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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 Center Houston, 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 Physicist An introduction to the technology of digital imaging in radiation oncology: its fundamentals, current clinical utilization, and initial Quality Assurance effort Li 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.” … 1 Acknowledgement of Collaborators Faculty - Clinical Physics Peter Balter Tina Briere Rajat Kudchadker Patricia Lindsay Rachel Liu Engineering Tom Diel Jermaul Prince Chad Fikes Sean Swiedom Quality Assurance Robert Session Craig Martin Andrea Robason Rationale for Radiation Oncology Imaging System QA: Successful targeting of tumor volumes depends greatly on optimized imaging Target and margin definition Target position determination and verfication From: ICRU News, Dec. 1999 2 Rationale 1: Accurate Target Definition SM The IGTV: Internal Margin Definition CTV IG’TV Target Volumes and Internal and Setup Margins Rationale 2: Verification of Treatment Delivery Patricia Lindsay, Ph.D., Tina Briere, Ph.D. 3 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 Fundamentals Clinical Uses of Technology Imaging System Quality Assurance Imaging system performance areas and Quality Control (QC) parameters QC procedures and tools 4 Current Imaging Technologies in Radiation Oncology Technology addressed: PET/CT Computed Tomography (CT) Electronic Portal Imaging Devices (EPIDs) MV MV systems kV systems Computed Radiography (CR) Systems CR kV CT in Radiation Oncology CT Technology Multi-Slice (4, 8, 16, 64) Spiral Pitch Imaging requirements differ from Diagnostic Imaging Patient group Purpose of imaging Target definition and localization, dose calculation D(r ) = ∫ r' μ (r ' ) × Ψ (r ') × Κ (r ' → r ) ρ Imaging procedure Bore size, couch, lasers, software Optimized CT imaging protocols 5 Optimized Imaging Protocols Optimization of CT imaging protocols for: High contrast resolution Low contrast resolution CATPHAN CTP528 line pair module 1% low contrast CTP515 module Presence of artifacts Optimized protocols for 2.5-3.0 mm slice thickness images Unit Pitch Detector Configuration Philips MX8000 0.688 16 × 0.75 Philips AcQSim 1 1 channel × 3mm GE LS16 1.375 16 × 1.25 GE LS RT16 0.938 16 × 1.25 GE LS RT 0.75 4 × 1.25 Rachel Liu, Ph.D., et. al., AAPM Abstract (submitted), 2007 Electronic Portal Imaging Devices - EPIDs (DR Systems) Current generation EPIDs use Flat Panel Amorphous Silicon Arrays (aSi) A large glass plate is divided into an array of 1 million photo diodes/capacitors Light is created in a detector (scintillation) layer Light is converted to charge by the photodiode The charge is digitized by a collection A/D converters X-rays light Scintillator Photodiodes A/D A/D A/D A/D A/D A/D A/D A/D A/D Converters Peter Balter, Ph.D. 6 Amorphous Silicon Detector Cross Section of an A-Si Detector Pixel copper metal plate 1mm Phosphor 0.4 mm e- photodiode λ MeV x-ray x-ray converter a-Si:H 1.5um glass substrate 1mm TFT switch scatter photon • Conversion of x-ray to optical signal • Conversion of optical signal from the phosphor to charge • Storage of charge on pixel (photo-diode) capacitance 13 wjw 12/01 From: Varian Medical Systems Flat Panel a-Si Arrays From: Digital Radiographic Technology, John Yorkston, Ph.D. 7 Computed Radiography Systems Plate scans down Aperture 43 cm Laser Stimulated Emission A Photostimulable Storage 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. Computed Radiography (CR) Systems Image Formation Photostimulable Storage Phosphors (PSPs) Latent image is created at F-centers (electron metastable states) produced by x-rays AAPM Report 93 8 Computed Radiography Systems Image Readout Plate is scanned by laser along a slit Luminescent signal captured by PMT Intensity is proportional to dose Position determined by laser position Plate is ‘erased’ by emptying meta-stable traps using high-intensity light AAPM Report 93 QA of Radiation Oncology Imaging Systems: Overview Current Imaging Technologies in Radiation Oncology Fundamentals Clinical Uses of Technology Imaging System Quality Assurance Imaging system performance areas and Quality Control (QC) parameters QC procedures and tools 9 Virtual Simulation and Treatment Planning Target definition, organ segmentation, beam and isocenter determination Visualization, contouring, multimodality fusion Beam definition and isocenter localization DRRs Verification of Treatment Delivery: Traditional Methods MV (and now kV) Images Anatomy and reticule based With digital imaging can use imageenhancement tool Histogram Equalization 10 Verification