Cardiac Computed Tomography (CT), Coronary Artery Calcium Scoring
Computed Tomography IIComputed Tomography II C … · Conventional CT Dosimetry • Computed...
Transcript of Computed Tomography IIComputed Tomography II C … · Conventional CT Dosimetry • Computed...
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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
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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
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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
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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
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“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
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Mobile Isocentric C-Arm
Siemens PowerMobil
MotorizedOrbit Replace XRII with
Flat-Panel Detector
GeometricControl System
Calibration
Tube + CollimatorModification (FOV)
Image Acquisition3D Reconstruction
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Cone-Beam CT
Projection data Volume reconstructionjMultiple projections
over ~180oSub-mm spatial resolution
+ soft tissue visibility
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Image Quality:Key Characteristics
• Large volumetric FOV• Single orbit about the patient
• Sub-Millimeter Spatial Resolution• Sub Millimeter Spatial Resolution• Soft-Tissue Visibility
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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
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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
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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
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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
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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
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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
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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
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Spatial Resolution( H f h PS )(FWHM of the PSF)
m)
HM
(mm
FWH
Sm
ooth
Shar
p
S S
Filter Param (hwin)
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Spatial Resolution(li i )(line-pairs per mm)
Minimum resolvableline-pair group
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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 =
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Spatial Resolution
Axial Stapes Crura
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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)
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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)
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Noise-Power Spectrum
• The NPS describes– Frequency content of the noise:
– Magnitude of the noise:– Magnitude of the noise:
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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
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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)
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Noise-Power Spectrum
NPS(fx, fy, fz)
•Transverse domain:“Filtered-ramp”Green NPS
•Axial domain:“Band-limited”Red NPSRed NPS
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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%
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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
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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
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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
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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
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Artifacts
Rings Shading MotionStreaks
LagMetal “Cone-Beam”Truncation
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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)
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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
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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
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Geometric CalibrationCalibration Comparison
Full Geometric Calibration
Assume Semi-Circular Orbit
“Single BB” Calibration
1 mm1 cm
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R di i DRadiation Dose
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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.)
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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
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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:
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Image Quality and Radiation Doseg y
mGy
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C-Arm CBCT Dosimetryy
A
DosimetryPhantom
C
A
D
PancakeDetector
Farmer Chamber
C D
B
StyrofoamSupport
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C-Arm CBCT Dosimetry
mAs mA T N
y
0.16
0.20 "Tube-Under""Tube-Over"A
s
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mAs = mA × TX × Nproj
A
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ose
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“Eyes” Central Dose
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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
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ual
iz-T
issu
entr
ast
Soft
Con
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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
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ApplicationsApplicationsin Image-Guided Surgeryin Image Guided Surgery
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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
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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
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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
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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
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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
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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
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Head & Neck Surgery
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Skull Base Surgery:Target Abation in the Clivus
Intra-Operative CBCT
Critical
TARGET lTARGET volumeNORMAL volume
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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
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ty T
arget
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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)
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Translation to Clinical Trials
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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
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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
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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
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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
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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.