Ultrasound Bursts Research Articles

Ultrasound-Induced Calcium Oscillations and Waves in Chinese Hamster Ovary Cells in the Presence of Microbubbles

This study investigated the effects of ultrasound on the intracellular [Ca 2+] of Chinese hamster ovary cells in the presence of albumin-encapsulated Optison microbubbles. Cells were exposed to 1 MHz ultrasound (tone burst of 0.2 s duration, 0.45 MPa peak pressure) while immersed in solution of 0.9 mM Ca 2+. Calcium imaging of the cells was performed using digital video fluorescence microscopy and Ca 2+-indicator dye fura-2AM. Experimental evidence indicated that ultrasound caused a direct microbubble-cell interaction resulting in the breaking and eventual dissolution of the microbubble and concomitant permeabilization of the cells to Ca 2+. These cells exhibited a large influx of Ca 2+ over 3–4 s and did not return to their equilibrium levels. Subsequently, some cells exhibited one or more Ca 2+ oscillations with the onset of oscillations delayed by 10–80 s after the ultrasound pulse. A variety of oscillations were observed including decaying oscillations returning to the baseline value over 35–100 s, oscillations superimposed on a more gradual recovery over 150–200 s, and oscillations continued with increased amplitude caused by a second ultrasound tone burst. The delays in onset appeared to result from calcium waves that propagated across the cells after the application of the ultrasound pulse.

Ultrasound-modulated optical tomography with intense acoustic bursts

Ultrasound-modulated optical tomography (UOT) detects ultrasonically modulated light to spatially localize multiply scattered photons in turbid media with the ultimate goal of imaging the optical properties in living subjects. A principal challenge of the technique is weak modulated signal strength. We discuss ways to push the limits of signal enhancement with intense acoustic bursts while conforming to optical and ultrasonic safety standards. A CCD-based speckle-contrast detection scheme is used to detect acoustically modulated light by measuring changes in speckle statistics between ultrasound-on and ultrasound-off states. The CCD image capture is synchronized with the ultrasound burst pulse sequence. Transient acoustic radiation force, a consequence of bursts, is seen to produce slight signal enhancement over pure ultrasonic-modulation mechanisms for bursts and CCD exposure times of the order of milliseconds. However, acoustic radiation-force-induced shear waves are launched away from the acoustic sample volume, which degrade UOT spatial resolution. By time gating the CCD camera to capture modulated light before radiation force has an opportunity to accumulate significant tissue displacement, we reduce the effects of shear-wave image degradation, while enabling very high signal-to-noise ratios. Additionally, we maintain high-resolution images representative of optical and not mechanical contrast. Signal-to-noise levels are sufficiently high so as to enable acquisition of 2D images of phantoms with one acoustic burst per pixel.

Changes in microbubble dynamics near a boundary revealed by combined optical micromanipulation and high-speed imaging

The authors report optical observations of the change in the dynamics of one and the same ultrasound contrast agent microbubble due to the influence of interfaces and neighboring bubbles. The bubble is excited by a 2.25MHz ultrasound burst and its oscillations are recorded with an ultrahigh-speed camera at 15 million frames per second. The position of an individual bubble relative to a rigid wall or second bubble is precisely controlled using optical tweezers based on Laguerre-Gaussian laser beams [P. Prentice et al., Opt. Express 12, 593 (2004); V. Garbin et al., Jpn. J. Appl. Phys. 44, 5773 (2005)]. This allows for repeated experiments on the very same bubble and for a quantitative comparison of the effect of boundaries on bubble behavior.

Photorefractive detection of tissue optical and mechanical properties by ultrasound modulated optical tomography

Ultrasound-modulated optical tomography is a developing hybrid imaging modality that combines high optical contrast and good ultrasonic resolution for imaging soft biological tissue. We developed a photorefractive-crystal-based, time-resolved detection scheme with the use of a millisecond long ultrasound burst to image both the optical and the mechanical properties of biological tissues, with improved detection efficiency of ultrasound-tagged photons.

