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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 228 out of 330
| Title |
Simulations of the thermo-acoustic lens effect during focused ultrasound surgery. |
| Author |
Hallaj IM, Cleveland RO, Hynynen K. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
2001 |
| Abstract |
Laboratory measurements of soft tissue properties show a dependence of background propagation properties on temperature. For typical focused ultrasound surgery (FUS) applications, only the slow variations in tissue background parameters need to be accounted for when computing the outcome of a FUS sonication. The cumulative effect of slowly varying sound speed has been referred to in the literature as a thermal lens, or a thermo-acoustic lens because of its beam-distorting properties. An algorithm to solve the coupled acoustic-thermal problem is described, and numerical results are presented to illustrate the effects of dynamic sound-speed profiles in layered tissues undergoing FUS. The results of simulations in liver with and without a fat layer indicate that the thermal-acoustic interaction results in more complex dynamics in FUS than a simple model will predict. Both the size and the position of the lesions predicted from the simulations are affected by the thermo-acoustic lens effect. However, the overall effect from short sonications at high power from sharply focused single element sources (F-no. from 0.8 to 1.3) around 1 MHz similar to those used in clinical setups is found to be small. |
| Title |
Simultaneous algebraic reconstruction technique (SART): A superior implementation of the art algorithm. |
| Author |
Andersen AH, Kak AC. |
| Journal |
Ultrason Imaging |
| Volume |
|
| Year |
1984 |
| Abstract |
In this paper we have discussed what appears to be a superior implementation of the Algebraic Reconstruction Technique (ART). The method is based on 1) simultaneous application of the error correction terms as computed by ART for all rays in a given projection; 2) longitudinal weighting of the correction terms back-distributed along the rays; and 3) using bilinear elements for discrete approximation to the ray integrals of a continuous image. Since this implementation generates a good reconstruction in only one iteration, it also appears to have a computational advantage over the more traditional implementation of ART. Potential applications of this implementation include image reconstruction in conjunction with ray tracing for ultrasound and microwave tomography in which the curved nature of the rays leads to a non-uniform ray density across the image. |
| Title |
Simultaneous backscatter and attenuation estimation using a least squares method with constraints. |
| Author |
Nam K, Zagzebski JA, Hall TJ. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2011 |
| Abstract |
Backscatter and attenuation variations are essential contrast mechanisms in ultrasound B-mode imaging. Emerging quantitative ultrasound methods extract and display absolute values of these tissue properties. However, in clinical applications, backscatter and attenuation parameters sometimes are not easily measured because of tissues inhomogeneities above the region-of-interest (ROI). We describe a least squares method (LSM) that fits the echo signal power spectra from a ROI to a three-parameter tissue model that simultaneously yields estimates of attenuation losses and backscatter coefficients. To test the method, tissue-mimicking phantoms with backscatter and attenuation contrast as well as uniform phantoms were scanned with linear array transducers on a Siemens S2000. Attenuation and backscatter coefficients estimated by the LSM were compared with those derived using a reference phantom method (Yao et al. 1990). Results show that the LSM yields effective attenuation coefficients for uniform phantoms comparable to values derived using the reference phantom method. For layered phantoms exhibiting nonuniform backscatter, the LSM resulted in smaller attenuation estimation errors than the reference phantom method. Backscatter coefficients derived using the LSM were in excellent agreement with values obtained from laboratory measurements on test samples and with theory. The LSM is more immune to depth-dependent backscatter changes than commonly used reference phantom methods. |
| Title |
Simultation of ultrasonic pulse propagation, distortion, and attenuation in the human chest wall. |
| Author |
Mast DT, Hinkelman LM, Metlay LA, Orr MJ, Waag RC. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1999 |
| Abstract |
