Electronics

Maintain consistent and high image qualityMinimize chances for artifactsCatch potential problems earlyMaintain quantitative accuracyEliminate unnecessary repeat scans

Data Flow in a PET System PET Detectors

Most modern PET system use a detectortechnology referred to as Block Detectors.

A large number of scintillation crystals arecoupled to a smaller number of PMTs.

In the block detector, a matrix of cuts aremade into a solid block of scintillator materialt d fi th d t t l tto define the detector elements.

The depth of the cuts are adjusted to directthe light to the PMTs.

The light produced in each crystal, willproduce a unique combination of signals inthe PMTs, which will allow the detector to beidentified.

Page 2: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

PET Detectors

To identify the detector elements, the following positioning indeces are calculated:

Below is the flood response (i.e., X and Y density

A B C D A B C DX YA B C D A B C D+ − − − + −

= =+ + + + + +

p ( , ydistribution when exposed to a flood source of 511 keV photons) for a block detector from the ECAT EXACT HR.

Element (4,4)

Element (5,3)

PET Detector Calibration OverviewPET Detector Calibration Overview

Well CounterCorrection

(Q)Update Gains

U d t P iti

RMS < x

RMS > x

Installation/Replacement of

Detector block, circuitBoard, etc.

O. Mawlawi MDACC

Set the Baseline

Update PositionMap

Energy Update

CTC

DQA Check

Geometric Calibration

(Q)

12 hour Norm(Q)

Coincident Calibrations

Singles Calibrations

Pixelated Detector System

ContinuousContinuous--pixelatedpixelateddetectordetector

Page 3: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Sinogram

θ

All coincidence lines that are parallel at a given angleform a projection in the sinogram.

r

Sinogram

θ

r

All coincidence lines (or lines of response) for a givendetector form a diagonal trace in the sinogram.

Daily QC

Daily Detector CheckTransmission/Rotating Rod SourcesUniform cylinder phantom (20 cm Ø)

Water Phantom Solid 68Ge Phantom

Visually Inspect the sinograms for:

Apparent streaks (hot or cold)

Daily QC / Blank Scan

Consistency – Comparison to a reference scan

Page 4: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Normal Detector Failure

Detector Controller Failure Sorting Memory Failure

Page 5: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Detector Block Failure Detector Block Failure

PET Daily QA ScanPET Daily QA Scan

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PET Daily QA ScanPET Daily QA Scan

O. Mawlawi MDACC

Page 6: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Daily QC and Normalization

Before Normalization After Normalization

Normalization

Normalization

Corrected Non-corrected Difference

Daily / Weekly Quantification ScanScan uniform 20 cm Ø 68Ge or 18F Cylinder

Emission & Transmission / CT

Reconstruct:

All corrections applied

Standard reconstruction parameters

Vi l i tiVisual inspection

Compare image ROI activity to calibrated activity

Always perform after:

Service

Re-tuning

Re-normalization

Page 7: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Quantification Scan

2D Cylinder0.0299 μCi/ml

FBP OSEM

Calculated Atten. Corr. 0.0296 -1.1% 0.0295 -1.5%

Meas. Atten. Corr. CT 0.0285 -4.7% 0.0284 -4.0%

Meas. Atten. Corr. Rods 0.0242 -19.2% 0.0239 -20.2%

Quality ControlLongitudinal uniformity

Austin Health

Comparison of mean SUVs of VOIs (Allegro CsAC 576FOV vs. 256FOV)

Quarterly QC ProceduresDetector setup (if needed)

PMT tuning

Detector setup

Coincidence timing

(TOF)

Normalization

Other cross calibrations (well counter, etc)

Gantry alignment (for PET/CT)

Always after Service

Software upgrades

Page 8: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Annual QC Procedures

Perform a sub-set of the acceptance (NEMA) tests:UniformityResolutionCount Rate TestDead Time CorrectionSensitivityQuantificationBed motion

Spatial Resolution Count Rate Test

1.5 106

2.0 106

2.5 106

Prompts [1/s]Randoms [1/s]Trues [1/s]NEC [1/s]

Rat

e [c

ps]

Septa Extended (2D)

0.0

5.0 105

1.0 106

0 1 2 3 4 5 6 7

Cou

nt R

Activity [μC/ml]

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Count Rate Test

1.5 106

2.0 106

2.5 106Count Rate Test

Prompts [1/s]Randoms [1/s]Trues [1/s]NEC [1/s]

te [c

ps]

Septa Extended (2D)