of Treatment Delivery: CT-Based IGRT Courtesy of: Lei Dong, Ph.D., and Associates Newer Image Guidance AP “2D” Match Image (kV) registration to plan DRR Couch shift computed (and can be applied) Lat DRR kV Varian Tina Briere, Ph.D., Patricia Lindsay, Ph.D. 11 Newer Image Guidance Fiducial-Marker Match DRR kV Isoloc Tina Briere, Ph.D., Patricia Lindsay, Ph.D. QA of Radiation Oncology Imaging Systems: Overview Current Imaging Technologies in Radiation Oncology Fundamentals Clinical Uses of Technology Imaging System Quality Assurance Imaging system performance areas and Quality Control (QC) parameters QC procedures and tools 12 Imaging System Performance Areas and Quality Control Image Quality and Image Quality Measures Geometric Accuracy Requirements Regulatory Requirements Image Quality Measures In general: Density (darkness) Contrast (Low) Sharpness Spatial Resolution (MTF) Signal/Noise (SNR) Dose relationship Latitude Noise Artifacts For digital images: Uniformity Linearity Spatial distortion Lag Image processing Display …… Based on: AAPM Monograph 30, 2004 13 Image Quality / Measures Uniformity High Contrast Spatial Resolution QC-3 MTF or LSF Low Contrast Resolution Noise (References) Artifacts PIPSpro Las Vegas Geometric Accuracy Requirements Margins and relationships between treatment plan (1), treatment-unit (2), and imaging system (3) isocenters (3) (2) (1) RadAc Winston-Lutz Test (2) Rajat Kudchadker, Ph.D. 14 Imaging Systems Regulatory Requirements State Regulations 25 TEXAS ADMINISTRATIVE CODE, §289.227, “Use of Radiation Machines in the Healing Arts” General Requirements Technique charts, operating and safety procedures; operator / medical physicist credentialing, … Radiographic entrance exposure limits Radiographic / Fluoroscopic CT Beam quality: half-value layer, kVp accuracy Beam limitation: x-ray field vs. indication Exposure reproducibility; linearity…… Radiation output of the CT system (CTDI) Additional requirements applicable to CT x-ray systems…… CT Quality Control (1) Baseline: Daily QC Manufacturer’s specifications References: AAPM TG Reports 66 & 39 High Contrast; Spatial integrity Low Contrast resolution Uniformity CT ring artifact (each channel) Monthly QC Localization laser accuracy Lap laser movement accuracy CT number accuracy …… Rachel Liu, Ph.D. 15 CT Quality Control (2) Monthly QC (cont.) Laser Position Scale Laser (external) alignment with imaging plane … Annual QC CTDI measurements Tube output linearity Radiation Profile Image quality evaluation: uniformity, CT number accuracy, resolution, slice sensitivity profile, etc. Couch movement accuracy …… 10-cm CT Ion Chamber Radcal, 9095 Multi-Purpose Analyzer EPID and OBI Quality Control MV / kV Imagers: Requirements Baseline Daily QC Monthly QC Annual QC Varian 16 OBI Quality Control kV Imagers: Baseline Daily QC Acceptance Specs. Baseline performance Functional: Isocenter verification with shift Monthly QC Annual QC Rajat Kudchadker, Ph.D., Robert Session, B.S. QBI Quality Control Monthly QC Isocenter coincidence (increased precision) Image quality: Contrast (Leeds) Resolution (Line pair) Annual QC Regulatory x ray Image quality Safety, mechanical QCkV-1 Leeds 17 EPID Quality Control MV Imagers: Las Vegas Baseline Acceptance Specs. Baseline performance Daily QA Monthly QA Re-acquire dark and flood fields Functional Image Quality Annual QA QC-3 Mechanical / Safety Image Quality Karl Prado, Ph.D., Andrea Robason, B.S. CR System Quality Control Baseline Daily QC Monthly QC Startup calibration (self test) Cleaning imaging plates, cassettes, scanners Image quality … Annual QC Uniformity Resolution Distortion Artifacts … 15% Rachel Liu, Ph.D., Craig Martin, B.S. 18 Data Flow Can be complex Reliance on IT Image storage and retrieval Multi-vendor environment Security: In-house solutions Firewalls Security updates … Ron Zhu, Ph.D., et al References: General Specifications, 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. 19 References (Continued) CT Simulation AAPM Task Group 66. Quality Assurance for Computed Tomography Simulators and the Computed-Tomography-Simulation Process. Med. Phys. 30 (10), 27622792 (2003) References (Continued) Electronic Portal Imaging Systems AAPM Task Group 58. Clinical Use of Electronic Portal Imaging. Med. Phys. 28 (5), 712-737 (2001) 20 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, 2006 Rajapakshe, R. … A Quality Control Test for Electronic Portal Imaging Devices. Med. Phys. 23 (7), 1237-1244, 1996 21 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 learn CT, EPID, and OBI Systems Principles, Practice Quality Assurance Program Initial Effort To be continued … 22 Thank you! 23