An Ultrasound In-Process-Measuring System to Ensure a Minimum Roundness Deviation for Rings During Turning

The turning process requires a clamping of the workpiece. If the fixing of a thin-walled workpiece (i.e. a ring) is realised with a three-jaw-chuck, distortion of the outer diameter of the workpiece occurs, leading to a varying circumferential wall thickness on unclamping. This paper presents an ultrasound system for the in-process-measurement of these workpiece distortions. The measurement principle is based on the determination of phase differences between echoes of an ultrasound burst. The measured in-process values can be used to control the turning process with a fast tool servo.

Focal disruption of the blood–brain barrier due to 260-kHz ultrasound bursts: a method for molecular imaging and targeted drug delivery

The goal of this study was to explore the feasibility of using low-frequency magnetic resonance (MR) image-guided focused ultrasound as a noninvasive method for the temporary disruption of the blood-brain barrier (BBB) at targeted locations. Rabbits were placed inside a clinical 1.5-tesla MR imaging unit, and sites in their brains were targeted for 20-second burst sonications (frequency 260 kHz). The peak pressure amplitude during the burst varied between 0.1 and 0.9 MPa. Each sonication was performed after an intravenous injection of an ultrasound contrast agent (Optison). The disruption of the BBB was evaluated with the aid of an injection of an MR imaging contrast agent (MAG-NEVIST). Additional tests involving the use of MION-47, a 20-nm magnetic nanoparticle contrast agent, were also performed. The animals were killed at different time points between 3 minutes and 5 weeks postsonication, after which light or electron microscopic evaluation was performed. The threshold for BBB disruption was approximately 0.2 MPa. More than 80% of the brain sites sonicated showed BBB disruption when the pressure amplitude was 0.3 MPa; at 0.4 MPa, this percentage was greater than 90%. Tissue necrosis, ischemia, and apoptosis were not found in tissue in which the pressure amplitude was less than 0.4 MPa; however, in a few areas of brain tissue erythrocytes were identified outside blood vessels following exposures of 0.4 MPa or higher. Survival experiments did not show any long-term adverse events. These results demonstrate that low-frequency ultrasound bursts can induce local, reversible disruption of the BBB without undesired long-term effects. This technique offers a potential noninvasive method for targeted drug delivery in the brain aided by a relatively simple low-frequency device.

Destruction of Microbubbles in the Ventricular Cavities Affects the Quantitative Assessment of Myocardial Perfusion During Real-time Myocardial Contrast Echocardiography

Background. Myocardial perfusion can be measured by assessing the replenishment of bubbles destroyed by transient high power ultrasound waves (burst) during real-time myocardial contrast echocardiography (MCE). However, the burst procedure may destroy microbubbles in ventricular cavities as well as in the myocardium, which can interfere with the accurate measurement of myocardial perfusion. The goal of the present study was to determine the effect of burst exposure to the ventricular cavities on the replenishment curve parameters during real-time MCE.Methods. The myocardial opacification of the left ventricular (LV) short-axis view was observed using SIEMENS Sequoia-512 (mechanical index = 0.1) during infusion of Optison in 11 open-chest dogs. A 1-second or 6-second ultrasound burst was applied to the right ventricle (RV-burst), lateral wall (Myo-burst), or in the usual fashion (General-burst). The LV cavity was insonified by the General-burst but not by the RV-burst or Myo-burst. The time course of myocardial opacification of the lateral wall after burst was fitted to an exponential function: y=a(1-e-βt)+c.Results. Although the opacification inside the RV cavity recovered soon after RV-burst regardless of burst duration (1-second burst, 0.88±0.42 sec; 6-second burst, 0.82±0.37 sec), the recovery time of opacification inside the LV cavity was significantly prolonged after the 6-second RV-burst (1-second burst, 0.49±0.39 sec; 6-second burst, 4.30±1.14 sec). Further, in the General-burst, the β-value was significantly lower with the 6-second burst than with the 1-second burst (1-second burst, 0.29±0.10 vs. 6-second burst, 0.11±0.05). However, with the Myo-burst, the β-value was similar when comparing the two burst durations (1-second burst, 0.38±0.13 vs. 6-second burst, 0.35±0.09)Conclusions. In real-time MCE, the replenishment curve was significantly influenced by the burst procedure area, possibly due to bubble destruction in the ventricular cavities. However, this effect was minimized by the use of a short burst duration.

