Objectives
Emerging XX-ray Fluoroscopic
Guidance Technologies
for
Challenging Cardiovascular
Interventions
Michael A. Speidel
University of Wisconsin - Madison
AAPM 2009 Annual Meeting
1. Review the demands and limitations of x-ray fluoroscopy
(XRF) in guided cardiac interventions
- Lack of tissue contrast and depth information
- X-ray dose concerns
2. Understand the principles of Inverse Geometry XRF
- Scanning-Beam Digital X-ray (SBDX) prototype system
- Reduction of patient x-ray dose
- 3D tracking of catheter devices
3. Discuss x-ray fluoroscopy combined with 3D roadmaps
- Visualization of 3D soft tissue targets
- Endocardial stem cell therapy
X-Ray Fluoroscopic (XRF) Guidance
Basic demands on a guidance
system in the cardiac cath lab:
1. XX-ray Guidance in
Cardiac Interventions
1. Real time continuous feedback
2. High spatial, temporal resolution
3. Device position relative to anatomy
4. Simple to set up and use
5. Compatible with catheter devices
XRF meets these requirements
well in many types of interventions
Coronary
Angioplasty
1
Lack of Tissue Contrast and Depth Focus
Ablation of Atrial Fibrillation
Left
Atrium
Device:
RF ablation
catheter
Pulm.
veins
Target:
line around
pulmonary
vein ostia
Target anatomy lacks contrast
Catheter position difficult to
determine relative to 3D target
Endomyocardial Cell Therapy
Device:
injection
catheter
Target:
viable
peri-infarct
zone
Left
ventricle
X-ray Radiation Dose in the Cath Lab
Deterministic risk of skin injury ( > 2 Gy to skin)
Case reports of skin injury, 1996-2001
Coronary intervention vs. Cardiac RF ablation
Koenig, T. et al. AJR 177, 3-11 (2001).
Chida, K. et al. AJR 186, 774-778 (2006).
Coronary angiography & intervention:
Cardiac radiofrequency ablation:
TIPS placement:
Neuroradiologic intervention:
Other:
44
11
6
2
3
PCI
RF Abl.
Fluoro time (min): 37 +/- 23
121 +/- 63
Cine runs (#): 35 +/- 17
18 +/- 12
Max skin dose (Gy): 1.45 +/- 0.99 0.64 +/- 0.55
Stochastic risk of cancer induction
Infarct
zone
Requires delineation of soft tissue
based on functional status
Experimental procedure
Obesity and Radiation Dose in RF ablation of Atrial Fibrillation
Ector, J. et al. JACC 50, 234-242 (2007).
BMI
< 25
25-30
≥ 30
n
28
41
16
Age
48 +/- 10
51 +/- 7
46 +/- 10
Calc. Effective Lifetime Attributable Risk
Dose (mSv)
of cancer incidence
15.2 +/- 7.9
1/1000
26.8 +/- 11.6
1/633
39.0 +/- 14.7
1/405
Guidance Solutions for the Cath Lab
2. Inverse Geometry XRF
Pursue non-fluoroscopic technologies
E.g. Electroanatomic mapping systems (EAM)
- 3D tracking of specialized catheters
- Point-by-point endocardial surface mapping
- Cardiac ablation guidance
ScanningScanning-Beam Digital XX-ray (SBDX) Prototype
Operating Principles
Dose Reduction
Catheter Tracking
Or seek to modify / enhance XRF guidance by:
1) Reducing x-ray dose while maintaining image quality
2) Adding 3D context to the live image display
2
SBDX Operating Principles
Photon-counting
Detector Array
Real-time
Reconstructor
~40,000 images
in 1/15 sec
15-30 fps
16 planes
Dose Reduction Principles
1. Beam scanning and large airgap
reduces detected x-ray scatter
1500
3-7% SF
Thick CdTe
2. Thick CdTe detector maintains high
DQE at high source kVp
1000
500
0
-50
0
50
-50
0 50
25-50% SF
Thin CsI
X-ray beam
Multi-hole
Collimator
Collimator
Transmission
Target
100 x 100
positions
Dose Reduction Principles
SBDX Prototype Performance (2006)
3. Inverse geometry reduces x-ray
fluence at the patient entrance
Large-area SNR
25
SBDX at
120 kVp
20
SBDX at 120 kVp
SNR
15
1/r2
10
SBDX at
70 kVp
5
~2x larger
entrance
field
Conventional
Entrance Exposure
140
II/CCD cine
SBDX at equal kVp
1/r2
SBDX
Conventional
Electron beam
0
Entrance exposure (R/min)
Scanned
Focal spot
123 R/min
123 kVp
II/CCD cine
120
SBDX at 120 kVp
SBDX at equal kVp
100
80
60
40
18 R/min
62 kVp
9.3
R/min
20
0
18
20
22
24
26
28
30
32
phantom thickness (cm acrylic)
34
36
18
20
22
24
26
28
30
32
34
36
phantom thickness (cm acrylic)
SBDX operating at equal SNR: 15% - 31% entrance exposure
• Greatest dose reduction for largest phantoms
SBDX
Speidel, M. et al. Comparison of entrance exposure and signal to noise ratio for an SBDX prototype. Med Phys 33, 2728-2743 (2006).
