Lasse Lovstakken

Coherent Plane Wave Compounding for Very High Frame Rate Ultrasonography of Rapidly Moving Targets

Coherent plane wave compounding is a promising technique for achieving very high frame rate imaging without compromising image quality or penetration. However, this approach relies on the hypothesis that the imaged object is not moving during the compounded scan sequence, which is not the case in cardiovascular imaging. This work investigates the effect of tissue motion on retrospective transmit focusing in coherent compounded plane wave imaging (PWI). Two compound scan sequences were studied based on a linear and alternating sequence of tilted plane waves, with different timing characteristics. Simulation studies revealed potentially severe degradations in the retrospective focusing process, where both radial and lateral resolution was reduced, lateral shifts of the imaged medium were introduced, and losses in signal-to-noise ratio (SNR) were inferred. For myocardial imaging, physiological tissue displacements were on the order of half a wavelength, leading to SNR losses up to 35 dB, and reductions of contrast by 40 dB. No significant difference was observed between the different tilt sequences. A motion compensation technique based on cross-correlation was introduced, which significantly recovered the losses in SNR and contrast for physiological tissue velocities. Worst case losses in SNR and contrast were recovered by 35 dB and 27-35 dB, respectively. The effects of motion were demonstrated in vivo when imaging a rat heart. Using PWI, very high frame rates up to 463 fps were achieved at high image quality, but a motion correction scheme was then required.

Simultaneous quantification of flow and tissue velocities based on multi-angle plane wave imaging

A quantitative angle-independent 2-D modality for flow and tissue imaging based on multi-angle plane wave acquisition was evaluated. Simulations of realistic flow in a carotid artery bifurcation were used to assess the accuracy of the vector Doppler (VD) technique. Reduction in root mean square deviation from 27 cm/s to 6 cm/s and 7 cm/s to 2 cm/s was found for the lateral (vx) and axial (vz) velocity components, respectively, when the ensemble size was increased from 8 to 50. Simulations of a Couette flow phantom (vmax = 2.7 cm/s) gave promising results for imaging of slowly moving tissue, with root mean square deviation of 4.4 mm/s and 1.6 mm/s for the x- and z-components, respectively. A packet acquisition scheme providing both B-mode and vector Doppler RF data was implemented on a research scanner, and beamforming and further post-processing was done offline. In vivo results of healthy volunteers were in accordance with simulations and gave promising results for flow and tissue vector velocity imaging. The technique was also tested in patients with carotid artery disease. Using the high ensemble vector Doppler technique, blood flow through stenoses and secondary flow patterns were better visualized than in ordinary color Doppler. Additionally, the full velocity spectrum could be obtained retrospectively for arbitrary points in the image.

Eigen-based clutter filter design for ultrasound color flow imaging: a review

Proper suppression of tissue clutter is a prerequisite for visualizing flow accurately in ultrasound color flow imaging. Among various clutter suppression methods, the eigen- based filter has shown potential because it can theoretically adapt its stopband to the actual clutter characteristics even when tissue motion is present. This paper presents a formative review on how eigen-based filters should be designed to improve their practical efficacy in adaptively suppressing clutter without affecting the blood flow echoes. Our review is centered around a comparative assessment of two eigen-filter design considerations: 1) eigen-component estimation approach (single-ensemble vs. multi-ensemble formulations), and 2) filter order selection mechanism (eigenvalue-based vs. frequencybased algorithms). To evaluate the practical efficacy of existing eigen-filter designs, we analyzed their clutter suppression level in two in vivo scenarios with substantial tissue motion (intra-operative coronary imaging and thyroid imaging). Our analysis shows that, as compared with polynomial regression filters (with or without instantaneous clutter downmixing), eigen-filters that use a frequency-based algorithm for filter order selection generally give Doppler power images with better contrast between blood and tissue regions. Results also suggest that both multi-ensemble and single-ensemble eigen-estimation approaches have their own advantages and weaknesses in different imaging scenarios. It may be beneficial to develop an algorithmic way of defining the eigen-filter formulation so that its performance advantages can be better realized.