The application of acoustic radiation force for molecular imaging and drug delivery

Acoustic radiation force is exerted on objects in an acoustic field. By optimizing acoustic parameters to maximize this force, ultrasound contrast agents can be manipulated and concentrated with an acoustic field. This technique can be used to enhance molecular imaging and drug delivery. Ultrasonic molecular imaging employs contrast agents such as microbubbles, nanoparticles, or liposomes coated with ligands specific for receptors expressed on cells at sites of angiogenesis, inflammation, or thrombus. Concentration of these highly echogenic contrast agents at a target site enhances the ultrasound signal received from that site, promoting ultrasonic detection and analysis of disease states. It is demonstrated that acoustic radiation force can be used to displace targeted contrast agents to a vessel wall, greatly increasing the number of agents binding to available surface receptors. Radiation force can also be used for microparticle-carrier based drug delivery. Acoustically active drug-carrier vehicles can be concentrated with radiation force, and then disrupted with high-intensity ultrasound bursts, resulting in local delivery of the therapeutic agent. This talk will discuss simulations and experimental results demonstrating these applications.

High-speed optical observations of contrast agent destruction

Ultrasound contrast agents are now available since a few years and used for diagnostic purposes. Improved diagnostic decisions have been made possible with new imaging methods that are mainly based on the nonlinear properties of gas microbubbles. Since it is well known that contrast agents are destroyed by ultrasound when the acoustic pressure exceeds a threshold, extremely low acoustic pressures were applied to achieve enhanced contrast image quality. However, destruction of contrast microbubbles is not necessarily undesirable, since it is beneficial in, for example, destruction/reperfusion imaging and recently in drug delivery. We investigate in this experimental study the destruction dynamics of a contrast agent consisting of nitrogen bubbles encapsulated in a double polymer/albumin wall shell. This is accomplished using an ultrafast camera Brandaris that operates at a frame rate of 25 MHz and records 128 frames. The measurements were performed with an ultrasound sine burst of 10 cycles at 1.7 MHz. Different acoustic pressures were applied and various microsphere sizes were examined. The results show three different zones depending on the applied pressure and bubble size: these are nondestruction zone, transient zone and destruction zone. The nondestruction zone is reached for either very small microspheres or low mechanical indices (MI) (<0.3). In the destruction zone lie either large microspheres (5 μm or higher) even when irradiated at low MIs or small microspheres (< 5 μm) when the MI is above 0.6. The optical observations revealed that the destruction of the microspheres is characterized by shell rupture and gas release. The release of the gas gives rise to new free microbubble that lasts for a few milliseconds and then disappears due to dissolution. In the transient zone, the microspheres are mainly compressed in the first few cycles but no expansion is induced. After intense compressions, the shell fissures and gas escapes in the last cycles of the burst or during a second burst depending on the initial size and MI. These optical recordings are important to investigate contrast bubble destruction and can help in amplifying or minimizing this process. Indeed, bubble disruption remains the basis of most current sensitive methods for detecting perfusion with contrast agents and is an essential component of perfusion quantification with microbubbles, in addition to drug delivery applications and pressure measurements. (E-mail: bouakaz@med.univ-tours.fr)

