Virtual CT Simulator QA – 2
topics Electromechanical
Virtual Simulator CT QA
Virtual CT simulation software
Performance and
Image quality
Alex Markovic, PhD
Director, Medical Radiation Physics Program,
Rosalind Franklin University, North Chicago, IL
Lead Therapeutic Physicist,
NorthShore University Health System, Evanston, IL
CT operator console
Alex
Prasad
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Original Simulators
What is Virtual CT Simulator
(VSim)?
TG66:Quality assurance for computed-tomography
simulators and the computed tomographysimulation process
X ray tube
“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.”
Image intensifier
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VSim Software Features
Examples of VSim software
packages:
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• Varian SomaVision
• GE Advantage SIM
• Elekta Group - CMS
Focal
• Phillips AcQSim3
• Siemens syngo RT
Oncologist
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Phillips
AcQSim
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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
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Relevant Guidance Documents
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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• 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
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Treatment Planning
Why Perform QA on the VSim?
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VSim Quality Management
Program
Acceptance testing - manufacturer procedure , if it
exists
• Ensure data to and from VSim
transfers correctly
• Reduce possibility of treatment errors
due to inaccurate transfer of data
• Ensure complete dataset is sent
• 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 from CT to VSim
Node Setup
VSim IP address
Data transfer
VSim AE Title
CT scanner DICOM export configuration screen
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Data Transfer Issues
Data Transfer from VSim to TPS
trouble in paradise
DICOM RT
Information standard to move images, etc
Images: CTs, DRRs
• 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!
Structure sets: Points of interest, bolus, 3D objects
Plan parameters: Tx fields, tolerance tables, Rx,
patient orientation, fractionation, machine ID,
accessories
VSim hosts file
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Data Transfer issues from VSim
TPS
Eg: Beam to
energy
does not transfer
Network Transfer Glossary
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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)
GE Advantage sim DICOM conformance
statement
XiO TPS DICOM conformance
statement
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VSim Acceptance Testing
VSim Acceptance Testing
patient orientation
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Patient orientation
Machine definition
Movable lasers
Structure
segmentation
• DRR accuracy
• 4DCT
• SRS
HFS
• 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
FFS
HFP
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VSim Acceptance Testing
VSim Acceptance Testing
patient orientation
machine definition
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CT screen
TPS screen
Incorrect patient orientation
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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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VSim Acceptance Testing
VSim Acceptance Testing
machine definition
machine definition
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VSim Acceptance Testing
VSim Acceptance Testing
machine definition - verification
movable lasers
• 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
1) Set up beams in VSim
2) Transfer to TPS
3) Verify Parameters:
• Machine/energy
• scale
• gantry angle
• rotations
• field size
• etc
TPS
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VSim Acceptance Testing
VSim Acceptance Testing
movable lasers
structure delineation
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Scales
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
Radiopaque markers
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VSim Acceptance Testing
VSim Acceptance Testing
structure delineation – contour interpolation
structure delineation – double contours
Ensure correct
position
TPS – single contour
Does the TPS allow
double contours?
VSim - Before interpolation
TPS - Shifted external contour
axial
TPS - Shifted external contour
sagittal
VSim -After interpolation
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VSim Acceptance Testing
VSim Acceptance Testing
structure delineation – volume reporting
DRR accuracy
• Contour object of known
dimensions in VSim
Physicians rely on accurate DRRs
to:
VSim
• Verify volume, dimensions
•
delineate blocking
• Ensure same volume,
dimensions are reported in
TPS
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set field size dimensions
•
view organ projections
Incorrect
windowing
• Set CT image level to ½ of
object’s max, set window to
<50
Implications:
• Under or over coverage of
disease
TPS
•Inadequate sparing of healthy
tissue
• Implication:
• incorrect DVH reporting
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VSim Acceptance Testing
VSim acceptance testing
DRR accuracy
4DCT
Scan plate between plastic slabs
• Acquires >10 images per
couch position
• Accurate tracking of motion
surrogate
• Analyze motion in three
planes
Radiopaque markers
AP DRR on VSim workstation
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VSim Acceptance testing
VSim Acceptance testing
4DCT
SRS/SBRT
• 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)
11 mm
measure
d
deflection
• Image quality
– High contrast resolution
– Low contrast detectability, noise
• Image fusion
– Accuracy of fusion module
– MRI distortion
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Use of VSim in SRS
Use of VSim in SRS
Image fusion - distortion
Image 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
• Implications of image
distortion:
- Incorrect anatomical
rendering
Acceptable registration
MRI
CT
MRI test phantom
Poor registration
Fused image sets
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VSim Periodic QA
Questions?
• Low likelihood of VSim
parameters changing over
time
• Yearly: repeat acceptance
tests
• Focus on testing after
upgrades and external system
changes
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Next Talk:
CT Sim: Electromechanical, image
quality
Guru Prasad
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