Physical phenomena affecting quantitative imaging of ultrasound contrast agents

Microbubbles stabilized by a surfactant or polymer coating are the most effective form of contrast agent available for ultrasound imaging. They have shown great potential as a means of quantifying tissue perfusion, in particular determining physiologically significant parameters such as relative vascular volume and flow velocity. Clinical implementation of quantitative imaging procedures, however, has been hindered by poor characterisation of the complex relationship between microbubble concentration, scattering and image intensity. The aim of this paper is to describe theoretical and experimental investigations of the physical phenomena underlying these effects, such as the time, pressure and frequency dependence of microbubble behaviour, the influence of the bubble coating, size distribution and concentration; and to discuss the challenges involved in developing accurate quantitative imaging protocols.     

准高斯声束对超声造影剂的声辐射力研究*

超声造影剂的定向输运在超声医学成像领域有着极为重要的意义,而声辐射力作用是实现该过程的关键,相比于高斯声束,准高斯声束是无源亥姆霍兹方程的精确解,可以使用标准波分解法简化计算。因此,本文研究了准高斯声束对超声造影剂的声辐射力作用。文章首先分析了准高斯声束与高斯声束之间的相关性;随后通过数值计算求得了准高斯声束对超声造影剂模型的声辐射力函数与无量纲频率之间的关系;最后,本文研究了不同造影剂气泡情况下的声辐射力。研究结果表明:声辐射力函数随无量纲频率变化将在不同位置出现共振峰,不同的波束宽度值将改变辐射力强度,但不改变共振峰的位置。相关结果可为利用声辐射力定向输运超声造影剂至靶向位置提供理论参考。     

超声造影剂研究进展

本文综述超声造影剂的现状及新进展，在简要介绍造影剂物理原理的基础上，对超声造影剂在医学超声领域尤其是低速血流测量与组织灌注成像中的应用进行了总结与讨论，并介绍了用于实验研究和第二期临床试验的国际上最新的超声造影剂的特点，制备技术与评价方法。     

Scaling of the viscoelastic shell properties of phospholipid encapsulated microbubbles with ultrasound frequency

Phospholipid encapsulated microbubbles are widely employed as clinical diagnostic ultrasound contrast agents in the 1–5 MHz range, and are increasingly employed at higher ultrasound transmit frequencies. The stiffness and viscosity of the encapsulating “shells” have been shown to play a central role in determining both the linear and nonlinear response of microbubbles to ultrasound. At lower frequencies, recent studies have suggested that shell properties can be frequency dependent. At present, there is only limited knowledge of how the viscoelastic properties of phospholipid shells scale at higher frequencies. In this study, four batches of in-house phospholipid encapsulated microbubbles were fabricated with decreasing volume-weighted mean diameters of 3.20, 2.07, 1.82 and 1.61 μm. Attenuation experiments were conducted in order to assess the frequency-dependent response of each batch, resulting in resonant peaks in response at 4.2, 8.9, 12.6 and 19.5 MHz, respectively. With knowledge of the size measurements, the attenuation spectra were then fitted with a standard linearized bubble model in order to estimate the microbubble shell stiffness S_p and shell viscosity S_f, resulting in a slight increase in S_p (1.53–1.76 N/m) and a substantial decrease in S_f (0.29 × 10⁻⁶–0.08 × 10⁻⁶ kg/s) with increasing frequency. These results performed on a single phospholipid agent show that frequency dependent shell properties persist at high frequencies (up to 19.5 MHz).     

超声造影剂微泡非线性动力学响应的机理及相关应用

随着生命科学及现代医学的发展, 一体化无创精准诊疗已经日益成为人们关注的焦点问题, 而关于超声造影剂微泡的非线性效应的相关机理、动力学建模及其在超声医学领域中的应用研究也得到了极大的推动. 本文对下列课题进行了总结和讨论, 包括: 1)基于Mie散射技术和流式细胞仪对造影剂微泡参数进行定征的一体化解决方案; 2)通过对微泡包膜的黏弹特性进行非线性修正, 构建新的包膜微泡动力学模型; 3)探索造影剂惯性空化阈值与其包膜参数之间的相关性; 以及4)研究超声联合造影剂微泡促进基因/药物转染效率并有效降低其生物毒性的相关机理.     

