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Quality Assurance of Radiation Oncology Imaging Systems
Karl L. Prado, Ph.D.Department of Radiation Physics,
University of Texas MD Anderson Cancer CenterHouston, Texas
Mexican Federation of Medical Physics Organizations,Southwest Chapter of the American Association of Physicists in Medicine,
Queretaro, Mexico, March, 2007
Introductory Remarks:Training and Quality Assurance
Imaging Physics for the Therapy PhysicistAn introduction to the technology of digital imaging in radiation oncology: its fundamentals, current clinical utilization, and initial Quality Assurance effortLi and Hendee. JACR (4), 40, 2007: Radiation Oncology Physicists Will Need to Better Understand Medical Imaging
… “A major effort is required for radiation oncology physicists to raise the quality assurance of image guidance to a level comparable with that achieved in the maintenance of dosimetric performance.” …
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Acknowledgement of Collaborators
Faculty - Clinical Physics
Peter BalterTina BriereRajat KudchadkerPatricia LindsayRachel Liu
EngineeringTom DielJermaul PrinceChad FikesSean Swiedom
Quality AssuranceRobert SessionCraig MartinAndrea Robason
Rationale for Radiation Oncology Imaging System QA:
Successful targeting of tumor volumes depends greatly on optimized imaging
Target and margin definitionTarget position determination and verfication
From: ICRU News, Dec. 1999
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Rationale 1:Accurate Target Definition
The IGTV: Internal Margin Definition
IG’TV
CTV
SM
Target Volumes and Internal and
Setup Margins
Rationale 2:Verification of Treatment Delivery
Patricia Lindsay, Ph.D., Tina Briere, Ph.D.
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Rationale 2: Verification of Treatment Delivery
Vertebral Body IMRT:
Plan and Verification
QA of Radiation Oncology Imaging Systems: Overview
Current Imaging Technologies in Radiation Oncology
FundamentalsClinical Uses of Technology
Imaging System Quality AssuranceImaging system performance areas and Quality Control (QC) parametersQC procedures and tools
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Current Imaging Technologies in Radiation Oncology
Technology addressed:Computed Tomography (CT)Electronic Portal Imaging Devices (EPIDs)
MV systemskV systems
Computed Radiography (CR) Systems
PET/CTMV
kVCR
CT in Radiation OncologyCT Technology
Multi-Slice (4, 8, 16, 64)Spiral Pitch
Imaging requirements differ from Diagnostic Imaging
Patient groupPurpose of imaging
Target definition and localization, dose calculation
Imaging procedureBore size, couch, lasers, software
Optimized CT imaging protocols
( ) ( ) ( )rrrrrD r →Κ×Ψ×= ∫ '')'(' ρμ
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Optimized Imaging ProtocolsOptimization of CT imaging protocols for:
High contrast resolution
CATPHAN CTP528 line pair module
Low contrast resolution
1% low contrast CTP515 module
Presence of artifacts
Rachel Liu, Ph.D., et. al., AAPM Abstract (submitted), 2007
4 × 1.250.75GE LS RT
16 × 1.250.938GE LS RT16
16 × 1.251.375GE LS16
1 channel ×3mm
1Philips AcQSim
16 × 0.750.688Philips MX8000
DetectorConfiguration
PitchUnit
Optimized protocols for 2.5-3.0 mm slice thickness images
Electronic Portal Imaging Devices - EPIDs (DR Systems)
Current generation EPIDsuse Flat Panel Amorphous Silicon Arrays (aSi)A large glass plate is divided into an array of 1 million photo diodes/capacitorsLight is created in a detector (scintillation) layerLight is converted to charge by the photodiodeThe charge is digitized by a collection A/D converters
Peter Balter, Ph.D.
X-rays
light
Photodiodes
A/D
A/D
A/D
A/D
A/D
A/D
A/D
A/DA/D
Converters
Scintillator
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Amorphous Silicon Detector
13 wjw 12/01
Cross Section of an A-Si Detector Pixel
• Conversion of x-ray to optical signal• Conversion of optical signal from the phosphor to charge• Storage of charge on pixel (photo-diode) capacitance
photodiode
copper metal plate 1mm
a-Si:H 1.5um
MeV x-ray
scatter photon
Phosphor 0.4 mm
TFT switch
x-ray converter
glass substrate 1mm
e-
λ
From: Varian Medical Systems
Flat Panel a-Si Arrays
From: Digital Radiographic Technology, John Yorkston, Ph.D.