3
SBDX System Development
Detector Re-design
1% SFxray
6% SFxray
3% SFxray
14
Next Gen
Iodine
SNR
12
CineCine-quality
120 kVp, 24.3 kWp, 90% DQE
1500
120 kVp, 24.3 kWp, 71% DQE
10
8
FluoroFluoro-quality
6
Shift-and-add
backprojection
at
multiple planes
10.6 cm
x 5.3 cm
area
Phantom: 28 cm acrylic
16
High Speed Multiplanar Tomosynthesis
‘04 -’06
100 kVp,12.6 kWp, 62% DQE
scan line
Source
&
Detector
Specs
1000
‘98 -’03
4
2
500
70 kVp, 4.2 kWp, ~40% DQE
1996
0
0
1
2
3
4
5
6
7
8
9
0
10
-50
0
50
-50
0 50
X-ray Beam Solid Angle Ω
16 planes per frame
12 mm spacing
(relative units)
SNR ∝ (1 − SF ) DQE (mAs ) Ω
Depth Focus Property
Plane Selection Algorithm
Multiplane Composite Display
Rays through object
originate from
different spot positions
Pixel-by-pixel plane selection:
Plane stack
In-plane
High contrast,
sharp appearance
object
position
Out-of-plane:
Low contrast,
blurry
“Score stack”
Gradient
filtering
Display
pixel from
plane
with highest
object focus
metric
4
3D Catheter Tracking Algorithm
3D Localization
Generate MIP
along z axis
Helix of 1-mm Pt spheres
Calculate center-ofmass along z
Extract score
vs. z distribution
Z
80
Rawscore at fixed(x,y)
70
object 1
object 2
Z
Y
3.0
(z)
(x,y)
x,y)
Perform 2D
connected
component
labeling
60
50
40
30
threshold
20
10
Z error: -0.56 +/- 0.65 mm
2.0
354
402
450
498
Planeposition
Position ZZ
(mm)
Plane
(mm)
Output is a set of (x,y,z) coordinates for each image frame
sphere
size
2σ
1σ
1.0
0.0
-1.0
-2.0
-3.0
0
0
X
Tracking Accuracy & Precision
SBDX Prototype Geometry
Z-coordinate Error
(mm)
Segmentation
Score
Image
Stack
Tracking Simulation Study (2008)
5
10
15
20
25
30
Source power (kWp)
12 mm plane-to-plane spacing
28 cm acrylic, 120 kVp
Stationary helix
Speidel M. et al. Frame-by-frame 3D catheter tracking methods for an inverse geometry [...] Proc SPIE 6913 (2008)
Tracking Phantom Study
3D Tracking Demonstration
3M chest phantom
Ablation catheter in trans-septal sheath
AngiogramSam
cardiac chamber
phantom
Linear stage for
catheter pullback
10 mm/sec
pullback rate
Catheter sheath
Fiducials with
screw mounts
SBDX
source
Tracking performed
in software using
stored detector
images
15 frame/sec
SBDX imaging
1850 photons/mm2
at isocenter
5
Comparison with CT
Inverse Geometry XRF & 3D Tracking
SBDX
Tracked tip to sheath centerline: 1.0 +/- 0.8 mm
(Tip diameter = 2.5 mm)
82% of tracked positions inside sheath volume
catheter
tip
confining
sheath
Well-suited to long, complex cardiac interventions
Real-time 3D tracking
at end diastole
Fluoroscopy at
15% skin dose rate
470
ring in
sheath
ABL tip
(in focus)
Left
Atrium
XYThresh=4, ZThresh=2
Frame 15
480
tracked tip positions
sheath volume
460
CS tip
450
440
lasso
cath.
430
CS tip
(blurred)
Z
410
Y
CT scan
Z plane = 427 mm
ABL
tip
420
X
Endocardial
target
lasso
400
40
20
0
-20
-40
40
20
0
RF ablation
catheter
-20
Tracking works with standard catheters, any number of
elements, and uses a single gantry angle, automatically
registered to XRF system without calibration
Targeted CellCell-based Therapy for MI
3. XRF / 3D Roadmap Fusion
Laboratory of
Amish Raval,
Raval, M.D.