0.0 100

5.0 105

1.0 106

0 1 2 3 4 5 6 7

Cou

nt R

at

Activity [uC/ml]

Other Test Phantoms

Page 10: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

CT QC Phantoms CT Daily QA ScanCT Daily QA Scan

• Normal operations include the following 3 tasks (in order):

• Tube Warmup- A built-in prep scan that gradually increases heat loading in the X-ray tube in order to prevent thermal cracking and eliminate the potential for an arc to occur It includes a series of

O. Mawlawi MDACC

eliminate the potential for an arc to occur. It includes a series of exposures made at incrementing kVp

• Daily Air Cals- A built-in prep scan that performs a series of exposures at varying techniques in order to normalize the detector response using air as the attenuating media. These scans essentially adjust the detector gains to achieve a uniform response

• Daily QA Phantom scan - Provides data for 3 areas of concern in daily quality assurance

• Scan water layer

Quality ControlCT daily regimen

Austin Health

– Measure water HU• CTAC

– Check for artifacts• Ring artifacts

– Redo conditioning

– Redo Air Cal

Courtesy: Stefan Eberl, RPA

CT Daily QA ScanCT Daily QA Scan

• Artifact Analysis•Looking for the presence of artifact•Ring artifact is the most clinically prevalent in QA scans•Caused by non-uniformity in detector response due to gain imbalance or beam obstruction

O. Mawlawi MDACC

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CT Daily QA ScanCT Daily QA Scan

• Uniformity– ROIs distributed in

homogeneous material should indicate consistent signal (HUs) and noise

O. Mawlawi MDACC

CT Daily QA ScanCT Daily QA Scan

• Linearity– Linear attenuation

coefficients track linearly with material density

– Remember that CT numbers are defined WRT the attenuation coefficient of water:

O. Mawlawi MDACC

of water:

– The mean CT numbers of air (-1000 HU), water (0 HU), and acrylic (120 HU) displayed within an ROI should be consistent with the defined value +/-manufacture specified tolerance

(x,y) water

water

μ -μCT(x,y) = 1000 ( )

μ

CT Daily QA ScanCT Daily QA Scan

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Quality ControlCT weekly regimen

• HU calibration check– Water– Air– Teflon

Austin Health

Page 12: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

x 0.495

x 0.406

H.U. → μ70 keV

Quality ControlCT weekly regimen

• Hounsfield Unit Calibration– ROI means– ROI standard deviation range

• mAs setting accuracy

Austin Health

• kVp, mAs exercising– Filament adaptation– Collimation

• MTF & Slice thickness– Physics layer

• Check error log

• Low Contrast Resolution–Qualitative assessment of smallest resolvable hole in a membrane with a CT number similar to that of water

CT Daily QA Scan

O. Mawlawi MDACC

Dosimetry - CTDI

Page 13: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

Daily CT QC

Accuracy of Water CalibrationImage NoiseUniformityArtifacts

Monthly/Semi-annual CT QC

Slice ThicknessSlice PositioningLaser AlignmentCT ScaleCT ScaleResolutionLow Contrast ResolutionDosimetry

CT PET

PET/CT Alignment

CT PETTransmission

Scan

Page 14: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

PET/CT Alignment

CT PETEmission

Scan

PET/CT Alignment

Jaszczak/ACR Phantom PET/CT Gantry Alignment

CT PET

Page 15: PET Detector Calibration Overview - AAPM: The American ... · PET Detector Calibration Overview Well Counter ... (for PET/CT) Always after Service ... Resources for PET and PET/CT

PET/CT Gantry Alignment System Misregistration

Resources for PET and PET/CT QC

Manufacturers’ manual

NEMA NU-2 Publications 2007, 2001 & 1994 AAPM rpt. 72ACR

Karp J.L. et. al., JNM 32 (12), 1991Buchert R. et. al., JNM 40 (10), 1999Geworski L., JNM 43 (5), 2002

Bailey et. al. “Positron Emission Tomography – Basic Science”Cherry SR & Dahlbom M, in Phelps ME “Molecular Imaging”

Summary

An effective QC/QA program for PET and PET/CT can be implemented with a few relatively simple daily and weekly phantom scans

With training and experience, potential problems can be gidentified and possibly rectified

A regular QC program will ensure consistent image quality as well as quantitative accuracy

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Acknowledgments

Osama Mawlawi, Ph.D.Department of Imaging PhysicsMD Anderson Cancer CenterHouston

G O’ fGraeme O’Keefe, Ph.D.Centre for PETAustin HealthMelbourne

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