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Computed Radiography Systems
A PhotostimulableStorage Phosphor (PSP) plate stores a ‘latent image’ (analogous to film)The PSP plate is subsequently “read” by detecting the emissions produced by laser stimulation
Peter Balter, Ph.D.
43 cm
Plate scansdown
Laser
Aperture
Stimulated Emission
Computed Radiography (CR) Systems
AAPM Report 93
Image FormationPhotostimulableStorage Phosphors (PSPs)Latent image is created at F-centers (electron meta-stable states) produced by x-rays
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Computed Radiography Systems
AAPM Report 93
Image ReadoutPlate is scanned by laser along a slitLuminescent signal captured by PMT
Intensity is proportional to dosePosition determined by laser position
Plate is ‘erased’ by emptying meta-stable traps using high-intensity light
QA of Radiation Oncology Imaging Systems: Overview
Current Imaging Technologies in Radiation Oncology
FundamentalsClinical Uses of Technology
Imaging System Quality AssuranceImaging system performance areas and Quality Control (QC) parametersQC procedures and tools
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Virtual Simulation and Treatment Planning
Target definition, organ segmentation, beam and isocenterdetermination
Visualization, contouring, multi-modality fusionBeam definition and isocenter localization
DRRs
Verification of Treatment Delivery: Traditional MethodsMV (and now kV) ImagesAnatomy and reticule basedWith digital imaging can use image-enhancement tool
Histogram Equalization
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Courtesy of: Lei Dong, Ph.D., and Associates
Verification of Treatment Delivery: CT-Based IGRT
Newer Image Guidance
“2D” MatchImage (kV) registration to plan DRRCouch shift computed (and can be applied)
Tina Briere, Ph.D., Patricia Lindsay, Ph.D.
AP Lat
DRR
kV
Varian
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Newer Image Guidance
Fiducial-Marker Match
Tina Briere, Ph.D., Patricia Lindsay, Ph.D.
DRR
kV
Isoloc
QA of Radiation Oncology Imaging Systems: Overview
Current Imaging Technologies in Radiation Oncology
FundamentalsClinical Uses of Technology
Imaging System Quality AssuranceImaging system performance areas and Quality Control (QC) parametersQC procedures and tools
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Imaging System Performance Areas and Quality Control
Image Quality and Image Quality MeasuresGeometric Accuracy RequirementsRegulatory Requirements
Image Quality MeasuresIn general:
Density (darkness)Dose relationshipLatitude
Contrast (Low)Sharpness
Spatial Resolution (MTF)Signal/Noise (SNR)
NoiseArtifacts
For digital images:
Uniformity
LinearitySpatial distortion
LagImage processing
Display……
Based on: AAPM Monograph 30, 2004
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Image Quality / Measures
UniformityHigh Contrast Spatial Resolution
MTF or LSFLow Contrast ResolutionNoise (References)Artifacts
Las Vegas
QC-3
PIPSpro
Geometric Accuracy RequirementsMargins and relationships between treatment plan (1), treatment-unit (2), and imaging system (3) isocenters
Rajat Kudchadker, Ph.D.
RadAc
Winston-Lutz Test
(1)
(2)
(3)
(2)
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Imaging Systems Regulatory Requirements
State Regulations25 TEXAS ADMINISTRATIVE CODE, §289.227, “Use of Radiation Machines in the Healing Arts”
General RequirementsTechnique charts, operating and safety procedures; operator / medical physicist credentialing, …Radiographic entrance exposure limits
Radiographic / FluoroscopicBeam quality: half-value layer, kVp accuracyBeam limitation: x-ray field vs. indication Exposure reproducibility; linearity……
CTRadiation output of the CT system (CTDI) Additional requirements applicable to CT x-ray systems……
CT Quality Control (1)Baseline:
Manufacturer’s specificationsReferences: AAPM TG Reports 66 & 39
Daily QCHigh Contrast; Spatial integrityLow Contrast resolutionUniformityCT ring artifact (each channel)
Monthly QCLocalization laser accuracy
Lap laser movement accuracyCT number accuracy……
Rachel Liu, Ph.D.
