Maartje M. Nillesen

Ultrafast vascular strain compounding using plane wave transmission

Deformations of the atherosclerotic vascular wall induced by the pulsating blood can be estimated using ultrasound strain imaging. Because these deformations indirectly provide information on mechanical plaque composition, strain imaging is a promising technique for differentiating between stable and vulnerable atherosclerotic plaques. This paper first explains 1-D radial strain estimation as applied intravascularly in coronary arteries. Next, recent methods for noninvasive vascular strain estimation in a transverse imaging plane are discussed. Finally, a compounding technique that our group recently developed is explained. This technique combines motion estimates of subsequently acquired focused ultrasound images obtained at various insonification angles. However, because the artery moves and deforms during the multi-angle acquisition, errors are introduced when compounding. Recent advances in computational power have enabled plane wave ultrasound acquisition, which allows 100 times faster image acquisition and thus might resolve the motion artifacts. In this paper the performance of strain imaging using plane wave compounding is investigated using simulations of an artery with a vulnerable plaque and experimental data of a two-layered vessel phantom. The results show that plane wave compounding outperforms 0° focused strain imaging. For the simulations, the root mean squared error reduced by 66% and 50% for radial and circumferential strain, respectively. For the experiments, the elastographic signal-to-noise and contrast-to-noise ratio (SNR(e) and CNR(e)) increased with 2.1 dB and 3.7 dB radially, and 5.6 dB and 16.2dB circumferentially. Because of the high frame rate, the plane wave compounding technique can even be further optimized and extended to 3D in future.

2H-1 In Vivo 3D Cardiac and Skeletal Muscle Strain Estimation

In this study, BiPlane imaging was adapted for measuring strain in actively deforming tissue in three orthogonal directions. BiPlane imaging assures a sufficient frame rate (75-120 Hz) for accurate strain estimation. A coarse-to-fine iterative 2D strain algorithm using spatial correction and local stretching was implemented. Considering the huge amount of generated data, a fast interpolation scheme was implemented for measuring sub-sample and sub-line displacements. Assuming a 2D parabolic shape of the cross-correlation function, a straightforward and direct calculation of the displacements is possible. The strain estimation method was validated by means of a simulation study and phantom experiments. Rf-data were acquired with a 3D X4 matrix array transducer (Philips Sonos 7500) in BiPlane mode. In vivo verification in human skeletal muscle was performed. Furthermore, cardiac strain imaging was conducted using cardiac BiPlane data of dogs. In a pilot animal study, beagles with an induced valvular aortic stenosis were monitored. The Field II simulation was used for determining the accuracy and detectibility of the algorithm and revealed excellent correlation between applied and measured axial strain (SNR = 43 dB) for a window of 0.60 mm. Obviously, a lower SNR was found in lateral and elevational direction. The in vivo verification experiment in the skeletal muscles revealed similar cumulative axial strain curves (up to 8%) in both the azimuth and elevational direction. The shape of the strain curve matched perfectly with the curve of the measured force. The lateral strain values parallel to the direction of the muscle fibers matched the axial strain curves, whereas the shape of the lateral strain in the perpendicular plane differed due to anisotropy. Finally, strain images of the beagles were calculated. The beagle with the most excessive pressure gradient revealed a decrease of the radial strain. Furthermore, an elongated plateau in the radial strain indicated hypertrophy. In conclusion, 3D cardiac and strain estimation is feasible using a real-time 3D scanner. Additional validation studies of full 3D imaging modes are required to fully validate the technique

Quantitative Ultrasound Imaging of Healthy and Reconstructed Cleft Lip: A Feasibility Study:

Objective: To investigate the feasibility of echographic imaging of healthy and reconstructed cleft lip and to estimate tissue dimensions and normalized echo level. Methods: Echographic images of the upper lip were made on three healthy subjects and two patients using a linear array transducer (7 to 11 MHz bandwidth) and a noncontact gel coupling. Tissue dimensions were measured using calipers. Echo levels were calibrated and were corrected for beam characteristics, gel path, and tissue attenuation using a tissue-mimicking phantom. Results: At the central position of the philtrum, mean thickness (SD) of lip loose connective tissue layer, orbicularis oris muscle, and dense connective layer was 4.0 (0.1) mm, 2.3 (0.7) mm, and 2.2 (0.7) mm, respectively, in healthy lip at rest; and 4.1 (0.9) mm, 3.8 (1.7) mm, and 2.6 (0.6) mm, respectively, in contracted lip. Mean (SD) echo level of muscle and dense connective tissue layer with respect to echo level of lip loose connective tissue layer was −19.3 (0.6) dB and −10.7 (4.0) dB, respectively, in relaxed condition and −20.7 (1.5) dB and −7.7 (2.3) dB, respectively, in contracted state. Color mode echo images were calculated, showing lip tissues in separate colors and highlighting details like discontinuity of the orbicularis oris muscle and presence of scar tissue. Conclusions: Quantitative assessment of thickness and echo level of various lip tissues is feasible after proper echographic equipment calibration. Diagnostic potentials of this method for noninvasive evaluation of cleft lip reconstruction outcome are promising.

