Karl Thiele

Towards pointwise motion tracking in echocardiographic image sequences--comparing the reliability of different features for speckle tracking.

In this paper, we studied the problem of feature-based motion tracking in echocardiographic image sequences. We described the relation between possible feature variations and different kinds of tissue motion using a linear convolution model. We also showed that motion-feature decorrelation (which means that the motion parameters estimated using feature tracking fail to represent the underlying tissue motion) compensation is an ill-posed inverse problem. Instead of finding a method that may provide better compensation results than previous approaches, we used an quantitative measure to compare the reliability of tracking features. Experiment results showed that the use of the reliability measure improved the robustness of displacement estimation. With the help of the reliability measure, we compared the performance of different features using simulations and phantom examples. While we noticed that the radio frequency (RF) signal outperforms the B-mode (BM) signal in the analysis of small deformation (e.g., less than 0.1% compression), we also found out that the BM signal works better than the RF signal in the analysis of large deformation (e.g., larger than 2% compression). The use of a band-passed filtered feature does not result in significant improvement in tracking.

Ultrasound imaging system and method for spatial compounding

An ultrasound imaging system is provided along with a method for spatially compounding a plurality of ultrasound images in an elevation dimension. The ultrasound imaging system and method reduce image speckle. The ultrasound imaging system may include a transducer in electrical communication with an ultrasound system controller configured to generate, transmit, and receive a series of ultrasound energy pulses. The ultrasound system controller is further configured to recover image information using beamforming techniques from steered or focused ultrasound imaging planes prior to spatial compounding of a plurality of image frames. The present invention may also be broadly viewed as providing a method for ultrasound imaging. Briefly stated, the method comprises the following steps: generating a transmit scan beam; generating a plurality of ultrasound response scan beams originating from spatially separated vantage points such that corresponding response scan beams converge at the transmit scan beam; deriving image information from the plurality of ultrasound response scan beams; and compounding the image information in the elevation direction.

Defining Left Ventricular Apex-to-Base Twist Mechanics Computed From High-Resolution 3D Echocardiography: Validation Against Sonomicrometry

OBJECTIVES To compute left ventricular (LV) twist from 3-dimensional (3D) echocardiography. BACKGROUND LV twist is a sensitive index of cardiac performance. Conventional 2-dimensional based methods of computing LV twist are cumbersome and subject to errors. METHODS We studied 10 adult open-chest pigs. The pre-load to the heart was altered by temporary controlled occlusion of the inferior vena cava, and myocardial ischemia was produced by ligating the left anterior descending coronary artery. Full-volume 3D loops were reconstructed by stitching of pyramidal volumes acquired from 7 consecutive heart beats with electrocardiography gating on a Philips IE33 system (Philips Medical Systems, Andover, Massachusetts) at baseline and other steady states. Polar coordinate data of the 3D images were entered into an envelope detection program implemented in MatLab (The MathWorks, Inc., Natick, Massachusetts), and speckle motion was tracked using nonrigid image registration with spline-based transformation parameterization. The 3D displacement field was obtained, and rotation at apical and basal planes was computed. LV twist was derived as the net difference of apical and basal rotation. Sonomicrometry data of cardiac motion were also acquired from crystals anchored to epicardium in apical and basal planes at all states. RESULTS The 3D dense tracking slightly overestimated the LV twist, but detected changes in LV twist at different states and showed good correlation (r = 0.89) when compared with sonomicrometry-derived twist at all steady states. In open chest pigs, peak cardiac twist was increased with reduction of pre-load from inferior vena cava occlusion from 6.25 degrees +/- 1.65 degrees to 9.45 degrees +/- 1.95 degrees . With myocardial ischemia from left anterior descending coronary artery ligation, twist was decreased to 4.90 degrees +/- 0.85 degrees (r = 0.8759). CONCLUSIONS Despite lower spatiotemporal resolution of 3D echocardiography, LV twist and torsion can be computed accurately.

Radial Basis Functions for Combining Shape and Speckle Tracking in 4D Echocardiography

Quantitative analysis of left ventricular deformation can provide valuable information about the extent of disease as well as the efficacy of treatment. In this work, we develop an adaptive multi-level compactly supported radial basis approach for deformation analysis in 3D+time echocardiography. Our method combines displacement information from shape tracking of myocardial boundaries (derived from B-mode data) with mid-wall displacements from radio-frequency-based ultrasound speckle tracking. We evaluate our methods on open-chest canines (N=8) and show that our combined approach is better correlated to magnetic resonance tagging-derived strains than either individual method. We also are able to identify regions of myocardial infarction (confirmed by postmortem analysis) using radial strain values obtained with our approach.

Accuracy of real-time, three-dimensional Doppler echocardiography for stroke volume estimation compared with phase-encoded MRI: an in vivo study.

