Computed Tomography II
C-Arm Cone-Beam CT:
Principles and Applications
Jeff Siewerdsen1 and Guang-Hong Chen2
1. Department of Biomedical Engineering, Johns Hopkins University
2. Department of Medical Physics, University of Wisconsin
Johns Hopkins University
Schools of Medicine and Engineering
University of Wisconsin
Institutes for Medical Research
Overview
Part 1: (Siewerdsen)
-
Cone-beam CT image quality
Radiation dose
Applications (non-vascular)
Sustained applause
Part 2: (Chen)
-
3D CBCT reconstruction
Artifacts
Applications (cardiovascular)
Thunderous ovation
Not Your Mama’s C-Arm
Some Essential Science and Practicalities
for the New Generation of Cone-Beam CT-Capable C’s
Jeff Siewerdsen, PhD
Department of Biomedical Engineering
Johns Hopkins University
Johns Hopkins University
Schools of Medicine and Engineering
The New C-Arm
• Fluoroscopy + Cone-Beam CT
- 3D imaging capability
capabilit
3D filtered backprojection (FDK)
FOV ~(20x20x20) cm3
from a single
g half-rotation
• Flat-Panel Detector
- Replacement to XRII
Larger FOV
Better 2D image quality
Distortionless
- High
High-performance
performance CBCT
Sub-mm spatial resolution
Soft-tissue visibility
“C-Arms” for IGI
Key Characteristics
• Real-time
(or near
near-real-time)
real time)
• Radiation dose
~1/10 – 1/2 of Dx CT
• Sub-mm resolution
• Soft-tissue visibility
Mobile Isocentric C-Arm
Siemens PowerMobil
Motorized
Orbit
Control System
Replace XRII with
Flat-Panel Detector
Geometric
Calibration
Tube + Collimator
Modification (FOV)
Image Acquisition
3D Reconstruction
Cone-Beam CT
Projection
j
data
Multiple projections
over ~180o
Volume reconstruction
Sub-mm spatial resolution
+ soft tissue visibility
Image Quality:
Key Characteristics
• Large volumetric FOV
• Single orbit about the patient
• Sub
Sub-Millimeter
Millimeter Spatial Resolution
• Soft-Tissue Visibility
Image Quality
• Key Image Quality Metrics
- Image uniformity / stationarity
Shading, view aliasing
- CT # accuracy
HU calibration, shading artifacts
- Spatial resolution
LP/mm, FWHM wire, MTF
- Contrast
Signal difference (HU),
(HU) SDNR
- Noise
Voxel noise, NPS
- SNR
N i equivalent
Noise
i l t quanta
t (NEQ)
- Artifacts
Truncation, scatter, metal, etc.
• C-arm System Parameters
- System configuration
Geometry, grid, bowtie
FPD readout mode
- Geometric calibration
Mechanical flex, reproducibility
Degrees of freedom
- Acquisition parameters
Number of projections
kVp, mAs
Dose
- Reconstruction parameters
Reconstruction filter
Voxel size (axy and az)
2D/3D sampling
Cone-Beam Geometry
S t
System
geometry
t dictated
di t t d by
b the
th application
li ti
Geometry affects every aspect of image quality
Uniformity / Stationarity
• Signal Uniformity
(3.8 ± 4.2)
- Stationarity of the mean
Shading artifacts
Beam-hardening
Truncation
- Stationarity of the noise
- WSS of second-order statistics
Physical effects:
Quantum noise
Bowtie filter
Sampling effects:
Intrinsic to FBP
Number of projections
View aliasing
((5.6 ± 2.4))
((-1.3 ± 6.2))
(4.6 ± 3.2)
= 3.3 HU
(4.4 ± 4.2)
Mean Signa
al (/mm)
• Noise Uniformity
ΔHU = (4.6-1.3) HU
0.20
SPR ~0
SPR ~100%
0.00
-10
0
Distance (mm)
+10
Uniformity / Stationarity
Variance Maps