A finite-difference time-domain model for ultrasonic pulse propagation through soft tissue has been extended to incorporate absorption effects as well as longitudinal-wave propagation in cartilage and bone. This extended model has been used to simulate ultrasonic propagation through anatomically detailed representations of chest wall structure. The inhomogeneous chest wall tissue is represented by two-dimensional maps determined by staining chest wall cross sections to distinguish between tissue types, digitally scanning the stained cross sections, and mapping each pixel of the scanned images to fat, muscle, connective tissue, cartilage, or bone. Each pixel of the tissue map is then assigned a sound speed, density, and absorption value determined from published measurements and assumed to be representative of the local tissue type. Computational results for energy level fluctuations and arrival time fluctuations show qualitative agreement with measurements performed on the same specimens, but show significantly less waveform distortion than measurements. Visualization of simulated tissue-ultrasound interactions in the chest wall shows possible mechanisms for image aberration in echocardiography, including effects associated with reflection and diffraction caused by rib structures. A comparison of distortion effects for varying pulse center frequencies shows that, for soft tissue paths through the chest wall, energy level and waveform distortion increase markedly with rising ultrasonic frequency and that arrival-time fluctuations increase to a lesser degree. |
| Title |
Single crystal PZN/PT-polymer composites for ultrasound transducer applications. |
| Author |
Ritter T, Geng X, Kirk Shung K, Lopath PD, Park SE, Shrout TR. |
| Journal |
IEEE Trans Ultrason Ferroelectr Freq Control |
| Volume |
|
| Year |
2000 |
| Abstract |
Single crystal relaxor ferroelectrics of PZN-8%PT were investigated for potential application in ultrasound transducers. The full set of electromechanical properties was determined using combined resonance and laser interferometry techniques. Ultra-high length extensional coupling (k(33)) of 0.94 was observed, a 25% increase over Navy Type VI PZT ceramics. The thickness extensional coupling (k(t)) of 0.48 was comparable to PZT compositions, and the compliance S(33)(E) was a factor of six greater. To maximize height extensional coupling (k'(33)), while minimizing length extensional coupling k(31) in array elements, it was necessary to align the elements along the 100 crystallographic direction in the x-y plane. Mode coupling plots and test samples for array elements determined that width-to-height ratios of less than 0.5 were desired, similar to the requirement for polycrystalline PZT ceramics. Modeling of 1-3 composites and experimental results demonstrated that thickness coupling greater than 0.80 could be achieved with a 40% to 70% volume fraction of PZN-PT. Although this is a substantial increase over PZT 1-3 composites, with a thickness coupling coefficient of 0.66, it represents a smaller fraction of the length extensional coupling k(33). This reduction may be a consequence of the increased compliance of PZN-PT, which results in significant clamping by the polymer matrix. Ultrasonic transducers fabricated using PZN-8%PT 1-3 composites achieved experimental bandwidths as high as 141%. The pulse-echo responses displayed good agreement with modeled results using the Redwood equivalent circuit. |
| Title |
Single microbubble response using pulse sequences:Initial results. |
| Author |
Thomas DH, Butler MB, Anderson T, Steel R, Pye SD, Poland M, Brock -Fisher T, McDicken WN, Sboros V. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2009 |
| Abstract |
The study of acoustic scattering by single microbubbles has the potential to offer improved signal processing techniques. A microacoustic system that employs a hydrodynamically-focused flow was used to detect radiofrequency (RF) backscatter from single microbubbles. RF data were collected using a commercial scanner. Results are presented for two agents, namely Definity® (Lantheus Medical Imaging, N. Billerica, MA, USA) and biSphere® (Point Biomedical Corp, San Carlos, CA, USA). The agents were insonified with amplitude-modulated pulses, and it was observed in both agents that a subpopulation of microbubbles did not produce a measurable echo from the first-half amplitude pulse, but did produce a response from the full amplitude pulse and from a subsequent half amplitude pulse. The number of microbubbles in this subpopulation was seen to increase with increasing transmit amplitude. These results do not bear out the simple theory of microbubble-pulse sequence interaction and invite a reassessment of signal processing approaches. |
| Title |
Single strand breaks in CHO cell DNA induced by ultrasonic cavitation in vitro. |
| Author |
Miller DL, Thomas RM, Frazier ME. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1991 |
| Abstract |
