Intravascular ultrasound tissue harmonic imaging: a simulation study

Recently, the in vivo feasibility of tissue harmonic imaging (THI) with a mechanically-rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, both pulse inversion (PI) and analog filtering were used. In the present paper, we report the development of a simulation tool to investigate nonlinear IVUS beams and the influence of rotation on the efficiency of PI signal processing. Nonlinear 20 MHz beams were simulated in a homogeneous tissue-mimicking medium, resulting in second harmonic pressure fields at 40 MHz. The acoustic response from tissue was simulated by summing radio-frequency (RF) pulse-echo responses from many point-scatterers. When the transducer was rotated with respect to the point-scatterers, the fundamental frequency suppression using PI degraded rapidly with increasing inter-pulse angles. The results of this study will aid in the optimization of harmonic IVUS imaging systems.     

Detection procedures of ultrasound contrast agents

In the early days, it was believed that ultrasound contrast agents (UCA) could be sufficiently detected and imaged with the conventional imaging methods nowadays referred to as fundamental imaging. Newer imaging techniques proved to be more sensitive and are based on specific properties of the UCA. In general, these new characteristics involve non-linear and transient characteristics of contrast agents that appear at the high end of the diagnostic acoustic intensity. Imaging modalities used today for UCA are, besides fundamental imaging, second harmonic imaging, power Doppler, harmonic power Doppler, pulse inversion and pulse inversion Doppler, multi-pulse imaging and subharmonic imaging. Although the results of conventional second harmonic imaging are still not optimal for perfusion imaging applications, in combination with Doppler techniques (colour Doppler, power Doppler) it is one of the most sensitive techniques currently available in terms of agent-to-tissue ratio. Further improvements in current and future detection methods demand a complete understanding of the ultrasound-UCA interaction.     

Nonlinear response to ultrasound of encapsulated microbubbles

The acoustic backscatter of encapsulated gas-filled microbubbles immersed in a weak compressible liquid and irradiated by ultrasound fields of moderate to high pressure amplitudes is investigated theoretically. The problem is formulated by considering, for the viscoelastic shell of finite thickness, an isotropic hyperelastic neo-Hookean model for the elastic contribution in addition to a Newtonian viscous component. First and second harmonic scattering cross-sections have been evaluated and the quantitative influence of the driving pressure amplitude on the harmonic resonance frequencies for different initial equilibrium bubble sizes and for different encapsulating physical properties has been determined. Conditions for optimal second harmonic imaging have been also investigated and some regions in the parameters space where the second harmonic intensity is dominant over the fundamental have been identified. Results have been obtained for albumin, lipid and polymer encapsulating shells, respectively.     

基于声透镜成像系统的光声层析成像

光声成像是采用“光激发-超声波成像”的新型成像技术，它是检测强散射介质内部光吸收分布的一种有效医学影像技术.用短脉冲激光照射强散射介质(如生物组织)，强散射介质由于光声效应产生超声信号，使用具有成像能力的声透镜把声压分布成像于像面上，然后利用具有空间分辨能力的阵列光声传感器，把同一像面上的光声信号强度记录下来，最后根据光声信号强度的空间分布进行图像重组.根据成像系统物像共轭原理，同一物平面的光声信号到达像面的时延相等，而不同物面的光声信号到达同一个探测器平面的时延各不相同，因此，利用BOXCAR的门控积分 关键词： 光声层析成像 声透镜 光声信号     

Nonlinear propagation delay and pulse distortion resulting from dual frequency band transmit pulse complexes

A method of acoustic imaging is discussed that potentially can improve the diagnostic capabilities of medical ultrasound. The method, given the name second order ultrasound field imaging, is achieved by the processing of the received signals from transmitted dual frequency band pulse complexes with at least partly overlapping high frequency (HF) and low frequency (LF) pulses. The transmitted HF pulses are used for image reconstruction whereas the transmitted LF pulses are used to manipulate the elastic properties of the medium observed by the HF imaging pulses. In the present paper, nonlinear propagation effects observed by a HF imaging pulse due to the presence of a LF manipulation pulse is discussed. When using dual frequency band transmit pulse complexes with a large separation in center frequency (e.g., 1:10), these nonlinear propagation effects are manifested as a nonlinear HF propagation delay and a HF pulse distortion different from conventional harmonic distortion. In addition, with different transmit foci for the HF and LF pulses, nonlinear aberration will occur.     

