Xiufen Gong

Acoustic nonlinearity parameter tomography for biological specimens via measurements of the second harmonics

The acoustic nonlinearity parameter B/A is a new parameter in ultrasound tissue characterization. In this paper, on the basis of a theoretical analysis of finite amplitude ultrasonic wave propagation in fluidlike media, an experimental system for nonlinearity parameter tomography is developed. Using this system, the amplitude of the second harmonic wave cumulated in the propagation path of the primary wave is measured and the finite amplitude insert-substitution method is used in the conventional computer tomography (CT) technique. The filtered convolution algorithm is implemented using the amplitude of the second harmonic as the projection data, then the B/A tomography can be reconstructed. The B/A images for several biological samples are obtained. Besides, the image of porcine pathological liver tissue is studied and is compared with that of porcine normal liver tissue. These results show the potential applications of the nonlinearity parameter B/A in medical diagnosis.

Enhancement of subharmonic emission from encapsulated microbubbles by using a chirp excitation technique

Subharmonic contrast imaging promises to improve ultrasound-imaging quality by taking advantage of an increased contrast to tissue signal. However, acoustic pressures beyond the subharmonic generation threshold using common ultrasound pulses may induce significant contrast microbubble destruction. In this work, a chirp excitation technique is presented to enhance the subharmonic emission from encapsulated microbubbles. Chirp signals with a center frequency of 5 MHz, variable frequency range and duration time are employed to drive microbubbles in numerical simulation and experimental studies. We provide a theoretical evaluation of the chirp excitation pressure threshold and the acoustic pressure dependence of subharmonic based on Churchs model and demonstrate that the amplitude and axial resolution of the subharmonic can be optimized by proper selection of the frequency range and time duration of the chirp signal. Measurements are qualitatively in agreement with the simulation. Moreover, we demonstrate that chirp excitation may be able to improve the amplitude of the subharmonic component up to 22 dB over the pulse excitation. The chirp excitation technique could potentially be used for improving the subharmonic contrast imaging quality.

Study of acoustic nonlinearity parameter imaging methods in reflection mode for biological tissues

Three novel methods for acoustic nonlinearity parameter B/A imaging in reflection mode are developed in this paper. They are: (1) the acoustic nonlinearity parameter B/A tomography by detecting reflective second harmonic wave, (2) the B/A tomography in reflection mode via the measurement of the difference frequency wave generated by a parametric array, and (3) the C-scan imaging of B/A via the measurement of the echo second-harmonic signal. A theoretical analysis and the experimental imaging of normal and pathological biological tissues by using these methods are also present and discussed. Results show that using the acoustic nonlinearity parameter imaging we can more easily distinguish the diseased tissue from the normal one than using the linear acoustic parameters.

Acoustic nonlinearity parameter tomography for biological tissues via parametric array from a circular piston source—Theoretical analysis and computer simulations

The acoustic nonlinearity parameter B/A describes the nonlinear features of a medium and may become a novel parameter for ultrasonic tissue characterization. This paper presents a theoretical analysis for acoustic nonlinear parameter tomography via a parametric array. As two primary waves of different frequencies are radiated simultaneously from a circular piston source, a secondary wave at the difference frequency is generated due to the nonlinear interaction of the primary waves. The axial and radial distributions of sound pressure amplitude for the generated difference frequency wave in the near field are calculated by a superposition of Gaussian beams. The calculated results indicated that the difference frequency component of the parametric array grows linearly with distance from the piston source. It therefore provides a better source to do the acoustic nonlinearity parameter tomography because the fundamental and second harmonic signals both have a near field that goes through many oscillations due to diffraction. By using a finite-amplitude insert substitution method and a filtered convolution algorithm, a computer simulation for B/A tomography from the calculated sound pressure of the difference frequency wave is studied. For biological tissues, the sound attenuation is considered and compensated in the image reconstruction. Nonlinear parameter computed tomography (CT) images for several biological sample models are obtained with quite good quality in this study.