Methodical study on the estimation of strain in shearing and rotating structures using radio frequency ultrasound based on 1-D and 2-D strain estimation techniques

This simulation study is concerned with: 1) the feasibility of measuring rotation and 2) the assessment of the performance of strain estimation in shearing and rotating structures. The performance of 3 different radio frequency (RF) based methods is investigated. Linear array ultrasound data of a deforming block were simulated (axial shear strain = 2.0, 4.0, and 6.0%, vertical strain = 0.0, 1.0, and 2.0%). Furthermore, data of a rotating block were simulated over an angular range of 0.5° to 10°. Local displacements were estimated using a coarse-to-fine algorithm using 1-D and 2-D precompression kernels. A new estimation method was developed in which axial displacements were used to correct the search area for local axial motion. The study revealed that this so-called free-shape 2-D method outperformed the other 2 methods and produced more accurate displacement images. For higher axial shear strains, the variance of the axial strain and the axial shear strain reduced by a factor of 4 to 5. Rotations could be accurately measured up to 4.0 to 5.0°. Again, the free-shape 2-D method yielded the most accurate results. After reconstruction of the rotation angle, the mean angles were slightly underestimated. The precision of the strain estimates decreased with increasing rotation angles. In conclusion, the proposed free-shape 2-D method enhances the measurement of (axial shear) strains and rotation. Experimental validation of the new method still has to be performed.

2-D Versus 3-D Cross-Correlation-Based Radial and Circumferential Strain Estimation Using Multiplane 2-D Ultrafast Ultrasound in a 3-D Atherosclerotic Carotid Artery Model

Three-dimensional (3-D) strain estimation might improve the detection and localization of high strain regions in the carotid artery (CA) for identification of vulnerable plaques. This paper compares 2-D versus 3-D displacement estimation in terms of radial and circumferential strain using simulated ultrasound (US) images of a patient-specific 3-D atherosclerotic CA model at the bifurcation embedded in surrounding tissue generated with ABAQUS software. Global longitudinal motion was superimposed to the model based on the literature data. A Philips L11-3 linear array transducer was simulated, which transmitted plane waves at three alternating angles at a pulse repetition rate of 10 kHz. Interframe (IF) radio-frequency US data were simulated in Field II for 191 equally spaced longitudinal positions of the internal CA. Accumulated radial and circumferential displacements were estimated using tracking of the IF displacements estimated by a two-step normalized cross-correlation method and displacement compounding. Least-squares strain estimation was performed to determine accumulated radial and circumferential strain. The performance of the 2-D and 3-D methods was compared by calculating the root-mean-squared error of the estimated strains with respect to the reference strains obtained from the model. More accurate strain images were obtained using the 3-D displacement estimation for the entire cardiac cycle. The 3-D technique clearly outperformed the 2-D technique in phases with high IF longitudinal motion. In fact, the large IF longitudinal motion rendered it impossible to accurately track the tissue and cumulate strains over the entire cardiac cycle with the 2-D technique.

10B-4 4D Cardiac Strain Imaging: Methods and Initial Results

In this study, four-dimensional (3D+t) ultrasound imaging techniques were used for the development and in vivo verification of 3D strain imaging. Two different iterative coarse- to-fine 3D strain estimation methods were developed. One method was based on measuring displacements using 3D kernels and a 3D cross-correlation function. The second method used 2D kernels and cross-correlation, and estimated 3D displacements in an iterative process. A 3D or 2D parabolic interpolation was used for sub-sample displacement estimates. The strain estimation methods were experimentally validated using a gelatin phantom with a hard cylindrical inclusion (four times stiffer). The phantom was compressed with a plate in steps of 0.5 mm up to 3.0 mm (3% strain). Rf-data were acquired with a 3D matrix array transducer (X4, Philips Sonos 7500) in ECG-triggered 3D full volume mode. Preliminary in vivo validation was performed by acquiring 3D full volume data (frame rate = 19 Hz) of the left ventricle of a trained athlete. Both methods were able to produce high quality elastograms of the inclusion model up to an applied compression of 3% strain (resulting in 0.5% - 5% axial strain in the phantom). No significant difference in elastographic signal-to- noise ratio (SNRe) was found between the two methods. The iterative 2D algorithm is favored for the shorter computation time. The signal- and contrast-to-noise ratios (SNRe, CNRe) of the axial elastograms increased to 28 and 53 dB, respectively (compared to previously described BiPlane axial strain images). Lateral and elevational elastograms were also in accordance with finite element solutions of the phantom model. However, the SNRe and CNRe were considerably lower (16 and 33 dB), which is presumably caused by the lower in-plane spatial resolution of the 3D full volume data. Initial in vivo results revealed mean strain profiles in three orthogonal directions comparable with our previous studies, although, the maximum radial strain was lower than expected (20%). Hence, 3D cardiac strain imaging is feasible even at a relatively low frame rate.

