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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 33 out of 330
| Title |
Alkane sonochemistry. |
| Author |
Suslick KS, Gawienowski JJ, Schubert PF, Wang HH. |
| Journal |
J Phys Chem |
| Volume |
|
| Year |
1983 |
| Abstract |
The chemical effects of high-intensity ultrasound on alkane solutions are reported. Primary products are HP,CH,, C2H2, and smaller 1-alkenes. Strong similarities to high-temperature (>1200 "C) alkane pyrolysis are observed and a radical chain mechansim is proposed. The principal sonochemical process appears to be C-C bond cleavage with secondary abstractions and rearrangements. In order to probe the factors which affect sonochemical yields, we have used two very different chemical dosimeters in alkane solutions: radical trapping by diphenylpicrylhydrazyl and decomposition of Fe(CO)5. In both cases, good correlation is found between the log of the sonochemical rate and the solvent vapor pressure. This result is justifiable in terms of the cavitation
"hot-spot" explanation of sonochemistry. Thus, decreasing solvent vapor pressure increases the intensity of cavitational collapse, the peak temperature reached during such collapse, and, consequently, the rates of sonochemical reactions.
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| Title |
Amplitude modulated chirp excitation to reduce grating lobes and maintain ultrasound intensity at the focus of an array. |
| Author |
Karunakaran CP, Oelze ML |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
2013 |
| Abstract |
During application of high intensity focused ultrasound (HIFU) with therapy arrays, the existence of grating lobes can cause heating at unintended tissue regions. Therefore, the reduction of grating lobes in therapeutic arrays is an important goal. One way to reduce the grating lobes in therapy arrays is to excite the arrays with broadband signals (defined here as >10% fractional bandwidth). To achieve a reduction in grating lobe levels in an ultrasonic array, coded waveforms can be utilized that reduce the grating lobe levels while maintaining the spatial peak temporal average intensity. In this study, a 5-MHz, 9-element,1.25 mm inter-elemental spacing linear array was excited by a sinusoidal waveform, a conventional linear chirp, and a modified linear chirp. Both chirps spanned the 3-dB bandwidth of the transducer. The conventional chirp was a broadband signal with a linear sweep of frequencies between 2.5 and 7.5 MHz,with all frequency components excited with equal amplitude. The modified chirp signal also swept the frequencies between 2.5 and 7.5 MHz, but the amplitude was weighted such that the edges (low and high frequencies of the band) were excited with more energy than the center of the band. In simulations, the field patterns for the sinusoidal, conventional chirp and modified chirp excitations were produced from the array using Field II and compared. For experiments, the beam pattern from a 5-MHz single-element transducer was mapped using a hydrophone for the sinusoidal, conventional chirp and modified chirp excitation. Each field from the transducer was repeated and summed to produce a field from an array of 9 elements. The difference in the time averaged intensity (in dB) in the main lobe and grating lobes were estimated for each excitation and compared. The results demonstrated that the chirp signals resulted in decreases in grating lobe levels compared to the main lobe, i.e. 10 dB down for focusing and 6 dB down for focusing and steering. A further 1 dB decrease in grating lobe levels was observed for the modified chirp excitation compared to the conventional chirp excitation, which corresponds to 21% reduction in energy deposition at the grating lobe location. |
| Title |
Amplitude, isobar and grey-scale imaging of ultrasonic shadows behind rigid, elastic and gaseous spheres. |
| Author |
Filipczynski L, Kujawska T, Tymkiewicz R, Wojcik J. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1996 |
| Abstract |
The theory of wave reflection from spherical obstacles was applied for determination of the cause of the shadow created by plane wave pulses incident on rigid, steel, gaseous spheres and on spheres made of kidney stones. The spheres were immersed in water which was assumed to be a tissue-like medium. Acoustic pressure distributions behind the spheres with the radii of 1 mm, 2.5 mm and 3.5 mm were determined at the frequency of 5 MHz. The use of the exact wave theory enabled us to take into account the diffraction effects. The computed pressure distributions were verified experimentally at the frequency of 5 MHz for a steel sphere with a 2.5-mm radius. The experimental and theoretical pulses were composed of about three ultrasonic frequency periods. Acoustic pressure distributions in the shadow zone of all spheres were shown in the amplitude axonometric projection, in the grey scale and also as acoustic isobar patterns. Our analysis confirmed existing simpler descriptions of the shadow from the point of view of reflection and refraction effects; however, our approach is more general, also