Quality Assurance for
image-guidance
technologies
J.-P. Bissonnette, D. Moseley, T.
Purdie, M. Sharpe, D. Jaffray
IGRT with Cone-beam CT
• Introduction
• Geometric QA
• Image quality
• Patterns of failure
Cone-beam CT: Infrastructure QA
• Safety
• Geometric
• System stability
• Image quality
• System infrastructure
• Dose
Image-guidance: MV Geometry
• Imaging and
treatment beams
coincide
Image-guidance: MV/kV Coincidence
• Treatment is
orthogonal to
imaging
kV/MV Calibration Concept
BB (Reconstruction
Iso-centre)
MV Mechanical
isocentre
kV
y
MV Radiation isocentre
x
Calibrated isocentre
z
Geometric Calibration
• Analogous to the Winston-Lutz test
used for brain stereotactic QA
– Lutz, Winston, & Maleki, IJROBP 14, pp. 373-81 (1988)
1. MV Localization (0o) of BB;
collimator at 0 and 90o.
2. Repeat MV Localization of BB for
gantry angles of 90o, 180o, and 270o.
3. Analyze images and adjust BB to
Treatment Isocentre (± 0.3 mm)
+1mm
θgantry
θgantry
u
-1mm
-180
v
θgantry
+180
Reconstruction
4. Measure BB Location in kV
radiographic coordinates (u,v) vs. θgantry.
5. Analysis of ‘Flex Map’ and Storage
for Future Use.
6. Employment of ‘Flex Map’ During
Routine Clinical Imaging.
MV/kV Calibration Procedure
v
u
Flexmap
• A plot of the apparent travel of a point
as a function of gantry angle.
• Removes the effect of component flexes
and torques prior to reconstructions.
• Ties the 3D image matrix to the
radiation isocentre of the accelerator.
Flexmap
Elekta
• Flexes are corrected for in
reconstruction algorithm.
Varian
• Flexes are compensated for by robotic
arm motion.
Flexmap
Elekta
+ “Burns” radiation isocentre into CBCT
dataset
- Vulnerable to file corruption
Varian
+ No software vulnerability
- Robotic arm motions need monitoring
Results for Six Units
5
4.5
4
3.5
Absolute U displacement (mm)
3
2.5
2
1.5
Unit A
1
Unit B
0.5
Unit C
0
Unit D
-0.5
Unit F
-1
Unit G
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
-5
-180
-135
-90
-45
0
Gantry angle (degrees)
45
90
135
180
Residual Error
1.50
Residual U displacement (mm)
1.25
1.00
0.75
0.50
Unit A
Unit B
Unit C
Unit D
Unit F
Unit G
0.25
0.00
-0.25
-0.50
-0.75
-1.00
-1.25
-1.50
-180
-135
-90
-45
0
45
Gantry Angle (degrees)
90
135
180
Results for Six Units
5
4.5
4
3.5
3
Absolute V displacement (mm)
2.5
2
1.5
Unit A
1
Unit B
0.5
Unit C
0
Unit D
-0.5
Unit F
-1
Unit G
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
-5
-180
-135
-90
-45
0
Gantry angle (degrees)
45
90
135
180
Residual Error
Residual V displacement (mm)
3
2
Unit A
Unit B
Unit C
Unit D
Unit F
Unit G
1
0
-1
-2
-3
-180
-135
-90
-45
0
45
Gantry Angle (degrees)
90
135
180
Long-term Stability: Flexmap
1.25
1
R e s id u a l d is p la c e m e n t (m m )
0.75
v
0.5
0.25
u
0
-0.25
-0.5
12
calibrations
over 28
months
-0.75
95% confidence
interval = 0.25 mm
-1
-1.25
-1.5
-180
-135
-90
-45
0
Gantry angle (degrees)
45
90
135
180
Effect of Incorrect Calibration
Lessons Learned: Geometric
Calibration
• Takes ~ 2 hours to perform per unit
• Flexmaps are stable on Elekta
– Accruing long term data on Varian platform
• Residual flexmaps are comparable on
both platforms
– Ultimately affects geometric accuracy of
volumetric imaging
• Considering changing test frequency
from monthly to semi-annually
Daily Geometry QA
• Align phantom with
lasers
• Acquire portal images
(AP & Lat) & assess
central axis
• Acquire CBCT
• Difference between
predicted couch
displacements (MV &
kV) should be < 2 mm
http://www.modusmed.com/igrt.htm
Daily Geometry QA
• Align phantom with
lasers
• Acquire portal images
(AP & Lat) & assess
central axis
• Acquire CBCT
• Difference between
predicted couch
displacements (MV &
kV) should be < 2 mm
1. Shift BB embedded in cube from
isocentre.
2. MV Localization of BB for gantry angles
of 0o and 90o.
θgantry
Reconstruction
3. kV Localization with cone-beam CT
4. Compare kV and MV localizations;
tolerance is ± 2 mm
5. Use automatic couch to place BB to
isocentre; verify shift with imaging
Compare Portal Image & DRR
0.25
X
Y
Z
Deviation from isocentre (cm)
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
A
B
C
Unit
D
G
Lessons Learned: Daily QA
• In a single set-up, therapists:
– Warm-up tube
– Software/hardware integrity
– Sufficient disk/storage space
– Lasers, ODI, field size indicators, etc.
