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Quality Assurance for Quality Assurance for imageimage--guidance guidance
technologiestechnologiesJ.-P. Bissonnette, D. Moseley, T.
Purdie, M. Sharpe, D. Jaffray
IGRT with ConeIGRT with Cone--beam CTbeam CT
• Introduction
• Geometric QA
• Image quality
• Patterns of failure
ConeCone--beam CT: Infrastructure QAbeam CT: Infrastructure QA
• Safety
• Geometric
• System stability
• Image quality
• System infrastructure
• Dose
ImageImage--guidance: MV Geometryguidance: MV Geometry
• Imaging and treatment beams coincide
ImageImage--guidance: MV/kV Coincidenceguidance: MV/kV Coincidence
• Treatment is orthogonal to imaging
kV/MV Calibration ConceptkV/MV Calibration Concept
kV
BB (Reconstruction Iso-centre)
MV Mechanical isocentre
x
y
z
MV Radiation isocentre
Calibrated isocentre
Geometric CalibrationGeometric Calibration
• Analogous to the Winston-Lutz test used for brain stereotactic QA– Lutz, Winston, & Maleki, IJROBP 14, pp. 373-81 (1988)
θgantry
4. Measure BB Location in kV radiographic coordinates (u,v) vs. θgantry.
+180θgantry-180
uv
5. Analysis of ‘Flex Map’ and Storage for Future Use.
+1mm
-1mm
Reconstruction
θgantry
6. Employment of ‘Flex Map’ During Routine Clinical Imaging.
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)
MV/kV Calibration Procedure MV/kV Calibration Procedure
u
v
FlexmapFlexmap
• 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.
FlexmapFlexmap
Elekta• Flexes are corrected for in
reconstruction algorithm.
Varian• Flexes are compensated for by robotic
arm motion.
FlexmapFlexmap
Elekta+ “Burns” radiation isocentre into CBCT
dataset- Vulnerable to file corruption
Varian+ No software vulnerability- Robotic arm motions need monitoring
Results for Six UnitsResults for Six Units
-5-4.5
-4-3.5
-3-2.5
-2-1.5
-1-0.5
00.5
11.5
22.5
33.5
44.5
5
-180 -135 -90 -45 0 45 90 135 180
Gantry angle (degrees)
Abs
olut
e U
dis
plac
emen
t (m
m)
Unit AUnit BUnit CUnit DUnit FUnit G
Residual ErrorResidual Error
-180 -135 -90 -45 0 45 90 135 180-1.50
-1.25
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
1.25
1.50
R
esid
ual U
dis
plac
emen
t (m
m)
Gantry Angle (degrees)
Unit A Unit B Unit C Unit D Unit F Unit G
Results for Six UnitsResults for Six Units
-5-4.5
-4-3.5
-3-2.5
-2-1.5
-1-0.5
00.5
11.5
22.5
33.5
44.5
5
-180 -135 -90 -45 0 45 90 135 180
Gantry angle (degrees)
Abso
lute
V d
ispl
acem
ent (
mm
)
Unit AUnit BUnit CUnit DUnit FUnit G
Residual ErrorResidual Error
-180 -135 -90 -45 0 45 90 135 180
-3
-2
-1
0
1
2
3
R
esid
ual V
dis
plac
emen
t (m
m)
Gantry Angle (degrees)
Unit A Unit B Unit C Unit D Unit F Unit G
LongLong--term Stability: Flexmapterm Stability: Flexmap
12 calibrations over 28 months
u
v
-1.5
-1.25
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
-180 -135 -90 -45 0 45 90 135 180
Gantry angle (degrees)
Resi
dual
dis
plac
emen
t (m
m)
95% confidence interval = 0.25 mm
Effect of Incorrect CalibrationEffect of Incorrect Calibration
Lessons Learned: Geometric Lessons Learned: Geometric CalibrationCalibration
• 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 QADaily 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 QADaily 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