A Comparison Between Compounding Techniques Using Large Beam-Steered Plane Wave Imaging for Blood Vector Velocity Imaging in a Carotid Artery Model

Conventional color Doppler imaging is limited, since it only provides velocity estimates along the ultrasound beam direction for a restricted field of view at a limited frame rate. High-frame-rate speckle tracking, using plane wave transmits, has shown potential for 2-D blood velocity estimation. However, due to the lack of focusing in transmit, image quality gets reduced, which hampers speckle tracking. Although ultrafast imaging facilitates improved clutter filtering, it still remains a major challenge in blood velocity estimation. Signal dropouts and poor velocity estimates are still present for high beam-to-flow angles and low blood flow velocities. In this paper, ultrafast plane wave imaging was combined with multiscale speckle tracking to assess the 2-D blood velocity vector in a common carotid artery (CCA) flow field. A multiangled plane wave imaging sequence was used to compare the performance of displacement compounding, coherent compounding, and compound speckle tracking. Zero-degree plane wave imaging was also evaluated. The performance of the methods was evaluated before and after clutter filtering for the large range of velocities (0-1.5 m/s) that are normally present in a healthy CCA during the cardiac cycle. An extensive simulation study was performed, based on a sophisticated model of the CCA, to investigate and evaluate the performance of the methods at different pulse repetition frequencies and signal-to-noise levels. In vivo data were acquired of a healthy carotid artery bifurcation to support the simulation results. In general, methods utilizing compounding after speckle tracking, i.e., displacement compounding and compound speckle tracking, were least affected by clutter filtering and provided the most robust and accurate estimates for the entire velocity range. Displacement compounding, which uses solely axial information to estimate the velocity vector, provided most accurate velocity estimates, although it required sufficiently high pulse repetition frequencies in high blood velocity phases and reliable estimates for all acquisition angles. When this latter requirement was not met, compound speckle tracking was most accurate, because it uses the possibility to discard angular velocity estimates corrupted by clutter filtering. Similar effects were observed for in vivo data obtained at the carotid artery bifurcation. Investigating a combination of these two compounding techniques is recommended for future research.Conventional color Doppler imaging is limited, since it only provides velocity estimates along the ultrasound beam direction for a restricted field of view at a limited frame rate. High frame rate speckle tracking, using plane wave transmits, has shown potential for 2D blood velocity estimation. However, due to the lack of focusing in transmit, image quality gets reduced, which hampers speckle tracking. Although ultrafast imaging facilitates improved clutter filtering, it still remains a major challenge in blood velocity estimation. Signal drop-outs and poor velocity estimates are still present for high beam-to-flow angles and low blood flow velocities. In this work, ultrafast plane wave imaging was combined with multi-scale speckle tracking to assess the 2D blood velocity vector in a common carotid artery (CCA) flow field. A multi-angled plane wave imaging sequence was used to compare the performance of displacement compounding, coherent compounding and compound speckle tracking. Zero-degree plane wave imaging was also evaluated. The performance of the methods was evaluated before and after clutter filtering for the large range of velocities (0 to 1.5 m/s) that are normally present in a healthy CCA during the cardiac cycle. An extensive simulation study was performed, based on a sophisticated model of the CCA, to investigate and evaluate the performance of the methods at different pulse repetition frequencies and signal-to-noise levels. In vivo data were acquired of a healthy carotid artery bifurcation to support the simulation results. In general, methods utilizing compounding after speckle tracking, i.e., displacement compounding and compound speckle tracking, were least affected by clutter filtering and provided the most robust and accurate estimates for the entire velocity range. Displacement compounding, which uses solely axial information to estimate the velocity vector, provided most accurate velocity estimates, although it required sufficiently high pulse repetition frequencies in high blood velocity phases and reliable estimates for all acquisition angles. When this latter requirement was not met, compound speckle tracking was most accurate, because it uses the possibility to discarded angular velocity estimates corrupted by clutter filtering. Similar effects were observed for in vivo data obtained at the carotid artery bifurcation. Investigating a combination of these two compounding techniques is recommended for future research.