Tracking of regions-of-interest in myocardial contrast echocardiography

Analysis of intramyocardial perfusion by contrast echocardiography provides quantitative parameters for the assessment of ischemic disease. This analysis can be achieved by applying an ultrasound (US) burst of high mechanical index to destroy contrast bubbles, measuring various myocardial refilling parameters from the time curves obtained from regions-of-interest (ROIs) within the myocardial wall. To obtain reliable intensity curves, the position of the ROIs must be tracked to compensate for the heart motion along the sequence. In this work, we studied the use of optical flow techniques for ROI repositioning. Two block-matching and one differential technique were evaluated for this purpose. Performance was measured by comparing the result of automatic tracking with results of ROI repositioning by a human expert. This evaluation was carried out on experimental data from animals as well as on sequences from clinical studies. Results are considered to be accurate enough for clinical purposes, and computation times may allow for a real-time processing if incorporated into a US scanner. (E-mail: desco@mce.hggm.es)

Tracking the moveable feast: sonomicrometry and gastrointestinal motility.

Ultrasonomicrometry measures distance between piezoelectric crystals based on transmission time of ultrasound bursts. It allows monitoring of coordinated motion of small and delicate tissues, including gastrointestinal sphincters. Its suitability for motility studies in small animals such as mice suggests that its use in gastrointestinal studies will increase in coming years.

Ultrasound-mediated cavitation thresholds of liquid perfluorocarbon droplets in vitro

This study was undertaken to measure the ultrasound (US)-mediated cavitation threshold of microdroplets as a function of its content and US parameters (frequency, amplitude and burst length). Albumin-coated droplets were prepared with perfluoropropane, perfluorohexane or perfluoromethylcyclohexane contents. The filtered suspensions were diluted to 1:1000 (v) and compared with Optison“. The formulations were injected into an acoustically transparent vessel and sonicated with a single focused transducer. The frequencies employed were 0.74, 1.1, 2.18 and 3.3 MHz and the burst length and acoustic pressure were varied. The inertial cavitation threshold for each experiment was monitored through passive acoustic detection. The formation of droplet emulsion of the perfluorocarbon increased the natural boiling point of the perfluorocarbon. However, perfluorocarbon droplets having contents with higher molecular weights and boiling points did not have detectably higher inertial cavitation thresholds and, thus, the droplets do not need to be in a superheated state to be cavitated by US bursts. Therefore, higher boiling point perfluorocarbons should be investigated for this purpose and may prove to be useful for both imaging and therapy. The inertial cavitation threshold of perfluorocarbon droplets increases with frequency, and was approximately 0.7 MPa at 0.74 MHz and 1.75 MPa at 3.3 MHz. Optison”, already in a gaseous state, has the lowest cavitation threshold of all formulations studied. Results show that, for the frequencies tested, there is no dependence between inertial cavitation threshold and burst lengths between 20 and 100 ms. As a conclusion, the inertial cavitation threshold of albumin-coated microdroplets of several perfluorocarbons was determined in vitro. The results indicate that the physical properties of these droplets are such that they may be useful for localized US therapies. (E-mail: kullervo@bwh.harvard.edu)

Real-time perfusion imaging: a new echocardiographic technique for simultaneous evaluation of myocardial perfusion and contraction.

Myocardial contrast echocardiography (MCE) with high acoustic energy and triggered harmonic imaging is the best established ultrasound technique to date for the assessment of myocardial perfusion. With this technique, however, the ultimate goal of MCE (noninvasive real-time simultaneous assessment of myocardial perfusion and function after an intravenous injection of microbubbles) is not met. Recently, technologic advances have enabled myocardial opacification to be visualized during low-energy real-time imaging. During real-time perfusion imaging, wall motion and myocardial perfusion may be assessed simultaneously, obviating the need of the presently time-consuming combination of different imaging modalities. When high-energy ultrasound bursts are periodically transmitted to produce bubble destruction during low-power imaging, the consecutive frames after destruction delineate the restoration of contrast intensity. Microbubble replenishment rate and peak intensity may be determined subsequently, and provide reliable quantitative parameters of regional microcirculatory flow. This review will introduce the modalities used for real-time perfusion imaging with focus on power pulse inversion imaging and quantitative analysis. Furthermore, we will describe the clinical role the technique may have in the identification of coronary artery disease, quantification of coronary stenosis severity, assessment of myocardial viability, determination of infarction size, and evaluation of reflow and no- or low-reflow after acute myocardial infarction.