On the possibility of soliton-like double-frequency propagation of femtosecond pulses in an optical fiber under conditions of second harmonic generation

A soliton-like mode of double-frequency propagation of femtosecond pulses during second harmonic generation (SHG) in optical fibers is predicted on the basis of computer simulation. The essence of this mode is that, under certain conditions, the central part of the pulses of both the fundamental and the second harmonic waves propagate without a change in the intensity of either wave. This mode can be realized by either of two schemes of the SHG process. In the first scheme, the second harmonic enters a nonlinear medium with some definite input amplitude and definite phase shift with respect to the fundamental wave. In the other scheme, the second harmonic has a zero input amplitude and a phase shift is introduced into the interacting waves in particular cross sections of the medium.     

参变过程中的群速延迟对晶体匹配长度的影响

基于能量守恒和三波耦合波方程,建立了超短脉冲在参变过程中二次谐波产生时的I类和II类相位匹配条件、基波与谐波之间的群速延迟时间、以及群速失配对晶体长度限制的理论基础。以负单轴非线性光学晶体CsLiB6O10为例,分析和数值计算了超短脉冲宽度为100fs时,谐波的群速匹配长度随基波波长变化的规律。研究结果表明在I类相位匹配条件下,基波波长为642nm时,群速延迟最小,相应的群速匹配晶体长度最长为19.1mm;在II类相位匹配条件下,基波波长为767nm,群速延迟最小,群速匹配长度最长为0.89mm。     

利用调制的偏振态门控制阿秒脉冲的产生

提出一种利用多光周期驱动脉冲获得极紫外宽带超连续谱的新方法.利用波长为1600nm的基频场组成的偏振态门增强高次谐波产生效率对椭扁率的依赖,并叠加上波长为800nm的倍频场来调制电离比率.计算结果表明,采用脉冲宽度为6个光周期(32.4fs)的基频脉冲就能够将高次谐波辐射限制在0.5个光周期内,获得了带宽为280eV的超连续谱,这个谱宽支持傅里叶极限为10as的单个脉冲输出.直接选取超连续谱上的一段,可以获得100as的脉冲输出.此外,在调制的偏振态门中可以使用相对延迟较小的两束基频光组成偏振态门,提高了 关键词： 阿秒脉冲 超连续谱 偏振态门     

A simulation study on tissue harmonic imaging with a single-element intravascular ultrasound catheter

Recently, in vivo feasibility of tissue harmonic imaging with a mechanically rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, a combination of pulse inversion and analog filtering was used. In this paper the development of a simulation tool to investigate nonlinear IVUS beams is reported, and the influence of transducer rotation and axial catheter-to-tissue motion on the efficiency of PI signal processing is evaluated. Nonlinear beams were simulated in homogeneous tissue-mimicking media at a transmit frequency of 20 MHz, which resulted in second harmonic pressure fields at 40 MHz. The competing effects of averaging and decorrelation between neighboring rf lines on the signal-to-noise ratio (SNR) were studied for a single point scatterer. An optimal SNR was achieved when lines were combined over 3 degrees - 3.75 degrees. When the transducer was rotated with respect to point scatterers, simulating the acoustic response of tissue, the fundamental frequency suppression using PI degraded rapidly with increasing interpulse angles. The effect of axial catheter-to-tissue motion on the efficiency of pulse inversion seemed to be of less influence for realistic motion values. The results of this study will aid in the optimization of harmonic IVUS imaging systems.     