Cardiac motion estimation using ultrafast ultrasound imaging tested in a finite element model of cardiac mechanics

Recent developments in ultrafast ultrasound imaging allow accurate assessment of 3D cardiac deformation in cardiac phases with high deformation rates. This paper investigates the performance of a multiple spherical wave (SW) ultrasound transmission scheme in combination with a motion estimation algorithm for cardiac deformation assessment at high frame rates. Ultrasound element data of a realistically deforming 3D cardiac finite element model were simulated for a phased array transducer, transmitting five SWs (PRF 2500 Hz). After delay-and-sum beamforming, coherent compounding of multiple SW transmissions was performed to generate radiofrequency data (frame rate 500 Hz). Axial and lateral displacements were determined using a normalized cross-correlation-based technique. Good agreement was obtained between estimated and ground truth displacements derived from the model over the cardiac cycle. This study indicates that high frame rate displacement estimation using multiple SWs is feasible and serves as an important step towards high frame rate 3D cardiac deformation imaging.

2D versus 3D cross-correlation-based radial and circumferential strain imaging in a 3D atherosclerotic carotid artery model using ultrafast plane wave ultrasound

Three-dimensional vascular strain estimation is crucial for assessment of the location of high strain regions in the carotid artery (CA) and the identification of vulnerable plaque features. This study compares 2D vs. 3D displacement estimation in terms of radial and circumferential strain using simulated ultrasound images of a 3D atherosclerotic CA model at the bifurcation embedded in surrounding tissue. The 3D finite element model (FEM) of a patient-specific, pulsating atherosclerotic CA (pulse pressure 60 mmHg) was generated with ABAQUS FEM software. Global longitudinal motion was superimposed to the model. Radiofrequency (RF) ultrasound data were simulated in Field II by moving point scatterers (vessel wall) according to the deformation patterns of the model. A linear array transducer (fc = 9 MHz, pitch = 198 μm, 192 elements) was used which transmitted plane waves at 3 alternating angles (+19.5°, 0°, -19.5°) at a pulse repetition rate of 10 kHz. Simulations with 20 ms (systole) and 100 ms (diastole) inter-frame (IF) time were performed for 191 equally spaced (0.1 mm) longitudinal positions of the internal CA containing fatty and calcified areas. After delay-and-sum beamforming, IF axial displacements were estimated using a coarse-to-fine normalized cross-correlation method. The axial displacement at 0° was used as the vertical displacement component. Projection of the -19.5° and +19.5° axial displacements yielded the horizontal displacement component. A polar grid and the lumen center were determined in the end-diastolic frame of each longitudinal position and used to convert the tracked vertical and horizontal displacements into radial and circumferential displacements. Least squares strain estimation was performed to determine accumulated radial and circumferential strain. The performance of the 2D and 3D method was compared by calculating the root-mean-squared error (RMSE) of the estimated strains with respect to the reference strains obtained from the model. More accurate strain images were obtained using the 3D displacement estimation for the entire cardiac cycle. The 3D technique clearly outperforms the 2D technique in phases with high IF longitudinal motion.

New developments in paediatric cardiac functional ultrasound imaging

Ultrasound imaging can be used to estimate the morphology as well as the motion and deformation of tissues. If the interrogated tissue is actively deforming, this deformation is directly related to its function and quantification of this deformation is normally referred as ‘strain imaging’. Tissue can also be deformed by applying an internal or external force and the resulting, induced deformation is a function of the mechanical tissue characteristics. In combination with the load applied, these strain maps can be used to estimate or reconstruct the mechanical properties of tissue. This technique was named ‘elastography’ by Ophir et al. in 1991. Elastography can be used for atherosclerotic plaque characterisation, while the contractility of the heart or skeletal muscles can be assessed with strain imaging. Rather than using the conventional video format (DICOM) image information, radio frequency (RF)-based ultrasound methods enable estimation of the deformation at higher resolution and with higher precision than commercial methods using Doppler (tissue Doppler imaging) or video image data (2D speckle tracking methods). However, the improvement in accuracy is mainly achieved when measuring strain along the ultrasound beam direction, so it has to be considered a 1D technique. Recently, this method has been extended to multiple directions and precision further improved by using spatial compounding of data acquired at multiple beam steered angles. Using similar techniques, the blood velocity and flow can be determined. RF-based techniques are also beneficial for automated segmentation of the ventricular cavities. In this paper, new developments in different techniques of quantifying cardiac function by strain imaging, automated segmentation, and methods of performing blood flow imaging are reviewed and their application in paediatric cardiology is discussed.