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Computed Tomography IIComputed Tomography IIComputed Tomography IIC-Arm Cone-Beam CT:
Computed Tomography IIC-Arm Cone-Beam CT:
Principles and ApplicationsPrinciples and Applications
Jeff Siewerdsen1 and Guang-Hong Chen21. Department of Biomedical Engineering, Johns Hopkins University
2. Department of Medical Physics, University of Wisconsin
Johns Hopkins UniversitySchools of Medicine and Engineering
University of WisconsinInstitutes for Medical Research
Overview
Part 1: (Siewerdsen)- Cone-beam CT image quality- Radiation doseRadiation dose- Applications (non-vascular)- Sustained applause
Part 2: (Chen)- 3D CBCT reconstruction- Artifacts- Artifacts- Applications (cardiovascular)- Thunderous ovation
Not Your Mamas C-ArmNot Your Mamas C-ArmSome Essential Science and Practicalities
for the New Generation of Cone-Beam CT-Capable CsSome Essential Science and Practicalities
for the New Generation of Cone-Beam CT-Capable Cs
Jeff Siewerdsen, PhDDepartment of Biomedical EngineeringDepartment of Biomedical Engineering
Johns Hopkins University
Johns Hopkins UniversitySchools of Medicine and Engineering
The New C-Arm
Fluoroscopy + Cone-Beam CT3D imaging capabilit- 3D imaging capability
3D filtered backprojection (FDK)FOV ~(20x20x20) cm3from a single half-rotationg
Flat-Panel Detector- Replacement to XRII
Larger FOVLarger FOVBetter 2D image qualityDistortionless
- High-performance CBCTHigh performance CBCTSub-mm spatial resolutionSoft-tissue visibility
C-Arms for IGIKey Characteristics Real-time
(or near-real-time)(or near real time)
Radiation dose~1/10 1/2 of Dx CT
Sub-mm resolution
Soft-tissue visibility
Mobile Isocentric C-Arm
Siemens PowerMobil
MotorizedOrbit Replace XRII withFlat-Panel Detector
GeometricControl System
Calibration
Tube + CollimatorModification (FOV)
Image Acquisition3D Reconstruction
Cone-Beam CT
Projection data Volume reconstructionjMultiple projections
over ~180oSub-mm spatial resolution
+ soft tissue visibility
Image Quality:Key Characteristics
Large volumetric FOV Single orbit about the patient
Sub-Millimeter Spatial Resolution Sub Millimeter Spatial Resolution Soft-Tissue Visibility
Image Quality
C-arm System Parameters Key Image Quality Metrics C-arm System Parameters- System configuration
Geometry, grid, bowtieFPD readout mode
- Geometric calibration
- Image uniformity / stationarityShading, view aliasing
- CT # accuracyHU calibration, shading artifacts
- System configurationGeometry, grid, bowtieFPD readout mode
- Geometric calibrationMechanical flex, reproducibilityDegrees of freedom
- Acquisition parametersNumber of projections
HU calibration, shading artifacts- Spatial resolution
LP/mm, FWHM wire, MTF- Contrast
Signal difference (HU) SDNR
Mechanical flex, reproducibilityDegrees of freedom
- Acquisition parametersNumber of projectionsNumber of projectionskVp, mAsDose
- Reconstruction parametersReconstruction filter
Signal difference (HU), SDNR- Noise
Voxel noise, NPS- SNR
N i i l t t (NEQ)
Number of projectionskVp, mAsDose
- Reconstruction parametersReconstruction filterReconstruction filterVoxel size (axy and az)2D/3D sampling
Noise equivalent quanta (NEQ)- Artifacts
Truncation, scatter, metal, etc.
