CLINICAL EVALUATION OF AN ULTRASOUND BASED IMAGING SYSTEM FOR GUIDING CARDIAC ABLATION 

Patrick D. Wolf, Stephanie A. Eyerly, Douglas M. Dumont, Gregg E. Trahey, and Tristram D. Bahnson

Duke UniversityDepartment of Biomedical Engineering

Duke University Medical Center

IntroductionNormal Sinus Rhythm (NSR)

Atrial Fibrillation (AF)

Thousands of cardiac ablation procedures performed daily to treat tachy-arrhythmias

Increase in the number of procedures that are anatomically guided rather than electrically guided

Atrial Fibrillation 3 M now 7 M by 2050

Introduction

Single lesions directly ablate arrhythmogenic tissue AVNRT, WPW, MAT

Lines of lesion isolate regions (pulmonary veins) - AF

Treating Arrhythmias: Transcatheter Cardiac Ablation (TCA)

http://www.heart.org/HEARTORG/Conditions/Arrhythmia/PreventionandTreatmentofArrhythmia/Ablation_UCM_301991_Article.jsp

Introduction

Radiofrequency Ablation (RFA) in TCA procedures Lines of RFA lesions must be transmural and continuous to block conduction Electrical reconnection occurs at unablated gaps Blood flow around catheter tip and tip-tissue contact affect lesion formation

Lesion size not predictable from delivery parameters Real-time image based evaluation would confirm transmurality and line

contiguity

Non-transmural lesionTransmural

lesion

Unablated Gap

RFA Catheter

Transmural lesion

Endocardium

Epicardium

Blood flow

Introduction Radiofrequency Ablation (RFA)

Young’s Modulus of RFA treated tissue

Pernot et al. Mapping Myocardial Elasticity Changes After RF-Ablation Using Supersonic Shear Imaging. Computers in Cardiology. 2009; 36:793-796

In vitro

In vitro In vivo diastole

Un-treated ~ 27 kPa ~ 10 kPa

RF-treated ~ 54 kPa ~ 20-30 kPa

• RF current heats the tissuecreating a discrete and stiff lesion volume

Introduction Acoustic Radiation Force

Impulse (ARFI) Imaging An ultrasound pulse applies radiation

force F [kg/cm2·s2] over a small region at the focus[1]

absorbed power = Wabsorbed, [W/cm3]absorption coefficient = α ~ 0.00072 [Np/cm][2]

speed of sound = c ~ 1540 or 1615±15 [m/s][3]

Temporal average intensity = I [W/cm2]

Radiation force creates μm scale tissue displacements mechanical properties

Ultrasound scan lines spatially and temporally monitor tissue response

RF Lines

Dis

plac

emen

t [μ

m]

t4

TimeARFI! t0 t1 t2 t3

Axial Response

Time (t)[1] Trahey, G.E., et al., Acoustic radiation force impulse imaging of the mechanical properties of arteries: in vivo and ex vivo results. Ultrasound in Medicine & Biology, 2004.

[2] Sagar, K.B., et al., Quantitative ultrasonic assessment of normal and ischaemic myocardium with an acoustic microscope: relationship to integrated backscatter. Cardiovascular research, 1990.

[3] Masugata, H., et al., Relationship between myocardial tissue density measured by microgravimetry and sound speed measured by acoustic microscopy. Ultrasound in Medicine and Biology, 1999.

In vitro proof of concept ARFI for RFA Lesion Assessment

(in vitro proof of concept) AccuNav ICE catheter and

Siemens Anteres scanner

RFA lesion differentiable from the surrounding myocardium in vitro by measured ARFI-induced displacements

2D ARFI able to provide lesion assessment

Lateral [cm]

PathologyB-M

odeARFI

4

2

0 μm

Axi

al [c

m]

Eyerly et. al. In vitro assessment of ARFI imaging for cardiac visualizing RFA lesions . J Cardiovasc Electrophysiol. Vol 21, 5: 557-563.

In vivo obstacles Three (3) major obstacles to performing

this imaging in vivo:

1. Heart stiffens and softens during each beat

2. Small ARFI induced displacements must be measured in the presence of substantial gross motion

3. A blind search of the myocardium to find lesions is extremely difficult

Eyerly et. al. In vitro assessment of ARFI imaging for cardiac visualizing RFA lesions . J Cardiovasc Electrophysiol. Vol 21, 5: 557-563.

Problem 1 Stiffness Contrast ARFI images must acquired in diastole when there is minimal cardiac

motion and there is maximum elasticity contrast between the RFA lesion and the surrounding myocardium.

Eyerly SA, Hsu SJ, Trahey GE, Wolf PD. In vivo differentiation of myocardial ablation lesions via a stiffness ratio with acoustic radiation force impulse imaging. Eighth International Conference on the Ultrasonic Measurements and Imaging of Tissue Elasticity. Vlissingen, Netherlands: September 2009.