including diffraction effects and assuming the pulse mode. The analysis has shown that gaseous spherical inclusions caused shadows with very high dynamics of acoustic pressures that were about 15 dB higher in relation to all the other spheres. The shadow length, determined as the length at which one observes a 6-dB drop of the acoustic pressure, followed the relation r-6dB = 3.7a2/lambda with the accuracy of about 20% independent of the sphere type. lambda denotes the wavelength and a the sphere radius. Thus, a theoretical possibility of differentiating between gaseous and other inclusions and of estimation of the inclusion size in the millimeter range from the shadow was shown. The influence of the frequency-dependent attenuation on the shadow will be considered in the next study. |
| Title |
Amplitude-dependent losses in ultrasound exposure measurement. |
| Author |
Duck FA, Perkins MA. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
1988 |
| Abstract |
Energy losses resulting from the nonlinear propagation of ultrasonic pulses in water have been measured using a polyvinylidene diflouride (pVdF) membrane hydrophone and a radiation-force balance. The focused ultrasonic transducers used were of low focal gain operating at source intensities and frequencies typical of those used in medical diagnostic applications. Energy transfer into harmonic components has been quantified by hydrophone measurements at the focus. At values of shock parameter theta > pie/2 total loss of intensity was observed, the greatest loss reaching 2.75 dB of the intensity predicted by linear extrapolation from low-amplitude measurements. A similar but smaller magnitude reduction in the radiation force measured by a force balance was observed. These results are related to ranges of acoustic parameters obtained from surveys on clinical equipment. It is concluded that a significant majority of contemporary clinical scanners can generate ultrasonic pulses which will lose energy during transmission through water due to amplitude-dependent nonlinear losses, and that it is necessary to consider these, and other nonlinear phenomena, when predicting exposure conditions in vivo. |
| Title |
Amplitude-dependent losses in ultrasound exposure measurement. |
| Author |
Duck FA, Perkins MA. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
1988 |
| Abstract |
Energy losses resulting from the nonlinear propagation of ultrasonic pulses in water have been measured using a polyvinylidene difluoride membrane hydrophone and a radiation-force balance. The focused ultrasonic transducers used were of low focal gain operating at source intensities and frequencies typical of those used in medical diagnostic applications. Energy transfer into harmonic components has been quantified by hydrophone measurements at the focus. At values of shock parameter ?>?/2, total loss of intensity was observed, with the greatest loss reaching 2.75 dB of the intensity predicted by linear extrapolation from low-amplitude measurements. A similar but smaller-magnitude reduction in the radiation force measured by a force balance was observed. These results are related to ranges of acoustic parameters obtained from surveys on clinical equipment. It is concluded that a significant majority of contemporary clinical scanners can generate ultrasonic pulses which will lose energy during transmission through water due to amplitude-dependent nonlinear losses, and that it is necessary to consider these, and other nonlinear phenomena, when predicting exposure conditions in vivo. |
| Title |
Amplitude-modulation chirp imaging for contrast detection. |
| Author |
Li M,Kuo Y,Yeh C. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2010 |
| Abstract |
We propose an amplitude-modulation chirp imaging method for contrast detection with high-frequency ultrasound. Our proposed method detects microbubbles by extracting and then selectively compressing the component of the backscattered chirp signal modulated by changes in the radii of microbubbles at their resonance frequency. Microbubbles are sonicated simultaneously with a narrowband, low-frequency pumping signal at their resonance frequency and a wideband, high-frequency imaging chirp signal. Changes in the radii of the resonant microbubbles result in periodic changes in their acoustic cross section that modulate the amplitude of the backscattered imaging chirp signal, forming pumping and imaging frequency sum-and-difference chirp terms. The frequency-sum or -difference chirp component is then extracted by a bandpass filter (BPF). Because a long imaging pulse duration is required to obtain a sufficient modulation depth on the chirp for contrast detection and to facilitate frequency-sum-and-difference signal extraction with the BPF, a chirp with a longer-than-usual waveform is used so pulse compression of the extracted chirp signal can then be performed to maintain the axial resolution, and even further improve the signal-to-noise ratio and contrast-to-tissue ratio. Experiments performed on flow phantoms with and without a speckle-generating background were performed to demonstrate the efficacy of the proposed technique. These results indicate that our proposed method can potentially provide high-resolution contrast detection in the microvasculature. |
