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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
6
• 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
7
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
8
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
9
• 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