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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 113 out of 330
| Title |
Estimating concentration of ultrasound contrast agents with backscatter coefficients: Experimental and theoretical aspects. |
| Author |
Leithem SM, Lavarello RJ, O'Brien WD Jr, Oelze ML. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
2012 |
| Abstract |
Ultrasound contrast agents (UCAs) have been explored as a means to enhance therapeutic techniques.Because the effectiveness of these techniques relies on the UCA concentration at a target site,it would be beneficial to estimate UCA concentration noninvasively. In this study, a noninvasive method for estimating UCA concentration was developed in vitro. Backscatter coefficients (BSCs) estimated from measurements of DefinityVR UCAs were fitted to a theoretical scattering model in the 15–25 MHz range using a Levenberg-Marquardt regression technique. The model was defined by the UCA size distribution and concentration, and therefore concentration estimates were extracted directly from the fit. Calculation of the BSC was accomplished using planar reference measurements from the back wall of a PlexiglasVR chamber and an average of 500 snapshots of ultrasonic backscatter from UCAs flowing through the chamber. In order to verify the ultrasonically derived UCA concentration estimates, a sample of the UCAs was extracted from the flow path and the concentration was estimated with a hemacytometer. UCA concentrations of 1, 2, and 5 times the dose recommended by the manufacturer were used in experiments. All BSC-based estimates were within one standard deviation of hemacytometer based estimates for peak rarefactional pressures of 100–400 kPa. |
| Title |
Estimating in situ exposure in the presence of acoustic nonlinearity. |
| Author |
Duck FA. |
| Journal |
J Ultrasound Med |
| Volume |
|
| Year |
1999 |
| Abstract |
Evidence is presented for excess attenuation of pulsed ultrasound due to finite amplitude effects in water. Measurements on a modern scanner are used to demonstrate that linear derating can underestimate many exposure quantities, including all safety indices apart from the cranial thermal index. More appropriate methods for estimating in situ exposure are reviewed. A preferred procedure that requires exposure measurements to be made in water under "small signal" conditions is selected. A spectra index is defined that is proposed as and indicator of finite amplitude effects, where spectral index=0.1 defines the threshold between nonlinear and quasi-linear conditions. |
| Title |
Estimating localized oscillatory tissue motion for assessment of the underlying mechanical modulus. |
| Author |
Konofagou EE, Ottensmeyer M, Agabian S, Dawson SL, Hynynen K. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
2004 |
| Abstract |
The technique of harmonic motion imaging (HMI) uses the localized stimulus of the oscillatory ultrasonic radiation force as produced by two overlapping beams of distinct frequencies, and estimates the resulting harmonic displacement in the tissue in order to assess its underlying mechanical properties. In this paper, we studied the relationship between measured displacement and stiffness in gels and tissues in vitro. Two focused ultrasound transducers with a 100 mm focal length were used at frequencies of 3.7500 MHz and either 3.7502 (or 3.7508 MHz), respectively, in order to produce an oscillatory motion at 200 Hz in the gel or tissue. A 1.1 MHz diagnostic transducer (Imasonics, Inc.) was also focused at 100 mm and acquired 5 ms RF signals (pulse repetition frequency (PRF)=3.5 kHz) at 100 MHz sampling frequency during radiation force application. First, three 50 × 50 mm2 acrylamide gels were prepared at concentrations of 4%, 8% and 16%. The resulting displacement was estimated using crosscorrelation techniques between successively acquired RF signals with a 2 mm window and 80% window overlap at 1260 W/cm2. A normal 1-D indentation instrument (TeMPeST) applied oscillatory loads at 0.1–200 Hz with a 5 mm-diameter flat indenter. Then, 12 displacement measurements in 6 porcine muscle specimens (two measurements/case, as above) were made in vitro, before and after ablation which was performed for 10 s at 1260 W/cm2. In all gel cases, the harmonic displacement was found to linearly increase with intensity and exponentially decrease with gel concentration. The TeMPeST measurements showed that the elastic moduli for the 4%, 8% and 16% gels equaled 3.93 ± 0.06, 17.1 ± 0.2 and 75 ± 2 kPa, respectively, demonstrating that the HMI displacement estimate depends directly on the gel stiffness. Finally, in the tissues samples, the mean displacement amplitude showed a twofold decrease between non-ablated and ablated tissue, demonstrating a correspondence between the HMI response and an increase in stiffness measured with the TeMPeST instrument. |
| Title |
