Szymon Cygan

Regional cardiac motion and strain estimation in three-dimensional echocardiography: a validation study in thick-walled univentricular phantoms

Automatic quantification of regional left ventricular deformation in volumetric ultrasound data remains challenging. Many methods have been proposed to extract myocardial motion, including techniques using block matching, phase-based correlation, differential optical flow methods, and image registration. Our lab previously presented an approach based on elastic registration of subsequent volumes using a B-spline representation of the underlying transformation field. Encouraging results were obtained for the assessment of global left ventricular function, but a thorough validation on a regional level was still lacking. For this purpose, univentricular thick-walled cardiac phantoms were deformed in an experimental setup to locally assess strain accuracy against sonomicrometry as a reference method and to assess whether regions containing stiff inclusions could be detected. Our method showed good correlations against sonomicrometry: r2 was 0.96, 0.92, and 0.84 for the radial (εRR), longitudinal (εLL), and circumferential (εCC) strain, respectively. Absolute strain errors and strain drift were low for εLL (absolute mean error: 2.42%, drift: -1.05%) and εCC (error: 1.79%, drift: -1.33%) and slightly higher for εRR (error: 3.37%, drift: 3.05%). The discriminative power of our methodology was adequate to resolve full transmural inclusions down to 17 mm in diameter, although the inclusion-to-surrounding tissue stiffness ratio was required to be at least 5:2 (absolute difference of 39.42 kPa). When the inclusion-to-surrounding tissue stiffness ratio was lowered to approximately 2:1 (absolute difference of 22.63 kPa), only larger inclusions down to 27 mm in diameter could still be identified. Radial strain was found not to be reliable in identifying dysfunctional regions.

A Dual-Chamber, Thick-Walled Cardiac Phantom for Use in Cardiac Motion and Deformation Imaging by Ultrasound

Determination of the mechanical properties of the myocardium is crucial for cardiac diagnosis. Cardiac strain and strain rate imaging may enable such quantification. To further develop these methodologies, an experimental setup allowing the recording of ultrasonic deformation data in a reproducible manner is necessary. Such setup with biventricular polyvinyl alcohol heart phantoms has been built. To test this setup, segmental longitudinal, radial and circumferential displacement, velocity, strain and strain rate in the phantoms were measured using a clinical ultrasound scanner and commercially available deformation imaging algorithms (based on both tissue velocity imaging and speckle tracking). The model deformation was close to that observed in the human left ventricular wall and was highly reproducible (e.g., the average peak longitudinal strain for the mid- and apical phantom segments equals -15.32 +/- 0.53% and -19 +/- 6% for the ventricle wall). The experimental setup is a valuable source of data for the development of algorithms for deformation estimation.

Detailed Evaluation of Five 3D Speckle Tracking Algorithms Using Synthetic Echocardiographic Recordings

A plethora of techniques for cardiac deformation imaging with 3D ultrasound, typically referred to as 3D speckle tracking techniques, are available from academia and industry. Although the benefits of single methods over alternative ones have been reported in separate publications, the intrinsic differences in the data and definitions used makes it hard to compare the relative performance of different solutions. To address this issue, we have recently proposed a framework to simulate realistic 3D echocardiographic recordings and used it to generate a common set of ground-truth data for 3D speckle tracking algorithms, which was made available online. The aim of this study was therefore to use the newly developed database to contrast non-commercial speckle tracking solutions from research groups with leading expertise in the field. The five techniques involved cover the most representative families of existing approaches, namely block-matching, radio-frequency tracking, optical flow and elastic image registration. The techniques were contrasted in terms of tracking and strain accuracy. The feasibility of the obtained strain measurements to diagnose pathology was also tested for ischemia and dyssynchrony.

Three-dimensional cardiac motion and strain estimation: A validation study in thick-walled univentricular phantoms

Automatic quantification of regional left ventricular function in volumetric ultrasound data remains challenging. Our lab previously presented such an approach based on elastic registration of subsequent volumes using a b-spline representation of the underlying transformation field. Good results were obtained for the assessment of global function, but a thorough validation on a regional level was still lacking. For this purpose, univentricular thick-walled cardiac phantoms were designed and mounted in an experimental setup to (i) locally assess strain accuracy against two reference methods: sonomicrometry and simulations through finite element modeling (FEM); and (ii) to assess whether regions containing stiff inclusions could be detected. Our method showed good correlations against sonomicrometry: R2 was 0.95, 0.92 and 0.84 in the radial (R), longitudinal (L) and circumferential (C) direction respectively. Furthermore, the predicted apex-base gradients by FEM were best resolved in the circumferential direction. Finally, the discriminative power of our methodology was adequate to resolve inclusions up to 17mm in diameter, although enough stiffness difference with the surrounding tissue was needed.

Elastic registration vs. block matching for quantification of cardiac function with 3D ultrasound: Initial results of a direct comparison in silico based on a new evaluation pipeline

This paper presents a comparison between block matching and elastic registration for the estimation of cardiac deformation and strain from 3D ultrasound. The comparison study exploits a new evaluation pipeline recently developed by the authors. The pipeline generates synthetic sequences that are extremely similar to real ultrasound recording while the synthetic cardiac motion is fully controlled by an electro-mechanical model of the heart. Hereto, five synthetic sequences were generated corresponding to one healthy heart and four ischemic ones. Elastic registration returned both the smaller tracking errors and the most robust strain estimates. Although with limitations, this study brings further evidence that the new technique might be ready for extensive clinical testing.

