A microfluidic device based on a pair of slant-finger interdigital transducers (SFITs) is developed to achieve a selective and flexible manipulation of microbubbles (MBs) by surface acoustic waves (SAWs). The resonance frequency of SAWs generated by the SFITs depends on the location of its parallel pathway; the particles at different locations of the SAWs' pathway can be controlled selectively by choosing the frequency of the excitation signal applied on the SFITs. By adjusting the input signal continuously, MBs can be transported along the acoustic aperture precisely. The displacement of MBs has a linear relationship with the frequency shift. The resolution of transportation is 15.19 ± 2.65 μm when the shift of input signal frequency is at a step of 10 kHz. In addition, the MBs can be controlled in a two-dimensional plane by combining variations of the frequency and the relative phase of the excitation signal applied on the SFITs simultaneously. This technology may open up the possibility of selectively and flexibly manipulating MBs using a simple one-dimensional device.
This paper proposes a novel strain estimator using scale-invariant keypoints tracking (SIKT) for ultrasonic elastography. This method is based on tracking stable features between the pre- and post-compression A-lines to obtain tissue displacement estimates. The proposed features, termed scaleinvariant keypoints, are independent of signal scale change according to the scale-space theory, and therefore can preserve their patterns while undergoing a substantial range of compression. The keypoints can be produced by searching for repeatedly assigned points across all possible scales constructed from the convolution with a one-parameter family of Gaussian kernels. Because of the distinctive property of the keypoints, the SIKT method could provide a reliable tracking over changing strains, an effective resistance to anamorphic noise and sonographic noise, and a significant reduction in processing time. Simulation and experimental results show that the SIKT method is able to provide better sensitivity, a larger dynamic range of the strain filter, higher resolution, and a better contrast- to-noise ratio (CNRe) than the conventional methods. Moreover, the computation time of the SIKT method is approximately 5 times that of the cross-correlation techniques.
A microfluidic device was developed to precisely transport a single cell or multiple microbubbles by introducing phase-shifts to a standing leaky surface acoustic wave (SLSAW). The device consists of a polydimethyl-siloxane (PDMS) microchannel and two phase-tunable interdigital transducers (IDTs) for the generation of the relative phase for the pair of surface acoustic waves (SAW) propagating along the opposite directions forming a standing wave. When the SAW contacts the fluid medium inside the microchannel, some of SAW energy is coupled to the fluid and the SAW becomes the leaky surface wave. By modulating the relative phase between two IDTs, the positions of pressure nodes of the SLSAW in the microchannel change linearly resulting in the transportation of a single cell or microbubbles. The results also reveal that there is a good linear relationship between the relative phase and the displacement of a single cell or microbubbles. Furthermore, the single cell and the microbubbles can be transported over a predetermined distance continuously until they reach the targeted locations. This technique has its distinct advantages, such as precise position-manipulation, simple to implement, miniature size, and noninvasive character, which may provide an effective method for the position-manipulation of a single cell and microbubbles in many biological and biomedical applications.
Determining a multidimensional velocity field within microscale opaque fluid flows is needed in areas such as microfluidic devices, biofluid mechanics and hemodynamics research in animal studies. The ultrasonic particle image velocimetry (EchoPIV) technique is appropriate for measuring opaque flows by taking advantage of PIV and B-mode ultrasound contrast imaging. However, the use of clinical ultrasound systems for imaging flows in small structures or animals has limitations associated with spatial resolution. This paper reports on the development of a high-resolution EchoPIV technique (termed as micro-EPIV) and its application in measuring flows in small vessel-mimic phantoms and vessels of small animals. Phantom experiments demonstrate the validity of the technique, providing velocity estimates within 4.1% of the analytically derived values with regard to the flows in a small straight vessel-mimic phantom, and velocity estimates within 5.9% of the computationally simulated values with regard to the flows in a small stenotic vessel-mimic phantom. Animal studies concerning arterial and venous flows of living rats and rabbits show that the micro-EPIV-measured peak velocities within several cardiac cycles are about 25% below the values measured by the ultrasonic spectral Doppler technique. The micro-EPIV technique is able to effectively measure the flow fields within microscale opaque fluid flows.
Echo-PIV technique was used to obtain 2D velocity vectors of flows in a 50% stenosis model. The experiment system was constructed, including a regionally stenotic channel that mimic artery with thrombosis, a fluid circulation system, and an ultrasound imaging equipment. The stenotic channel was fabricated using a pair of male and female plastic rods, a mold, and tissue mimic phantom materials. Ultrasound contrast agents was added into fully developed laminar flow, and Echo-PIV technique was used to obtain the flow velocity pattern in the post stenosis region of the stenosis model. The simulation result of Fluent software was applied for validation. Echo-PIV technique is confirmed to be a reliable 2D velocimetry technique for blood flow in artery with thrombosis.
