锯齿波信号激励增强超声造影剂的次谐波信号

提出了一种利用锯齿波激励以增强超声造影剂微气泡激发的次谐波信噪比的方法。基于修正的Church方程,采用数值计算分析了锯齿波激励下微气泡产生的次谐波的声压阈值及变化特性,并与正弦波激励进行了比较。实验中采用声压相等的基频为2 MHz的锯齿波及正弦波激发一种自制的包膜微气泡造影剂,测量了产生次谐波的激励声压阈值及与激励声压的关系。结果表明,锯齿波信号激励可以增强次谐波信号的强度,相对于正弦波激励增强约13 dB,并且激发声压阈值较低。实验结果与数值计算结果相符。     

调频超声脉冲驱动微气泡运动偏移的研究

提出调频超声辐射力技术驱动微泡群,以加强微泡的吸附效率.基于改进的RP方程及粒子轨迹方程研究了微泡群整体的运动位移与调频信号的中心频率、调频范围、信号声压,以及微泡半径分布关系.研究结果表明调频信号在驱动半径具有宽泛分布的气泡群,以及半径分布远离谐振半径的气泡群时,作用效果好于传统正弦波信号.例如中心频率1 MHz、调频范围0.75 MHz的调频脉冲作用高斯分布（平均半径3.5 μm、均方差为1）的微泡群200 μs,可比同等声压的正弦波多约12%的微气泡产生位移30 μm. 关键词： 超声辐射力 调频波 高斯分布     

微气泡激发的声微流对细胞声孔效应的影响

理论及实验研究了微气泡激发的声微流对声孔效应的影响。实验采用低幅度(0.05~0.3MIPa)连续超声波信号照射MCF-7细胞,PEI:DNA的复合质粒和造影剂气泡的混合溶液,通过扫描电子显微镜测量细胞膜声孔效应。结果表明声孔大小随着激励声压的幅度和照射时间的增加而增大,平均孔径范围为100 nm~1.25μm。基于Marmottant微气泡振动模型的理论计算结果表明微气泡振动所产生的声微流引起的剪切力在低幅度超声引发声致穿孔作用中起着关键作用。      

基于调频信号发射的超声造影剂谐波成像方法

提出了一种能够提取超声造影剂回波中二次谐波的非线性成像方法,从数学原理、仿真计算和声学实验三个方面对造影剂血流灌注成像方法进行了研究。首次将调频信号发射技术用于医学超声造影剂谐波成像领域,仿真计算和声学实验结果表明:为了获得较好的图像质量,发射调频信号应该具有合适的包络;发射信号带宽与解码匹配滤波器带宽之间必须有一个带宽间隔;适当降低发射声压能够降低生物组织的谐波。应用编码发射方法和解码滤波器,得到了造影剂二次谐波图像,此图像具有较好的信号噪声比和造影剂组织比。声学实验结果和理论计算结果完全吻合。     

脉冲占空比对磁性微泡介导的聚焦超声温升效应的影响

集合多种诊断和治疗功能的声/磁造影剂微泡的研究与开发已经成为当前医学超声、生物医学工程及临床应用领域共同关注的热点问题.超顺磁氧化铁纳米颗粒具有独特的磁性特征和良好的生物相容性,可被用作核磁共振造影剂来提升影像对比度、空间分辨率及临床诊断准确性.我们的前期工作表明,通过将超顺磁氧化铁纳米颗粒挂载于常规超声造影剂微泡表面,可以成功构建多模态诊断及治疗介质,显著改变超声造影剂微泡的尺度分布及包膜粘弹系数等物理特性,进而影响微泡造影剂的声散射特性及其声空化效应和热效应.然而,此前的研究仅考虑了声场强度和微泡浓度等影响因素,对于脉冲超声时间特性对磁性微泡造影剂动力学响应的影响的相关研究仍有所欠缺.本文通过热电偶对凝胶仿体血管模型中流动的双模态磁性微泡在不同占空比超声脉冲信号作用下,产生温升效应开展了系统的实验测量,并基于有限元模型对实验结果进行了仿真验证.结果显示,脉冲信号占空比的提升是增强血管中磁性微泡在聚焦超声作用下温升效果的关键性时间影响因素.本文的研究成果将有助于更好地理解不同超声作用参数对双模态磁性微泡的热效应的影响机制,对保障双模态磁性微泡在临床热疗应用中的安全性和有效性具有重要的...     

