Øyvind Standal

SURF imaging: In vivo demonstration of an ultrasound contrast agent detection technique

A dual-band method for ultrasound contrast agent detection is demonstrated in vivo in an animal experiment using pigs. The method is named Second-order UltRasound Field Imaging, abbreviated SURF Imaging. It relies on simultaneously transmitting two ultrasound pulses with a large separation in frequency. Here, a low-frequency pulse of 0.9 MHz is combined with a high-frequency pulse of 7.5 MHz. The low-frequency pulse is used to manipulate the properties of the contrast agent, and the high- frequency pulse is used for high-resolution contrast detection and imaging. An annular array capable of transmitting the low- and high-frequency pulses simultaneously was constructed and fitted to a mechanically scanned probe used in a GE Vingmed System 5 ultrasound scanner. The scanner was modified and adapted for the dual-band transmit technique. In-house software was written for post-processing of recorded IQ-data. Contrast-processed B-mode images of pig kidneys after bolus injections of 1 mL of Sonovuereg are presented. The images display contrast detection with contrast-to-tissue ratios ranging from 15-40 dB. The results demonstrate the potential of SURF Imaging as an ultrasound contrast detection technique for clinically high ultrasound frequencies. This may allow ultrasound contrast imaging to be available for a wide range of applications.

Effect of Ultrasound Parameters on the Release of Liposomal Calcein

The ultrasound exposure parameters that maximize drug release from dierucoyl-phosphatidylcholine (DEPC)-based liposomes were studied using two transducers operating at 300 kHz and 1 MHz. Fluorescent calcein was used as a model drug, and the release from liposomes in solution was measured using a spectrophotometer. The release of calcein was more efficient at 300 kHz than at 1 MHz, with thresholds of peak negative pressures of 0.9 MPa and 1.9 MPa, respectively. Above this threshold, the release increased with increasing peak negative pressure, mechanical index (MI), and duty cycle. The amount of drug released followed first-order kinetics and increased with exposure time to a maximal release. To increase the release further, the MI had to be increased. The results demonstrate that the MI and the overall exposure time are the major parameters that determine the drugs release. The drugs release is probably due to mechanical (cavitation) rather than thermal effects, and that was also confirmed by the detection of hydroxide radicals.

Coherent compounding in doppler imaging

Coherent compounding can provide high frame rates and wide regions of interest for imaging of blood flow. However, motion will cause out-of-phase summation, potentially causing image degradation. In this work the impact of blood motion on SNR and the accuracy of Doppler velocity estimates are investigated. A simplified model for the compounded Doppler signal is proposed. The model is used to show that coherent compounding acts as a low-pass filter on the coherent compounding Doppler signal, resulting in negatively biased velocity estimates. Simulations and flow phantom experiments are used to quantify the bias and Doppler SNR for different velocities and beam-to-flow (BTF) angles. It is shown that the bias in the mean velocity increases with increasing beam-to-flow angle and/or blood velocity, whereas the SNR decreases; losses up to 4 dB were observed in the investigated scenarios. Further, a 2-D motion correction scheme is proposed based on multi-angle vector Doppler velocity estimates. For a velocity of 1.1 vNyq and a BTF angle of 75°, the bias was reduced from 30% to less than 4% in simulations. The motion correction scheme was also applied to flow phantom and in vivo recordings, in both cases resulting in a substantially reduced mean velocity bias and an SNR less dependent on blood velocity and direction.

Nonlinear propagation acoustics of dual-frequency wide-band excitation pulses in a focused ultrasound system.

In this article, acoustic propagation effects of dual-frequency wide-band excitation pulses in a focused ultrasound system are demonstrated in vitro. A designed and manufactured dual-frequency band annular array capable of transmitting 0.9/7.5 MHz center frequency wide-band pulses was used for this purpose. The dual-frequency band annular array, has been designed using a bi-layer piezo-electric stack. Water tank measurements demonstrate the function of the array by activating the low- and high-frequency layers individually and simultaneously. The results show that the array works as intended. Activating the low- and high-frequency layers individually, results in less than -50 dB signal level from the high- and low-frequency layers respectively. Activating both layers simultaneously, produce a well defined dual-frequency pulse. The presence of the low-frequency pulse leads to compression, expansion, and a time delay of the high-frequency pulse. There is a phase shift between the low- and high-frequency pulse as it propagates from the array to the focus. This makes the latter described effects also dependent on the array configuration. By varying the low-frequency pressure, a shift of up to 0.5 MHz in center frequency of a 8.0 MHz transmitted high-frequency pulse is observed at the array focus. The results demonstrate the high propagation complexity of dual-frequency pulses.

