Ultrasonic Superharmonic Imaging

Medical ultrasound is one of the most prevalent imaging techniques used for diagnosing patients. The technique allows for the visualization of tissues in the human body. Compared to competing imaging techniques such as CT or MRI, medical ultrasound has numerous advantages: it is real-time, safe (no radiation is involved), inexpensive and portable. Over the years medical ultrasound equipment has become smaller and the image quality has improved considerably. Similar to CT or MRI, contrast agents are also used in medical ultrasound. Their use comprises e.g. endocardial border delineation, the improvement of blood flow detection using Doppler methods in cardiology and radiology, and the visualization of malignant tumors (Emmer, 2009; Vos, 2010). Ultrasound is also commonly used for therapeutic applications such as High Intensity Focused Ultrasound (HIFU), which is used for the noninvasive treatment of tumors by ablation, and extracorporeal shock wave lithotripsy, which is used for the noninvasive treatment of urinary calculosis and biliary calculi. The therapeutic applications for contrast agents are currently under research and range from the stimulated uptake of bioactive materials (van Wamel et al., 2006), to drug delivery (Kooiman et al., 2009) and the treatment of tumors by bloodvessel destruction (Skyba et al., 1998)

Data interpolation beyond aliasing

Proper spatial sampling is critical for many applications. If the sampling criterion is not met, artifacts appear for example in images. Last year an iterative approach was presented using wave field extrapolation to interpolate spatially aliased signals. The main idea behind this approach is that after inverse wave field extrapolation the signal is concentrated in a small region with a high amplitude, while the aliasing artifacts are spread-out through the domain. Inverse wave field extrapolation focusses optimally at one depth, making the performance of the reconstruction depth dependent. Obviously the method can be repeated for several depths. This year we show an alternative approach using an imaging/inverse-imaging approach. The demonstration of this approach is extended to 2D-arrays where the sampling limitations are even more critical. Moreover we show in this paper that the interpolation approach is not limited to near-field data, but can also be used on far-field data (plane waves). The Radon transform can be used for plane waves to focus the data. The approach is demonstrated using modeled and measured data for linear and 2D arrays

A study of nanoparticle manipulation using ultrasonic standing waves

There has been considerable interest in the noninvasive manipulation of particles in dispersions during the last 15 years. The manipulation techniques based on acoustic radiation forces are particularly interesting as they allow for the manipulation of particles based on their density, compressibility and/or size. The majority of the acoustic research has been focused on particles above a few μm. In the current work we investigate the manipulation of particles in the range of 100 nm to 10 μm using ultrasonic standing waves. The setup consisted of a small rectangular cell. Within the cell, two sides of piezo material were meticulously aligned (square: 30 × 30 mm2, center frequency 2 MHz). The distance between the piezo slabs was 9 mm. One piezo slab was excited using continuous wave signals with frequencies around 2 MHz; the frequency was optimized such that the power transfer was maximized. The liquid in the tank consisted of SiO2 nanoparticle dispersions with varying mean sizes: 100 nm, 500 nm, 1000 nm and 10 μm. The concentration of the particles was 0.03 mass%. If the ultrasound was switched off, the nanoparticle dispersion appeared as a homogeneous milky white substance. If the ultrasound was switched on, the particles moved to the nodes of the standing wave field. The distance between the planes in which the particles were localized corresponded to half the wavelength. Good localization was achieved in roughly ≪1s, 5s and 10s for the 10 μm, the 1 μm and the 500 nm particles, respectively. No particle localization was observed for the 100 nm particles, which was likely caused by mixing due to acoustic streaming of the liquid and compounded by Brownian motion. The peak power dissipated into the piezo slabs was 6W. Due to the small size of the cell and the fairly high losses in the piezo material, the temperature increased considerably during the experiments (1.3°C/min). Nanoparticles down to 500 nm were successfully manipulated using the radiation force induced by ultrasonic standing waves. Regarding 100 nm particles, the mixing effects due to acoustic streaming prevented successful manipulation. © 2013 IEEE

Iterative trace reconstruction of aliased radio-frequency data obtained using harmonic imaging: A feasibility study

<p>It is critical to use a proper spatial sampling, otherwise images suffer from grating lobes. However, the cost of a medical ultrasound scanner is strongly related to the channel count of the receive electronics. This has led to channel reduction using multiplexing or in-probe pre-beamforming methods at the cost of image quality or frame rate. An alternative is to reduce the receive channel count and reconstruct the non-aliased data from spatially aliased data. Last year we reported on a wavenumber frequency domain mapping based iterative trace reconstruction method developed for fundamental imaging. However, harmonic imaging is often used in medical imaging to further improve the image quality. As the reconstruction method assumes linearity, it is not a-priori clear whether the reconstruction will work satisfactory in combination with harmonic imaging. Here, the feasibility of using the method for harmonic imaging is investigated using in-vivo linear array data. The reconstruction algorithm operates by iteratively focusing and defocusing of the data using an imaging algorithm and uses intermittent thresholding to suppress the aliasing artifacts in the imaging domain. Properly sampled plane wave transmission datasets were recorded of the right common carotid artery of a healthy volunteer using a linear array transducer attached to a research system. The reconstruction technique significantly improved the image quality of all aliased datasets for both the fundamental and second harmonic imaging modalities. In fact, the reconstruction quality was slightly better for the second harmonic imaging case.</p

