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