Axial Displacement Estimation Research Articles

Synthetic aperture with high lateral sampling frequency for ultrasound elastography.

Ultrasound elastography is a non-invasive technique for detecting pathological alterations in tissue. It is known that pathological alteration of tissue often has a direct impact on its elastic modulus, which can be revealed using elastography. For estimating elastic modulus, we need to estimate both axial and lateral displacement accurately. Current state of the art elastography techniques provide a substantially less accurate lateral displacement field as compared to the axial displacement field. One of the most important factors in poor lateral estimation is a low sampling frequency in the lateral direction. In this paper, we use synthetic aperture beamforming to benefit from its capability of high sampling frequency in the lateral direction. We compare highly sampled data and focused line per line beam formed data by feeding them to our recently published elastography method, OVERWIND [1]. According to simulation and phantom experiments, not only the lateral displacement estimation is substantially improved, but also the axial displacement estimation is improved.

Diverging beam with synthetic aperture technique for rotation elastography: preliminary experimental results

Rotation Elastogram (RE) is a 2D spatial distribution map of the estimated local rigid-body rotation undergone by a target when subjected to an external compression, which is one of the recent variants in elastographic imaging. A recent study has shown that inclusion-contrast in RE is independent of inclusion-background modulus contrast and thus may be helpful in distinguishing between barely-stiff benign and malignant lesions. However, estimation of quality RE requires not only precise axial displacement estimates but also lateral displacement estimates. The widely used conventional focused beamforming technique using linear array (CFB-LA) provides better lateral resolution only over the depth of focus, which still results in poorer quality lateral displacement estimates compared to the axial displacement estimates. As an alternative to overcome this depth-dependent lateral resolution and obtain an improved lateral resolution, synthetic aperture-based approaches have been proposed in literature. Recently, we developed a synthetic aperture-based method, diverging beam with synthetic aperture technique (DB-SAT) that was aimed to not only reduce the ultrasound system complexity, but also provide improved lateral resolution throughout the depth of imaging and at higher frame-rate than that is possible in CFB-LA. In this paper, we report the preliminary experimental findings on the use of DB-SAT on RE and compare the resultant image quality against that obtained using often-employed CFB-LA and the synthetic transmit aperture (STA) technique. The investigation was done on tissue-mimicking phantoms and using contrast-to-noise ratio (CNR) as the metric for performance evaluation. The estimated CNR values from the REs obtained using CFB-LA, STA, and DB-SAT were 2.69 ± 0.81, 1.35 ± 0.22, and 14.71 ± 9.83, respectively, for inclusion present at 55 mm depth. The obtained results clearly demonstrated that the quality of RE can be improved significantly, especially at larger depth, using DB-SAT compared to that obtained using CFB-LA and STA technique.

Multi-Plane Ultrafast Compound 3D Strain Imaging: Experimental Validation in a Carotid Bifurcation Phantom

Strain imaging of the carotid artery (CA) has demonstrated to be a technique capable of identifying plaque composition. This study assesses the performance of volumetric strain imaging derived from multi-plane acquisitions with a single transducer, with and without displacement compounding. These methods were compared to a reference method using two orthogonally placed transducers. A polyvinyl alcohol phantom was created resembling a stenotic CA bifurcation. A realistic pulsatile flow was imposed on the phantom, resulting in fluid pressures inducing 10% strains. Two orthogonally aligned linear array transducers were connected to two Verasonics systems and fixed in a translation stage. For 120 equally spaced elevational positions, ultrasound series were acquired for a complete cardiac cycle and synchronized using a trigger. Each series consisted of ultrafast plane-wave acquisitions at 3 alternating angles. Inter-frame displacements were estimated using a 3D cross-correlation-based tracking algorithm. Horizontal displacements were acquired using the single probe lateral displacement estimate, the single probe compounded by axial displacement estimates obtained at angles of 19.47 and −19.47 degrees, and the dual probe registered axial displacement estimate. After 3D tracking, least squares strain estimations were performed to compare compressive and tensile principal strains in 3D for all methods. The compounding technique clearly outperformed the zero-degree method for the complete cardiac cycle and resulted in more accurate 3D strain estimates.