Estimating mean scatterer spacing with the frequency-smoothed spectral autocorrelation function. |
| Author |
Varghese T, Donohue KD. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
1995 |
| Abstract |
The quasiperiodicity of regularly spaced scatterers results in characteristic patterns in the spectra of backscattered ultrasonic signals from which the mean scatterer spacing can be estimated. The mean spacing has been considered for classifying certain biological tissue. This paper addresses the problem of estimating the mean scatterer spacing from backscattered ultrasound signals using the frequency-smoothed spectral autocorrelation (SAC) function. The SAC function exploits characteristic differences between the phase spectrum of the resolvable quasiperiodic scatterers and the unresolvable uniformly distributed (diffuse) scatterers to improve estimator performance over other estimators that operate directly on the magnitude spectrum. Mean scatterer spacing estimates are compared for the frequency-smoothed SAC function and a cepstral technique using an AR model. Simulation results indicate that SAC-based estimates converge more reliably over smaller amounts of data than cepstrum-based estimates. An example of computing an estimate from liver tissue scans is also presented for the SAC function and the AR cepstrum. |
| Title |
Estimating of ultrasound attenuation along the propagation path by applying multiple filters to backscattered echoes. |
| Author |
Bigelow TA. |
| Journal |
Proc Ultrason Symp IEEE |
| Volume |
|
| Year |
2009 |
| Abstract |
Quantifying the correlation length of the tissue microstructure has shown potential for differentiating between benign and malignant tumors. To implement these advances in the clinic, the total frequency-dependent attenuation along the propagation path must be determined on a patient specific basis. Previously, an algorithm was developed to estimate this attenuation using echoes from multiple sources. In this study, the developed algorithm was extended to echoes from a single source by filtering the echoed signal into multiple frequency bands. This step was needed because it would be challenging to scan exactly the same tissue region using multiple sources in the clinic. Computer simulations and phantom experiments were conducted to verify the attenuation could be determined by filtering the echoes from a single source. The simulations utilized a spherically focused single-element source (5 cm focal length, f/4, 14 MHz center frequency, 50% bandwidth) exposing a homogeneous tissue region (Gaussian scattering structures with effective radii of 5 to 55 mum at a density of 250/mm(3), attenuation of 0.1 to 0.9 dB/cm.MHz). The phantom experiments utilized a spherically focused single-element source (5.08 cm focal length, f/4, 7.5 MHz center frequency) exposing a 0.5 dB/cm.MHz homogeneous glass bead phantom. The computer simulations and phantom experiment confirmed that the total attenuation along the propagation path can be determined by appropriately applying multiple filters to the backscattered echoes from a single source. |
| Title |
Estimating reflected ultrasound spectra from quantized signals. |
| Author |
Kuc R. |
| Journal |
IEEE Trans Biomed Eng |
| Volume |
|
| Year |
1985 |
| Abstract |
Quantization errors are inevitably introduced whenever analog-to-digital conversion of data is employed. The effect of these errors on estimating power spectra from ultrasound signals reflected from within the body is examined. Reflected data were stimulated by convolving a pulse sequence with a filter model of the random reflectors encountered within the liver. The data were then qauntized with an N-bit quantizer model, with 1*N*8, multiplied by a Hamming data window and used to estimte the power spectrum. For typical diagnostic signals reflected from 1 cm of tissue, the results indicte that the quantization errors limit the frequency range over which unbiased estimates of the spectra are observed for N<8. For N*8, the sidelobes in the Hamming window spectrum limit this range. To illustrate the implication of these results, the problem of estimating the slope of the acoustic attenuation coefficient, denoted by ?, for liver tissue from reflected ultrasound signals is examined. Three ? estimators are considered: 1) a new correlation method, 2) a zero-crossing count analysis, and 3) the slope of the log-spectral difference. The simulated results indicate under which conditions a particular estimator should be employed. |
| Title |
Estimating sound attenuation coefficient from backscattering echo for animal soft tissue. [Article in Chinese.] |
| Author |
Feng R, Mao W, Chen Z. |
| Journal |
Acta Acust |
| Volume |
|
| Year |
1992 |
| Abstract |