In vitro evaluation of the impact of ultrasound scanner settings and contrast bolus volume on time-intensity curves

The objective of this study was to assess in vitro the impact of ultrasound scanner settings and contrast bolus volume on time-intensity curves formed from dynamic contrast-enhanced ultrasound image loops. An indicator-dilution experiment was developed with an in vitro flow phantom setup used with SonoVue contrast agent (Bracco SpA, Milan, Italy). Imaging was performed with a Philips iU22 scanner and two transducers (L9-3 linear and C5-1 curvilinear). The following ultrasound scanner settings were investigated, along with contrast bolus volume: contrast-specific nonlinear pulse sequence, gain, mechanical index, focal zone depth, acoustic pulse center frequency and bandwidth. Four parameters (rise time, mean transit time, peak intensity, and area under the curve) were derived from time-intensity curves which were obtained after pixel by pixel linearization of log-compressed data (also referred to as video data) included in a region of interest. Rise time was found to be the parameter least impacted by changes to ultrasound scanner settings and contrast bolus volume; the associated coefficient of variation varied between 0.7% and 6.9% while it varied between 0.8% and 19%, 12% and 71%, and 9.2% and 66%, for mean transit time, peak intensity, and area under the curve, respectively. The present study assessed the impact of ultrasound scanner settings and contrast bolus volume on time-intensity curve analysis. One should be aware of these issues to standardize their technique in each specific organ of interest and to achieve accurate, sensitive, and reproducible data using dynamic contrast-enhanced ultrasound. One way to mitigate the impact of ultrasound scanner settings in longitudinal, multi-center quantitative dynamic contrast-enhanced ultrasound studies may be to prohibit any adjustments to those settings throughout a given study. Further clinical studies are warranted to confirm the reproducibility and diagnostic or prognostic value of time-intensity curve parameters measurements in a particular clinical scenario of interest, for example that of cancer patients undergoing vascular targeting therapies.     

Discussion of the application of finite Volterra series for the modeling of the oscillation behavior of ultrasound contrast agents

Ultrasound contrast agents consist of microbubbles with diameters in the micrometer range. Excited by ultrasound, these bubbles exhibit highly nonlinear oscillation. While well developed physical models for microbubble oscillation exist, the efficiency of pulse sequences for sensitive microbubble detection is discussed based on simple mathematical models of general nonlinearity. Typically, Taylor series are used to model microbubble nonlinearity for the development of detection schemes. Recently, pulse sequences were proposed which exploit nonlinear memory of microbubbles, a property that cannot be modeled by a Taylor series but can be explained using a Volterra series. Therefore, this paper discusses and evaluates the usage of Volterra series for the modeling of the scattering behavior of contrast agent microbubbles. A numerically stable linear estimation algorithm is implemented to determine a third order Volterra model for a free gas bubble with a resting radius . For insonification pressures up to 100 kPa, the identified model allowed for a mean-square error of less than −16 dB with respect to the reference signal. Analysis of the response to narrowband signals showed that the achievable mean-square error is further reduced for the bandwidth available to typical ultrasound transducers used for clinical diagnostics.     

Carotid atherosclerotic plaque characterisation by measurement of ultrasound sound speed in vitro at high frequency, 20 MHz

This study aimed to utilise a tissue mimicking material (TMM) in order to embed in vitro carotid plaque tissue so that its acoustic properties could be assessed. Here, an International Electrotechnical Commission (IEC) agar-based TMM was adapted to a clear gel by removal of the particulates. This clear TMM was measured with sound speed at 1540 ms⁻¹ and an attenuation coefficient of 0.15 dB cm⁻¹ MHz⁻¹. Composite sound speed was then measured through the embedded material using a scanning acoustic microscope (SAM). Both broadband reflection and transmission techniques were performed on each plaque specimen in order to ensure the consistency of the measurement of sound speed, both at 21 °C and 37 °C. The plaque was measured at two temperatures to investigate any effect on the lipid content of the plaque. The contour maps from its associated attenuation plots were used to match the speed data to the photographic mask of the plaque outline. This physical matching was then used to derive the sound speed from the percentage composition seen in the histological data by solution of simultaneous equations. Individual speed values for five plaque components were derived; TMM, elastin, fibrous/collagen, calcification and lipid. The results for derived sound speed in the TMM were consistently close to the expected value of soft tissue, 1540 ms⁻¹. The fibrous tissue showed a mean value of 1584 ms⁻¹ at 37 °C. The derived sound speeds for elastic and lipid exhibited large inter-quartile ranges. The calcification had higher sound speed than the other plaque components at 1760–2000 ms⁻¹. The limitations here lay in the difficulties in the matching process caused by the inhomogeneity of the plaque material and shrinkage during the histological process. Future work may concentrate on more homogeneous material in order to derive sound speed data for separate components. Nevertheless, this study increases the known data ranges of the individual components within a plaque. This information may be used help to assess the mechanical properties and structural integrity and its associated vulnerability or risk of embolization in future diagnostic ultrasound techniques.     