纳米包膜造影微泡的小波检测技术研究

用小波变换的方法检测人体组织的微小血流灌注包膜造影微泡回波。结合超声造影剂微泡振动的物理模型,构造了不同外加声场条件下的气泡小波,对原始信号进行小波变换,将变换后得到的小波系数分离;进而从微小血流灌注组织杂波中检测出造影剂微泡回波信号。心肌条件下的计算机模拟以及对灌注组织的仿体实验结果表明:与普通母小波相比,基于气泡振动模型的小波,因为是通过理论模型构造所得,已先验表征了造影剂微泡在声场中的回波特性,因此与实验中产生的回波信号具有更为紧密的相关性,从而经小波变换后,造影剂微泡回波信号与组织杂波产生了更高强度的信噪比,及更明显的可分离和对比效果。同时,构造了一个完备的母小波函数库,进一步提高了本方法的适用性以及健壮性。     

岩石特性的编码激励超声参数表征及分析

编码激励技术是提高超声穿透能力和分辨率的有效手段。为分析岩石特性对编码信号脉冲压缩性能的影响,通过数值模拟以及不同岩石的超声检测实验,研究并比较了几类编码信号的检测效果。结果表明幅度加权线性调频信号(TLFM)调制的Barker码信号(BTL)受到的信噪比损失低于正弦调制Barker码信号(BS),而分辨率损失要低于TLFM信号。实验结果验证了编码激励技术的有效性,为岩石超声检测中编码参数的设置提供了依据。      

生物组织成像中用反相位脉冲技术提高二次谐波信噪比的研究

本文基于有限振幅声波在介质中的非线性传播理论,分析了反相位脉冲技术对生物组织中二次谐波增强的原理.实验中利用反相位脉冲激发超声换能器,对生物组织中传播的非线性信号相加分析.结果表明反相位脉冲技术可有效抑制基波及奇次谐波信号,而可增强偶次谐波信号6dB.与滤波器滤波法相比,反相位脉冲技术在抑制基波信号的同时,可有效地提高二次谐波的信噪比,因而在生物组织的二次谐波成像中具有广阔的应用前景.     

超声造影剂微气泡的包膜黏弹特性的定量表征研究

包膜黏弹特性显著影响微气泡超声造影剂的诊断及治疗应用效果. 本文结合原子力显微镜技术及声衰减特性测量提出了一种对微气泡造影剂包膜黏弹特性定量表征的新方法. 首先采用原子力显微镜技术进行机械特性分析得到包膜微气泡的有效硬度及体弹性模量; 然后测量声衰减特性, 基于微气泡动力学理论, 计算包膜微气泡的体黏度系数. 为验证方法的有效性, 实验制备了直径为1-5 μm的白蛋白包膜微气泡造影剂, 原子力显微镜测量的有效硬度和体弹性模量分别为0.149±0.012 N/m和8.31±0.667 MPa, 并与粒径无关. 声衰减特性测量和动力学理论拟合的包膜微气泡的体黏度系数为0.374±0.003 Pa·s. 该方法可推广至其他种类包膜微气泡的黏弹特性表征, 对超声造影剂的制备及其诊断和治疗应用有积极意义.     

超声无损检测中的数字匹配滤波技术

本文在超声无损检测技术中采用线性调频脉冲信号,并用数字化了的电激励信号自身作为数字匹配滤波器,在不同的时延带宽乘积的系统中得到了声脉冲的压缩信号。实验结果与理论预期值符合得很好.对于Hamming和Gaussian时间加权的信号,其旁瓣抑制可分别达到—41.9dB和—41.5dB.数字匹配滤波技术的引入可以有力地改善超声检测系统的性能。     

Parameter optimization of pulse compression in ultrasound imaging systems with coded excitation

