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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 144 out of 330
| Title |
Imaging parameters on third harmonic transmit phasing for tissue harmonic generation. |
| Author |
Shen C-C, Wang Y-C, Yeh CK. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2008 |
| Abstract |
In third harmonic (3f0) transmit phasing, transmit waveforms comprising fundamental (f0) signal and 3f0 signal are used to generate both frequency-sum and frequency-difference components for manipulation of tissue harmonic amplitude. Nevertheless, the acoustic propagation of 3f0 transmit signal suffers from more severe attenuation and phase aberration than the f0 signal and hence degrades the performance of 3f0 transmit phasing. Besides, 3f0 transmit parameters such as aperture size and signal bandwidth are also influential in 3f0 transmit phasing. In this study, extensive simulations were performed to investigate the effects of these imaging parameters. Results indicate that the harmonic enhancement and suppression in 3f0 transmit phasing are compromised when the magnitude of frequency-difference component decreases in the presence of tissue attenuation and phase aberration. To compensate for the reduced frequency-difference component, a higher 3f0 transmit amplitude can be used. When the transmit parameters are concerned, a smaller 3f0 transmit aperture can provide more axially uniform harmonic enhancement and more effective suppression of harmonic amplitude. In addition, the spectral leakage signal also interferes with tissue harmonics and degrades the efficacy of 3f0 transmit phasing. Our results suggest that, in the method of 3f0 transmit phasing, the transmit amplitude, phase and aperture size of 3f0 signal should remain adjustable for optimization of clinical performance. Besides, multipulse sequences such as pulse inversion are also favorable for leakage removal in 3f0 transmit phasing. |
| Title |
Imaging with unfocused regions of focused ultrasound beams. |
| Author |
Zemp R, Insana MF. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
2007 |
| Abstract |
This article gives an analytical, computational, and experimental treatment of the spatial resolution encoded in unfocused regions of focused ultrasound beams. This topic is important in diagnostic ultrasound since ultrasound array systems are limited to a single transmit focal point per acoustic transmission, hence there is a loss of spatial resolution away from the transmit focus, even with the use of dynamic receive focusing. It is demonstrated that the spatial bandwidth of a Gaussian-apodized beam is approximately constant with depth, which means that there is just as much encoded spatial resolution away from the transmit focus as there is in the focal region. The practical application of this principle is discussed, an algorithm for retrospectively focusing signals from unfocused regions of fixed-focus beams is presented, and a quantitative comparison between the authors' methods and dynamic-receive beamforming is provided. |
| Title |
Immunological effects of microwaves, ultrasound, and hyperthermia: B-lymphocyte capping. |
| Author |
Sultan MF. |
| Journal |
Thesis(PhD): Univ of Illinois |
| Volume |
|
| Year |
1982 |
| Abstract |
No abstract available. |
| Title |
Impact of myocardial contrast echocardiography on vascular permeability: An in vivo dose response study of delivery mode,pressure amplitude and contrast dose. |
| Author |
Li P,Cao L,Dou C, Armstrong WF, Miller D. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2003 |
| Abstract |
An in vivo rat model of myocardial contrast echocardiography (MCE) was defined and used to examine the dose range response of microvascular permeabilization and premature ventricular contractions (PVCs) with respect to method of imaging, peak rarefactional pressure amplitude (PRPA) and agent dose. A left ventricular short axis view was obtained on anesthetized rats at 1.7 MHz using a diagnostic ultrasound system with simultaneous ECG recording. Evans blue dye, a marker for microvascular leakage, and a bolus of Optison were injected i.v. Counts of PVCs were made from video tape during the 3 min of MCE. Hearts were excised 5 min after imaging and petechial hemorrhages, Evans blue colored area and Evans blue content were determined. No PVCs or microvascular leakage were seen in rats imaged without contrast agent followed by contrast agent injection without imaging. When PVCs were detected during MCE, petechial hemorrhages and Evans blue leakage were also found in the myocardium. Triggering 1:4 at end-systole produced the most PVCs per frame and most microvascular leakage, followed by end-systole 1:1, continuous scanning and end-diastole triggering 1:1. All effects increased with increasing Optison dosage in the range 25 to 500 µL kg(-1). Ultrasound PRPA was important, with apparent thresholds for PVCs at 1.0 MPa and for petechiae at 0.54 MPa. PVCs, petechial hemorrhages and microvascular leakage in the myocardium occur as a result of MCE in rats. |