Right ventricular wall before and after ablation. Movie made with multi-beat synthesis using a transthoracic probe positioned on the RV epicardium.

Problem 1 & 2: Contrast and Motion

Trigger the scanner to start the ARFI sequence during the passive filling phase of diastole.

Maximum stiffness contrast with softened myocardium Minimum motion related to contraction

Problem 2: Motion Take multiple scan lines before and after the ARFI ‘push’

Track bulk motion Fit to a quadratic function and interpolate Subtract bulk motion Find ‘Peak Displacement’ Confirm motion filter with ‘zero push’ sequences

Hsu, S., Acoustic Radiation Force Impulse Imaging of Myocardial Performance , in Biomedical Engnieering2009, Duke University.

Problem 3: Find the Lesions Imaging and Guidance Tools Used in TCA Procedures

Fluoroscopy Visualize catheter

position/orientation in the body Can not visualize soft

tissue

Intracardiac Echo (B-Mode) Identifies anatomical

structures Assess catheter-tissue

contact Can not differentiate lesion

from normal tissue

http://emedicine.medscape.com/article/151907-overview

http://www.touchcardiology.com/img/Image/acunav.gif

Find the Lesions

Electroanatomical Mapping (EAM): CARTO (Biosense/Webster) Magnetic fields determine the 3D location of catheters

NaviStar™ mapping/ablation catheter: contains a location coil that measures magnetic field strength

All points plotted relative to a reference patch on the patient’s back

Real-time guidance for TCA procedures

http://www.ipej.org/0801/bhakta.htm http://www.pages.drexel.edu/~gn52/MEDICAL_ROBOTICS/Downloads_files/Carto%20XP%20EP%20Navigation%20System.pdf

Find the Lesions

Electroanatomical Mapping (EAM): CARTO (Biosense Webster) Construct 3D cartoon of chamber geometry Mapping catheter collects endocardial EGs, local activation times (LAT) Visualization of electrical propagation, identifies arrhythmogenic regions

Real-time guidance for TCA procedures

Find the Lesions

Tracking and 2D ICE based imaging Magnetic tracking of imaging tip

Registered display of B-mode images on cartoon Shows “where you are looking”

64 element phased array

http://www.biosensewebster.com/products/navigation/cartosound.aspx

Hsu, S.J., et al., Challenges and implementation of radiation-force imaging with an intracardiac ultrasound transducer. Ieee Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2007. 54(5): p. 996-1009.

In vivo testing

Paced from coronary sinus catheter

Mapped electrical activation using CARTO ARFI imaged normal tissue

Created a lesion line with ≈1 cm gap ARFI imaged line and gap

Closed the gap ARFI imaged line Poster by Eyerly

Right atrium mapped using CARTO EAM system No

Lesion

Lesion with Gap

Gap Closed

Normal and Gap Images

RFA Lesion Gap

RFA Lesion

Gap

Linear RFA ARFI image lesion assessments correlate with electrical block in CARTO LAT maps

In vivo Results

CARTO LAT Maps

TA

81ms

24ms

TA

RFA LesionGap at TA

ContinuousRFA Lesion

RFA Lesion

86.44.8

1.63.2

0μm

Moving to the Clinic - ARFIi Clinical Tool

Research Tool

Acquire ECG synchronous ARFI data on S2000 scanner

Download data to laptop

Calculate and display ARFI displacements on laptop screen

Clinical Tool

Acquire ECG synchronous ARFI data on S2000 scanner

Process and display ARFI data on S2000 screen

Done by Duke in collaboration with Siemens Medical

Moving to the Clinic – FDA Considerations

Mechanical Index (MI)

FDA limit 1.9

Measured peak for our sequences <1.6

Face Heating

FDA limit < 6 °C in standing water

Siemens internal limit <2 °C

Measured change for our sequences <2 °C

Moving to the Clinic Clinically, we seek to understand

• The elasticity contrast for lesions in senescent and diseased hearts

• The ability to access all lesions in both the right and left atria with a limited field of view

• The effect of atrial fibrillation on ARFIi contrast

• The correlation of ARFIi evaluation with electrical outcome in atrial flutter and atrial fibrillation ablation procedures

Clinical Study Outline Patients undergoing AFl ablation or AF ablation

Following each round of ablation in the normal therapeutic procedure image normal and lesioned tissue with ARFI

blinded results to the physician

Characterize “normal” and ablated tissue

In sinus rhythm

In AF

ARFI imaging of ablation lesions can be implemented clinically with significant but minor modifications to existing equipment commonly used in the EP lab (CARTO, ICE).

Lesion boundary correlates with ARFI imaged stiffness boundary

Electrical block correlates with ARFIi determined contiguous and transmural lesions

The tools for clinical evaluation have been created

Summary

Thank you to Dr. Kapur et al for organizing workshop

NIH Grant: R21-EB-007741NIH Grant: R01-EB-012384

Biosense Webster and Siemens Medical Solutions USA, Inc. for their hardware and system support

Acknowledgements