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Daily kV Localization:Factors Affecting Image Quality

Daily kV Localization:Factors Affecting Image Quality

J. H. Siewerdsen, Ph.D.

University Health Network

Princess Margaret HospitalOntario Cancer Institute, Princess Margaret Hospital

University of TorontoDept. of Medical Biophysics

Dept. of Radiation OncologyDept. of Otolaryngology – Head and Neck Surgery

Institute for Biomaterials and Biomedical EngineeringInstitute of Medical Science

University of Toronto

Factors Affecting Image Quality

2Dor3D

SpatialResolution

ImageNoise

IQMetrics Artifacts

Modulation

Transfer

Function

(MTF)Noise-Power

Spectrum

(NPS)

Pixel Noise

Voxel Noise

“Contrast”

or CNR

Noise-Equivalent

Quanta

(NEQ)

Detectability

ROC

Pixel Defects

X-Ray ScatterRings

Cupping

Truncation

DosePixel Size

Voxel Size

Motion

Recon Filter

• Factors affecting spatial resolution– Focal spot size– Detector configuration

• X-ray converter• Pixel pitch

– System geometry• Magnification

– Recon parameters• Recon filter• Voxel size

Spatial Resolution

Converter

Pixel Matrixapix

Voxel Matrix

apixavox~ M

2

FW

HM

(mm

)

Voxel Size (mm)

0

0.5

1

1.5

2

2.5

3

3.5

0.8 0.6 0.4 0.2 0.1 (mm)1/8 ¼ ½ Full Res

‘Resolution Length’(FWHM of the PSF)

FW

HM

(mm

)

Sm

oo

th

Sh

arp

Axial Stapes Crura

Effect of Reconstruction Filter

Modulation Transfer Function (MTF)

JJJJJJJJJJJJJJJJJJJJJJJJJJ

JJ

JJ

JJ

JJ

JJ

JJ

JJ

JJ

JJ

JJ

JJ

JJ

JJJJ

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0

SpatialFrequency(mm-1)

Steel Wire

Measured

System MTF

( ) ( )[ ],, yxLSFFTffMTF yx =

x (mm) y (mm)

Sig

nal(m

m-1

)

127 µm Wire in H2O

Image Noise

Noise is characterized by its:Magnitude (standard deviation)Spatial-frequency content (‘texture’)

3

• CT image noise depends on– Dose– Detector efficiency– Voxel size

• Axial, axy

• Slice thickness, az

– Reconstruction filter

Barrett, Gordon, and Hershel (1976)

zxy

xy

o

E

aa

K

D

k 13

2

ησ =

oD

1∝σ 3

1

xya∝

za

1∝

Image Noise Image Noise

Sm

oo

th

Sh

arp

X

ba +~σ

0

10

20

30

40

50

60

0 0.5 1.0 1.5 2.0 2.5 3.0Dose (mGy)

Nois

e(C

T#

)

Benchtop CBCT Scanner(1998)

Dose Reconstruction Filter

( )[ ]N2

ixdFTNPS ∆≡

Noise-Power SpectrumAxial Plane (x,y)

• The NPS describes– Noise magnitude

– Frequency content

( )∫∫∫= zyxzyx dfdfdffffNPS ,,2σ

S(fx, fy)Sagittal Plane (x,z)

S(fx, fz)

Noise-Power Spectrum

4

Image Quality

Imaging performance depends on:Spatial resolution and noise (MTF and NPS)Structure of interest (task)Artifacts

∫∫∫=Nyq

fdd '( )

( )fNPS

fMTF2 ( )fWtask2

DetectorConfigurationReconstruction Filter

Voxel SizeDose

Noise-EquivalentQuanta(NEQ)

“Frequencies of Interest”ObjectsizeObjectcontrast

Task(Template)

Figure of Merit: Detectability

Reconstruction Filter (hwin)

Dete

ctabili

tyIn

dex

,d’

Detection of a Point Object (δ-function)

Detection of a Gaussian Sphere

Smooth Sharp

Figure of Merit: DetectabilitySoft-Tissue Visualization

Variable

Contr

ast (10–100) HU

12.7 mm

Variable

Dia

met

er 88 HU

(2–13) mm

170 mAs

0.6 mGy0.02 mSv

2.9 mGy0.1 mSv

9.6 mGy0.35 mSv

23.3 mGy0.8 mSv

Image Quality

5

Bony Visualization

50 mAs

Para

nasa

lSin

use

sSku

llBase

0.6 mGy0.02 mSv

2.9 mGy0.1 mSv

9.6 mGy0.35 mSv

23.3 mGy0.8 mSv

Image Quality Plane of VisualizationAxial (x,y) Sagittal (x,z)

HanningFilter

(“Smooth”)

Ram-LakFilter

(“Sharp”)