To the editor: Accurate measurement of cardiac output (CO) is important in clinical medicine. Current methods of calculation use two-dimensional echocardiography and ventricle volumetry or spectral Doppler pulse wave. Phase-encoded MRI is an accurate technique for volume assessment; however, is not suitable for many clinical situations, such as in those who are critically ill. Previous work by our group has shown that real-time, three-dimensional Doppler echocardiography (RT3DDE) can accurately compute stroke volume (SV) and CO in an animal model against a “gold standard” of aortic flow probe using the left ventricular outflow tract colour Doppler signal,1 and this technique can be used in patients transthoracically.2 3 We tested the accuracy of RT3DDE of the left ventricular outflow tract for calculation of SV in a series of patients and healthy volunteers against a “gold standard” of phase-encoded MRI. Two patients and 12 healthy volunteers took part in the study. The patients were undergoing MRI as part of their clinically indicated investigations for cardiac disease (both patients for aortic coarctation). All subjects gave written informed consent in accordance with the local Institutional Review Board. The Philips Live 3D 7500 Sonos echocardiography system with a 3000-element 2–4 MHz xMATRIX transthoracic probe (Philips Medical Systems, Andover, Massachusetts, USA) was used for imaging. Subjects underwent echocardiography immediately before MRI. Between three and five ECG-gated, three-dimensional colour Doppler volumes were acquired from …

Aberration correction apparatus and methods

An ultrasound system and method for aberration correction processing in conjunction with finely pitched transducer elements and/or aberration correction processing in conjunction with a hierarchical control scheme. Results obtained from known aberration correction algorithms may be improved with the use of finely pitched transducer elements wherein the pitch of the elements (or subgroup of elements) is less than or equal to the wavelength of an ultrasound signal at the fundamental frequency. Elements of a transducer may be grouped into subgroups with aberration correction algorithms being applied to the output of each subgroup.

Motion analysis of 3D ultrasound texture patterns

We model the process of imaging soft tissues with a 3D ultrasound probe using a linear convolution model, and obtain analytical expressions of both the ultrasound image and its spectrum. We use this model to study the ultrasound decorrelation caused by tissue motion both in the spatial domain and spectral domain. Finally, we propose a spectral-feature-based algorithm to analyze tissue motion. The comparison with intensity-based algorithm shows promising results.

3D Elasticity imaging using principal stretches on an open-chest dog heart

Ultrasound strain imaging has demonstrated its ability to quantitatively assess myocardial viability and contractility altered by myocardial ischemia. However, current ultrasound strain imaging methods still use lower dimensional methods to monitor 3D heart motion. Some 3-D tracking algorithms have also been developed recently in different groups. Quantitative analysis using current methods depends on ultrasound probe orientation and selection of a centroid. To address this, 3D elasticity imaging derived using principal stretches independent of the centroid point is used to assess the contractility of myocardial fibers with 3D data from a commercial 3D scanner on an open-chest dog heart. In this study, an open-chest dog was performed according to a Yale institutional animal protocol and 3D radio frequency (RF) volume data were acquired using an commercial 2D phased array (iE33, Philips, Andover, MA) placed in front of the anterior wall of the left-ventricle with a small water stand-off. 3D speckle tracking was applied to estimate 3D displacement with tracking resolution of 1.2 mm in the axial direction and 4.5 mm in azimuthal and zenithal directions. Volume-to-volume tracking results were accumulated referenced to the heart geometry at the end of diastole until the end of systole. Three principal stretches were estimated using eigenvalue decomposition of the derived right Cauchy deformation tensor at each point. Initial results of strains based on principal stretch and the principal direction has successfully demonstrated that heart wall is thickening along the radial direction and shortening along the longitudinal direction during systole. Unlike 1D or 2D methods, 3D speckle tracking can estimate myocardiums three-dimensional motion. Three strains based on the principal stretches were derived from 3D displacements to assess myocardial contractility independent of probe position and the selection of the centroid point.

Detection and display of acoustic window for guiding and training cardiac ultrasound users

Successful ultrasound data collection strongly relies on the skills of the operator. Among different scans, echocardiography is especially challenging as the heart is surrounded by ribs and lung tissue. Less experienced users might acquire compromised images because of suboptimal hand-eye coordination and less awareness of artifacts. Clearly, there is a need for a tool that can guide and train less experienced users to position the probe optimally. We propose to help users with hand-eye coordination by displaying lines overlaid on B-mode images. The lines indicate the edges of blockages (e.g., ribs) and are updated in real time according to movement of the probe relative to the blockages. They provide information about how probe positioning can be improved. To distinguish between blockage and acoustic window, we use coherence, an indicator of channel data similarity after applying focusing delays. Specialized beamforming was developed to estimate coherence. Image processing is applied to coherence maps to detect unblocked beams and the angle of the lines for display. We built a demonstrator based on a Philips iE33 scanner, from which beamsummed RF data and video output are transferred to a workstation for processing. The detected lines are overlaid on B-mode images and fed back to the scanner display to provide users real-time guidance. Using such information in addition to B-mode images, users will be able to quickly find a suitable acoustic window for optimal image quality, and improve their skill.

Novel method of measuring valvular regurgitation using three-dimensional nonlinear curve fitting of doppler signals within the flow convergence zone

Mitral valve regurgitation (MR) is among the most prevalent and significant valve problems in the Western world. Echocardiography plays a significant role in the diagnosis of degenerative valve disease. However, a simple and accurate means of quantifying MR has eluded both the technical and clinical ultrasound communities. Perhaps the best clinically accepted method used today is the 2-D proximal isovelocity surface area (PISA) method. In this study, a new quantification method using 3-D color Doppler ultrasound, called the field optimization method (FOM), is described. For each 3-D color flow volume, this method iterates on a simple fluid dynamics model that, when processed by a model of ultrasound physics, attempts to agree with the observed velocities in a least-squares sense. The output of this model is an estimate of the regurgitant flow and the location of its associated orifice. To validate the new method, in vitro experiments were performed using a pulsatile flow loop and different geometric orifices. Measurements from the FOM and from 2-D PISA were compared with measurements made with a calibrated ultrasonic flow probe. Results show that the new method has a higher correlation to the truth data and has lower inter- and intra-observer variability than the 2-D PISA method.