A pMUT based flowmeter: a feasibility study

pMUTs are efficient ultrasound sources and receivers in gasses, and are cheap to produce due to their lithographic production process. Making use of their 1st resonance frequency, a good signal to noise ratio (SNR) may be obtained using limited power. This is of interest for low cost-low power gas flow meters. However, using pMUTs in this application also poses challenges; 1) pMUTs are almost omnidirectional and 2) pMUTs are very narrow-banded. A pMUT's resonance frequency mainly depends on the membrane radius, which varies due to production. In the current work the suitability of pMUTs for a flowmeter is investigated. To match the resonance frequencies of the transmitting and receiving pMUTs, a DC bias voltage (from -4V to +4V) was applied to the transmitting pMUT. The flow setup consisted of two pMUTs (radius 0.4 mm, resonance frequency 343 kHz) mounted on the same side of a pipe (crosssection 3.6 cm) with an inter-pMUT distance of 1.7 cm. The transmitting pMUT was excited using 2.5 V Gaussian apodized sine bursts 10 cycles in length. The signals produced by the receiving pMUT were amplified by a custom made pre-amplifier. Pressurized air was used to create a gas flow past the pMUTs. The gas flow speed was calculated using a time delay based algorithm. By varying the DC bias voltage the pMUTs resonance frequency could be shifted by ∼5 kHz, which was sufficient to match the resonance frequencies of both pMUTs. The application of a bias Voltage increased the intensity of the emitted sound by 12 dB. The SNR of a single received trace was ∼18 dB. The SNR was improved by averaging 1000x for each measurement. The direct sound path between the pMUTs provided a good estimate of the flow velocity. The first arrival in the time signal (at 50 μs) was the direct path between both pMUTs. 55 μs later the echo bouncing off the sidewalls of the rectangular pipe arrived. It was shown that a flow speed of 2 m/s could be measured with an accuracy of ∼±0.1 m/s. An initial investigation of the suitability of pMUTs for gas flow measurements was conducted, and encouraging initial results were obtained. © 2013 IEEE

Ultrasound transmission spectroscopy: In-line sizing of nanoparticles

Nanoparticles are increasingly used in a number of applications, e.g. coatings or paints. To optimize nanoparticle production in-line quantitative measurements of their size distribution and concentration are needed. Ultrasound-based methods are especially suited for in-line particle sizing. These methods can be used for opaque dispersions and at high concentrations. However, using ultrasound to measure nanoparticles is challenging: despite the use of high frequencies the scattering is close to the Rayleigh regime (ka 蠐 1) and the information contained in the measurements is limited. In this work the performance of an ultrasonic particle sizing method is evaluated using SiO2 nanoparticles. The measurement method is based on ultrasound transmission spectroscopy. The presence of nanoparticles affects the propagation of ultrasound in the medium, which is measured over a frequency band of 50 - 250 MHz. The wave propagation effects are then interpreted using the inversion of a physics model. The investigated dispersions consisted of SiO2 nanoparticles (1.4 and 2.0 vol%) dispersed in water. Four batches, provided by Nano-H S.A.S., had monomodal size distributions with mean sizes 150, 300, 420 and 440 nm. Two bimodal size distributions were investigated: 1) a mix of 50% 302 nm and 50% 422 nm particles, and 2) a mix of 50% 150 nm and 50% 422 nm particles. As a reference the size distributions were measured using an optics based Malvern Zetasizer. The mean particle sizes and concentrations were similar to the reference, with differences between 4.5 and 19% and between 3 and 15%, respectively. The shape of the particle size distributions obtained by the ultrasonic instrument were similar to that of the reference. Also, the ultrasound instrument was able to produce correct results for both mono- and bimodal size distributions. The temperature of the mixture did not have a significant influence on the results

Iterative trace reconstruction of aliased radio-frequency data obtained using harmonic imaging: A feasibility study

It is critical to use a proper spatial sampling, otherwise images suffer from grating lobes. However, the cost of a medical ultrasound scanner is strongly related to the channel count of the receive electronics. This has led to channel reduction using multiplexing or in-probe pre-beamforming methods at the cost of image quality or frame rate. An alternative is to reduce the receive channel count and reconstruct the non-aliased data from spatially aliased data. Last year we reported on a wavenumber frequency domain mapping based iterative trace reconstruction method developed for fundamental imaging. However, harmonic imaging is often used in medical imaging to further improve the image quality. As the reconstruction method assumes linearity, it is not a-priori clear whether the reconstruction will work satisfactory in combination with harmonic imaging. Here, the feasibility of using the method for harmonic imaging is investigated using in-vivo linear array data. The reconstruction algorithm operates by iteratively focusing and defocusing of the data using an imaging algorithm and uses intermittent thresholding to suppress the aliasing artifacts in the imaging domain. Properly sampled plane wave transmission datasets were recorded of the right common carotid artery of a healthy volunteer using a linear array transducer attached to a research system. The reconstruction technique significantly improved the image quality of all aliased datasets for both the fundamental and second harmonic imaging modalities. In fact, the reconstruction quality was slightly better for the second harmonic imaging case.ImPhys/Acoustical Wavefield Imagin