Progress in phase angle thermography

Phase angle thermography is basically spatially multiplexed photothermal radiometry. It provides images for remote detection and imaging of damage. For energy deposition one can use external heat sources (e.g., light or convective heating) or internal heat generation (e.g., electric current, microwaves, eddy current, or elastic wave). More complete information about defects is obtained by combining information from external and internal heating methods: Effects of intact thermal structure can be distinguished from defect specific thermal signatures. Our recent results relate to multifrequency excitation allowing for depth profiling from one lock-in-thermography measurement and, additionally, to modulated ultrasound frequency burst phase angle thermography where effects of standing elastic waves are eliminated. This way the applications for nondestructive evaluation become more reliable and faster.

Microembolic signal description: a reappraisal based on a customized digital postprocessing system

The high variability in presence and signature of microembolic signals (MES), detected with transcranial Doppler sonography (TCD) in the middle cerebral artery (MCA), cannot be explained with the currently available published data. We applied customized postprocessing on the radiofrequency (RF) signal of a standard TCD system. The spatial resolution was on the order of 2 mm, depending only on the length of the ultrasound (US) burst emitted. The amplitude of clutter-filtered RF signals was color-coded and plotted as a function of time and depth (range 30 mm). Additionally, 128 point fast Fourier transforms (FFTs) (50% temporal overlap) were calculated, visualizing both the background Doppler spectrum and the MES. We evaluated 122 gaseous MES from two patients during cardiac surgery and 52 particulate MES from four patients after carotid endarterectomy. Both MES categories showed comparable properties: they appeared in the RF amplitude plot as rather straight lines of increased intensity, indicating that the velocity remained approximately the same while they passed the US beam. The velocity calculated from the amplitude plot never exceeded that of the background Doppler spectrum. Various “MES patterns” could be identified with respect to the depth range at which the MES were visible. A quarter of the gaseous MES changed their direction at a specific depth, suggesting that the MES entered a branch ( e.g., an M2 artery or the anterior cerebral artery). In the FFT analysis, these MES contained both positive and negative frequencies. It is concluded that MES show consistent signature patterns in the amplitude-time plots and that the previously reported variability of MES appearance in conventional Doppler systems is an artefact caused by relatively large signal amplitudes and sample volumes. (E-mail: Max@fknf.azm.nl)

MRI guided focused ultrasound opening of blood brain barrier

Previously, we demonstrated that focused ultrasound can focally open the blood brain barrier (BBB) reversibly without damage to the neurons. In this study, we varied the ultrasound pulse parameters to investigate their influence on BBB opening and any resulting tissue damage. Under magnetic resonance imaging (MRI) guidance, a series of targeted ultrasound bursts (1.5 MHz, burst duration: 5 cycles to 100 ms, repetition frequency: 1 Hz–1 kHz) were delivered into rabbit brains at low pressure levels (up to approximately 5 MPa) after intravenous administration of a commercially available ultrasound contrast agent containing preformed gas bubbles. Focal MRI signal intensity changes were observed during the deposition of acoustic energy. BBB opening at the correct target was confirmed by detecting the localized uptake of an MRI contrast agent. In some animals Trypan Blue was injected intravenously after the sonications. The animals were sacrificed at time intervals ranging from 2 h to 7 days. Light and electron microscopy evaluation was performed. No neuronal damage was observed at the lowest power levels that produced BBB opening. No neuronal damage was observed at the lowest power levels that produced BBB opening. Correlation of focal MRI signal changes to the uptake of a large molecular size contrast agent was observed demonstrating the potential of image guided drug delivery.

