computed tomography iicomputed tomography ii c-arm … · noise-power spectrum • the nps...
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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 Chen2
1. Department of Biomedical Engineering, Johns Hopkins University2. 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 Mama’s C-ArmNot Your Mama’s C-ArmSome Essential Science and Practicalities
for the New Generation of Cone-Beam CT-Capable C’sSome Essential Science and Practicalities
for the New Generation of Cone-Beam CT-Capable C’s
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) cm3
from 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 with
Flat-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)
ΔHU = (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 / Stationarityσ2
• Signal Uniformity- Stationarity of the mean
Variance Maps σ2(x,y)σ2
(/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( d l i T f i )
127 μm Wire in H2O
(Modulation Transfer Function)
JJ
JJ
JJ
JJ
JJJJ
0.8
1.0
Steel Wire
-1)
μ 2
JJ
JJ
JJ
JJ
JJ
0.4
0.6System MTF
nal
(m
m
JJJJJJJJJJJJJJJJJJJJJJJ
JJ
0.2
0.4
Measured
Sig
JJJJJJJJJJJJ
0.00.0 0.5 1.0 1.5 2.0
Spatial Frequency (mm-1)
( ) ( )[ ] ,, yxLSFFTffMTF yx =
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 +~σ50
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
(μ2 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 (μ
2 mm
3 ) 2 mAs4 mAs
NP
Spatial Frequency, fz (mm-1)
Noise-Power Spectrum
NPS(fx, fy, fz)
•Transverse domain:“Filtered-ramp”Green NPS
•Axial domain:“Band-limited”Red NPSRed NPS
Contrast
A “large-area transfer characteristic”Defined: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-2
orC = |-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
Geometric CalibrationTwo-Circle Phantom
u
16 Tungsten BBsv
v uyi
zi
φ
θη
yw
xwzwxi
*
y
Y. B. Cho et al. Med. Phys. 32(4) (2005)
Geometric CalibrationCalibration Parameters (10 Trials Overlaid)
Source Position (mm)
DetectorPosition (mm)
DetectorAngle (o)
Detector Distances (mm)
2
0
2
-5
0
5
-5
0
5
Position (mm) ( ) g ( )
φΔXs ΔXd
-10
0
10
( )
50-5
ΔSDD100
-10
20-2
50-5
50-5
0 90 180
-2
10 2
0 90 180
-5
5
0 90 180
-5
5 θΔYs ΔYdΔU
0 90 180
-10 5
0 90 180 0 90 180 0 90 180 0 90 180
10
10
-2
25
-5
5
-5
0 90 180
-10
0
0 90 180
-2
0
0 90 180
-5
0
0 90 180
-5
00-10
0-2
0-5
0-5
0 90 180 0 90 180 0 90 180 0 90 180
0
1
0
10
0
2
0
2
ηZs ZdΔV10010
2
0
2
0
1
0
0 90 180-1
0 90 180
-10
0 90 180-2
0 90 180-2
Gantry Angle (o) Gantry Angle (o) Gantry Angle (o) Gantry Angle (o)
-10
0 90 180 0 90 180 0 90 180-2-2 -1
0 90 180
Geometric Calibration
Xs Xd φFull Xs Xd φFull
Sensitivity Analysis (“Knockout”)
FWHM = 0.63 mm
Xs Xd φFull Xs Xd φFull
Ys Yd θU
1 mm
Ys Yd θU
1 mmZs Zd ηV Zs Zd ηV
Wire = 0.16 mm diameteravox = (0.2 x 0.2 x 0.2) mm3
Geometric CalibrationCalibration Comparison
Full Geometric Calibration
Assume Semi-Circular Orbit
“Single BB” Calibration
1 mm1 cm
C-Arm CBCT DosimetryyAAPM REPORT NO. To-Be-Determined
Comprehensive Methodology for the Evaluation of Radiation Dose in X-ray Computed TomographyA new measurement paradigm based on a unified theory for axial, helical, fan-beam, or cone-beam
scanning with or without longitudinal translation of the patient tableReport of AAPM Task Group 111: The Future of CT Dosimetry
(R. L. Dixon et al.)