Ultrasound simulation of complex flow velocity fields based on computational fluid dynamics

In this work, a simulation environment for the development of flow-related ultrasound algorithms is presented. Ultrasound simulations of realistic Doppler signals require accurate modeling of blood flow. Instead of using analytically described flow behavior, complex blood movement can be derived from velocity fields obtained with computational fluid dynamics (CFD). By further modeling blood as a collection of point scatterers, resulting RF-signals can be efficiently retrieved using an existing ultrasound simulation model. The main aim of this paper is to elaborate on creating CFD-based phantoms for ultrasound simulations. The coupling of a computed flow field with an ultrasound model offers flexible control of flow and ultrasound imaging parameters, beneficial for improving and developing imaging algorithms. The proposed method was validated in a straight tube with a stationary parabolic velocity profile and further demonstrated by an eccentrically stenosis carotid bifurcation. The estimated flow velocities are in good agreement with the CFD reference, both for color flow imaging and pulsed-wave doppler simulations. The presented method can also be extended to include wall mechanics simulations in future work.

Impact of competitive flow on wall shear stress in coronary surgery: computational fluid dynamics of a LIMA–LAD model

AIMS Competitive flow from native coronary vessels is considered a major factor in the failure of coronary bypass grafts. However, the pathophysiological effects are not fully understood. Low and oscillatory wall shear stress (WSS) is known to induce endothelial dysfunction and vascular disease, like atherosclerosis and intimal hyperplasia. The aim was to investigate the impact of competitive flow on WSS in mammary artery bypass grafts. METHODS AND RESULTS Using computational fluid dynamics, WSS was calculated in a left internal mammary artery (LIMA) graft to the left anterior descending artery in a three-dimensional in vivo porcine coronary artery bypass graft model. The following conditions were investigated: high competitive flow (non-significant coronary lesion), partial competitive flow (significant coronary lesion), and no competitive flow (totally occluded coronary vessel). Time-averaged WSS of LIMA at high, partial, and no competitive flow were 0.3-0.6, 0.6-3.0, and 0.9-3.0 Pa, respectively. Further, oscillatory WSS quantified as the oscillatory shear index (OSI) ranged from (maximum OSI = 0.5 equals zero net WSS) 0.15 to 0.35, <0.05, and <0.05, respectively. Thus, high competitive flow resulted in substantial oscillatory and low WSS. Moderate competitive flow resulted in WSS and OSI similar to the no competitive flow condition. CONCLUSION Graft flow is highly dependent on the degree of competitive flow. High competitive flow was found to produce unfavourable WSS consistent with endothelial dysfunction and subsequent graft narrowing and failure. Partial competitive flow, however, may be better tolerated as it was found to be similar to the ideal condition of no competitive flow.

Two-dimensional blood velocity estimation with ultrasound: speckle tracking versus crossed-beam vector doppler based on flow simulations in a carotid bifurcation model

Detailed imaging of complex blood flow may improve early diagnosis of cardiovascular disease. In clinical practice, non-invasive flow imaging has been limited to one-dimensional Doppler techniques. Searching for multi-dimensional estimators, research has given attention to speckle tracking (ST) and vector Doppler (VD). However, these techniques have yet to be validated for complex flow patterns as may arise in diseased arteries. In this work, the properties of ST and crossed-beam VD are compared with a ground truth for clinically relevant flow using an ultrasonic simulation environment coupled with the output from computational fluid dynamics (CFD). The statistical properties (n = 80) of ST and VD were first evaluated for stationary flow in a tube for varying vessel positions and angles, and for varying noise levels. The parameter study demonstrated VD to be a more robust axial velocity estimator, and similar results were obtained overall for the lateral velocity component. As an example, the relative standard deviation was 15% and 8% for ST compared with 3% and 10% for VD, for the axial and lateral velocity component, respectively. Further, performance was evaluated for pulsatile flow conditions in a stenosed carotid bifurcation model. A linear regression analysis showed that both methods overall had a good agreement to the CFD reference, however VD suffered from more spurious artifacts and was severely hampered by aliasing in parts of the cardiac cycle. ST was less accurate in estimating the axial component, but prevailed in estimating velocities well beyond the Nyquist range. Based on our simulations, both methods may be used to image complex flow behavior in the carotid bifurcation, however, considering also the scanning limitations of VD, ST may provide a more consistent and practical approach. Future work will entail in vitro and in vivo validation of these results.

