Biomedical subjects

Shigao Chen

Publications and source records attributed to Shigao Chen.

12 recordsLinked to original sources

PubMed2006

Difference frequency and its harmonic emitted by microbubbles under dual frequency excitation.

Vibro-acoustography is an elasticity imaging method that uses two ultrasound beams of slightly different frequency to excite an object and detects the resulting acoustic emission (AE) at the difference frequency. This method is especially sensitive to bubbles due to their nonlinearity. This study explores the harmonic acoustic emission (HAE) at twice the difference frequency emitted from bubbles. A perturbation method based on the dynamic bubble equation is used to derive the AE and HAE from a single bubble excited by dual frequency waves. Simulation shows that HAE is generated only by microbubbles whose resonant frequencies match the incident ultrasound frequencies. In contrast, AE is more sensitive to resonance at the difference frequency, which is relevant to sub-millimeter bubbles. This finding was confirmed by experiments where HAE was produced from Optison microbubbles, but not from larger air bubbles which are off resonance at the incident ultrasound frequency. In conclusion, harmonic acoustic emission is present for microbubbles. It is very sensitive to the size of the bubble and may be used for selective detection of microbubbles.

Albumins↗

PubMed2006

Parametric amplification of the dynamic radiation force of acoustic waves in fluids.

We report on parametric amplification in dynamic radiation force produced by a bichromatic acoustic beam in a fluid. To explain this effect we develop a theory taking into account the nonlinearity of the fluid. The theory is validated through an experiment to measure the dynamic radiation force on an acrylic sphere. Results exhibit an amplification of 66 dB in water and 80 dB in alcohol as the difference of the frequencies is increased from 10 Hz to 240 kHz.

Journal Article↗

PubMed2006

[Sustainable rice pest management and its economic benefits].

With rice varieties Jiahua 1 (resistant to the white backed plant hopper Sogatella furcifera) and Shanyou 63 (susceptible) as test materials, this paper studied the effects of different insecticide treatments on the pest population and quality and yield of rice grain, and evaluated their actual economic benefits (AEB). The results indicated that in the control plot, the peak density of S. furcifera on Jiahua 1 was only 1/10 of that on Shanyou 63. Throughout the whole rice growth period, the S. furcifera population on Jiahua 1 kept below economic injury level, while Shanyou 63 suffered heavy infestation by S. furcifera. The spider density in the control plot was 3 -7 times higher than that in the plots treated with insecticide following farmers' practice. No application of insecticide decreased the grain yield of Jiahua 1 and Shanyou 63 by 11. 8% and 43.4% , respectively. Compared with the control, spraying with insecticide one time decreased the AEB of Jiahua 1 and Shanyou 63 by 0.9% - 2.6% and 2.6% - 4.7% , respectively. Without insecticide application, the AEB of Shanyou 63 decreased by 32.9% - 36.1% , while that of Jiahua 1 increased by 2.2% - 4.8%. The income of planting Jiahua 1 without insecticide application was 9403 yuan x hm(-2) , which was remarkably higher than that (8632 yuan hm (-2)) of Shanyou 63 protected by spraying insecticide 3 times. It was worthwhile to point out that insecticide treatment did not affect the processing of rice grain and its commercial and edible quality, but decreased its protein content of Jiahua 1 and Shanyou 63.

Animals↗

PubMed2005

Dynamic ultrasound radiation force in fluids.

The subject of this paper is to present a theory for the dynamic radiation force produced by dual-frequency ultrasound beams in lossless and nondispersive fluids. An integral formula for the dynamic radiation force exerted on a three-dimensional object by a dual-frequency beam is obtained stemming from the fluid dynamics equations. The static radiation force due to a monochromatic wave appears as a particular case of this theory. Dependence of the dynamic radiation force to nonlinear effects of the medium is analyzed. We calculate the dynamic radiation force exerted on solid elastic spheres of two different materials by a low-amplitude dual-frequency plane wave. The static and dynamic radiation forces exhibited approximately same magnitude. Resonance patterns observed in the dynamic radiation force are similar to those present in the static radiation force.

Journal Article↗

PubMed2005

Measurement of dynamic and static radiation force on a sphere.

Dynamic radiation force from ultrasound has found increasing applications in elasticity imaging methods such as vibro-acoustography. Radiation force that has both static and dynamic components can be produced by interfering two ultrasound beams of slightly different frequencies. This paper presents a method to measure both static and dynamic components of the radiation force on a sphere suspended by thin threads in water. Due to ultrasound radiation force, the sphere deflects to an equilibrant position and vibrates around it. The static radiation force is estimated from the deflection of the sphere. The dynamic radiation force is estimated from the calculated radiation impedance of the sphere and its vibration speed measured by a laser vibrometer. Experimental results on spheres of different size, vibrated at various frequencies, confirm the theoretical prediction that the dynamic and static radiation force on a sphere have approximately equal magnitudes [G. T. Silva, Phys. Rev. E 71, 056617 (2005)].

Acoustics↗

PubMed2005

Evaluating the dynamic performance of a fibre optic pressure microsensor.

The dynamic performance of a new fibre optic sensor intended for measuring physiological fluid pressures is assessed in water. The sensor's sensitivity is evaluated at 23 degrees, 35 degrees and 37 degrees C against a Millar pressure catheter for sinusoidal pressure inputs with frequency ranging from 0.5 to 10 Hz. We found that sensitivity versus frequency is flat to 6 Hz and decreases slightly between 6 and 10 Hz. The sensitivity is slightly lower at 23 degrees C than at 37 degrees C. The reproducibility of measurements is excellent (two separate calibration tests in two consecutive days). The output of the fibre optic system used shows a constant time delay (0.13 s) for all frequencies tested. Experiments suggest that, with current sensor design, its immersion in degassed water prior to use ensures a reliable performance.