A linear array imaging system with coded excitation is considered, where the proposed excitation/compression scheme maximizes the signal-to-noise ratio (SNR) and minimizes sidelobes at the output of the compression filter. A pulse with linear frequency modulation (LFM) is used for coded excitation. The excitation/compression scheme is based on the fast digital mismatched filtering. The parameter optimization of the excitation/compression scheme includes (i) choice of an optimal filtering function for the mismatched filtering; (ii) choice of an optimal window function for tapering of the chirp amplitude; (iii) optimization of a chirp-to-transducer bandwidth ratio; (iv) choice of an appropriate n-bit quantizer. The simulation results show that the excitation/compression scheme can be implemented as a Dolph–Chebyshev filter including amplitude tapering of the chirp with a Lanczos window. An example of such an optimized system is given where the chirp bandwidth is chosen to be 2.5 times the transducer bandwidth and equals 6 MHz: The sidelobes are suppressed to −80 dB, for a central frequency of 4 MHz, and to −94 dB, for a central frequency of 8 MHz. The corresponding improvement of the SNR is 18 and 21 dB, respectively, when compared to a conventional short pulse imaging system. Simulation of B-mode images demonstrates the advantage of coded excitation systems of detecting regions with low contrast.     

A preliminary in vitro assessment of polymer-shelled microbubbles in contrast-enhanced ultrasound imaging

This paper focuses on the use of poly (vinyl alcohol)-shelled microbubbles as a contrast agent in ultrasound medical imaging. The objective was an in vitro assessment of the different working conditions and signal processing methods for the visual detection (especially in small vessels) of such microbubbles, while avoiding their destruction. Polymer-shelled microbubbles have recently been proposed as ultrasound contrast agents with some important advantages. The major drawback is a shell that is less elastic than that of the traditional lipidic microbubbles. Weaker echoes are expected, and their detection at low concentrations may be critical. In vitro experiments were performed with a commercial ultrasound scanner equipped with a dedicated acquisition board. A concentration of 100 bubbles/mm³, excitation pressure amplitudes from 120 kPa to 320 kPa, and a central frequency of 3 MHz or 4.5 MHz were used. Three multi-pulse techniques (i.e., pulse inversion, contrast pulse sequence based on three transmitted signals, and contrast pulse sequence in combination with the chirp pulse) were compared. The results confirmed that these microbubbles produce a weaker ultrasound response than lipidic bubbles with a reduced second-order nonlinear component. Nevertheless, these microbubbles can be detected by the contrast pulse sequence technique, especially when the chirp pulse is adopted. The best value of the contrast-to-tissue ratio was obtained at an excitation pressure amplitude of 230 kPa: although this pressure amplitude is higher than what is typically used for lipidic microbubbles, it does not cause the rupture of the polymeric contrast agent.     

Subharmonic behavior of phospholipid-coated ultrasound contrast agent microbubbles

Coated microbubbles, unlike tissue are able to scatter sound subharmonically. Therefore, the subharmonic behavior of coated microbubbles can be used to enhance the contrast in ultrasound contrast imaging. Theoretically, a threshold amplitude of the driving pressure can be calculated above which subharmonic oscillations of microbubbles are initiated. Interestingly, earlier experimental studies on coated microbubbles demonstrated that the threshold for these bubbles is much lower than predicted by the traditional linear viscoelastic shell models. This paper presents an experimental study on the subharmonic behavior of differently sized individual phospholipid coated microbubbles. The radial subharmonic response of the microbubbles was recorded with the Brandaris ultra high-speed camera as a function of both the amplitude and the frequency of the driving pulse. Threshold pressures for subharmonic generation as low as 5 kPa were found near a driving frequency equal to twice the resonance frequency of the bubble. An explanation for this low threshold pressure is provided by the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499-3505 (2005)]. It is shown that the change in the elasticity of the bubble shell as a function of bubble radius as proposed in this model, enhances the subharmonic behavior of the microbubbles.     

Experimental evaluation of a non-linear coded excitation method for contrast imaging

Previously, we have shown that for a single bubble, using chirps as the excitation signal improves both the linear and the non-linear response. Computer simulations of randomly distributed contrast agent bubbles show an increase of 10-13 dB in response when comparing pulse excitations with chirp excitations that have equal bandwidths and peak amplitudes. Second harmonic compression of simulated bubble echoes with chirp excitation shows low side-lobe levels and limited loss of axial resolution when compared to pulse excitation. Experimental results from water tank measurements with SonoVue contrast agent are in agreement with computer simulations showing increased signal-to-noise ratio and an increase of approximately 12 dB at the second harmonic when comparing pulse and chirp excitation.     