Ultrasound stimulated release of liposomal calcein

Ultrasound exposure parameters that maximize drug release from liposomes were studied using two ultrasound transducers (300 kHz and 1 MHz). Variations in acoustic peak negative pressure (260–2037 kPa), temporary average intensity (0.05–6.08 W/cm2), mechanical index (MI) (0.4–3.0), insonation time (0.5–20 minutes), pulse repetition frequency (PRF) (100–1000 Hz) and pulse length (0.05–0.4 ms) were studied. Drug release was more efficient at 300 kHz compared to 1 MHz. A certain threshold in peak negative pressure had to be overcome to obtain drug release, and the pressure needed was lower at 300 kHz (0.72 MPa) than at 1 MHz (1. 39 MPa) which corresponds to MI values of 1.30 and 1.39 respectively. Above the threshold the release increased with increasing temporal average intensity, peak negative pressure, MI and duty cycle (i.e PRF and pulse length). The release was found to increase with exposure time, where the profile followed a first-order kinetics. The first-order rate constant for the release increased linearly with MI. This indicates that the release of the drug from liposomes was caused by mechanical rather than thermal effects. The results demonstrate that ultrasound has a potential in enhancing drug release from liposomes and can potentially improve cancer therapy.

P2D-4 A Phase Based Approach for Estimation and Tracking of Locally Variable Delays

Time delay estimation is important for several ultrasound imaging methods, such as Doppler, elastography and SURF Imaging. In this paper a new phase based approach is presented which makes no assumption on the local delay variation. The method is computationally fast and performs well compared to other methods.

On the accuracy of coherent compounding Doppler imaging

The influence of blood motion on the accuracy of coherent compounding Doppler imaging is investigated. A simplified model for the compounded Doppler signal is proposed. The model is used to show that coherent compounding acts as a low pass filter, inferring a negative shift of the Doppler spectrum and a bias in mean frequency estimates. Simulations and in vitro experiments were used to quantify the bias and signal-to-noise ratio (SNR) for different velocities and beam-to-flow angles. Further, a multi-angle vector Doppler approach is proposed for 2D correction of blood motion during coherent compounding, which was verified in simulations and in vitro to improve SNR and substantially reduce the mean velocity bias.

Ultrasonic detection of spark eroded notches in steel plates

A long-standing challenge in integrity management of gas pipelines is stress corrosion cracking (SCC). Such cracks are difficult to detect, calling for sensitive detection methods with good coverage. Recent advances in gas-coupled broad band ultrasound provide a potential method that is based on guided plate waves. The objective of this work is to describe and experimentally evaluate this method based on Halfwaves ART Scan® tool. A water-immersed 12 mm thick steel plate with machined crack-like, spark eroded notches (SEN) of 2 and 6 mm depth, and 10 and 20 mm lengths, were acoustically investigated with broadband (0.3–1.3 MHz) transducers in a pitch-catch setup. Received signals were acquired at 2D grid locations, and pointwise processing parameters were extracted. Summed signal power and a spectral parameter demonstrated the detection of the SENs. The received energy in a selected frequency band was 2.5 and 5.5 dB lower for the 2 and 6 mm deep SENs, respectively, compared to that of an undamaged part of the plate. Using a tomographic image inversion method, the received signals were combined and converted into images. Results of the 6 mm deep SEN showed good agreement with the actual size and location of the SEN. In conclusion, the potential to detect SCC by using an ART Scan® based set-up, combined with pointwise and image formation processing, was successfully demonstrated on test plates with machined crack-like defects.

A visualization tool for improved assessment and follow-up with ultrasound

Ultrasound has been shown to be a powerful imaging tool for the evaluation of musculoskeletal disease (e.g. rheumatoid arthritis or carpal tunnel syndrome). Power Doppler imaging in particular plays an important role in treatment monitoring. However it is also known that ultrasound is operator dependent and has low intra- and inter-observer repeatability. The aim of this work was to reduce the uncertainty of the follow-up process of a typical musculoskeletal exam by ensuring that the same anatomy is visualized at baseline and follow-up. A visualization tool, which receives a continuous stream of ultrasound data, has been implemented in Qt. The streamed images are compared on the fly against a pre-loaded baseline image. To compensate for in plane translation and rotation differences, 2D rigid registration is performed. Normalized mutual information was used as a similarity metric and is color coded from red (0%) to green (100%) to show the amount of similarity among the current images. Several display alternatives (e.g. semitransparent overlay, color coded overlay) have been implemented and tested. Optional anisotropic filtering can be applied to both images before registration to reduce the impact of the speckle noise on the final result. Two healthy volunteers and a phantom have been used for validation purposes. Both the reference and follow-up ultrasound data have been acquired with a Vivid E9 scanner, with an ML6-15 probe (GE Vingmed Ultrasound). At the same time the position of the probe with regards to a reference frame was tracked with an accurate optical position sensor (Polaris, NDI). It was shown that the highest similarity scores corresponded to the lowest translation offsets and angle differences. The average offset and angle errors for the patient data were: [0.68 ± 0.97, 0.47 ± 0.79, -2.28 ± 1.38]mm, [2.92 ± 2.35, -0.17 ± 1.2, 1.24 ± 2.38]°. A novel method for real-time fusion of ultrasound images aimed at musculoskeletal disease monitoring has been presented. Being provided with instant feedback, the user is more confident during the acquisition process.