| Title |
Impact of local attenuation approximations when estimating correlation length from backscattered ultrasound echoes. |
| Author |
Bigelow T, O'Brien WD Jr. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
2006 |
| Abstract |
Estimating the characteristic correlation length of tissue microstructure from the backscattered power spectrum could improve the diagnostic capability of medical ultrasound. Previously, size estimates were obtained after compensating for source focusing, the frequency-dependent attenuation along the propagation path (total attenuation), and the frequency-dependent attenuation in the scattering region (local attenuation). In this study, the impact of approximations of the local attenuation on the scatterer size estimate was determined using computer simulations and theoretical analysis. The simulations used Gaussian impedance distributions with an effective radius of 25 ?m randomly positioned in a homogeneous half-space sonified by a spherically focused source (f/1 to f/4). The approximations of the local attenuation that were assessed neglected local attenuation (i.e., assume 0 dB/cm-MHz) neglected frequency dependence of the local attenuation, and assumed a finite frequency dependence (i.e., 0.5 dB/cm-MHz) independent of the true attenuation of the medium. Errors in the scatterer size estimate due to the local attenuation approximations increased with increasing window length, increasing true local attenuation and increasing f number. The most robust estimates were obtained when the local attenuation was approximated by a tissue-independent attenuation value that was greater than 70% of the largest attenuation expected in the tissue region of interest. |
| Title |
Impact of microbubbles on shockwave-mediated DNA uptake in cells in vitro. |
| Author |
Bekerdjian R,Bohris C,Hansen A,Katus HA,Kuecherer HF,Hardt SE. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2007 |
| Abstract |
Gas-filled microbubbles have been successfully used as gene delivery reagents in combination with diagnostic ultrasound. Although shock wave exposure has been shown to transfect cells with naked DNA in vitro, it has not been tested whether the addition of microbubbles would augment DNA uptake under those conditions. Therefore, the aim of this study was to test the impact of microbubbles on transgene expression in vitro under shock wave exposure conditions. HEK 293 cells were treated with 60 or 120 pulses of shock waves at varying energy levels. Cells were mixed with either 100 µg/mL luciferase expressing plasmid DNA or with microbubbles that were produced with the same amount of this DNA. Cell death was evaluated after 1 h and transgene expression, after 24 h. In the presence of microbubbles, transgene expression was significantly higher (as much as 29-fold) relative to that obtained without microbubbles. Cells exposed to 120 pulses demonstrated higher transgene expression (as high as 2.7-fold) compared with cells exposed to 60 pulses. The use of microbubbles resulted in greater cell death, varying from 26% (low energy) to 78% (high energy). In conclusion, DNA-loaded microbubbles can significantly increase shock wave mediated gene transfer. However, this effect is associated with increased levels of cell destruction. |
| Title |
Implementation of a quality assurance program for ultrasound B-scanners. |
| Author |
Lopez H. |
| Journal |
Rep U S Dept HEW |
| Volume |
|
| Year |
1979 |
| Abstract |
No abstract available. |
| Title |
Implementation of ultrasound time-domain cross-correlation blood velocity estimators. |
| Author |
Jensen JA. |
| Journal |
IEEE Trans Biomed Eng |
| Volume |
|
| Year |
1993 |
| Abstract |
The implementation of real-time blood velocity estimators using time-domain cross-correlation is investigated. The basic algorithm is presented for doing stationary echo canceling, cross-correlation estimation and subsequent velocity estimation. Sampled data acquired at rates of approximately 20 MHz are used in the algorithm imposing a heavy burden on the signal processing hardware. The algorithm is analyzed with regard to the high sampling frequency, and a method for performing real-time high-speed data movement and cross-correlation is suggested. Implementation schemes based on using the sign of the data as well as the full precision are proposed. From an analysis of the process it is concluded that the sign data implementation can attain real-time processing. This can also be obtained for the full precision data, however, at the expense of using a number of dedicated signal processing chips. Both implementations suggested can handle the estimation of velocities for A-lines acquired from multiple directions. .. |
| Title |