Experimental investigation of the pulse inversion technique for imaging ultrasound contrast agents

The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.     

Contrast harmonic imaging

The behavior of ultrasound contrast agents depends highly on the acoustic pressure of the insonified ultrasound wave. For low pressure the expansion and compression is linear to the pressure, for medium acoustic pressure nonlinear behavior starts to occur and for high pressures, but still in the diagnostic range transient scattering can be noticed, resulting in an enhanced scattering followed by a disappearance of the bubble. The nonlinear and transient regime can be utilized for imaging of the contrast agent in or nearby tissue. The magnitude of the nonlinear signal from the contrast has to compete with the nonlinear component of the ultrasound wave, which is generated during propagation. It is shown that contrast is superior to tissue when using low frequencies and imaging the third or fourth harmonic of the transmitted frequency.     

Nonlinear pulsed ultrasound beams radiated by rectangular focused diagnostic transducers

A numerical model for simulating nonlinear pulsed beams radiated by rectangular focused transducers, which are typical of diagnostic ultrasound systems, is presented. The model is based on a KZK-type nonlinear evolution equation generalized to an arbitrary frequency-dependent absorption. The method of fractional steps with an operator-splitting procedure is employed in the combined frequency-time domain algorithm. The diffraction is described using the implicit backward finite-difference scheme and the alternate direction implicit method. An analytic solution in the time domain is employed for the nonlinearity operator. The absorption and dispersion of the sound speed are also described using an analytic solution but in the frequency domain. Numerical solutions are obtained for the nonlinear acoustic field in a homogeneous tissue-like medium obeying a linear frequency law of absorption and in a thermoviscous fluid with a quadratic frequency law of absorption. The model is applied to study the spatial distributions of the fundamental and second harmonics for a typical diagnostic ultrasound source. The nonlinear distortion of pulses and their spectra due to the propagation in tissues are presented. A better understanding of nonlinear propagation in tissue may lead to improvements in nonlinear imaging and in specific tissue harmonic imaging. Published in Russian in Akusticheskiĭ Zhurnal, 2006, Vol. 52, No. 4, pp. 560–570. This article was translated by the authors.     

Fundamental study on subharmonic imaging by irradiation of amplitude-modulated ultrasound waves

The second harmonic and subharmonic components, the frequencies of which are twice and one half the fundamental frequency, are included in echoes from contrast agents. An imaging method, which employs a second harmonic (second harmonic imaging), is widely used in medical diagnoses. On the other hand, subharmonic is expected to provide a higher contrast between biological tissues and blood flow because echo signals are generated only from blood containing the contrast agents. However, the subharmonic component echo signal power from contrast agents is relatively low. This has resulted in little progress in the field of subharmonic imaging. In this study, a new imaging method is proposed using amplitude-modulated waves as transmitted waves combined with the pulse inversion method to enhance subharmonic echo signals. Two optimal frequencies are set, including the modulated waves, F(1) and F(2), so that the subharmonic frequency of F(1) and the second harmonic frequency of F(2) may result in the same value. This allows a more powerful signal at the frequency band because the second harmonic and subharmonic components are integrated. Furthermore, a B-mode ultrasound image of an agar phantom that imitated biological tissue and showed the effectiveness of our method was reconstructed. As a result, the echo power of the subharmonic component was enhanced by approximately 11.8 dB more than the conventional method and the signal to noise ratio showed an improvement of 7.6 dB.     

Generation of frequency shifted picosecond pulses with low temporal jitter

Transient stimulated Raman scattering is used for the generation of a frequency shifted picosecond light pulse; part of this Raman shifted pulse is subsequently coherently scattered at a material excitation of a second Raman cell. Starting with the second harmonic pulse (t_p = 4 ps) of a mode-locked Nd : glass laser system, both the stimulated and the coherently produced pulses have durations of 2.3 ps at different wavelengths. By the appropriate choice of the Raman medium pulses between 13 000 and 21 000 cm^-1 can be generated. The coherent generation process minimizes the temporal jitter between the two pulses and allows to obtain a high time resolution of better than 0.3 ps in excite and probe experiments.     