Excitation of the Morse oscillator by an ultrashort chirped pulse

The excitation of the classic Morse oscillator by an ultrashort electromagnetic pulse with a linear frequency chirp is studied theoretically. Formulas are derived for the oscillation amplitude and the radiation power averaged over a period as functions of the excitation energy for free oscillations of the Morse oscillator. Analytical expressions for describing the oscillator motion after the end of the pulse are obtained in the harmonic limit. In the general case of arbitrary parameters of the problem, the specific features of an excited Morse oscillator are analyzed numerically. Prominence is given to the effect of chirp on the excitation energy. The consideration is performed in terms of dimensionless variables, which makes it possible to apply the results obtained to a wide range of molecular systems and exciting-pulse parameters.     

Modeling subharmonic response from contrast microbubbles as a function of ambient static pressure

Variation of subharmonic response from contrast microbubbles with ambient pressure is numerically investigated for non-invasive monitoring of organ-level blood pressure. Previously, several contrast microbubbles both in vitro and in vivo registered approximately linear (5-15 dB) subharmonic response reduction with 188 mm Hg change in ambient pressure. In contrast, simulated subharmonic response from a single microbubble is seen here to either increase or decrease with ambient pressure. This is shown using the code BUBBLESIM for encapsulated microbubbles, and then the underlying dynamics is investigated using a free bubble model. The ratio of the excitation frequency to the natural frequency of the bubble is the determining parameter--increasing ambient pressure increases natural frequency thereby changing this ratio. For frequency ratio below a lower critical value, increasing ambient pressure monotonically decreases subharmonic response. Above an upper critical value of the same ratio, increasing ambient pressure increases subharmonic response; in between, the subharmonic variation is non-monotonic. The precise values of frequency ratio for these three different trends depend on bubble radius and excitation amplitude. The modeled increase or decrease of subharmonic with ambient pressure, when one happens, is approximately linear only for certain range of excitation levels. Possible reasons for discrepancies between model and previous experiments are discussed.     

Subharmonic imaging of contrast agents

Ultrasound contrast agents promise to improve the sensitivity and specificity of diagnostic ultrasound imaging. It is of great importance to adapt ultrasound equipment for optimal use with contrast agents e.g., by exploiting the nonlinear properties of the contrast microbubbles. Harmonic imaging is one technique that has been extensively studied and is commercially available. However, harmonic imaging is associated with problems, due to second harmonic generation and accumulation within the tissue itself. Given the lack of subharmonic generation in tissue, one alternative is the creation of subharmonic images by transmitting at the fundamental frequency (fo) and receiving at the subharmonic (fo/2). Subharmonic imaging should have a much better lateral resolution and may be suitable for scanning deep-lying structures owing to the higher transmit frequency and the much smaller attenuation of scattered subharmonic signals. In this paper, we will review different aspects of subharmonic imaging including implementation, in-vitro gray-scale imaging and subharmonic aided pressure estimation.     

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.     

Second-harmonic generation with traveling-wave pulses

We experimentally studied properties of traveling-wave excitation and traveling-wave pulse compression for a Nd : glass laser system with a streak camera. These properties were extended to second-harmonic generation with a type-11 KDP crystal. We found that the second-harmonic pulse generated from the chirped-fundamental pulse also has a good chirp linearity, but the spectral bandwidth and temporal duration were reduced. We demonstrated the effective subpulse suppression in the compressed pulse with the second-harmonic generation. The pulse with tilted wave front, short wavelength and high contrast ratio can be obtained by the second-harmonic generation of traveling-wave pulse.     

方差评估在幅值限制脉冲优化中的应用

改进了现有针对限制幅值磁共振脉冲的优化设计方法,在脉冲优化过程中加入对脉冲激发效率方差的评估．该方法能有效改善脉冲带宽激发的不均匀性．并探讨了改进方法中不同的方差权重系数以及脉冲幅值限制条件对于优化效果的影响,以获得最佳的脉冲优化效果．