Conventional CT Dosimetry• Computed Tomography Dose Index (CTDI)
• Developed in the context of axial CT- Average multiple scan dose profileg p p- Midpoint of scan length L- n axial slices of thickness T- Discrete contiguous axial scans
Electrometer(mGy / C)z
LCTDI = f X
TL Pencil
Ion Chamber
peripheryTL
- 100 mm pencil chamber spanning T- 16 cm “Head” phantom- 32 cm “Body” phantom- each ~14-15 cm long
center
g
• Insufficient for modern CT- Helical scanning- Multi-detector CT
16 or 32 cm DiameterAcrylic Cylinderwith or w/o
- Cone-beam CT table motion
Cone-Beam CT Dosimetry• Cumulative Dose for CBCT (without Table Motion)
• Cumulative dose is simply the dose profile: DN(z) = Nf(z)• Central cumulative dose is simply Nf(z=0) TG 111 Report:
The Future of CT Dosimetry
• CBCT Dosimetry• For cone-beam width a > Length of ion chamber
- f(0) determined from “point dose”
The Future of CT DosimetryR. L. Dixon et al.
measurement with IC located at z=0• For cone-beam width a <~ Length of ion chamber
- Necessitates a small (~point) dosimeter(e.g., solid state, Farmer, or TLD)
• For cone-beam width a > Length of the phantom- A long phantom to capture x-ray scatter tailsor
l “h d” “b d ” h h- Conventional “head” or “body” phantom with appropriate extrapolation to equilibrium(parameters α and Leq) Approach to Equilibrium:
C-Arm CBCT Dosimetryy
A
DosimetryPhantom
C
A
D
PancakeDetector
Farmer Chamber
C D
B
StyrofoamSupport
C-Arm CBCT Dosimetry
mAs mA T N
y
0.16
0.20 "Tube-Under""Tube-Over"A
s
100 kVp
mAs = mA × TX × Nproj
A
0.12
0.16 Tube-Over
mG
y)/m
A
C
A
D
0.04
0.08 D
ose
(m C D
A B C D0.00
B
“Eyes” Central Dose
Image Quality and Radiation Dose
0.6 mGy 2.9 mGy 9.6 mGy 23.3 mGy
g yny zation
0.6 mGy0.02 mSv
2.9 mGy0.1 mSv
9.6 mGy0.35 mSv
23.3 mGy0.8 mSv
Bon
Vis
ual
iz-T
issu
entr
ast
Soft
Con
Image Quality and Radiation Dose
B D t il S ft Ti
Task-Specific Imaging Techniques
Example Intra op Protocol
g y
Bony Detail Soft-Tissue Example Intra-op Protocol
Pre-Op 10 mGyIntra-Op 3Intra Op 3Intra-Op 3Intra-Op 10Intra-Op 3pIntra-Op 3Post-Op 10
170 mAs50 mAs
TOTAL 42 mGy
Typical DiagnosticCT Dose: >50 mGy
9.6 mGy0.35 mSv
50 mAs2.9 mGy0.1 mSv
y
A Mobile C-Armfor Intraoperative Cone-Beam CT
Multiple projection images acquired over ~180o
2D Image acquisition- Nominal: 60 s- High-speed motor: 10 s
3D Image reconstruction- Nominal: 60 s- High-speed recon: 10 s
Radiation dose- ~1/10th that of Dx CT
Applications in IG Surgerypp g y
Platform for optimizing / integrating imaging and
navigation
• Orthopedic Surgery• Spine Surgery B h th• Brachytherapy
• Ear Surgery• Interventional Radiology• Interventional Radiology• Urology• Lung Surgery• Lung Surgery• Breast Surgery• Head and Neck Surgeryg y
Applications in IG Surgerypp g y
In vivo studiesIn vivo studiesof image quality and geometric precision
• Orthopedic Surgery• Spine Surgery B h th• Brachytherapy
• Ear Surgery• Interventional Radiology• Interventional Radiology• Urology• Lung Surgery• Lung Surgery• Breast Surgery• Head and Neck Surgeryg y