Second harmonic beam analysis, a sensitive technique to determine the duration of single ultrashort laser pulses

The second harmonic beam generated in a noncollinear arrangement allows the observation of the autocorrelation functio: A compact optical beam splitter and imaging system eliminates alignment problems. Single pulses of 1 ps duration and approximately 10⁻⁸ joule energy are readily observed using an optical multichannel analyser.     

Combined optical and acoustical detection of single microbubble dynamics

A detailed understanding of the response of single microbubbles subjected to ultrasound is fundamental to a full understanding of the contrast-enhancing abilities of microbubbles in medical ultrasound imaging, in targeted molecular imaging with ultrasound, and in ultrasound-mediated drug delivery with microbubbles. Here, single microbubbles are isolated and their ultrasound-induced radial dynamics recorded with an ultra-high-speed camera at up to 25 million frames per second. The sound emission is recorded simultaneously with a calibrated single element transducer. It is shown that the sound emission can be predicted directly from the optically recorded radial dynamics, and vice versa, that the nanometer-scale radial dynamics can be predicted from the acoustic response recorded in the far field.     

Polar light and symmetry of electro-optical effects

General regularities of the interaction of a polar light wave synthesized from the Fourier harmonics of a fundamental frequency with a transparent centrosymmetric optically nonlinear medium are considered. The notion of light polarity is introduced and transverse electro-optical effects specific for the problem of interaction between polar light and isotropic nonlinear media (optical rectification and the linear electro-optical effect) are analyzed. Estimates of values of the effects in combination with published experimental data on optically encoded second harmonic generation in glasses indicate promising opportunities for practical application of the effects of the interaction of polar light and matter in optical systems of information recording and processing.     

Excitation of linear and nonlinear cavity modes upon interaction of femtosecond pulses with arrays of metallic nanowires

We investigate linear and nonlinear effects induced by the scattering of femtosecond optical pulses by cavity-shaped two-dimensional distributions of metallic cylinders. In particular, by employing a numerical method based on the multiple scattering matrix algorithm, we demonstrate that, at both the fundamental frequency and the second harmonic, such arrays of metallic nanowires support localized (cavity) surface plasmon–polariton modes with characteristic lifetime ranging from a few tens of femtoseconds to more than a hundred of femtoseconds.     

Correlation transfer and diffusion of ultrasound-modulated multiply scattered light

We develop a temporal correlation transfer equation (CTE) and a temporal correlation diffusion equation (CDE) for ultrasound-modulated multiply scattered light. These equations can be applied to an optically scattering medium with embedded optically scattering and absorbing objects to calculate the power spectrum of light modulated by a nonuniform ultrasound field. We present an analytical solution based on the CDE and Monte Carlo simulation results for light modulated by a cylinder of ultrasound in an optically scattering slab. We further validate with experimental measurements the numerical calculations for an actual ultrasound field. The CTE and CDE are valid for moderate ultrasound pressures and on a length scale comparable with the optical transport mean-free path. These equations should be applicable to a wide spectrum of conditions for ultrasound-modulated optical tomography of soft biological tissues.     

Scattering of time-harmonic elastic waves by an elastic inclusion with quadratic nonlinearity

This paper considers the scattering of a plane, time-harmonic wave by an inclusion with heterogeneous nonlinear elastic properties embedded in an otherwise homogeneous linear elastic solid. When the inclusion and the surrounding matrix are both isotropic, the scattered second harmonic fields are obtained in terms of the Green's function of the surrounding medium. It is found that the second harmonic fields depend on two independent acoustic nonlinearity parameters related to the third order elastic constants. Solutions are also obtained when these two acoustic nonlinearity parameters are given as spatially random functions. An inverse procedure is developed to obtain the statistics of these two random functions from the measured forward and backscattered second harmonic fields.