Effect of Transmural Extent of the Simulated Infarction in a Left Ventricular Model on Displacement and Strain Distribution Estimated from Synthetic Ultrasonic Data

The identification of a sub-endocardial infarction is of major interest in cardiology. This study evaluates the sensitivity of selected measures to the thickness of such an infarction. Synthetic ultrasonic data (long-axis view) of left ventricular models with inclusions were generated using Field II and meshes obtained from finite-element simulations, which also provided the reference for the estimates obtained from ultrasonic data. The displacements, the first and second component of the principal strain (ε1 and ε2), and several measures derived from these quantities were estimated. All estimates, except for the poorly estimated ε2, exhibited sensitivity to the presence and transmurality of the inclusion. The most sensitive was the gradient of the averaged transmural profiles of ε1, and ε1 averaged over the area corresponding to the transmural inclusion. The inflection point of the ε1 profile shifted toward the outer wall with increasing thickness of the non-transmural inclusion.

Influence of Polivinylalcohol Cryogel Material Model in FEM Simulations on Deformation of LV Phantom

One of the available tools for validation of strain imaging methods are physical phantoms, most frequently produced of polivinylalcohol cryogel (PVA). This material was often assumed to exhibit elastic properties, but it has more complex nature. In this work we examine the influence of the applied material model – elastic vs hyperelastic – on the strains within the numerical model of the phantom obtained from FEM. This influence appeared significant – hyperelastic model provides lower strain contrasts and also the ratios between radial, circumferential and longitudinal strains differ for both models.

Experimental setup with dual chamber cardiac phantom for ultrasonic elastography

Strain (S) and strain rate (SR) imaging may enable to detect and quantify changes in regional myocardial wall contractility and viability. However, the range and rate of cardiac wall deformation make development and evaluation of strain estimation algorithms difficult. A well-controlled experimental setup capable of producing the deformation patterns similar to those encountered in vivo may provide controlled data for this development. Such a setup was build, consisting of a 2-chamber biventricular homogeneous Polyvinyl Alcohol (PVA) heart phantom connected to a hydraulic pulsatile mock circulation system. The phantom was cyclically deformed using a computer controlled piston pump at a stroke volume near 83 ml, stroke rate of 1 Hz, and systole/diastole ratio 2/3. The internal LV pressure data were collected using a tip-catheter manometer (F 4.1, Sentron, Nl). Two dimensional color Doppler myocardial imaging (CDMI) data were recorded from the phantom using standard long and short axis views (∼200 frames/s, 2.4 MHz, Vivid 7, GE, Norway). Tissue velocity (TV), S and SR values for both radial and longitudinal phantom wall deformation were estimated. For radial deformation SR and S profiles were computed for anterior, lateral, posterior and inferior LV wall segments. For longitudinal deformation, they were determined for mid segment of the lateral LV wall. The obtained values of TV, SE and S are close to those found in clinical research. The experimental setup appears to be an interesting tool providing controlled data for testing new methods and algorithms for myocardial strain and strain rate estimation.

Left ventricular phantoms with inclusions simulating transmural and non-transmural infarctions: FEM and EchoPAC study

The cardiac elastography evolves to enable local strain estimation and identification of non-transmural infarctions. Below we compare the strain values obtained using EchoPAC in physical left ventricular phantoms made of PVA with results of the Finite Element Modelling (FEM) studies on their counterparts. Models had the form of half of an ellipsoid with 15 mm wall thickness. The homogenous model, transmural inclusion model and nontransmural inclusion (5mm thickness) model were designed. The inclusions were located in the mid segment. The material of the ventricle in the FEM studies was modeled as a hyperelastic, isotropic one. The material parameters came from measurements of the PVA samples for the homogenous case and were extrapolated to obtain stiffer inclusions. The model was deformed by applying 36 kPa pressure load to its inner surface. Peak systolic strain values were close to those observed in healthy subjects. A dedicated setup, the Vivid 6 scanner, probe M4S-RS and EchoPAC BT13 software were used in experiments. The values of strains from FEM models were averaged over nodes corresponding to the layers used in the EchoPAC software. The circumferential strain (CS) values from the FEM simulation and the physical experiment are qualitatively very close and correlate well with the clinical data. The experimental CS results also agree with expectations in terms of slope across the wall and effect of the inclusion. Segmental radial strains obtained from EchoPAC and FEM are close. The proposed approach (phantoms, setup) may be used for development of methods for identification of nontransmural infarctions.

Block matching and B-spline methods in deformation estimation in synthetic left ventricular model with nontransmural infarction

The cardiac elastography aims at identification of non-transmural infarctions. Two displacement estimation methods in such an application using synthetic ultrasonic data are studied. Reference was obtained from Finite Element Modelling. Models had the form of half of an ellipsoid with 15 mm wall thickness. The homogenous model, models with transmural and nontransmural inclusion were designed. Deformation of the models was simulated using Abaqus. Ultrasonic data of LAX and SAX views were generated using Field II. Radial (dR) and lateral (dL) displacements were estimated using a 2D correlation search with 2D stretching (2DCS) and B-spline (BS) method. Strains were estimated using least squares estimator. Mean Absolute Error (MAE) of the dR in the LAX view was approx. 6[μm] for 2DCS and 8[μm] for BS, that of the dL 30 and 24[μm] respectively. MAE of the second component of the principal strain (epsilon)2 was 0.10 and 0.14[%], respectively. Corresponding values for SAX view were 7, 10, 42, 52[μm] and 0.47 and 1.08[%]. In the LAX view both estimation methods result in the (epsilon)2 behavior coherent with the presence of the inclusion, with the 2DCS results closer to the reference. In the SAX view the BS approach results in high errors of the estimate. The (epsilon)2 profiles, LAX view, show minor discrepancies with respect to the reference and show the effect of the inclusion. The (epsilon)2 profiles, SAX view, obtained from displacements estimated using the BS method strongly deviate from the reference. Block matching performs better in application to the local strain estimation.