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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• Low likelihood of VSim parameters changing over time

• Yearly: repeat acceptance tests

• Focus on testing after upgrades and external system changes

VSim Periodic QA

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Questions?

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Next Talk:CT Sim: Electromechanical, image

qualityGuru Prasad