Real-time adaptive clutter rejection filtering in color flow imaging using power method iterations

We propose a new algorithm for real-time, adaptive-clutter-rejection filtering in ultrasound color flow imaging (CFI) and related techniques. The algorithm is based on regression filtering using eigenvectors of the signal correlation matrix as a basis for representing clutter, a method that previously has been considered too computationally demanding for real-time processing in general CFI applications. The data acquisition and processing scheme introduced allows for a more localized sampling of the clutter statistics and, therefore, an improved clutter attenuation for lower filter orders. By using the iterative power method technique, the dominant eigenvalues and corresponding eigenvectors of the correlation matrix can be estimated efficiently, rendering real-time operation feasible on desktop computers. A new adaptive filter order algorithm is proposed that successfully estimates the proper dimension of the clutter basis, previously one of the major drawbacks of this clutter-rejection technique. The filter algorithm performance and computational demands has been compared to that of conventional clutter filters. Examples have been included which confirms that, by adapting the clutter-rejection filter to estimates of the clutter-signal statistics, improved attenuation of the clutter signal can be achieved in normal as well as more excessive cases of tissue movement and acceleration

Blood flow imaging - a new real-time, flow imaging technique

This paper presents a new method for the visualization of two-dimensional (2-D) blood flow in ultrasound imaging systems called blood flow imaging (BFI). Conventional methods of color flow imaging (CFI) and power Doppler (PD) techniques are limited as the velocity component transversal to the ultrasound beam cannot be estimated from the received Doppler signal. The BFI relies on the preservation and display of the speckle pattern originating from the blood flow scatterer signal, and it provides qualitative information of the blood flow distribution and movement in any direction of the image. By displaying speckle pattern images acquired with a high frame rate in slow motion, the blood flow movement can be visually tracked from frame to frame. The BFI is easily combined with conventional CFI and PD methods, and the resulting display modes have been shown to have several advantages compared to CFI or PD methods alone. Two different display modes have been implemented: one combining BFI with conventional CFI, and one combining BFI with PD. Initial clinical trials have been performed to assess the clinical usefulness of BFI. The method especially has potential in vascular imaging, but it also shows potential in other clinical applications.

Intrasellar ultrasound in transsphenoidal surgery: a novel technique.

OBJECTIVEResidual tumor masses are common after transsphenoidal surgery. The risk of a residual mass increases with tumor size and parasellar or suprasellar growth. Transsphenoidal surgery is usually performed without image guidance. We aimed to investigate a new technical solution developed for intraoperative ultrasound imaging during transsphenoidal surgery, with respect to potential clinical use and the ability to identify neuroanatomy and tumor. METHODSIn 9 patients with pituitary macroadenomas, intrasphenoidal and intrasellar ultrasound was assessed during transsphenoidal operations. Ultrasound B-mode, power-Doppler and color-Doppler images were acquired using a small prototype linear array, side-looking probe. The long probe tip measures only 3 × 4 mm. We present images and discuss the potential of intrasphenoidal and intrasellar and ultrasound in transsphenoidal surgery. RESULTSWe present 2-dimensional, high-resolution ultrasound images. A small side-looking, high-frequency ultrasound probe can be used to ensure orientation in the midline for the surgical approach to identify important neurovascular structures to be avoided during surgery and for resection control and identification of normal pituitary tissue. The image resolution is far better than what can be achieved with current clinical magnetic resonance imaging technology. CONCLUSIONWe believe that the concept of intrasellar ultrasound can be further developed to become a flexible and useful tool in transsphenoidal surgery.

SHUNT FLOW EVALUATION IN CONGENITAL HEART DISEASE BASED ON TWO-DIMENSIONAL SPECKLE TRACKING

High-frame-rate ultrasound speckle tracking was used for quantification of peak velocity in shunt flows resulting from septal defects in congenital heart disease. In a duplex acquisition scheme implemented on a research scanner, unfocused transmit beams and full parallel receive beamforming were used to achieve a frame rate of 107 frames/s for full field-of-view flow images with high accuracy, while also ensuring high-quality focused B-mode tissue imaging. The setup was evaluated in vivo for neonates with atrial and ventricular septal defects. The shunt position was automatically tracked in B-mode images and further used in blood speckle tracking to obtain calibrated shunt flow velocities throughout the cardiac cycle. Validation toward color flow imaging and pulsed wave Doppler with manual angle correction indicated that blood speckle tracking could provide accurate estimates of shunt flow velocities. The approach was less biased by clutter filtering compared with color flow imaging and was able to provide velocity estimates beyond the Nyquist range. Possible placements of sample volumes (and angle corrections) for conventional Doppler resulted in a peak shunt velocity variations of 0.49-0.56 m/s for the ventricular septal defect of patient 1 and 0.38-0.58 m/s for the atrial septal defect of patient 2. In comparison, the peak velocities found from speckle tracking were 0.77 and 0.33 m/s for patients 1 and 2, respectively. Results indicated that complex intraventricular flow velocity patterns could be quantified using high-frame-rate speckle tracking of both blood and tissue movement. This could potentially help increase diagnostic accuracy and decrease inter-observer variability when measuring peak velocity in shunt flows.