Review the demands and limitations of x-ray fluoroscopy. (XRF) in guided
cardiac interventions. - Lack of tissue contrast and depth information. - X-ray dose
...
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
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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
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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).
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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)
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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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