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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


• Magnification

SAD– Detector configuration• X-ray converter

SDD

• Pixel pitch– Recon parameters

Recon filter• Recon filter• Voxel size

C tConverter

Pixel Matrixapix

Spatial Resolution


• 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 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 =

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 +~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:


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


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)


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

Artifacts

Rings Shading MotionStreaks

LagMetal “Cone-Beam”Truncation

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

R di i DRadiation Dose

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:

Image Quality and Radiation Doseg y

mGy

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

ApplicationsApplicationsin Image-Guided Surgeryin Image Guided Surgery

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


In vivo studiesIn vivo studiesof image quality and geometric precision




• 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


Resection ofsub-palpable lesions




Maximal target ablation and critical structure

avoidance



Head & Neck Surgery

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)

Translation to Clinical Trials

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.