Spatial Compounding Technique to Obtain Rotation Elastogram: A Feasibility Study

The perception of stiffness and slipperiness of a breast mass on palpation is used by physicians to assess the level of suspicion of a lesion as being malignant or benign. However, most current ultrasound elastography imaging methods provide only stiffness-related information. There is no existing approach that provides information about the local rigid body rotation undergone by only a loosely bonded, asymmetrically oriented lesion subjected to a small quasi-static compression. The inherent poor lateral resolution in ultrasound imaging poses a limitation in estimating the local rigid body rotation. Several techniques have been reported in the literature to improve the lateral resolution in ultrasound imaging, and among them is spatial compounding. In this study, we explore the feasibility of obtaining better-quality rotation elastograms with spatial compounding through simulations using Field II and experiments on tissue-mimicking phantoms. The phantom was subjected to axial compression (∼1%–2%) from the top, and the angular axial and lateral displacement estimates were obtained using a multilevel 2-D displacement tracking algorithm at different insonification angles. A rotation elastogram (RE) was obtained by taking half of the difference between the lateral gradient of the axial displacement estimates and the axial gradient of the lateral displacement estimates. Contrast-to-noise ratio was used to quantify the improvements in quality of RE. Contrast-to-noise ratio values were calculated by varying the maximum steering angle and the incremental angle, and its effects on RE quality were evaluated. Both simulation and experimental results corroborated and indicated a significant improvement in the quality of RE using compounding technique.

Intracardiac myocardial elastography in canines and humans in vivo.

Intracardiac echocardiography (ICE) is a useful imaging modality which is used during RF ablation procedures to identify anatomical structures. Utilizing ICE in conjunction with myocardial elastography (ME) can provide additional information on the mechanical properties of cardiac tissue and provide information on mechanical changes caused by ablation. The objective of this study was to demonstrate that ICE can be used at high frame rate using a diverging beam transmit sequence to image myocardial strain and differentiate myocardial tissue properties before, during, and after ablation for a clinical ablation procedure. In this feasibility study, three normal canines and eight patients with atrial fibrillation (AF) were studied in vivo. A 5.8-MHz ICE transducer was used to image the heart with a diverging beam transmit method achieving 1200 frames per second (fps). Cumulative axial displacement estimation was performed using 1-D cross-correlation with a window size of 2.7 mm and 95% overlap. Axial cumulative strains were estimated in the left atrium (LA) and right atrium (RA) using a least-squares estimator with a kernel of 2 mm on the axial displacements. In the canine case, radial thickening was detected in the lateral wall and in the interatrial septum during LA emptying. For AF patients, the mean absolute strain in the ablated region was lower (6.7 ± 3.1%) than before the ablation (17.4 ± 9.3%) in LA at the end of the LA emptying phase. In the cavotricuspid isthmus (CTI) region, mean absolute strain magnitude at the end of the RA emptying phase was found to be higher during ablation (43.0 ± 18.1%) compared with after ablation (33.7 ± 15.8%). Myocardial strains in the LA of an AF patient were approximately 2.6 times lower in the ablated region than before ablation. This initial feasibility indicates that ME can be used as a new imaging modality in conjunction with ICE in RF ablation guidance and lesion monitoring.