UWUW-Madison
Cardiology
Stem cell therapy may improve left
ventricle function after recent
myocardial infarction (acute MI) [1]
Endomyocardial Cell Therapy
Device:
injection
catheter
Left
ventricle
Direct endomyocardial cell injection
requires guidance system beyond
XRF in order to:
1) Target peri-infarct region
2) Avoid perforating friable infarct
XRF / 3D MRI fusion enables device
& target visualization while
minimizing tissue contact [2]
Target:
Viable
peri-infarct
zone
Avoid:
Infarct
[1] Segers, V. and Lee, R. Stem-cell therapy for cardiac disease. Nature 451, 937-942 (2008).
[2] de Silva, Gutierrez, et al. X-ray fused with magnetic resonance imaging to target endomyocardial injections. Circulation 114 (2006).
6
BiBi-plane XRF / 3D Fusion System
Conventional
Bi-plane
Fusion
Display
Control
Display
XRF / 3D Fusion Procedure
MRI
Scanner
X-Ray
Fluoroscopy
Segmentation
Workstation
Slice
Contours
Combine with
3D XRF Model
PC Workstation
Portable
Fusion System
Frame grabber
(Helios eA, Matrox)
DICOM
MR or CT data
3D Surface
Generation
C-arm
Calibration
(one-time)
Projection
Matrices
Custom fusion
software (C++)
Surface
Projection
and Overlay
Live
Video
Gantry
Orientation
Manual
Adjustments
Frames
Frame
Grabber
Fusion Display
Tomkowiak M. et al. Multimodality image merge to guide catheter based injection of biologic agents. RSNA, Chicago, IL, 2008.
Porcine Study: Segmentation
Pre-intervention MRI
LV Endocardial
contour
Epicardial
contour
3D Model
Red: LV endocardium
Yellow: infarct volume
Porcine Study: Registration
Manual Registration to Internal Anatomy
Biplane Ventriculogram (end diastole, end expiration)
bSSFP
scan
DHE
scan
Infarct contour
End diastole, end expiration
Frontal plane
Lateral plane
7
Porcine Study: Injections
Porcine Study: Targeting Accuracy
Post-procedure:
Cath lab:
Frontal plane
Lateral plane
Virtual 3D marker
Biplane XRF / 3D Fusion
MRI
Necropsy
Yellow:
infarct
Orange:
injection
Bullseye display
6 animal studies:
Study time: 24 +/- 12 min
Injected mixture
iodinated contrast : intra procedure myo. staining
iron oxide (SPIO) : MRI visualization of injections
tissue dye
: for post procedure necropsy
D1
injection point to infarct perimeter
Targeting accuracy depends on the quality of:
Fusion System Development
Desired features:
-
Respiratory and patient motion compensation
-
Ability to re-check registration throughout procedure
-
Cardiac gating
-
Automation, to the extent it is safe and reliable
(gantry calibration)
(depends on modality)
(landmarks)
MRI and X-ray fusion method feasible and safe for targeted
injections to the peri-infarct region
- No myocardial perforation
- Targeting error ~ MR slice thickness & in-plane resolution
Portability and vendor-independence
28 injection sites:
D2 – D1 = 3.6 +/- 2.3 mm
Supposed distance vs. Actual distance
XRF / MRI Roadmap Fusion
- Modeling of XRF system
- Segmentation of 3D images
- Registration of 3D surface to live x-ray
D2
Automated device and anatomic landmark tracking
-
Conventional XRF tracking
Ultrasound
EAM systems
(2D imaging)
(3D imaging)
(3D points)
-
Inverse Geometry XRF
(tomosynthesis, 3D tracking)
8
Emerging Fluoroscopic Technologies
XRF Guidance: Advantages and limitations
- High quality, real-time imaging
- Device compatibility
- Simple, easy use
Conclusion
- Poor 3D visualization of
devices and endocardial targets
- Radiation dose in long procedures
Inverse geometry XRF: Unique design and capabilities
- Narrow scanning x-ray beam
- Inverted x-ray field geometry
- High speed multiplane tomosynthesis
Low dose fluoroscopy
3D tracking capability
XRF / 3D Fusion: 3D anatomy & function in the cath lab
- Enables novel cardiac interventions
- Non-contact visualization of function
- 3D soft tissue anatomy
Acknowledgements
Financial support for this work
was provided by:
University of Wisconsin
Amish Raval, M.D.
Andrew Klein, M.D.
Douglas Kopp, M.D.
NHLBI R01 HL084022
NovaRay Medical, Inc.
Michael Van Lysel, Ph.D.
Michael Tomkowiak, M.S.
Karl Vigen, Ph.D.
Timothy Hacker, Ph.D.
Larry Whitesell
TripleRing Technologies, Inc.
Joseph Heanue, Ph.D.
Augustus Lowell, Ph.D.
Brian Wilfley, Ph.D.
Thank you!
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