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CT Quality Control (2)Monthly QC (cont.)
Laser (external) alignment with imaging plane …
Annual QCCTDI measurementsTube output linearityRadiation ProfileImage quality evaluation: uniformity, CT number accuracy, resolution, slice sensitivity profile, etc. Couch movement accuracy……
Radcal, 9095 Multi-Purpose Analyzer
10-cm CT Ion Chamber
Laser Position Scale
EPID and OBI Quality Control
MV / kV Imagers: Requirements
BaselineDaily QCMonthly QCAnnual QC
Varian
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OBI Quality ControlkV Imagers:
BaselineAcceptance Specs.Baseline performance
Daily QCFunctional: Isocenterverification with shift
Monthly QCAnnual QC
Rajat Kudchadker, Ph.D., Robert Session, B.S.
QBI Quality ControlMonthly QC
Isocenter coincidence (increased precision)Image quality:
Contrast (Leeds)Resolution (Line pair)
Annual QCRegulatory x rayImage qualitySafety, mechanical
Leeds
QCkV-1
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EPID Quality ControlMV Imagers:
BaselineAcceptance Specs.Baseline performance
Daily QAFunctional
Monthly QAImage Quality
Annual QAMechanical / SafetyImage Quality
Karl Prado, Ph.D., Andrea Robason, B.S.
Las Vegas
Re-acquire dark and flood fields
QC-3
CR System Quality ControlBaselineDaily QC
Startup calibration (self test)
Monthly QCCleaning imaging plates, cassettes, scannersImage quality …
Annual QCUniformity ResolutionDistortionArtifacts …
Rachel Liu, Ph.D., Craig Martin, B.S.
15%
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Data FlowCan be complex
Reliance on ITImage storage and retrievalMulti-vendor environment
In-house solutionsSecurity:
FirewallsSecurity updates
…Ron Zhu, Ph.D., et al
References: GeneralSpecifications, Performance Evaluation, and Quality Assurance of Radiographic and Fluoroscopic Systems in the Digital Era. AAPM Medical Physics Monograph No. 30, 2004.
Fundamentals: Samei, E. Saunders, R. The Nature of the Digital Image.Flat Panel Imagers: Yorkston, J. Flat Panel DR Detectors for Radiography and Fluoroscopy.CR Systems: Seibert, J. Computed Radiography Technology 2004.
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References (Continued)
CT SimulationAAPM Task Group 66. Quality Assurance for Computed Tomography Simulators and the Computed-Tomography-Simulation Process. Med. Phys. 30 (10), 2762-2792 (2003)
References (Continued)
Electronic Portal Imaging Systems
AAPM Task Group 58. Clinical Use of Electronic Portal Imaging. Med. Phys. 28 (5), 712-737 (2001)
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References (Continued)
Computed Radiography (CR) Sytems
AAPM Task Group 10. Acceptance Testing and Quality Control of Photostimulable Storage Phosphor Imaging Systems. AAPM Report Number 93, 2006.
http://www.aapm.org/pubs/reports/
References (Continued)
Radiation Oncology Imaging Systems Quality Assurance
Yoo, S. … A Quality Assurance Program for the On-Board Imager. Med. Phys. 33, (11), 4431-4447, 2006Rajapakshe, R. … A Quality Control Test for Electronic Portal Imaging Devices. Med. Phys. 23 (7), 1237-1244, 1996
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References (Continued)Radiation Oncology Quality Assurance
http://www.oncologymeetings.org/QA of Radiation Oncology Imaging Systems
Jaffray, D. QA Program for Radiographic, Fluoroscopic, and Cone-Beam CT Image Guided RT.Balter, J. QA for Radiographic On-Line and Off-Line Localization.
Imaging System QA: Summary
Overview of digital imaging systems in Radiation Oncology
Still much to learnCT, EPID, and OBI Systems
Principles, PracticeQuality Assurance Program
Initial Effort
To be continued …
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Thank you!