| Title |
An acoustic backscattering technique for the detection of transient cavitation produced by microsecond pulses of ultrasound. |
| Author |
Roy RA, Madanshetty SI, Apfel RE. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1997 |
| Abstract |
An acoustic backscattering technique for detecting transient cavitation produced by 10-µs-long pulses of 757-kHz ultrasound is described. The system employs 10-µs-long, 30-MHz center frequency tone bursts that scatter from cavitation microbubbles. Experiments were performed with suspensions of hydrophobic polystyrene spheres in ultraclean water. Transient cavitation threshold pressures measured with the active cavitation detector (ACD) were always less than or equal to those measured using a passive acoustic detection scheme. The measured cavitation thresholds decreased with increasing dissolved gas content and increasing suspended particle concentration. Results also show that ultrasonic irradiation of the polystyrene sphere suspensions by the ACD lowered the threshold pressure measured with the passive detector. A possible mechanism through which suspensions of hydrophobic particles might nucleate bubbles is presented. |
| Title |
An acoustic backscattering technique for the detection of transient cavitation produced by microsecond pulses of ultrasound. |
| Author |
Roy RA, Madanshetty SI, Apfel RE. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1990 |
| Abstract |
An acoustic backscattering technique for detecting transient cavitation produced by 10-µs-long pulses of 757-kHz ultrasound is described. The system employs 10-µs-long, 30-MHz center frequency tone bursts that scatter from cavitation microbubbles. Experiments were performed with suspensions of hydrophobic polystyrene spheres in ultraclean water. Transient cavitation threshold pressures measured with the active cavitation detector (ACD) were always less than or equal to those measured using a passive acoustic detection scheme. The measured cavitation thresholds decreased with increasing dissolved gas content and increasing suspended particle concentration. Results also show that ultrasonic irradiation of the polystyrene sphere suspensions by the ACD lowered the threshold pressure measured with the passive detector. A possible mechanism through which suspensions of hydrophobic particles might nucleate bubbles is presented. |
| Title |
An adaptive, noise tolerant, frequency extrapolation algorithm for diffraction corrected ultrasound tomography. |
| Author |
Stenger F, berggren MJ, Johnson SA, Li Y. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
1983 |
| Abstract |
No Abstract Available. |
| Title |
An all-ultrasound system for sonication and real-time monitoring of temperature and ablation. |
| Author |
Maleke C, Konofagou EE. |
| Journal |
Proc Ultrason Symp IEEE |
| Volume |
|
| Year |
2006 |
| Abstract |
Over the past years, researchers have investigated the potential of focused ultrasound surgery for non-invasive or minimally invasive modalities for cancer treatment. Focus ultrasound (FUS) transducer induces a high acoustic intensity at the localized focus for a short time period while the temperature at the focus rises significantly and reaches a thermal dose that causes local irreversible cell damage (coagulation necrosis). The key limitations of FUS surgery are the difficulty of monitoring temperature and tissue mechanical properties as well as potential requirement of interrupting the FUS exposure during surgery. Amplitude-Modulated Harmonic Motion Imaging (AM-HMI) technique for simultaneous monitoring and generation of ultrasound surgery using a single-element FUS transducer was formerly introduced [Maleke, C, et al., 2006]. The uniqueness of this technique is that the focused transducer is driven by a low AM wave with high modulation index. This produces a stable harmonic radiation force oscillating at a low modulation frequency (25 Hz). This paper presents the effect of thermal ablation using the AM-HMI technique in in vitro bovine liver. The temperature and RF echoes were recorded during FUS ablation. The tissue displacement was estimated using a speckle tracking technique based on the one-dimensional cross-correlation to estimate the resulting axial tissue displacement. The temperature elevation at the focal zone throughout sonication was over 50degC that produced tissue damage. During FUS ablation, the temperature rose significantly while the tissue stiffness decreased resulting in higher tissue displacements. The results show a linear relationship between the temperature and tissue displacements during lesion formation (<53degC). The AM-HMI technique was able to accurately detect the protein-denatured lesion according to variation in the tissue displacement. In this conclusion, the technique could be potentially used for realtime monitoring of the te- mperature and mechanical properties of tissues during FUS surgery |
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