Scanning electric conductivity gradients with ultrasonically-induced Lorentz force.

The ions in a fluid element oscillating under the effect of a sound wave in the presence of a magnetic field are submitted to Lorentz force. This gives rise to a bulk current density proportional to the medium's electric conductivity. In the present study, the integrality of this interaction current was collected using a pair of plane electrodes located on opposite sides of the sample. A focused transducer produced ultrasound bursts of 10 micros duration, 500 kHz frequency and 1.5 MPa peak pressure. The magnetic field was created by a purpose-built 0.35 T permanent magnet. Wiener inverse filtering was used to retrieve the system response from the recorded waveforms. The final signal was shown to be proportional to the gradient of sigma/rho along ultrasound propagation axis. Electric conductivity, sigma, predominantly controls this parameter since mass density, rho, does not vary in great proportions in biological media. Rectangular blocks of Agar gel and a layered bacon sample were used as models of biological media. The signals obtained in gel blocks had a longitudinal spatial resolution better than 1 mm. The successive layers of the bacon sample were clearly resolved. The advantages of this new modality for tissue characterization include the permeability of body tissue to magnetic field and ultrasound, the harmlessness of the applied fields and the improved spatial resolution in the measurement of a tissue's electric conductivity distribution.

Multi-frequency parametric phase conjugation of ultrasound beams in magnetic ceramics.

A new operation mode of a parametric phase conjugation of ultrasound waves above the threshold of absolute parametric instability of phonons is reported. A deviation of the carrier frequency in a sequence of incident ultrasound bursts and in a proper sequence of electromagnetic pumping is applied. The deviation avoids amplification of the reverberation noise in the active medium when the repetition time of the conjugation is short enough to be comparable with the relaxation time of acoustic reverberations. As a result, a decrease of repetition time and a correspondent increase of the average output power of a phase conjugator becomes available. The advantage of the new operation mode is demonstrated on an example of supercritical phase conjugation in magnetoacoustic ceramics.

The role of internal reflection in transskull phase distortion

Phase distortion due to reflection in transcranial ultrasound propagation is investigated. Understanding of these phase-dependent properties is motivated by efforts to construct a reliable prediction model for noninvasive ultrasound therapy in the brain. The present study measures the phase of an ultrasound wave after propagation through an ex vivo human skull and considers the dependence of this phase on reflections between the transducer and the skull surface in addition to reflections within the skull. Experiments are performed using a human calvarium fragment placed between an underwater ultrasonic transducer and a polyvinylidene difluoride hydrophone. Data are presented indicating the ultrasound phase dependence as a function of burst length and the distance of the transducer element from the skull at a driving frequency of 0.5 MHz. Experimental results are compared with predictions obtained from a propagation model which considers transmission at the skull interfaces as well as multiple reflections within the skull. It is concluded that by using short ultrasound bursts a distance may be indicated that beyond which the contributions of transducer reflections on the phase of the propagating wave may be neglected. Additionally, a comparison of the measurements with simulated data supports the contention that for reasonably small incident angles, reflection within the skull causes minimal phase shift.

Ultraschall-Burst-Phasen- Thermografie

Phased Ultrasonic Burst Thermography. Phased ultrasonic burst thermography is a new nondestructive technique originating from ultrasound lock-in thermography. It provides thermal wave imaging of defects by ultrasonic stimulation. Relatively short ultrasound bursts yield faster measurements and better reproducibility in relation to the well established lock-in method. The advantages of phase images remain valid: the recognition of defect depth and the suppression of temperature gradients. Application of phased ultrasonic burst thermography to typical defects of aircraft components and ceramic turbine blade coatings provide specific information on their nature.