In this paper a principle and method of estimating ultrasound attenuation spectrum from a backscattering echo are described. A series of experimental studies on fresh in vitro animal soft tissues have been completed by using the established in our laboratory automatic spectrum measuring system. The obtained results show that the measurement accuracy and precision are satisfactory. It is pointed out that this method of estimating attenuation spectrum, as one can be used for ultrasound tissue characterization, is of worth to study. |
| Title |
Estimating the acoustic attenuation coefficient slope for liver from reflected ultrasound signals. |
| Author |
Kuc R, Schwartz M. |
| Journal |
IEEE Trans Sonics Ultrason |
| Volume |
|
| Year |
1979 |
| Abstract |
The acoustic attenuation coefficient measured in dB/cm is known to increase linearly with frequency for liver tissue. The slope of this linear function, denoted by ?, has been shown by other investigators to be an indicator of tissue state. ? is usually measured from the change in the log spectrum experienced by an acoustic pulse when it is transmitted through the tissue. Research is presented in estimating ? from the reflected signals which are currently used in the clinical environment to generate diagnostic images. The reflected signals from internal tissue structures (in liver, the vascular and biliary system) are distorted by the irregular reflector shapes. By modeling the roughness of the typical acoustic reflector, the distribution of the values within each spectrum can be derived and the 95-percent confidence limits for the measured ? can be calculated. The confidence limits are presented in terms of the tissue size and average reflector density. Experimental results using the reflected signals from in vitro refrigerated and formalin-fixed liver sections are presented. It is predicted mathematically and verified experimentally that an uncertainty in the estimate exists due to the lack of knowing the exact reflector shapes within the liver. This uncertainty decreases as the amount of available data increases. For a large organ, such as liver, which provides large quantities of reflected data, our signal processing techniques may provide diagnostically valuable information. |
| Title |
Estimating the medical ultrasound in vivo power spectrum. |
| Author |
Bigelow TA. |
| Journal |
Thesis(PhD): Univ of Illinois |
| Volume |
|
| Year |
2004 |
| Abstract |
This thesis considered the estimation of the in vivo power spectrum from the backscattered waveforms by finding the total attenuation along the propagation path. The total attenuation was estimated by assuming model for the scatterers (i.e., spherically symmetric Gaussian impedance distributions of unknown size) and then solving for the size and total attenuation simultaneously from the frequency dependence of the backscattered spectrum. The attenuation and scatterer size could be accurately and precisely estimated provided that sufficient frequency data was available. The accuracy and precision were significantly improved by increasing the range of frequencies used in the estimate. In addition, some improvement could be obtained by increasing the length of the window used to gate the backscattered RF echoes (i.e., samples in frequency domain independent). Although only applied to the estimation of scatterer size in this thesis, the estimation of the in vivo power spectrum using the developed methods could be applied to other tissue characterization procedures as well as estimating the temperature increase in the tissue from ultrasound exposures. |
| Title |
Estimating the shell parameters of sono vue® microbubbles using light scattering. |
| Author |
Tu J, Guan J, Qiu Y,Matula TM. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
2009 |
| Abstract |
Experiments were performed to measure the dynamical response of individual SonoVue® microbubbles subjected to pulsed ultrasound. Three commonly used bubble dynamic models (i.e., Hoff's, Sarkar's, and linearized Marmottant's models) were compared to determine the most appropriate model for fitting to the experimental data. The models were evaluated against published optical microscopy data. The comparison suggests that it is difficult to rank these models for lipid-shelled microbubbles undergoing small-amplitude oscillations, because under these conditions the shell parameters in these models are closely related. A linearized version of the Marmottant model was used to estimate the shell parameters (i.e., shear modulus and shear viscosity) of SonoVue® microbubbles from the experimental light scattering data, as a function of ambient microbubble radius. The SonoVue® microbubble shell elasticity and dilatational viscosity increase with ambient bubble radius, in agreement with previously published data. The results suggest that light scattering, used in conjunction with one of several popular bubble dynamics models, is effective at characterizing microbubble response and evaluating shell parameters. ©2009 Acoustical Society of America. |
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