Chirp excitation technique to enhance microbubble displacement induced by ultrasound radiation force

Ultrasound radiation force has been proposed to increase the targeting efficiency in ultrasonic molecular imaging and drug delivery. A chirp excitation technique is proposed to increase the radiation force induced microbubble displacement and might potentially be used for enhancing the targeting efficiency of microbubble clouds. In this study, a modified Rayleigh-Plesset equation is used to estimate the radius-time behavior of insonified microbubbles, and the translation of insonified microbubbles is calculated by using the particle trajectory equation. Simulations demonstrate that the chirp excitation is superior to the sinusoidal one in displacing microbubbles with a wide-size distribution, and that the performance is dependent on the parameters of the chirp signal such as the center frequency and frequency range. For Gaussian size distributed microbubble clouds with mean diameter of 3.5 microm and variance of 1, a 2.25 MHz chirp with frequency range of 1.5 MHz induces about 59.59% more microbubbles over a distance of 10 microm during 200 micros insonification, compared to a 2.25 MHz sinusoidal excitation with equal acoustic pressure.

Relationship between the temperature and the acoustic nonlinearity parameter in biological tissues

Recently with the rapid development of the high-intensity focused ultrasound (HIFU) in biomedical ultrasound, much attention has been paid to the noninvasive temperature estimation in biological tissue in order to determine the region and degree of the ultrasound-induced lesions. In ultrasound hyperthermal therapy it is highly desirable to study the real-time noninvasive monitoring of temperature distribution in biological tissue. In this paper, the relationship between the nonlinearity parameterB/A and the temperature in biological tissue is studied and compared with the theoretical model as well as the experimental results from the thermocouple. Results indicated thatB/A could be used as an effective tool to monitor the temperature distribution in biological media.

Subharmonic and ultraharmonic emissions based on the nonlinear oscillation of encapsulated microbubbles in ultrasound contrast agents

Subharmonics or ultraharmonics provides better contrast-to-tissue ratio (CTR) than the fundamental or the second harmonics, having prospective application in medical diagnosis. In this paper, subharmonic and ultraharmonic emissions are theoretically studied through nonlinear oscillation of encapsulated bubbles. The optimized frequencies for emissions of the subharmonics and ultraharmonics are discussed. In addition, sound pressure dependences of the subharmonics and ultraharmonics are studied in theory as well as in measurement. Results reveal that the developments of both subharmonics and ultraharmonics have the same trend, i.e. occurrence, growth and saturation, but the generation of ultraharmonic is a little earlier than that of subharmonic.

Nonlinear effects of the finite amplitude ultrasound wave in biological tissues

Nonlinear effects will occur during the transmission of the finite amplitude wave in biological tissues. The theoretical prediction and experimental demonstration of the nonlinear effects on the propagation of the finite amplitude wave at the range of biomedical ultrasound frequency and intensity are studied. Results show that the efficiency factor and effective propagation distance will decrease while the attenuation coefficient increases due to the existence of nonlinear effects. The experimental results coincided quite well with the theory. This shows that the effective propagation distance and efficiency factor can be used to describe quantitatively the influence of nonlinear effects on the propagation of the finite amplitude sound wave in biological tissues.

Estimation of the tissue lesion induced by a transmitter with aluminium lens

A virtual source model is developed to describe the nonlinear acoustic field generated by a transmitter with aluminium lens. This model assumes the geometry focal length of the virtual concave transducer to be equal to the acoustic focal length of the real transducer. Thus the acoustic field in the focal area can be accurately presented by that generated by the virtual source. This model in combination with the bio-heat equation is applied to the size estimation of tissue lesion created by high intensity focused ultrasound. A transmitter with aluminium lens is used to create tissue lesions for bovine livers in vitro under three different focal acoustic intensities of 7, 13.1, 25.4 kW/cm2. The predicted tissue lesion volumes coincide well with those previously reported measured results for 7 kw/cm2, but are smaller those in the measurement for 13.1 and 25.4 kW/cm2 owing to effect of vapor bubbles.

Estimation Of Temperature Distribution In Biological Tissue By Acoustic Nonlinearity Parameter

In the rapid development of biomedical ultrasound, especially in the ultrasound therapy, it is desirable to monitor the temperature distribution of the tissues. In this paper the dependence of nonlinearity parameter on the temperature distribution in tissues can be estimated by measuring the nonlinearity parameter B/A. The obtained results are compared with the theoretical calculation of heat conductive model as well as the experiments by the thermocouple. Research work indicated that B/A could be used as an effective tool to monitor the temperature distribution in biological media.