Peter Hollender

Intracardiac Echocardiography Measurement of Dynamic Myocardial Stiffness with Shear Wave Velocimetry

Acoustic radiation force (ARF)-based methods have been demonstrated to be a viable tool for noninvasively estimating tissue elastic properties, and shear wave velocimetry has been used to measure quantitatively the stiffening and relaxation of myocardial tissue in open-chest experiments. Dynamic stiffness metrics may prove to be indicators for certain cardiac diseases, but a clinically viable means of remotely generating and tracking transverse wave propagation in myocardium is needed. Intracardiac echocardiography (ICE) catheter-tip transducers are demonstrated here as a viable tool for making this measurement. ICE probes achieve favorable proximity to the myocardium, enabling the use of shear wave velocimetry from within the right ventricle throughout the cardiac cycle. This article describes the techniques used to overcome the challenges of using a small probe to perform ARF-driven shear-wave velocimetry and presents in vivo porcine data showing the effectiveness of this method in the interventricular septum.

Single- and Multiple-Track-Location Shear Wave and Acoustic Radiation Force Impulse Imaging: Matched Comparison of Contrast, Contrast-to-Noise Ratio and Resolution

Acoustic radiation force impulse imaging and shear wave elasticity imaging (SWEI) use the dynamic response of tissue to impulsive mechanical stimulus to characterize local elasticity. A variant of conventional, multiple-track-location SWEI, denoted single-track-location SWEI, offers the promise of creating speckle-free shear wave images. This work compares the three imaging modalities using a high push and track beam density combined acquisition sequence to image inclusions of different sizes and contrasts. Single-track-location SWEI is found to have a significantly higher contrast-to-noise ratio than multiple-track-location SWEI, allowing for operation at higher resolution. Acoustic radiation force impulse imaging and single-track-location SWEI perform similarly in the larger inclusions, with single-track-location SWEI providing better visualization of small targets ≤ 2.5 mm in diameter. The processing of each modality introduces different trade-offs between smoothness and resolution of edges and structures; these are discussed in detail.

Intracardiac acoustic radiation force impulse (ARFI) and shear wave imaging in pigs with focal infarctions

Four pigs, three with focal infarctions in the apical intraventricular septum (IVS) and/or left ventricular free wall (LVFW), were imaged with an intracardiac echocardiography (ICE) transducer. Custom beam sequences were used to excite the myocardium with focused acoustic radiation force (ARF) impulses and image the subsequent tissue response. Tissue displacement in response to the ARF excitation was calculated with a phase-based estimator, and transverse wave magnitude and velocity were each estimated at every depth. The excitation sequence was repeated rapidly, either in the same location to generate 40 Hz M-modes at a single steering angle, or with a modulated steering angle to synthesize 2-D displacement magnitude and shear wave velocity images at 17 points in the cardiac cycle. Both types of images were acquired from various views in the right and left ventricles, in and out of infarcted regions. In all animals, acoustic radiation force impulse (ARFI) and shear wave elasticity imaging (SWEI) estimates indicated diastolic relaxation and systolic contraction in noninfarcted tissues. The M-mode sequences showed high beat-to-beat spatio-temporal repeatability of the measurements for each imaging plane. In views of noninfarcted tissue in the diseased animals, no significant elastic remodeling was indicated when compared with the control. Where available, views of infarcted tissue were compared with similar views from the control animal. In views of the LVFW, the infarcted tissue presented as stiff and non-contractile compared with the control. In a view of the IVS, no significant difference was seen between infarcted and healthy tissue, whereas in another view, a heterogeneous infarction was seen to be presenting itself as non-contractile in systole.

Micro-elasticity (μ-E): CNR and resolution of acoustic radiation force impulse imaging and single- and multiple track location shear wave elasticity imaging for visualizing small targets

Acoustic radiation force impulse (ARFI) imaging and shear wave elasticity imaging (SWEI) use the dynamic response of tissue to impulsive mechanical stimulus to characterize local elasticity. A variant of conventional, multiple track location SWEI (MTL-SWEI), denoted single track location SWEI (STL-SWEI) offers the promise of creating speckle-free shear wave images. This work compares the three imaging modalities using a high push and track beam density combined acquisition sequence to image stiff inclusions with diameters of 1.5 and 6 mm. STL-SWEI is shown to have significantly higher CNR than MTL-SWEI, allowing for operation at higher resolution. ARFI and STL-SWEI perform similarly in the 6 mm inclusions, but STL-SWEI images the 1.5 mm targets with the highest CNR and best resolution. The processing trade-offs between CNR and resolution for each modality are discussed.

Three-dimensional fusion of Shear Wave Imaging and electro-anatomical mapping for intracardiac radiofrequency ablation monitoring

Electroanatomical Mapping (EAM) provides electrophysiologists with a three-dimensional visualization of endocardial geometry and electrical activity, used to guide transcatheter ablation (TCA) treatment of arrhythmia. Acoustic Radiation Force Impulse (ARFI) and Shear Wave Imaging (SWI) are ultrasonic methods of imaging the mechanical properties of the myocardium that may allow direct visualization of ablation lesion formation, and can be acquired with Intracardiac Echocardiography (ICE) imaging catheters, which are commonly used to guide TCA. The mechanical substrate mapping holds promise to complement the electrical data to improve the safety and efficacy of the procedure. This work demonstrates the use of an image fusion system to stitch a series of in vivo planar ARFI or SWI images together into a 3-D volume of substrate elasticity.