측방향 움직임 보상을 이용한 초음파 의료용 변형률 영상의 화질개선

In order to improve the quality of strain images in medical ultrasound imaging, displacements need to be accurately estimated. In this paper, in order to apply one-dimensional displacement estimation methods to two-dimensional motion estimation, the axial and lateral displacements are separately estimated. In order to estimate lateral displacements, one-dimensional signals aligned in the lateral direction are converted to analytic signals, which are then crosscorrelated. Strain images are produced by first compensating two-dimensional displacements for lateral motion with lateral motion displacement estimates obtained from the proposed lateral displacement estimation algorithm and then estimating axial displacements. Both phantom and human data experiments show that the proposed method provides better signal-to-noise ratio and contrast-to-noise ratio characteristics than a conventional strain imaging method that utilizes axial displacement estimates only.

A hybrid displacement estimation method for ultrasonic elasticity imaging

Axial displacement estimation is fundamental to many freehand quasistatic ultrasonic strain imaging systems. In this paper, we present a novel estimation method that combines the strengths of quality-guided tracking, multi-level correlation, and phase-zero search to achieve high levels of accuracy and robustness. The paper includes a full description of the hybrid method, in vivo examples to illustrate the method's clinical relevance, and finite element simulations to assess its accuracy. Quantitative and qualitative comparisons are made with leading single- and multi-level alternatives. In the in vivo examples, the hybrid method produces fewer obvious peak-hopping errors, and in simulation, the hybrid method is found to reduce displacement estimation errors by 5 to 50%. With typical clinical data, the hybrid method can generate more than 25 strain images per second on commercial hardware; this is comparable with the alternative approaches considered in this paper.

Elasticity reconstruction from displacement and confidence measures of a multi-compressed ultrasound RF sequence

Ultrasound elasticity imaging shows promise as a new way for early detection of cancers by assessing the elastic characteristics of soft tissue. So far the commonly used approach involves solving the so-called inverse elasticity problem of recovering elastic parameters from displacement measurements. We propose a finite-elementbased nonlinear scheme to estimate the elasticity distribution of soft tissue from multi-compressed ultrasound radio frequency (RF) data. An experimental ultrasound workstation has been developed to acquire multi-compressed data. A composite probe was employed as the compression plate. The contact forces and torques were acquired at the same time as imaging. Axial displacements under different static loads are estimated from the RF data before and after deformation using a cross-correlation technique. The confidence of displacement estimates is employed as a weighting factor in solving the objective function describing the inverse elasticity reconstruction problem. A novel splitand- merge strategy is employed over the image sequence in which strain images are used to provide a priori knowledge of the relative stiffness distribution of the tissue to constrain the inverse problem solution. The experimental study has allowed us to investigate the performance of our approach in the controlled environment of simulated and phantom data. For a simulated single inclusion model with 5% axial displacement estimation error, the L2-error between the target and the reconstructed Young's modulus was found to be about 1%. In vivo validation of the proposed method has been carried out and some preliminary results are presented.

Beamforming Scheme for 2D Displacement Estimation in Ultrasound Imaging

We propose a beamforming scheme for ultrasound imaging leading to the generation of two sets of images, one with oscillations only in the axial direction and one with oscillations only in the lateral direction. Applied to tissue elasticity imaging, this leads to the development of a specific displacement estimation technique that is capable of accurate estimation of two components of the displacement. The mean standard deviation for the axial displacement estimates is 0.0219 times the wavelength of the axial oscillations, and for the lateral estimates, it is equal to 0.0164 times the wavelength of the lateral oscillations. The method is presented and its feasibility is clearly established by a simulation work.

Lateral speckle tracking using synthetic lateral phase

In traditional speckle tracking, lateral displacement (perpendicular to the beam direction) estimates are much less accurate than axial ones (along the beam direction). The accuracy of lateral tracking is very important whenever spatial derivatives of both axial and lateral displacements are required to give a full description of a two-dimensional (2-D) strain field. A number of methods have been proposed to improve lateral tracking by increasing the sampling rate in the lateral direction. We propose an alternate method using synthetic lateral phase (SLP). The algorithm, a direct analog of the phase zero-crossing approach used in axial displacement estimation, synthesizes the lateral phase first, then performs a zero-crossing detection on this synthetic phase to obtain lateral displacement estimates. The SLP is available by simply eliminating either the positive or negative half of the lateral spectrum of the original analytic signal. No new data need to be acquired for this procedure. This new algorithm was tested on both simulations and measurements from a cardiac phantom model. Results show that the method greatly improves the accuracy of lateral tracking, especially for low strain cases (< or =1%). The standard deviation of the estimation error of the lateral normal strain obtained with this approach has an approximate factor of 2-3 improvement for low strain cases. The conceptual and computational simplicity of this new method makes it a practical approach to improve lateral tracking for elasticity imaging.

