Stian Langeland

3-D Speckle Tracking for Assessment of Regional Left Ventricular Function

Speckle tracking in 2-D ultrasound images has become an established tool for assessment of left ventricular function. The recent development of ultrasound systems with capability to acquire real-time full volume data of the left ventricle makes it possible to perform speckle tracking in three dimensions, and thereby track the real motion of the myocardium. This paper presents a method for assessing local strain and rotation from 3-D speckle tracking in apical full-volume datasets. The method has been tested on simulated ultrasound data based on a computer model of the left ventricle, and on patients with myocardial infarction. When applied on simulated ultrasound data, the method showed good agreement with strain and rotation traces calculated from the reference motion, and the method was able to capture segmental differences in the deformation pattern, although the magnitudes of strains were systematically lower than the reference strains. When applied on patients, the method demonstrated reduced strain in the infarcted areas. Bulls-eye plots of regional strains showed good correspondence with wall motion scoring based on 2-D apical images, although the dyskinetic and hypokinetic regions were not apparent in all strain components.

Comparison of time-domain displacement estimators for two-dimensional RF tracking

Techniques have been described in the literature to enable multidimensional strain rate estimation. They are based on multidimensional velocity estimation. One of the problems in obtaining robust lateral strain rate estimates is the fact that lateral velocity estimates are intrinsically noisier than axial ones. The aim of this study was to find the optimal estimator for tracking of the radiofrequency patterns both in axial and lateral directions. Performances of the following estimators were investigated using simulations: cross-correlation, normalized cross-correlation, sum of absolute differences and sum of squared differences. Two-dimensional (2-D) velocity estimation was not feasible using cross-correlation. However, normalized cross-correlation, sum of absolute differences and sum of squared differences showed accurate axial and lateral results. For smaller window lengths, sum of squared differences was found to be the preferred estimator for 2-D velocity estimation using a 1-D kernel.

RF-based two-dimensional cardiac strain estimation: a validation study in a tissue-mimicking phantom

Strain and strain rate imaging have been shown to be useful techniques for the assessment of cardiac function. However, one of the major problems of these techniques is their angle dependency. In order to overcome this problem, a new method for estimating the strain (rate) tensor had previously been proposed by our lab. The aim of this study was to validate this methodology in a phantom setup. A tubular thick-walled tissue-mimicking phantom was fixed in a water tank. Varying the intraluminal pressure resulted in a cyclic radial deformation. The 2D strain was calculated from the 2D velocity estimates, obtained from 2D radio frequency (RF) tracking using a 1D kernel. Additionally, ultrasonic microcrystals were implanted on the outer and inner walls of the tube in order to give an independent measurement of the instantaneous wall thickness. The two methods were compared by means of linear regression, the correlation coefficient, and Bland-Altman statistics. As expected, the strain estimates dominated by the azimuth velocity component were less accurate than the ones dominated by the axial velocity component. Correlation coefficients were found to be r=0.78 for the former estimates and =0.83 was found for the latter. Given that the overall shape and timing of the 2D deformation were very accurate (r=0.95 and r=0.84), these results were within acceptable limits for clinical applications. The 2D RF-tracking using a 1D kernel thus allows for 2D, and therefore angle-independent, strain estimation.

Fast ultrasound imaging simulation in K-space

Most available ultrasound imaging simulation methods are based on the spatial impulse response approach. The execution speed of such a simulation is of the order of days for one heart-sized frame using desktop computers. For some applications, the accuracy of such rigorous simulation approaches is not necessary. This work outlines a much faster 3-D ultrasound imaging simulation approach that can be applied to tasks like simulating 3-D ultrasound images for speckle-tracking. The increased speed of the proposed simulation method is based primarily on the approximation that the point spread function is set to be spatially invariant, which is a reasonably good approximation when using polar coordinates for simulating images from phased arrays with constant aperture. Ultrasound images are found as the convolution of the PSF and an object of sparsely distributed scatterers. The scatterers are passed through an anti-aliasing filter before insertion into a regular beam-space grid to reduce the bandwidth and significantly reduce the amount of data. A comparison with the well-established simulation software package field II has been made. A simulation of a cyst image using the same input object was found to be in the order of 7000 times slower than the presented method. Following these considerations, the proposed simulation method can be a rapid and valuable tool for working with 3-D ultrasound imaging and in particular 3-D speckle-tracking.