Feasibility and safety of transthoracic cardiac acoustic radiation force impulse imaging methods

This work examines clinical feasibility of using noninvasive transthoracic echocardiography techniques to visualize temporal variations of stiffness through the cardiac cycle using acoustic radiation force impulse (ARFI) imaging. Custom M-mode ARFI sequences were implemented on a Verasonics Research Platform using a Philips/ATL P4-2 phased-array echocardiography transducer. The research systems robust power supply, full parallel-receive capability, and programmable interface enabled sustained excitations, rapid data acquisition, and real-time processing and display of images in the clinic. An extended radiation force pulse length of 480 μs was used to produce tissue displacements up to 12μm around a region of excitation focused at 3 cm. Quadratic motion filters were used to separate ARFI excitation-induced displacements from intrinsic cardiac and respiratory physiological motion artifacts. Acoustic intensity and face heating measurements, as well as finite element method tissue focal heating simulations, were completed. These measurements and simulations calibrated the sequences with respect to the FDA acoustic exposure limits for intensity, mechanical index (MI) and tissue heating. Tests were conducted in phantom and animal models in preparation for the clinical trial. A series of 7 healthy volunteers were scanned in accordance with an approved Duke University Medical Center Institutional Review Board (IRB) protocol. Measurements were acquired from the apical 4 chamber view of the apex, at power levels with MIs ranging from 1.9-3.0. During each M-mode ARFI acquisition, the matched ECG signal was acquired, enabling registration with cardiac cycle. The M-mode ARFI displacement images reflect the expected myocardial stiffness changes through the cardiac cycle, with greatest displacements in diastole and lowest in systole. In the 7 volunteers, the mean displacements throughout the cardiac cycle rose with increasing transmit power level. The ratio of diastolic-to-systolic displacement was examined as a possible indicator of myocardial health. In this study, the measured ratios were in range up to 3.1:1 for the 7 patients, showing agreement with previous ratios reported by an animal studies using transthoracic, intracardiac and epicardial imaging methods. These preliminary clinical results support the feasibility of real-time imaging of cardiac stiffness in vivo using transthoracic ARFI imaging.

Intracardiac shear wave velocimetry using Acoustic Radiation Force (ARF) excitations: In vivo results

Acoustic Radiation Force (ARF)-driven shear wave velocimetry is a validated way of quantitatively measuring tissue elasticity. Open-chest velocimetry experiments have been able to measure regional myocardial contractility, a potentially useful diagnostic parameter for heart disease, but a clinically-viable method of making this measurement is needed. Intracardiac Echocardiography (ICE) catheter-tip ultrasound probes are demonstrated here to measure myocardial contractility while remaining minimally invasive. Specialized techniques such as nonrigid elastic registration address the challenges of using ICE for ARF methods, and in vivo data are presented, demonstrating the effectiveness of this technique.

Eliminating speckle noise with three-dimensional single-track-location shear wave elasticity imaging (STL-SWEI)

Conventional multiple track location shear wave elasticity imaging (MTL-SWEI) is a powerful tool for noninvasively estimating tissue elasticity. The resolution and noise of MTL-SWEI systems, however, are limited by ultrasound speckle. Single track location SWEI (STL-SWEI) is a novel variant which fixes the position of the tracking beam and modulates the push location to effectively cancel out the effects of speckle bias. We present a 3D STL-SWEI system which provides full suppression of lateral and elevation speckle bias for high resolution 3D elasticity imaging, and requires no inherent spatial filtering to make precise measurements of shear wave speed. The system is demonstrated in a uniform elasticity phantom.

A multiresolution approach to shear wave image reconstruction

Shear wave imaging techniques build maps of local elasticity estimating the local group velocity of induced mechanical waves. Velocity estimates are formed using the time delay in the motion profile of the medium at two or more points offset from the shear wave source. Because the absolute time-of-flight between any pair of locations scales with the distance between them, there is an inherent trade-off between robustness to time-of-flight errors and lateral spatial resolution based on the number and spacing of the receive points used for each estimate. This work proposes a method of using the time delays measured between all combinations of locations to estimate a noise-robust, high-resolution image. The time-of-flight problem is presented as an overdetermined system of linear equations that can be directly solved with and without spatial regularization terms. Finite element method simulations of acoustic radiation force-induced shear waves are used to illustrate the method, demonstrating superior contrast-to-noise ratio and lateral edge resolution characteristics compared with linear regression of arrival times. This technique may improve shear wave imaging in situations where time-of-flight noise is a limiting factor.

A comparison of intracardiac ARFI and SWI for imaging radiofrequency ablation lesions

Radiofrequency ablation (RFA) is commonly used to treat cardiac arrhythmias, by generating a contiguous series of discrete radiofrequency ablation (RFA) lesions in the myocardium to destroy or isolate arrhythmogenic conduction pathways. The size of each lesion is controlled by the duration and power of the delivered RF energy, and by the temperature of the tissue at the surface, but feedback on the extent and transmurality of the generated lesion are unavailable with current technology. Intracardiac Echocardiography (ICE) may provide a solution through Acoustic Radiation Force Impulse (ARFI) imaging or Shear Wave Imaging (SWI), which each generate images of local mechanical compliance from very small ultrasonically-induced waves. This work compares ARFI and SWI in an ex vivo experiment for lesion boundary assessment and lesion gap resolution.