This paper presents a new algorithm for ultrasonic particle image velocimetry (Echo PIV) for improving the flow velocity measurement accuracy and efficiency in regions with high velocity gradients. The conventional Echo PIV algorithm has been modified by incorporating a multiple iterative algorithm, sub-pixel method, filter and interpolation method, and spurious vector elimination algorithm. The new algorithms' performance is assessed by analyzing simulated images with known displacements, and ultrasonic B-mode images of in vitro laminar pipe flow, rotational flow and in vivo rat carotid arterial flow. Results of the simulated images show that the new algorithm produces much smaller bias from the known displacements. For laminar flow, the new algorithm results in 1.1% deviation from the analytically derived value, and 8.8% for the conventional algorithm. The vector quality evaluation for the rotational flow imaging shows that the new algorithm produces better velocity vectors. For in vivo rat carotid arterial flow imaging, the results from the new algorithm deviate 6.6% from the Doppler-measured peak velocities averagely compared to 15% of that from the conventional algorithm. The new Echo PIV algorithm is able to effectively improve the measurement accuracy in imaging flow fields with high velocity gradients.
A microfluidic device was developed to precisely transport a single cell or multiple microbubbles by introducing phase-shifts to a standing leaky surface acoustic wave (SLSAW). The device consists of a polydimethyl-siloxane (PDMS) microchannel and two phase-tunable interdigital transducers (IDTs) for the generation of the relative phase for the pair of surface acoustic waves (SAW) propagating along the opposite directions forming a standing wave. When the SAW contacts the fluid medium inside the microchannel, some of SAW energy is coupled to the fluid and the SAW becomes the leaky surface wave. By modulating the relative phase between two IDTs, the positions of pressure nodes of the SLSAW in the microchannel change linearly resulting in the transportation of a single cell or microbubbles. The results also reveal that there is a good linear relationship between the relative phase and the displacement of a single cell or microbubbles. Furthermore, the single cell and the microbubbles can be transported over a predetermined distance continuously until they reach the targeted locations. This technique has its distinct advantages, such as precise position-manipulation, simple to implement, miniature size, and noninvasive character, which may provide an effective method for the position-manipulation of a single cell and microbubbles in many biological and biomedical applications.
Manual segmentation of ultrasound contrast images is time-consuming and inevitable to variability, and computer-based segmentation algorithms often require user interaction. This paper proposes a novel level set model for fully automated segmentation of vascular ultrasound contrast images. The initial contour of arterial boundaries is acquired based on an automatic procedure. The level set model moves the initial contour towards the boundaries of arterial inner wall based on minimization of the energy function. The traditional energy function is improved by introducing an edge detector based on image gradient and the standard difference image. Both spatial and temporal information of the image are considered, and the robustness and accuracy of the level set model is enhanced. Ultrasonic contrast images of living mouse are acquitted with high frequency ultrasound system. Images of carotid arteries are processed with our method. The segmentation results using the proposed method are evaluated against two observers' hand-outlined boundaries, showing that computer-generated boundaries agree well with the observers' hand-outlined boundaries as much as the different observers agree with each other.
Many cardiovascular diseases are closely associated with the mechanical properties of arterial wall and hemodynamic parameters. Simultaneous measurements of the arterial strain and flow pattern may aid diagnosis of cardiovascular diseases and may be useful to study fluid-structure interaction between blood and vessel. This paper proposes a 2D non-invasive ultrasonic method to simultaneously measure arterial strain and flow pattern with sub-pixel accuracy.The method uses a multiple iterative algorithm to estimate the geometrical transformations of arterial wall and high-velocity gradient flows simultaneously. The accuracy of the method was validated by an in vitro arterial phantom and in vivo common carotid arteries (CCAs) of 12 mice using a Sonix RP (10 MHz) and a VisualSonics Vevo 2100 (30 MHz) ultrasound imaging system, respectively.For the arterial phantom, the calculated elasticity modulus from the strain profile shows good agreement with the mechanical testing value, deviating no more than 9.3%. The calculated flow velocity agrees well with the value obtained from the rotameter, deviating only 4.3%. For the CCAs of mice, good agreement is found between the calculated flow velocity and the measured value by ultrasound Doppler. The mean elasticity modulus of CCAs is 134.62 ± 54.3 kPa, which is in accordance with published data.The proposed method is capable of measuring the arterial wall strain and flow velocity pattern. This may be clinically useful for early detecting and monitoring cardiovascular diseases and may provide an essential tool in modelling the fluid-structure interaction between the blood and blood vessel.
The biophysical properties of arteries and blood flow have played important roles in the development of cardiovascular diseases. Increased arterial stiffness causes reduced vasomotion capability of an artery and influences the local hemodynamics. Designing and fabricating elastic vessel phantoms for use in physiological flow condition can add valuable alternatives to intravital and computational studies. This paper used polyvinyl alcohol (PVA) cryogel to fabricate elastic artery phantoms and performed detailed acoustic and mechanical characterization. A PVA solution in water was prepared, injected into a custom-designed mould, and subjected to a number of freeze-thaw (f-t) cycles. The obtained vessel phantoms were empty cylinders with 6 mm inner diameter, 2mm wall thickness, and 160 mm length. Young's modulus was measured, and a linear increase was observed from one cycle to eight cycles. The sound speed and the attenuation coefficient of the cylindrical samples were determined using the pulse-echo substitution method. Six different transducers were used in this study, operating at six different frequencies. Sound speed is dependent on transmit frequency, but not number of f-t cycle. At fixed transmit frequency, an increase of sound speed can be observed against the number of f-t cycles. Sound attenuation coefficient are dependent on both the transmit frequency and the number of f-t cycles. The PVA-C vessel phantom can be used to construct physiological flow circulation and perform the hemodynamics parameter measurements using ultrasonic or MRI techniques.
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