θgantry
3. kV Localization with cone-beam CT
1. Shift BB embedded in cube from isocentre.
2. MV Localization of BB for gantry angles of 0o and 90o.
Reconstruction
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 & DRRCompare Portal Image & DRR
A B C D G-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
X Y Z
Dev
iatio
n fr
om is
ocen
tre
(cm
)
Unit
Lessons Learned: Daily QALessons 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
ConeCone--beam CT: QA of a Devicebeam CT: QA of a Device
• Safety
• Geometric
• System stability
• Image quality
• System infrastructure
• Dose
Image QualityImage Quality
CatPhan 500 phantom
ScaleScale
• Geometric calibration to tie isocentre to centre of volumetric reconstruction
• Scale to relate all pixels to isocentre
5 cm
ScaleScale
• Geometric calibration to tie isocentre to centre of volumetric reconstruction
• Scale to relate all pixels to isocentre
Linearity of CT NumbersLinearity of CT Numbers
Linearity of CT NumbersLinearity of CT Numbers
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Theoretical Housfield unit
Mea
sure
d Ho
unsf
ield
uni
t Unit 7Unit 8Unit 9Unit 10Unit 12Unit 16Unit 16 with annulusUnit 17
Linearity of CT NumbersLinearity of CT Numbers
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Theoretical Housfield Units
Mea
sure
d Ho
usfie
ld U
nits
Mean Elekta Mean Varian Identity
Add ScatterAdd Scatter
IEC standard 61675-1
Linearity of CT NumbersLinearity of CT Numbers
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Theoretical Housfield Units
Mea
sure
d Ho
usfie
ld U
nits
Mean Varian Identity Varian with annulus
Linearity of CT NumbersLinearity of CT Numbers
• Fairly linear (χ2 > 0.99) for all systems
• Beam hardening
• Scatter conditions
• Non-standard metric; use only as a baseline
Spatial ResolutionSpatial Resolution
0 2 4 6 8 100.0
0.2
0.4
0.6
0.8
1.0
1.2
MTF
Spatial frequency (cm-1)
Unit A Unit B Unit C Unit D Unit E Unit F Unit G Unit H Unit I Unit J
Droege, Radiology 146, pp. 244-246 (1983)
Spatial ResolutionSpatial Resolution
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
1.2
MTF
Spatial frequency (cm-1)
Mean Elekta Mean Varian
1.2 mm
1.4 mm
Effect of Scatter on MTFEffect of Scatter on MTF
Window & level identical
Acceptance Narrow x-ray field
0.8 mm
Spatial ResolutionSpatial Resolution
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
1.2
MTF
Spatial frequency (cm-1)
Mean Elekta Mean Varian Unit E small field
1.2 mm
Image Quality: Lessons LearnedImage 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: TruncationPatterns of Failure: Truncation
Truncation artefact, Zhang, IJROBP 2005
Patterns of Failure: Ring ArtefactsPatterns of Failure: Ring Artefacts
Patterns of Failure: Capping ArtefactPatterns of Failure: Capping Artefact
Patterns of Failure: StreakingPatterns of Failure: Streaking
Patterns of Failure: Motion ArtefactsPatterns of Failure: Motion Artefacts
ConclusionsConclusions
• 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
ConclusionsConclusions
• 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 ProgramDaily CBCT QA ProgramDimension Procedure Tolerance
Detector stability Dark image calibrationGeometry Localising lasers < 1 mm
MV/kV/laser alignment ± 2 mmAccuracy of shifts ± 2 mm
Safety Interlocks: interrupts or prevents irradiation
Functional
Warning lights FunctionalWarm-up Generator operation Functional
Detector operation FunctionalDetector signal Within expected rangeCollimator operational Functional
Clinical process issues Database integrityStorage space availability
Monthly CBCT QA ProgramMonthly CBCT QA ProgramDimension Procedure Tolerance
Imaging system Gain stability Replace or refreshperformance Defect maps Replace or refreshImage quality Scale and distances ± 0.5 mm
CT number linearity & stability
Baseline
Image uniformity BaselineHigh contrast spatial resolution
Baseline
Artefacts AbsenceGeometric Geometric calibration Replace / refresh
Accuracy of couch shifts
< 1 mm
Clinical process issues Review of daily test results
Annual CBCT QA Program (service)Annual CBCT QA Program (service)Dimension Procedure Tolerance
X-ray generator kVp accuracy Baselinestability mAs linearity Baseline
Radiation quality (HVL) BaselineAccuracy of mA & mAs Baseline
Geometry Couch scales 1 mmCouch motion accuracy (manual or remote)
1 mm
Detector tilt BaselineDetector skew BaselineDetector scale Baseline
Annual CBCT QA Program Annual CBCT QA Program (upgrades)(upgrades)
Dimension Procedure ToleranceData transfer Link to treatment
planningFunctional and
accurateLong term and short term storage
Functional
Dosimetry Axial and skin doses BaselineClinical process issues Database integrity and
maintenance Baseline
Documentation of imaging procedure
Up-to-date
Review of daily and monthly test results
Completeness