Reconstruction filterVoxel size (axy and az)2D/3D sampling
Cone-Beam GeometryS t t di t t d b th li tiSystem geometry dictated by the application
Geometry affects every aspect of image quality
Uniformity / Stationarity
Signal Uniformity- Stationarity of the mean
(3.8 4.2)
Shading artifactsBeam-hardeningTruncation
(4.6 3.2)(5.6 2.4) (-1.3 6.2)
HU = (4.6-1.3) HU= 3.3 HU
Noise Uniformity- Stationarity of the noise- WSS of second-order statistics
Physical effects:
(4.6 3.2)( ) ( )
(4.4 4.2)
0.20
Physical effects:Quantum noiseBowtie filter
Sampling effects:Intrinsic to FBP
SPR ~0
al (
/mm
)
Intrinsic to FBPNumber of projectionsView aliasing
Mea
n S
igna
SPR ~100%
0.00
Distance (mm)-10 0 +10
Uniformity / Stationarity2
Signal Uniformity- Stationarity of the mean
Variance Maps 2(x,y)2
(/mm)2
Shading artifactsBeam-hardeningTruncation
Noise Uniformity- Stationarity of the noise- WSS of second-order statistics
Physical effects:
Water CylinderCylinder + Bowtie
Physical effects:Quantum noiseBowtie filter
Sampling effects:Intrinsic to FBP Water Cylinder
Cylinder + Bowtie
aria
nce
Intrinsic to FBPNumber of projectionsView aliasing
Air
Air
Va
-10 0 +10Distance (mm)
10 0 +10
Spatial Resolution
Factors affecting spatial resolution Focal spot sizeFocal spot size System geometry
Magnification Detector configuration
X-ray converterSAD
Pixel pitch Recon parameters
Recon filterSDD
Recon filter Voxel size
Spatial Resolution
Factors affecting spatial resolution Focal spot sizeFocal spot size System geometry
Magnification
SAD Detector configuration X-ray converter
SDD
Pixel pitch Recon parameters
Recon filter Recon filter Voxel size
C tConverter
Pixel Matrixapix
Spatial Resolution
Factors affecting spatial resolution Focal spot sizeFocal spot size System geometry
Magnification Detector configuration
X-ray converter Pixel pitch
Recon parametersRecon filter Recon filter
Voxel size
Spatial Resolution( H f h PS )(FWHM of the PSF)
m)
HM
(mm
FWH
Sm
ooth
Shar
p
S S
Filter Param (hwin)
Spatial Resolution(li i )(line-pairs per mm)
Minimum resolvableline-pair group
Spatial Resolution( d l i T f i )
127 m Wire in H2O
(Modulation Transfer Function)
JJ
JJ
JJ
JJ
JJJJ
0.8
1.0
Steel Wire
-1)
2
JJ
JJ
JJ
JJ
JJ
0.4
0.6System MTF
nal
(m
m
JJJJJJJ
JJJJ
JJJJ
JJJJ
JJJ
JJ
J
0.2
0.4
Measured
Sig
JJJJJJJJJJJ
J
0.00.0 0.5 1.0 1.5 2.0
Spatial Frequency (mm-1)
( ) ( )[ ] ,, yxLSFFTffMTF yx =
Spatial Resolution
Axial Stapes Crura
Image Noise
CT image noise depends on Dose Detector efficiency
V l i Voxel size Axial, axy Slice thickness a Slice thickness, az
Reconstruction filter
Barrett, Gordon, and Hershel (1976)
Image Noise
Dose Reconstruction Filter
Xba +~50
60
) X
30
40
se (
CT#
)
Sm
ooth
Shar
p
10
20Nois
S S
00 0.5 1.0 1.5 2.0 2.5 3.0
Dose (mGy)Dose (mGy)
Noise-Power Spectrum
The NPS describes Frequency content of the noise:
Magnitude of the noise: Magnitude of the noise:
Noise-Power Spectrum
Axial Plane (x,y)S(f f )
Axial NPSS(fx, fy)
m3 )
0.4 mAs1 mAs2 mAs
NPS
(2 m
m 2 mAs4 mAs
N
Spatial Frequency, fx (mm-1)y fx
Noise-Power Spectrum
Sagittal Plane (x,z)
S(f f )Sagittal NPS
S(fx, fz)0.4 mAs
1 mAs
S (
2 mm
3 ) 2 mAs4 mAs
NP
Spatial Frequency, fz (mm-1)
Noise-Power Spectrum
NPS(fx, fy, fz)
Transverse domain:Filtered-rampGreen NPS
Axial domain:Band-limitedRed NPSRed NPS
Contrast
A large-area transfer characteristicDefined:Defined:
As an absolute difference in mean pixel values:ROI #1
ROI #2For example:C |0 18 cm-1 0 20 cm-1|C = |0.18 cm-1 0.20 cm-1|
= 0.02 cm-2orC = |-100 HU 0 HU|
100 HU
As a relative difference in mean pixel values:
= 100 HU
For example:C = |0.18 cm-1 0.20 cm-1|
0.19 cm-1~ 10% ~ 10%
Signal Difference-to-Noise Ratio
3.5100 kV 103 HU
23.3 mGySoft-Tissue-Simulating Spheres
2.53.0 100 kVp
88 HU
103 HU
1.52.0
CN
R
66 HU
9.6 mGy
0 51.0.C
45 HU
25 HU
y
0.00.5
11 HU
22 HU2.9 mGy
0 5 10 15 20 25Dose to Isocenter (mGy)
0.6 mGy
3D NEQ and DQENEQEffective number of quanta
DQE
Fraction of quanta used at each eachused at each spatial frequency(Efficiency x Fluence)
Fraction of quanta used at each each frequency.
Observations:3D DQE(0) ~ Projection DQE(0)
(f) d d i3D DQE(f) dependent on reconstruction parameters
3D NEQ
Axial NEQ4 mAs2 mAs1 mAs
Axial NEQ
mm
2 )
y(m
m-1
)
1 mAs0.4 mAs
hoto
ns/m
uenc
y, f y
NEQ
(ph
tial F
requ
Spatial Frequency, fx (mm-1)
N
Spatial Frequency, fx (mm-1)
Spat
3D NEQ
4 ASagittal NEQ 4 mAs2 mAs1 mAs
Sagittal NEQ
mm
2 )
z(m
m-1
)
0.4 mAs
hoto
ns/m
uenc
y, f z
NEQ
(ph
tial F
requ
N
Spatial Frequency, fz (mm-1)Spatial Frequency, fx (mm-1)
Spat
Artifacts
Rings Shading MotionStreaks
LagMetal Cone-BeamTruncation
Geome