• Signal Uniformity
σ2(x,y)
- Stationarity of the mean
Shading artifacts
Beam-hardening
Truncation
• Noise Uniformity
Cylinder + Bowtie
Water Cylinder
Cylinder + Bowtie
Va
ariance
- Stationarity of the noise
- WSS of second-order statistics
Physical effects:
Quantum noise
Bowtie filter
Sampling effects:
Intrinsic to FBP
Number of projections
View aliasing
Water Cylinder
Air
Air
-10
10
0
Distance (mm)
+10
σ2
(/mm)2
Spatial Resolution
• Factors affecting spatial resolution
– Focal spot size
– System geometry
• Magnification
– Detector configuration
• X-ray converter
• Pixel pitch
– Recon parameters
• Recon filter
• Voxel size
SAD
SDD
Spatial Resolution
• Factors affecting spatial resolution
– Focal spot size
– System geometry
• Magnification
– Detector configuration
• X-ray converter
• Pixel pitch
– Recon parameters
• Recon filter
• Voxel size
SAD
SDD
C
Converter
t
apix
Pixel Matrix
Spatial Resolution
• Factors affecting spatial resolution
– Focal spot size
– System geometry
• Magnification
– Detector configuration
• X-ray converter
• Pixel pitch
– Recon parameters
• Recon filter
• Voxel size
Sharp
S
Smooth
S
FWH
HM (mm
m)
Spatial Resolution
(
(FWHM
H off the
h PS
PSF))
Filter Param (hwin)
Spatial Resolution
(li
(line-pairs
i per mm))
Minimum resolvable
line-pair group
Spatial Resolution
( d l i T
(Modulation
Transfer
f Function)
i )
127 μ
μm Wire in H2O
1.0
J
J
JJ
J
J
J
Steel Wire
Signal (mm-1)
J
0.8
J
J
J
J
J
J
0.6
J
J
System MTF
J
J
J
0.4
0.2
J
J
J
J
J
J
J
J
Measured
J
J
JJ
JJ
J
JJ
JJ
JJ
JJ
JJJ
0.0
0.0
0.5
1.0
1.5
-1
Spatial Frequency (mm )
MTF ( f x , f y ) = FT [LSF ( x, y )]
JJJ
JJJJ
JJ
2.0
Spatial Resolution
Axial
Stapes Crura
Image Noise
• CT image noise depends on
– Dose
– Detector efficiency
– Voxel
V
l size
i
• Axial, axy
• Slice thickness,
thickness az
– Reconstruction filter
Barrett, Gordon, and Hershel (1976)
Image Noise
Dose
Reconstruction Filter
60
σ ~ a+
X
40
20
10
0
0
0.5 1.0 1.5 2.0 2.5 3.0
Dose (mGy)
Sharp
S
30
Smooth
S
Nois
se (CT#))
50
b
Noise-Power Spectrum
• The NPS describes
– Frequency content of the noise:
– Magnitude of the noise:
Noise-Power Spectrum
Axial NPS
NPS (μ2mm
N
m3)
Axial Plane (x,y)
S(fx, fy)
0.4 mAs
1 mAs
2 mAs
4 mAs
Spatial Frequency,
y fx (mm-1)
Noise-Power Spectrum
Sagittal NPS
NPS (μ2mm3)
Sagittal Plane (x,z)
S(fx, fz)
0.4 mAs
1 mAs
2 mAs
4 mAs
Spatial Frequency, fz (mm-1)
Noise-Power Spectrum
NPS(fx, fy, fz)
•Transverse domain:
“Filtered-ramp”
Green NPS
•Axial domain:
“Band-limited”
Red NPS
Contrast
A “large-area transfer characteristic”
Defined:
• As an absolute difference in mean pixel values:
For example:
C = |0.18
|0 18 cm-1 – 0.20
0 20 cm-1|
= 0.02 cm-2
or
C = |-100 HU – 0 HU|
= 100 HU
• As a relative difference in mean pixel values:
For example:
C = |0.18 cm-1 – 0.20 cm-1|
0.19 cm-1
~ 10%
ROI #1
ROI #2
Signal Difference-to-Noise Ratio
3.5
103 HU
100 kVp
kV
3.0
C
CNR
23.3 mGy
Soft-Tissue-Simulating Spheres
2.5
88 HU
2.0
66 HU
1.5
.
9.6 mGy
y
1.0
45 HU
05
0.5
25 HU
22 HU
0.0
2.9 mGy
11 HU
0
5
10
15
20
Dose to Isocenter (mGy)
25
0.6 mGy
3D NEQ and DQE
NEQ
Effective number of quanta
used at each spatial frequency
(Efficiency x Fluence)
DQE
Fraction of quanta used at each each
frequency.