Improved algorithm for estimation of attenuation along propagation path using backscattered echoes from multiple sources. |
| Author |
Bigelow TA |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
2010 |
| Abstract |
Accurately determining the attenuation along the propagation path leading to a region of interest could significantly improve diagnostic ultrasound tissue characterization since tissue characterization requires exact compensation for the frequency-dependent attenuation along the propagation path. In a previous study (JASA, 124:1367, 2008), it was shown that the total attenuation can be determined by using the backscattered echoes from multiple sources. The preliminary computer simulation results, had an average error between -0.3 and +0.2dB/MHz for the cases tested with a trend towards increasing error with increasing correlation length (i.e., characteristic size of the tissue microstructure of the scattering medium) and attenuation along the propagation path. Therefore, the goal of this study was to improve the accuracy of previously derived algorithm and reduce the dependence of the algorithm on correlation length and attenuation. In this study, the previous derivations were redone and the assumptions made by the algorithm regarding the scattering properties of the medium and the shape of the backscattered power spectrum were relaxed. The revised algorithm was then verified using computer simulations of five sources (6, 8, 10, 12, and 14MHz, 50% bandwidth) exposing a homogeneous tissue region. The simulated tissue had microstructure following a Gaussian spatial correlation function (i.e., exp(-0.827(ka(eff))(2)) where k is the wavenumber) with effective radii, a(eff), of 5-55microm (one size per simulated case) placed at a density of 250/mm(3) ( approximately 5 scatterers/resolution cell for 14MHz transducer). The attenuation of the tissue was also varied from 0.1 to 0.9dB/cm-MHz. The computer simulations demonstrated that the modifications significantly improved the accuracy of the algorithm resulting in average errors between -0.04 and 0.1dB/MHz which is three times better than the error performance of the original algorithm. |
| Title |
Improved axial resolution using pre-enhanced chirps and pulse compression. |
| Author |
Oelze ML. |
| Journal |
Proc Ultrason Symp IEEE |
| Volume |
|
| Year |
2006 |
| Abstract |
Improving the resolution of an ultrasound imaging device can have broad clinical impact. A novel pulse compression technique is developed that improves the axial resolution of an ultrasound imaging system and provides a boost in the echo signal-to-noise ratio (eSNR). The new technique, called the resolution enhancement compression (REC) technique, was validated with simulations and experimental measurements. The technique relies on exciting the source with a pre-enhanced frequency-modulated (FM) chirp that is found from application of convolution equivalence. Convolution equivalence is used to equate the pre-enhanced FM chirp convolved with the impulse response of the source of finite bandwidth with a linear FM chirp convolved with the impulse response of a source of larger bandwidth. The REC technique uses the linear chirp to compress the excitation waveform resulting in an impulse response with a bandwidth larger than the impulse response from conventional pulsing methods. Simulations and experimental measurements were conducted to validate the REC technique. Image quality was examined in terms of three metrics: the eSNR, the bandwidth, and the modulation transfer function (MTF). The simulations were conducted with a weakly-focused single-element ultrasound source with a center frequency of 5 MHz. The experimental measurements were carried out with a single-element transducer (f/3) with a center frequency of 2.25 MHz. Measurements were taken from a planar reflector and wire targets. In simulations, the axial resolution of the ultrasound imaging system was almost doubled using the REC technique versus conventional pulsing techniques. The axial resolution measured from MTF curves was 0.14 mm and 0.27 mm, respectively. The -3-dB bandwidth was almost doubled from 47% to 96% and maximum range sidelobes were -50 dB. Experimental measurements conducted using the single-element transducer also revealed an improvement in axial resolution using the REC technique versus conv- entional pulsing. The axial resolution from the MTF curves was 0.31 mm and 0.44 mm, respectively. The -3-dB bandwidth was doubled from 56% to 113% and maximum range sidelobes were observed at -45 dB. In addition, a significant gain in eSNR (9.2 to 16.2 dB) was achieved in both simulations and experiments. Improvement in axial resolution, doubling of the system bandwidth, and a gain in eSNR were achieved with the REC technique with range sidelobe levels compatible with ultrasound imaging systems. |
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