Artifacts

Rings Shading

Lag

Motion

Metal

Streaks

“Cone-Beam”Truncation

A big problem for cone-beam CT:SPR is very large for large cone angles (i.e., large FOV)

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0 20 40 60 80 100 120 140

Con

tras

t(m

m-1

)

SPR (%)

BR-SR1 Breastin Water

+

++=−

SPR1SPR1

ln1

'0 dC

oe

dCC

α

α

Artifacts: X-ray Scatter

1) Image artifactsCupping and streaks

2) Reduced contrastReduction of ∆CT#

3) Increased image noiseReduced DQE

Reduced soft-tissue detectability

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• Select– Good geometry (gap)– Limit FOVz to volume of interest

• Reject– Antiscatter grids– Bowtie filter

• Correct– Estimate and subtract φscatter(u,v)– Measurement and modeling

Managing Scatter

M*~1.5

1

10

100

0 5 10 15 20 25

SPR

(%)

Field of View (z) (cm)

Pelvis

Abdomen

Chest

Extremity

Air

• Select– Good geometry (gap)– Limit FOVz to volume of interest

• Reject– Antiscatter grids– Bowtie filter

• Correct– Estimate and subtract φscatter(u,v)– Measurement and modeling

No Grid

6:1 GR

8:1 GR

10:1 GR

12:1 GR

No Grid 10:1 Grid

Managing Scatter

• Select– Good geometry (gap)– Limit FOVz to volume of interest

• Reject– Antiscatter grids– Bowtie filter

• Correct– Estimate and subtract φscatter(u,v)– Measurement and modeling

400

600

200

800

0 50 150 250 350

1000

Distance along profile

(HU)

(HU)

0 50 150 250 350

200

400

600

800

1000

Distance along profile

Bowtie FilterNo Bowtie

HU

Managing Scatter

• Select– Good geometry (gap)– Limit FOVz to volume of interest

• Reject– Antiscatter grids– Bowtie filter

• Correct– Estimate and subtract φscatter(u,v)– Measurement and modeling

No

Corr

ection

Corr

ecte

d

Managing Scatter

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Pop-Quiz (1 of 2)• Which of the following is NOT associated

with the use of a bow-tie filter in cone-beam CT?

(a) Improved spatial resolution

(b) Improved contrast

(c) Reduced dose to the patient

(d) Reduced x-ray scatter artifacts

Pop-Quiz (1 of 2)

Which of the following is NOT associated with the use of a bow-tie filter in cone-

beam CT?

8%

33%

5%

54% (a) Improved spatial resolution

(b) Improved contrast

(c) Reduced dose to the patient

(d) Reduced x-ray scatter artifacts

Pop-Quiz (2 of 2)• CBCT image noise (standard deviation)

exhibits which of the following dependencies?

(a)

(b)

(c)

(d) is independent of filter selection

(e)

Dose1∝σ

ThicknessSlice1∝σ

sprojectionN∝σ

SPR∝σ

σ

Pop-Quiz (2 of 2)CBCT image noise (standard deviation)

exhibits which of the following dependencies?

Dose1∝σ

ThicknessSlice1∝σ

sprojectionN∝σ

SPR∝σ

(a) A.

(b) A.(c) .A(d) is independent of filter selection

(e). A

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Take-Home Points

SimpleScalar Metric

Fourier-Based

FWHM of theSpread Function

Standard Deviationin Pixel Values

ContrastCNR

MTF NPS NEQand Task

Spatial Resolution Image Noise Image Quality

Artifacts X-ray scatter:- Often a major factor in CBCT image quality- Artifacts, contrast reduction, and noise- Select / Reject / Correct

Other artifacts:- Object motion- Lateral truncation- Rings, beam hardening, cone-beam artifacts, …

Pop-Quiz (1 of 2)• Which of the following is NOT a factor in

the spatial resolution of CT images?

(a) Dose

(b) System geometry

(c) X-ray converter type and thickness

(d) Reconstruction filter

The 3-D Noise-Power Spectrum

NPS(fx, fy, fz)

•Transversedomain:“Filtered-ramp”GreenNPS

•Axial domain:“Band-limited”RedNPS

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• Calculation of dose

• Measurement of dose– Cylindrical phantoms (16 cm ‘Head’ or 32 cm ‘Body’)– Calibrated Farmer ionization chamber

Radiation Dose

SFefproj

mAsN

mAs

mRD R

oprojooµ−

=

Tube Output(measured) Total mAs

f-factor(cGy/R)

Attenuation

Scatter(measured

or MC)

Radiation Dose

DosimetryPhantom

PancakeDetector Farmer

ChamberC

A

B

D

Radiation Dose

C

A

BA B C D

0.00

0.04

0.08

0.12

0.16

0.20

"Tube-Under""Tube-Over"

Dose

(mG

y/m

As)

Soft-tissue: ~200 mAsBone: ~50 mAs

~12 mGy~3 mGy

D

Radiation Dose