Applications in IG Surgerypp g y
• Orthopedic Surgery• Spine Surgery B h th
Soft-tissue visualization and real-time planning• Brachytherapy
• Ear Surgery• Interventional Radiology
and real time planning
• Interventional Radiology• Urology• Lung Surgery• Lung Surgery• Breast Surgery• Head and Neck Surgeryg y
Applications in IG Surgerypp g y
Resection ofsub-palpable lesions
• Orthopedic Surgery• Spine Surgery B h th• Brachytherapy
• Ear Surgery• Interventional Radiology• Interventional Radiology• Urology• Lung Surgery• Lung Surgery• Breast Surgery• Head and Neck Surgeryg y
Applications in IG Surgerypp g y
Maximal target ablation and critical structure
avoidance
• Orthopedic Surgery• Spine Surgery B h th• Brachytherapy
• Ear Surgery• Interventional Radiology• Interventional Radiology• Urology• Lung Surgery• Lung Surgery• Breast Surgery• Head and Neck Surgeryg y
Skull Base Surgery:Target Abation in the Clivus
Intra-Operative CBCT
Critical
TARGET lTARGET volumeNORMAL volume
Skull Base Surgery:Target Abation in the Clivus
Intra-Operative CBCT Post-Operative CBCT1 0
0.8
1.0CBCT-Guided
Unguided(conventional)xc
ised
)
0.6
(conventional)
nsi
tivi
ty T
arget
Ex
Critical Critical0.2
0.4Sen
action o
f
TARGET l TARGET R i i
0.00.0 0.2 0.4 0.6 0.8 1.0
1-Specificity
(Fra
TARGET volumeNORMAL volume
TARGET RemainingNORMAL Remaining
1-Specificity(Fraction of Normal Excised)
C-Arm Trials: MandibulectomyS
can
1Sc
an 1
Sn
2S
Targetn 2
FibulaReconstruction
Sca
n g(Radionecrosis)
Scan
Sca
n 3
Scan
3n
4an
4S
ca ResectionSca
Plates
C-Arm Trials: Invasive TumorS
can
1Sc
an 1 Craniotomy
Sn
2S
n 2
Tumor P ki
Sca
n
ChondrosarcomaScan Tumor
resectionPacking
Sca
n 3
Scan
3 Tumormargins
Closure
n 4
n 4
Sca
Scan
Conclusions• Image Quality
- Uniformityy- Contrast and SDNR- Spatial resolution (FWHM and MTF)- Noise and NPS
NEQ- NEQStandardization underway
• Radiation Dose- A departure from conventional CTDI- Small dosimeters and long phantoms
Standardization underway (TG 111)
• Applications- Burgeoning scope of specialty applications- Technology development, optimization,
and streamlined integration
Acknowledgements
Collaborators and SupportCollaborators and Support• NIH R01-CA112163• NIH R01-CA127444• Siemens Healthcare (Erlangen AG)( g )• University Health Network, Toronto ON• Stanford University• California State University – Fullerton
Conventional CT Dosimetry• Cumulative Dose with Table Motion
• Superposition of single scans displaced in z• z-axis collimation width ≡ a
- Projection of collimator opening at the AOR• Total width of n slices ≡ nT
- A scanning parameter (not physical)- Nominal length of the volume scanned
• Note: a ≠ nT
• For a series of N scansS i f i b• Spacing of successive scans ≡ b
• Each with dose profile ≡ f(z)• Scan length ≡ L = Nb• Cumulative dose at the midpoint of the scan:
TG 111 Report:The Future of CT Dosimetry
R. L. Dixon et al.• “Equilibrium dose” ≡ Deq = lim(L ∞)
R. L. Dixon et al.