Ultrasonic cavitation induces a multiplicity of bioeffects in cell suspensions.exposed in a 72 RPM rotating-tube exposure system. Single strand DNA breaks.(SSBs) were found in cultured Chinese hamster ovary (CHO) cells exposed directly to 1.61 MHz ultrasound at a continuous 8 W/cm2 spatial peak temporal average (SPTA) intensity with cavitation for 10 min at 2 degrees C. Viability assessed by the trypan blue test was less than 1%, which indicates that these SSBs were in dead cells. Burst mode exposure with 10.5 microseconds bursts repeated each 21 microseconds not only caused SSBs at 5.6 W/cm2 and 8 W/cm2 (SPTA) for 10 min, but also allowed 20% and 7% viability, respectively. In order to determine if any of these breaks resided in the viable fraction of cells, the exposures were repeated with a 30 min postexposure incubation period at 37 degrees C to allow breaks in viable cells to repair. No significant repair occurred, relative to the samples which remained at 2 degrees C to prevent repair. A similar result was obtained with 10.5 microseconds bursts repeated each 42 microseconds at 4 W/cm2 (SPTA) with 46% viability. Thus, the observed.ultrasonically induced SSBs reside primarily in the nonviable fraction of cells. |
| Title |
Single strand DNA breaks in human leukocytes induced by ultrasound in vitro. |
| Author |
Miller DL, Reese JA, Frazier ME. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1989 |
| Abstract |
Single strand breaks (SSBs) in the DNA of fresh human leukocytes were analyzed after in vitro exposure to ultrasound in an ice bath. The alkaline elution method was used to quantify the frequency of SSBs. Five different ultrasonic exposures were employed: (1) 53 mW/cm2 spatial-peak temporal-average intensity (SPTA), 0.37:100 microseconds pulsed 6.2 MHz ultrasound presumably without cavitation, (2) 470 mW/cm2 (SPTA) continuous 1.7 MHz ultrasound with gas-body activation, which is a form of cavitation lacking in chemical activity, (3) 10.8 W/cm2 (SPTA), 3.0:100 microseconds pulsed 1.48 MHz ultrasound, presumably without cavitation, (4) 161 W/cm2 (SPTA) continuous 1.45 MHz ultrasound with transient cavitation, and (5) 94 W/cm2 (SPTA) continuous 8 MHz ultrasound presumably without cavitation. All of these exposures produced negative results, except (4), which yielded a significantly increased frequency of SSBs relative to sham exposed cells. For this condition, the SSB effect was comparable in magnitude to that of 1 Gy of 60Co gamma-rays, and may have resulted from the chemical activity associated with transient cavitation. The SSBs were detected in cells which remained intact after exposure; however, it is uncertain if any of the ultrasonically-induced SSBs occurred in viable cells. |
| Title |
Single transducer swept frequency ultrasonic reflection measurements. |
| Author |
Gammell PM. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
1979 |
| Abstract |
Time delay spectrometry (TDS) has been employed for transmission measurements with ultrasound. This swept frequency technique allows better control of the signal processing parameters. Several advantages can be realized by using TDS in a reflection mode. Reflection measurements from undersea sediments have been made using TDS, which permits discrimination not possible with pulse-echo methods. Isolation was achieved by using separate transmitting and receiving transducers. For non-destructive testing and medical applications it would be preferable to be able to use the same transducer for transmission and for reception. The unique features of TDS permit simultaneous transmission and reception with the same transducer. |
| Title |
Single-element focused ultrasound transducer method for harmonic motion imaging. |
| Author |
Maleke C, Pernot M, Konofagou. EE. |
| Journal |
Ultrason Imaging |
| Volume |
|
| Year |
2006 |
| Abstract |
The harmonic motion imaging (HMI) technique for simultaneous monitoring and generation of ultrasound therapy using two separate focused ultrasound transducer elements was previously demonstrated. In this study, a new HMI technique is described that images tissue displacement induced by a harmonic radiation force using a single focused-ultrasound element. A wave propagation simulation model first indicated that, unlike in the two-beam configuration, the amplitude-modulated beam produced a stable focal zone for the applied harmonic radiation force. The AM beam thus offered the unique advantage of sustaining the application of the spatially-invariant radiation force. Experiments were performed on gelatin phantoms and ex vivo tissues. The radiation force was generated by a 4.68 MHz focused ultrasound (FUS) transducer using a 50 Hz amplitude-modulated wave. A 7.5 MHz pulse-echo transducer was used to acquire rf echoes during the application of the harmonic radiation force. Consecutive rf echoes were acquired with a pulse repetition frequency (PRF) of 6.5 kHz and 1D cross-correlation was performed to estimate the resulting axial tissue displacement. The HMI technique was shown capable of estimating stiffness-dependent displacement amplitudes. Finally, taking advantage of the real-time capability of the HMI technique, temperature-dependent measurements enabled monitoring ofHIFU sonication in ex vivo tissues. The new HMI method may thus enable a highly-localized force and stiffness-dependent measurements as well as real-time and low-cost HIFU monitoring. |
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