– QC of image-guided process
• Takes ~ 20 mins in the morning
Cone-beam CT: QA of a Device
• Safety
• Geometric
• System stability
• Image quality
• System infrastructure
• Dose
Image Quality
CatPhan 500
phantom
Scale
• Geometric
calibration to tie
isocentre to centre
of volumetric
reconstruction
• Scale to relate all
pixels to isocentre
5 cm
Scale
• Geometric
calibration to tie
isocentre to centre
of volumetric
reconstruction
• Scale to relate all
pixels to isocentre
Linearity of CT Numbers
Linearity of CT Numbers
2000
1800
Measured Hounsfield unit
1600
1400
Unit 7
Unit 8
1200
Unit 9
Unit 10
1000
Unit 12
Unit 16
800
Unit 16 with annulus
Unit 17
600
400
200
0
0
200
400
600
800
1000
1200
Theoretical Housfield unit
1400
1600
1800
2000
Linearity of CT Numbers
Mean Elekta
Mean Varian
Identity
2000
1800
Measured Housfield Units
1600
1400
1200
1000
800
600
400
200
0
0
200
400
600
800
1000
1200
Theoretical Housfield Units
1400
1600
1800
2000
Add Scatter
IEC standard 61675-1
Linearity of CT Numbers
Mean Varian
Identity
Varian with annulus
2000
1800
Measured Housfield Units
1600
1400
1200
1000
800
600
400
200
0
0
200
400
600
800
1000
1200
Theoretical Housfield Units
1400
1600
1800
2000
Linearity of CT Numbers
• Fairly linear (χ2 > 0.99) for all systems
• Beam hardening
• Scatter conditions
• Non-standard metric; use only as a
baseline
Spatial Resolution
1.2
Unit A
Unit B
Unit C
Unit D
Unit E
Unit F
Unit G
Unit H
Unit I
Unit J
1.0
MTF
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
-1
Spatial frequency (cm )
Droege, Radiology 146, pp. 244-246 (1983)
10
Spatial Resolution
1.2
Mean Elekta
Mean Varian
1.0
1.4 mm
MTF
0.8
0.6
1.2 mm
0.4
0.2
0.0
0
2
4
6
8
-1
Spatial frequency (cm )
10
12
Effect of Scatter on MTF
Acceptance
Narrow x-ray field
Window & level identical
Spatial Resolution
1.2
Mean Elekta
Mean Varian
Unit E small field
1.0
0.8 mm
MTF
0.8
0.6
1.2 mm
0.4
0.2
0.0
0
2
4
6
8
-1
Spatial frequency (cm )
10
12
Image Quality: Lessons Learned
• Scale is accurate
• CBCT affected by beam hardening and
scatter
– CT linearity and MTF curves differ for
individual units
– Reasonable changes in scatter conditions
affect curves
• Track as baseline; CBCT not ready for
quantitative CT.
Patterns of Failure: Truncation
Truncation artefact, Zhang, IJROBP 2005
Patterns of Failure: Ring Artefacts
Patterns of Failure: Capping Artefact
Patterns of Failure: Streaking
Patterns of Failure: Motion Artefacts
Conclusions
• Geometric calibration can be within 0.25
mm
• Daily QA tolerance ± 2 mm
• Value if image quality QA?
– Debatable, except for scale
– Stick to baseline values for now – should
improve in the future
Conclusions
• Spatial resolution, contrast & uniformity
– Depend on unit and imaging conditions
• Scatter environment
• Imaging technique
• Beam hardening
• Artefacts can hint at defective
components or reveal limits of the
CBCT physics
Daily CBCT QA Program
Dimension
Procedure
Tolerance
Detector stability
Dark image calibration
Geometry
Localising lasers
< 1 mm
MV/kV/laser alignment
± 2 mm
Accuracy of shifts
± 2 mm
Safety
Warm-up
Interlocks: interrupts or
prevents irradiation
Functional
Warning lights
Functional
Generator operation
Functional
Detector operation
Functional
Detector signal
Collimator operational
Clinical process issues
Database integrity
Storage space
availability
Within expected range
Functional
Monthly CBCT QA Program
Dimension
Procedure
Tolerance
Imaging system
Gain stability
Replace or refresh
performance
Defect maps
Replace or refresh
Image quality
Scale and distances
± 0.5 mm
CT number linearity &
stability
Baseline
Image uniformity
Baseline
High contrast spatial
resolution
Baseline
Artefacts
Absence
Geometric
Geometric calibration
Accuracy of couch
shifts
Clinical process issues
Review of daily test
results
Replace / refresh
< 1 mm
Annual CBCT QA Program (service)
Dimension
Procedure
Tolerance
X-ray generator
kVp accuracy
Baseline
stability
mAs linearity
Baseline
Radiation quality (HVL)
Baseline
Accuracy of mA & mAs
Baseline
Geometry
Couch scales
1 mm
Couch motion accuracy
(manual or remote)
1 mm
Detector tilt
Baseline
Detector skew
Baseline
Detector scale
Baseline
Annual CBCT QA Program
(upgrades)
Dimension
Data transfer
Procedure
Link to treatment
planning
Tolerance
Functional and
accurate
Long term and short
term storage
Functional
Dosimetry
Axial and skin doses
Baseline
Clinical process issues
Database integrity and
maintenance
Baseline
Documentation of
imaging procedure
Up-to-date
Review of daily and
monthly test results
Completeness