Equipment Design↗

PubMed2005

Theory of dynamic acoustic radiation force experienced by solid cylinders.

A body insonified by a sound field is known to experience a steady force that is called the acoustic radiation force. This force can also be dynamic or oscillatory, knowing that the intensity of the incident sound field changes over time (amplitude modulation). The present paper develops the theory of dynamic radiation force experienced by a solid cylinder immersed in an ideal fluid. Analytical solutions of the equations for the dynamic force are derived. The equations provide analytical radiation force dependencies on the acoustic field and medium parameters. The case of compressional and shear waves' absorption in the solid material of the cylinder is also discussed. It is shown here that radiation force is no longer static and cannot be treated as a steady-state phenomenon.

Journal Article↗

PubMed2005

Stress field forming of sector array transducers for vibro-acoustography.

This paper presents a study of the stress field forming of sector array transducers for vibro-acoustography applications. The system point-spread function (PSF) is given in terms of the dynamic radiation stress exerted on a point target by a dual ultrasound beam with slightly different frequencies. The radiation stress is calculated by assuming that the resulting ultrasound beam is a plane wave. The stress is proportional to the product of the velocity potential of each incident ultrasound beam. The beamforming and stress field forming of sector array transducers are analyzed through linear acoustics. An expression for the velocity potential produced by sector array transducers is derived. The vibro-acoustography PSF is evaluated numerically. A comparison between the PSF of a sector array and a confocal transducers is presented. The compared characteristics of the PSF are sidelobe levels, transverse, and in-depth spatial resolution. Indeed, one motivation to study sector transducers is the fact the depth-of-field of these transducers should be smaller than that of same size confocal transducers. An experimental setup was used to validate the theoretical PSF of sector array transducers. Results show that the measured PSF is in good agreement with the theoretical predications. Vibro-acoustography images of a breast-phantom by both transducers are presented and discussed.

Acoustics↗

PubMed2004

Quantifying elasticity and viscosity from measurement of shear wave speed dispersion.

The propagation speed of shear waves is related to frequency and the complex stiffness (shear elasticity and viscosity) of the medium. A method is presented to solve for shear elasticity and viscosity of a homogeneous medium by measuring shear wave speed dispersion. Harmonic radiation force, introduced by modulating the energy density of incident ultrasound, is used to generate cylindrical shear waves of various frequencies in a homogeneous medium. The speed of shear waves is measured from phase shift detected over the distance propagated. Measurements of shear wave speed at multiple frequencies are fit with the theoretical model to solve for the complex stiffness of the medium. Experiments in gelatin phantoms show promising results validated by an independent method. Practical considerations and challenges in possible medical applications are discussed.

Adipose Tissue↗

PubMed2004

Spectral normalization for ultrasonic contrast microbubble detection.

Ultrasonic contrast agents consisting of microbubbles are used to assess tissue perfusion. The microbubbles are highly reflective and nonlinear and thus produce harmonics that are stronger than those from tissues. However, the magnitude of harmonic signals resulting from a region with microbubbles also depends on the acoustic pressure of incident ultrasound and the attenuation of intervening tissues in the ultrasound path. Therefore, the harmonic magnitude, as used in traditional harmonic imaging, may not be a reliable indicator of the presence or absence of microbubbles, and hence, tissue perfusion. To compensate for these effects, we present two parameters defined as the ratio of the harmonic to the fundamental component (HFR) and the ratio of the harmonic to squared fundamental (HSFR). A simplified model is used to illustrate the usefulness of these two parameters. Experiments show that both parameters improve detection of microbubbles and that HSFR performs better than HFR.

Contrast Media↗

PubMed2004

Comparison of stress field forming methods for vibro-acoustography.

Vibro-acoustography is a method that produces images of the acoustic response of a material to a localized harmonic motion generated by ultrasound radiation force. The low-frequency, oscillatory radiation force (e.g., 10 kHz) is produced by amplitude modulating a single ultrasound beam, or by interfering two beams of slightly different frequencies. Proper beam forming for the stress field of the probing ultrasound is very important because it determines the resolution of the imaging system. Three beam-forming geometries are studied: amplitude modulation, confocal, and x-focal. The amplitude of radiation force on a unit point target is calculated from the ultrasound energy density averaged over a short period of time. The profiles of radiation stress amplitude on the focal plane and on the beam axis are derived. The theory is validated by experiments using a small sphere as a point target. A laser vibrometer is used to measure the velocity of the sphere, which is proportional to the radiation stress exerted on the target as the transducer is scanned over the focal plane or along the beam axis. The measured velocity profiles match the theory. The theory and experimental technique may be useful in future transducer design for vibro-acoustography.

Acoustics↗

PubMed2002

Remote measurement of material properties from radiation force induced vibration of an embedded sphere.

A quantitative model is presented for a sphere vibrated by two ultrasound beams of frequency omega1 and omega2. Due to the interference of two sound beams, the radiation force has a dynamic component of frequency omega2-omega1. The radiation impedance and mechanical impedance of the sphere are then used to compute the vibration speed of the sphere. Vibration speed versus vibration frequency is measured by laser vibrometer on several spheres, both in water and in gel phantom. These experimental results are used to verify the model. This method can be used to estimate the material properties of the medium (e.g., shear modulus) surrounding the sphere.

Elasticity↗