高频聚焦超声声场和温度场的仿真研究*

为探究临床常用的7 MHz高频聚焦超声在多层生物组织中的声传播以及毫秒级时间内的生物传热规律问题,基于Westervelt方程和Pennes传热方程,使用有限元方法建立高频聚焦超声辐照多层组织的非线性热黏性声传播及传热模型。首先分析了线性模型和非线性模型之间的差异,然后在非线性模型下探究换能器的参数对声场和温度场的影响。仿真结果显示：在7 MHz频率下,当换能器输出声功率超过5 W时,声波传播的非线性效应不可忽视(p <0.05);当声功率从5 W增大到15 W时,非线性模型与线性模型预测的温度偏差从20%增加到34.703%;高频聚焦超声波的非线性行为比低频更加显著,基频能量向高次谐波转移的程度增大,声功率为10 W和15 W时4次谐波与基波之比分别达到7.33%和12.12%;高频换能器参数的改变对组织中声场和温度场分布的影响较大,换能器焦距从12 mm减小到11.2 mm,焦点处最高温度增加了77%。结果表明,7 MHz聚焦超声的非线性声传播需要考虑到4次谐波的影响。该文提出的多层组织非线性仿真模型可为高频聚焦超声换能器参数优化及制定安全、有效的术前治疗方案提供理论参考。     

Analytical and numerical calculations of optimum design frequency for focused ultrasound therapy and acoustic radiation force

Focused ultrasound therapy relies on acoustic power absorption by tissue. The stronger the absorption the higher the temperature increase is. However, strong acoustic absorption also means faster attenuation and limited penetration depth. Hence, there is a trade-off between heat generation efficacy and penetration depth. In this paper, we formulated the acoustic power absorption as a function of frequency and attenuation coefficient, and defined two figures of merit to measure the power absorption: spatial peak of the acoustic power absorption density, and the acoustic power absorbed within the focal area. Then, we derived “rule of thumb” expressions for the optimum frequencies that maximized these figures of merit given the target depth and homogeneous tissue type. We also formulated a method to calculate the optimum frequency for inhomogeneous tissue given the tissue composition for situations where the tissue structure can be assumed to be made of parallel layers of homogeneous tissue. We checked the validity of the rules using linear acoustic field simulations. For a one-dimensional array of 4 cm acoustic aperture, and for a two-dimensional array of 4 × 4 cm² acoustic aperture, we found that the power absorbed within the focal area is maximized at 0.86 MHz, and 0.79 MHz, respectively, when the target depth is 4 cm in muscle tissue. The rules on the other hand predicted the optimum frequencies for acoustic power absorption as 0.9 MHz and 0.86 MHz, respectively for the 1D and 2D array case, which are within 6% and 9% of the field simulation results. Because radiation force generated by an acoustic wave in a lossy propagation medium is approximately proportional to the acoustic power absorption, these rules can be used to maximize acoustic radiation force generated in tissue as well.     

高频调制电弧的声学特性及其细化焊缝组织的应用

以等离子体电弧作为声源,利用高频电流进行调制,激发电弧可以产生超声波。研究了高频调制电弧的激发特性和声场特征,发现调制电弧所发射超声波在激励频段内呈现平坦的幅频特性,激励电流是影响电弧超声强度的主要因素,声压幅值与电弧等离子体流力的变化量成正比,声场呈现轴向强两侧弱的特征。同时介绍了利用电弧激发超声波在改善焊缝组织性能方面的应用。     

Emulsification by high frequency ultrasound using piezoelectric transducer: Formation and stability of emulsifier free emulsion

Emulsifier free emulsion was developed with a new patented technique for food and cosmetic applications. This emulsification process dispersed oil droplets in water without any emulsifier. Emulsions were prepared with different vegetable oil ratios 5%, 10% and 15% (v/v) using high frequency ultrasounds generated by piezoelectric ceramic transducer vibrating at 1.7 MHz. The emulsion was prepared with various emulsification times between 0 and 10 h. Oil droplets size was measured by laser granulometry. The pH variation was monitored; electrophoretic mobility and conductivity variation were measured using Zêtasizer equipment during emulsification process. The results revealed that oil droplets average size decreased significantly (p < 0.05) during the first 6 h of emulsification process and that from 160 to 1 μm for emulsions with 5%, 10% and from 400 to 29 μm for emulsion with 15% of initial oil ratio.For all tested oil ratios, pH measurement showed significant decrease and negative electrophoretic mobility showed the accumulation of OH⁻ at oil/water interface leading to droplets stability in the emulsion. The conductivity of emulsions showed a decrease of the ions quantity in solution, which indicated formation of positive charge layer around OH⁻ structure. They constitute a double ionic layer around oil particles providing emulsion stability. This study showed a strong correlation between turbidity measurement and proportion of emulsified oil.     