A new method for two-dimensional myocardial strain estimation by ultrasound: an in-vivo comparison with sonomicrometry

At present, ultrasonic strain and strain rate imaging only give information on the deformation occurring along the image line. As a result, the techniques have been shown to be angle dependent. As a first step to overcome this problem, fully resolved two-dimensional (2D) strain estimates are required. We have developed a new methodology for the estimation of 2D strain based on in-silico and in-vitro experiments. The paper reports a study which further develops and validates this methodology in an in-vivo setting. Ultrasound RF data were acquired in a parasternal long axis view. Myocardial radial and longitudinal strain components were simultaneously estimated in the inferolateral wall using the new methodology from single RF data sets. After baseline acquisitions, the deformation was altered. Ultrasonically estimated peak systolic radial and longitudinal strain were validated against sonomicrometry by means of linear regression and Bland-Altman analysis. For both strain components, good agreement was found between the ultrasound and the sonomicrometry measurements. Simultaneous estimation of both in-plane myocardial strain components using our methodology gave accurate information on myocardial deformation in the in-vivo setting. Myocardial strain can thus be assessed in-plane, independent of insonation angle. This could potentially accelerate the clinical acceptance of deformation imaging in cardiology.

The ultrasonic assessment of radial, longitudinal and circumferential cardiac strain in normal pigs

Background: Ultrasound Strain Rate Imaging (SRI) is a technique that is used to assess regional myocardial function. To date, only the radial strain could be measured in a porcine model because the deformation could only be detected along the ultrasound image line in parasternal views. Nevertheless, the longitudinal and circumferential strain can contain a lot of information about cardiac (patho) physiology. Using a new technique, 2D strain, these two strain components can also be measured by measuring the strain along and perpendicular to the image line. The purpose of this study was to test the feasibility of assessing all strain components, (radial, longitudinal and circumferential) in a closed-chest pig model. Methods: 13 closed-chest pigs were anesthetized and im- aged using a Toshiba Aplio system. During brief apnea, radio- frequency data were acquired from parasternal long and short axis views. Subsequently, both in-plane myocardial strain com- ponents of the anteroseptal and inferolateral wall were extracted using custom made software (SPEQLE-2D). In this way, radial and longitudinal strain was obtained from the long axis data sets and radial and circumferential strain from the short axis images. Results: For all animals, all strain components could be esti- mated in both myocardial walls and showed normal physiological behavior. The average±standard deviation of the end-systolic radial, longitudinal and circumferential strain components were calculated. Conclusions: The non-invasive assessment of all strain com- ponents of the left ventricular anteroseptal and inferolateral wall was shown to be feasible. This might open interesting new possibilities to investigate the patho-physiology of different cardiac diseases.

Regional Differences in Systolic Active Stress Profiles in the Normal Beating Heart. Assesment Using an Ultrasound Based Mathematical Model

Active stress (˙<inf>A</inf>) developed by cardiac muscle has been measured in isolated muscle preparations, under physiological loading conditions, by subtracting the passive stress (˙<inf>P</inf>) from the total stress (˙<inf>T</inf>). We previously developed a mechanical model based on M-mode ultrasound imaging to calculate these stresses in beating hearts. However, this model was based on one-dimensional imaging information and could not estimate regional differences in ˙<inf>A</inf>. In the current study this model was improved by including two-dimensional B-mode echocardiographic data. Methods: In a porcine model a micro-manometer tipped catheter was used to measure left-ventricular pressure (LVP) and B-mode ultrasound images were recorded in a short-axis view. On the ultrasound image points in the mid-wall were selected and tracked to completely define the deformation of the myocardium. A kinematic model of the LV was then constructed from the displacement vectors of these points. ˙<inf>T</inf>was calculated from the LVP. The material parameters for an exponential stress/strain relation were estimated during the diastolic E-wave when it was assumed that ˙<inf>A</inf>= 0. These parameters were used to calculate ˙<inf>P</inf>during systole and by subtracting this from ˙<inf>T</inf>, ˙<inf>A</inf>was calculated. Results: The timing and shape of ˙<inf>A</inf>profiles match those obtained from isolated muscle experiments. ˙<inf>A</inf>was higher and peaked sooner in the posterior wall than in the anterior wall. Conclusion: Regional active stress estimation is possible in normal beating hearts.