Observations:
3D DQE(0) ~ Projection DQE(0)
3D DQE(f)
(f) ddependent
d on reconstruction
i parameters
Axial NEQ
NEQ (phhotons/m
N
mm2)
Spattial Frequuency, fy (mm-1)
3D NEQ
Spatial Frequency, fx (mm-1)
4 mAs
2 mAs
1 mAs
0.4 mAs
Spatial Frequency, fx (mm-1)
Sagittal NEQ
NEQ (phhotons/m
N
mm2)
Spattial Frequuency, fz (mm-1)
3D NEQ
Spatial Frequency, fx (mm-1)
4 mAs
A
2 mAs
1 mAs
0.4 mAs
Spatial Frequency, fz (mm-1)
Artifacts
Rings
Shading
Streaks
Motion
Metal
Lag
Truncation
“Cone-Beam”
Geometric Calibration
Two-Circle Phantom
u
v
16 Tungsten BBs
φ
θ
xi
u η
v
xw
yi
zi
zw
yw
*
Y. B. Cho et al. Med. Phys. 32(4) (2005)
Geometric Calibration
Calibration Parameters (10 Trials Overlaid)
Detector
Distances ((mm))
10
10
0
0
ΔSDD
-10
10-10
0
90
90
0
-10-10
0
0
ΔXs
00
5
5
0
0
0
90
90
180
180
90
90
ΔV
10 10
0
2
0
90
180
0
90
180
Gantry Angle (o)
0
0
ΔXd
00
5
5
0
0
90
90
180
180
00
2
2
0
0
0
-2-2
-2
-2
Zs
0
90
180
0
90
180
Gantry Angle (o)
Detector
g (o)
Angle
2
2
0
0
φ
-2 -22
90
90
180
180
ΔYd
-5 -5
-10
10-10
0
5
180
180
ΔYs
00
2
5
-5 -5
-5 -5
0
0
5
180
180
ΔU
10 10
0
5
5 -5
-5
0
Detector
Position ((mm))
Source
Position (mm)
00
2
2
0
0
90
90
180
180
θ
-2 -2
90
90
180
180
Zd
00
1
1
0
0
90
90
180
180
η
-1 -1
90
180
90
180
00
90
180
00
90
180
Gantry Angle (o)
Gantry Angle (o)
Geometric Calibration
Sensitivity Analysis (“Knockout”)
Full
Xs
Xd
φ
FWHM = 0.63 mm
1 mm
U
Ys
Yd
θ
V
Zs
1 mm
Zd
η
Wire = 0.16 mm diameter
avox = (0.2 x 0.2 x 0.2) mm3
Geometric Calibration
Calibration Comparison
Full Geometric
Calibration
1 mm
1 cm
“Single BB”
Calibration
Assume SemiCircular Orbit
R di i D
Radiation
Dose
C-Arm CBCT Dosimetryy
AAPM REPORT NO. To-Be-Determined
Comprehensive Methodology for the Evaluation of Radiation Dose in X-ray Computed Tomography
A 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 table
Report 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
g multiple
p scan dose p
profile
- Average
- Midpoint of scan length L
- n axial slices of thickness T
- Discrete contiguous axial scans
fX
CTDI =
L
T
-
100 mm pencil chamber spanning T
16 cm “Head” phantom
32 cm “Body” phantom
each ~14-15 cm long
g
• Insufficient for modern CT
- Helical scanning
- Multi-detector CT with or w/o
table motion
- Cone-beam CT
z
L
T
Pencil
Ion Chamber
Electrometer
(mGy / C)
periphery
center
16 or 32 cm Diameter
Acrylic Cylinder
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)
• CBCT Dosimetry
TG 111 Report:
The Future of CT Dosimetry
R. L. Dixon et al.
• For cone-beam width a > Length of ion chamber
- f(0) determined from “point dose”
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 tails
or
- Conventionall “h
“head”
d” or “b
“body”
d ” phantom
h
with
h
appropriate extrapolation to equilibrium
(parameters α and Leq)
Approach to Equilibrium:
Image
g Qualityy and Radiation Dose
mGy
C-Arm CBCT Dosimetryy
Dosimetry
Phantom
Pancake
Detector
Farmer
Chamber
A
C
B
Styrofoam
Support
D
C-Arm CBCT Dosimetryy
Dose (m
mGy)/mA
As
0.20
mAs = mA × TX × Nproj
100 kVp
"Tube-Under"
"Tube-Over"
Tube-Over
0.16
A
0.12
C
0.08
0.04
0.00
B
A
“Eyes”
B
C
Central Dose
D
D
Image
g Qualityy and Radiation Dose
Soft-Tissue
Con
ntrast
Bon
ny
Visualiz
zation
0.6 mGy
0.02 mSv
2.9 mGy
0.1 mSv
9.6 mGy
0.35 mSv
23.3 mGy
0.8 mSv
Image
g Qualityy and Radiation Dose
Task-Specific Imaging Techniques
B
Bony
Detail
D t il
50 mAs
2.9 mGy
0.1 mSv
S ft Ti
Soft-Tissue
170 mAs
9.6 mGy
0.35 mSv
Example Intra-op
Intra op Protocol
Pre-Op
Intra-Op
Intra
Op
Intra-Op
Intra-Op
Intra-Op
p
Intra-Op
Post-Op
10 mGy
3
3
10
3
3
10
TOTAL
42 mGy
Typical Diagnostic
CT Dose:
>50 mGy
y
Applications
in Image-Guided
Image Guided Surgery
A Mobile C-Arm
for 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
pp
in IG Surgery
g y
•
•
•
•
•
•
•
•
•
Orthopedic Surgery
Spine Surgery
B
Brachytherapy
h th
Ear Surgery
Interventional Radiology
Urology
Lung Surgery
Breast Surgery
g y
Head and Neck Surgery
Platform for optimizing /
integrating imaging and
navigation
Applications
pp
in IG Surgery
g y
•
•
•
•
•
•
•
•
•
Orthopedic Surgery
Spine Surgery
B
Brachytherapy
h th
Ear Surgery
Interventional Radiology
Urology
Lung Surgery
Breast Surgery
g y
Head and Neck Surgery
In vivo studies
of image quality and
geometric precision
Applications
pp
in IG Surgery
g y
•
•
•
•
•
•
•
•
•
Orthopedic Surgery
Spine Surgery
B
Brachytherapy
h th
Ear Surgery
Interventional Radiology
Urology
Lung Surgery
Breast Surgery
g y
Head and Neck Surgery
Soft-tissue visualization
and real
real-time
time planning
Applications
pp
in IG Surgery
g y
•
•
•
•
•
•
•
•
•
Orthopedic Surgery
Spine Surgery
B
Brachytherapy
h th
Ear Surgery
Interventional Radiology
Urology
Lung Surgery
Breast Surgery
g y
Head and Neck Surgery
Resection of
sub-palpable lesions
Applications
pp
in IG Surgery
g y
•
•
•
•
•
•
•
•
•
Orthopedic Surgery
Spine Surgery
B
Brachytherapy
h th
Ear Surgery
Interventional Radiology
Urology
Lung Surgery
Breast Surgery
g y
Head and Neck Surgery
Maximal target ablation
and critical structure
avoidance
Head & Neck Surgery
Skull Base Surgery:
Target Abation in the Clivus
Intra-Operative CBCT
Critical
TARGET volume
l
NORMAL volume
Skull Base Surgery:
Target Abation in the Clivus
Intra-Operative CBCT
10
1.0
Sen
nsitivity
(Fra
action of Target Ex
xcised)
Critical
Post-Operative CBCT
CBCT-Guided
Unguided
(conventional)
0.8
0.6
0.4
Critical
0.2
0.0
0.0
TARGET volume
l
NORMAL volume
0.2
0.4
0.6
0.8
TARGET1-Specificity
R
Remaining
i i
(Fraction of Normal Excised)
NORMAL Remaining
1.0
Translation to Clinical Trials
S
Scan
1
C-Arm Trials: Mandibulectomy
Target
g
(Radionecrosis)
Sca
an
n4
Scan
Scan 3
Scan
Scan
nn 22
Fibula
Reconstruction
Resection
Plates
Craniotomy
Scan
Scann 44
Scan
Scan 3
Scan
n2
S
Scan
1
C-Arm Trials: Invasive Tumor
Tumor Packing
P ki
resection
Chondrosarcoma
Tumor
margins
Closure
Conclusions
• Image Quality
- Uniformity
y
- Contrast and SDNR
- Spatial resolution (FWHM and MTF)
- Noise and NPS
- NEQ
J Standardization underway
• Radiation Dose
- A departure from conventional CTDI
- Small dosimeters and long phantoms
J Standardization underway (TG 111)
• Applications
- Burgeoning scope of specialty applications
- Technology development, optimization,
and streamlined integration
Acknowledgements
Collaborators and Support
•
•
•
•
•
•
NIH R01-CA112163
NIH R01-CA127444
g
AG))
Siemens Healthcare ((Erlangen
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 scans
•
•
•
•
Spacing
S
i
off successive
i
scans ≡ b
Each with dose profile ≡ f(z)
Scan length ≡ L = Nb
Cumulative dose at the midpoint of the scan:
• “Equilibrium dose” ≡ Deq = lim(LJ ∞)
TG 111 Report:
The Future of CT Dosimetry
R. L. Dixon et al.