Lateral speckle tracking using synthetic lateral phase

In traditional speckle tracking, lateral displacement (perpendicular to the beam direction) estimates are much less accurate than axial ones (along the beam direction). The accuracy of lateral tracking is very important whenever spatial derivatives of both axial and lateral displacements are required to give a full description of a two-dimensional (2-D) strain field. A number of methods have been proposed to improve lateral tracking by increasing the sampling rate in the lateral direction. We propose an alternate method using synthetic lateral phase (SLP). The algorithm, a direct analog of the phase zero-crossing approach used in axial displacement estimation, synthesizes the lateral phase first, then performs a zero-crossing detection on this synthetic phase to obtain lateral displacement estimates. The SLP is available by simply eliminating either the positive or negative half of the lateral spectrum of the original analytic signal. No new data need to be acquired for this procedure. This new algorithm was tested on both simulations and measurements from a cardiac phantom model. Results show that the method greatly improves the accuracy of lateral tracking, especially for low strain cases (/spl les/1%). The standard deviation of the estimation error of the lateral normal strain obtained with this approach has an approximate factor of 2-3 improvement for low strain cases. The conceptual and computational simplicity of this new method makes it a practical approach to improve lateral tracking for elasticity imaging.

Lateral Speckle Tracking Using Synthetic Lateral Phase

In traditional speckle tracking, lateral displacement (perpendicular to the beam direction) estimates are much less accurate than axial ones (along the beam direction). The accuracy of lateral tracking is very important whenever spatial derivatives of both axial and lateral displacements are required to give a full description of a two-dimensional (2-D) strain field. A number of methods have been proposed to improve lateral tracking by increasing the sampling rate in the lateral direction. We propose an alternate method using synthetic lateral phase (SLP). The algorithm, a direct analog of the phase zero-crossing approach used in axial displacement estimation, synthesizes the lateral phase first, then performs a zero-crossing detection on this synthetic phase to obtain lateral displacement estimates. The SLP is available by simply eliminating either the positive or negative half of the lateral spectrum of the original analytic signal. No new data need to be acquired for this procedure. This new algorithm was tested on both simulations and measurements from a cardiac phantom model. Results show that the method greatly improves the accuracy of lateral tracking, especially for low strain cases (/spl les/1%). The standard deviation of the estimation error of the lateral normal strain obtained with this approach has an approximate factor of 2-3 improvement for low strain cases. The conceptual and computational simplicity of this new method makes it a practical approach to improve lateral tracking for elasticity imaging.

Theoretical analysis of tissue axial stretching model in elastography*

Abstract The performance of axial displacement estimation in ultrasound elastography can be degraded by tissue lateral displacements, mainly because of the move-in and move-out of tissue scatterers. In our previous work, a tissue axial stretching model was proposed to separate the decorrelation effect induced by lateral displacements from other decorrelation sources. In this paper, the tissue axial stretching model is analyzed theoretically. The theoretical result in a closed form indicates that the peak value of the cross-correlation function between the pre- and post-compression echoes is determined mainly by the beam width, the beam position and the lateral strain. Computer simulations are carried out to verify the theoretical conclusion. The theoretical analysis and simulation results can help to understand more clearly the decorrelation effect of tissue lateral displacements and the 2-D spatial comprehensive cross-correlation method presented previously to reduce the decorrelation effect.