Virtual Simulator CT QAAlex Markovic, PhD
Director, Medical Radiation Physics Program, Rosalind Franklin University, North Chicago, IL
Lead Therapeutic Physicist, NorthShore University Health System, Evanston, IL
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Virtual CT Simulator QA – 2 topics
Electromechanical Performance and
Image quality
CT operator console
Virtual CT simulation software
Alex Prasad
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Original Simulators
X ray tube
Image intensifier
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What is Virtual CT Simulator (VSim)?
TG66:Quality assurance for computed-tomography simulators and the computed tomography-simulation process
“A virtual simulator is a set of software which recreates the treatment machine and which allows import, manipulation, display, and storage of images from CT and/or other imaging modalities.”
• Varian SomaVision• GE Advantage SIM• Elekta Group - CMS
Focal• Phillips AcQSim3
• Siemens syngo RT Oncologist
Examples of VSim software packages:
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Phillips AcQSim
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VSim Software Features
• DRR generation• Automatic structure segmentation• Interpolation of contours• Manual and semi-automatic (eg. “smart
brush”) contouring tools• Volume expansion to apply margins• 4D Visualization of organ motion • Isocenter placement, design of treatment
fields• Moving laser support
Advantages of VSim
• Efficient workflow• Doctors are usually present during the CT scan• Good time to have them complete their
contouring and planning work.• Poor scans can be repeated• Doctors’ work is less likely to be delayed• Vsim software can be installed on doctor’s PC or
can be accessed through Citrix
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Relevant Guidance Documents• No guidance document dedicated to Vsim QA• TG 2(1993): Specification and acceptance testing of CT
scanners• TG 53(1998) – QA of treatment planning• NCRP report #99: Quality Assurance for Diagnostic
Imaging • TG 66 (2003): QA for CT-simulators and CT simulation
process• TG 117 (in progress): The Use of MRI Data in Treatment
Planning and Stereotactic Procedures – Spatial Accuracy and Quality Control Procedures
• TG132 (In progress): Use of Image Registration and Data Fusion Algorithms and Techniques in Radiotherapy Treatment Planning
• Ensure data to and from VSim transfers correctly
• Reduce possibility of treatment errors due to inaccurate transfer of data
• Ensure complete dataset is sent
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Why Perform QA on the VSim?
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VSim Quality Management Program
• Acceptance testing - manufacturer procedure , if it exists
• Commissioning - resident physicist establishes procedures baseline values , mostly for electromechanical/imaging system components
• Periodic QA – ensures consistency of baseline values
• End of warranty tests - 6 weeks prior to end of warranty
• Post service/upgrade tests
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Data transfer
Data Transfer from CT to VSimNode Setup
CT scanner DICOM export configuration screen
VSim IP address
VSim AE Title
Data Transfer Issuestrouble in paradise
VSim hosts file
• VSim hosts file does not include CT
scanner IP
• Routers are mac layered
• Dicom service has stopped on VSim
• IT has secretly upgraded the router
• When in doubt – reboot!
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Information standard to move images, etc
Images: CTs, DRRs
Structure sets: Points of interest, bolus, 3D objects
Plan parameters: Tx fields, tolerance tables, Rx, patient orientation, fractionation, machine ID, accessories
Data Transfer from VSim to TPSDICOM RT
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Network Transfer Glossary
• SOP: service object pair (CT -> Vsim)• SCP: Service Class Provider (eg VSim)• SCU: Service Class User (eg CT)• ftp: file transfer protocol• UID: Unique identifier (transfer session
ID)
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XiO TPS DICOM conformance statement
GE Advantage sim DICOM conformance statement
Data Transfer issues from VSim to TPSEg: Beam energy does not transfer
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• Patient orientation• Machine definition• Movable lasers• Structure
segmentation• DRR accuracy• 4DCT• SRS
VSim Acceptance Testing
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• Ensure images are transferred to VSim and TPS correctly and they display correct orientation
• Implications:• Reversed beam orientation• Contours drawn on wrong
side of patient
VSim Acceptance Testingpatient orientation
HFS
FFS
HFP
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VSim Acceptance Testingpatient orientation
Incorrect patient orientation
CT screen TPS screen
VSim Acceptance Testing
• Available beam energies• Position and limits of jaws• Collimator rotation• MLC type (52,58, 82 120, 120 HD, m3, 160 leaf)• Angle convention (IEC 601-2-1, IEC 1217 or
non-IEC)• Tolerance levels should be tight• Implications are numerous: missing or incorrect
beam parameters transferred to TPS
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machine definition
VSim Acceptance Testing
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machine definitionVSim Acceptance Testing
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machine definition
VSim Acceptance Testing
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machine definition - verification
TPS
1) Set up beams in VSim 2) Transfer to TPS3) Verify Parameters:
• Machine/energy• scale• gantry angle• rotations• field size• etc
• Mechanical system• Absolute, relative position accuracy• Scan phantom with radio-opaque
reference point and mm markings • Test system through its maximum
mechanical extents• Implications: incorrect isocenter markings
which may not be caught until film is reviewed
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VSim Acceptance Testingmovable lasers
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VSim Acceptance Testingmovable lasers
Radiopaque markers
Scales
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• Bifurcating structures or disconnected structures• Contouring in sagittal cuts• Contour interpolating• Double contours (external skin)• Copy structures, expansion, Boolean operations• Unclosed contours• Accurate volume reporting
VSim Acceptance Testingstructure delineation
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VSim Acceptance Testingstructure delineation – contour interpolation
VSim - Before interpolation VSim -After interpolation
Ensure correct position
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VSim Acceptance Testingstructure delineation – double contours
TPS – single contourTPS - Shifted external contour
axial
TPS - Shifted external contour sagittal
Does the TPS allow double contours?
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VSim Acceptance Testingstructure delineation – volume reporting
VSim
TPS
• Contour object of known dimensions in VSim
• Verify volume, dimensions
• Ensure same volume, dimensions are reported in TPS
• Set CT image level to ½ of object’s max, set window to <50
• Implication:
• incorrect DVH reporting
Incorrect windowing
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VSim Acceptance TestingDRR accuracy
Physicians rely on accurate DRRs to:
• delineate blocking
• set field size dimensions
• view organ projections
Implications:
• Under or over coverage of disease
•Inadequate sparing of healthy tissue
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VSim Acceptance TestingDRR accuracy
AP DRR on VSim workstationRadiopaque markers
Scan plate between plastic slabs
• Acquires >10 images per couch position
• Accurate tracking of motion surrogate
• Analyze motion in three planes
VSim acceptance testing4DCT
• Test to verify that total deflection of movement is being acquired by 4DCT system
• Should be done in 3 axes
• Implications: incorrect size of MIP (maximum intensity projection)
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VSim Acceptance testing4DCT
11 mm measured deflection
• Image quality– High contrast resolution– Low contrast detectability, noise
• Image fusion– Accuracy of fusion module– MRI distortion
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VSim Acceptance testingSRS/SBRT
Use of VSim in SRSImage fusion - distortion
• TG 117 (in progress): The use of MRI Data in Treatment Planning and Stereotactic Procedures – Spatial Accuracy and Quality Control Procedures
• TG132 (In progress): Use of Image Registration and Data Fusion Algorithms and Techniques in Radiotherapy Treatment Planning
• Deformable/translational or translational only
• QA of MRI to test image distortion• Evaluate fusion with MRI/CT phantom
MRI test phantom
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Use of VSim in SRSImage fusion - distortion
• Implications of image distortion:
- Incorrect anatomical rendering
Poor registration
Acceptable registration
MRI CT
Fused image sets
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