Influence of modification time and high frequency ultrasound irradiation on self-assembling of alkylphosphonic acids on stainless steel: Electrochemical and spectroscopic studies

Self-assembly of alkylphosphonic acids on stainless steel was investigated under different conditions. Four different alkylphosphonic acids exhibiting alkyl chain of various size were synthesized and studied: butylphosphonic acid (C4P), octylphosphonic acid (C8P), decylphosphonic acid (C10P), and hexadecylphosphonic acid (C16P). Electrochemistry experiments were extensively carried out in order to determine electrochemical surface blocking of adsorbed layers in function of grafting time. In term of surface blocking, an 8 h modification time was optimal for all alkylphosphonic acids. Longer immersion times lead to degradation of adsorbed layers. For the first time, grafting of C16P was studied under high frequency ultrasound irradiation. Interestingly, grafting process is highly accelerated under sonication and well-covering C16P modified substrates are obtained after 1 h of immersion under ultrasound irradiation. This would allow to elaborate high-quality alkylphosphonic acids modified samples within much shorter times. Water contact angles measurements and X-ray Photoelectrons Spectroscopy (XPS) confirmed presence of adsorbed alkylphosphonic acids on stainless steel surface. A very tight link between electrochemical blocking, surface hydrophobicity and species chemical grafting was established.     

Control of the dynamics of a boiling vapour bubble using pressure-modulated high intensity focused ultrasound without the shock scattering effect: A first proof-of-concept study

Boiling histotripsy is a promising High-Intensity Focused Ultrasound (HIFU) technique that can be used to induce mechanical tissue fractionation at the HIFU focus via cavitation. Two different types of cavitation produced during boiling histotripsy exposure can contribute towards mechanical tissue destruction: (1) a boiling vapour bubble at the HIFU focus and (2) cavitation clouds in between the boiling bubble and the HIFU source. Control of the extent and degree of mechanical damage produced by boiling histotripsy is necessary when treating a solid tumour adjacent to normal tissue or major blood vessels. This is, however, difficult to achieve with boiling histotripsy due to the stochastic formation of the shock scattering-induced inertial cavitation clouds. In the present study, a new histotripsy method termed pressure-modulated shockwave histotripsy is proposed as an alternative to or in addition to boiling histotripsy without inducing the shock scattering effect. The proposed concept is (a) to generate a boiling vapour bubble via localised shockwave heating and (b) subsequently control its extent and lifetime through manipulating peak pressure magnitudes and a HIFU pulse length. To demonstrate the feasibility of the proposed method, bubble dynamics induced at the HIFU focus in an optically transparent liver tissue phantom were investigated using a high speed camera and a passive cavitation detection systems under a single 10, 50 or 100 ms-long 2, 3.5 or 5 MHz pressure-modulated HIFU pulse with varying peak positive and negative pressure amplitudes from 5 to 89 MPa and −3.7 to −14.6 MPa at the focus. Furthermore, a numerical simulation of 2D nonlinear wave propagation with the presence of a boiling bubble at the focus of a HIFU field was conducted by numerically solving the generalised Westervelt equation. The high speed camera experimental results showed that, with the proposed pressure-modulated shockwave histotripsy, boiling bubbles generated by shockwave heating merged together, forming a larger bubble (of the order of a few hundred micron) at the HIFU focus. This coalesced boiling bubble then persisted and maintained within the HIFU focal zone until the end of the exposure (10, 50, or 100 ms). Furthermore, and most importantly, no violent cavitation clouds which typically appear in boiling histotripsy occurred during the proposed histotripsy excitation (i.e. no shock scattering effect). This was likely because that the peak negative pressure magnitude of the backscattered acoustic field by the boiling bubble was below the cavitation cloud intrinsic threshold. The size of the coalesced boiling bubble gradually increased with the peak pressure magnitudes. In addition, with the proposed method, an oval shaped lesion with a length of 0.6 mm and a width of 0.1 mm appeared at the HIFU focus in the tissue phantom, whereas a larger lesion in the form of a tadpole (length: 2.7 mm, width: 0.3 mm) was produced by boiling histotripsy. Taken together, these results suggest that the proposed pressure-modulated shockwave histotripsy could potentially be used to induce a more spatially localised tissue destruction with a desired degree of mechanical damage through controlling the size and lifetime of a boiling bubble without the shock scattering effect.     

The dependence of relaxation rates and chemical shift on the size of the imaged molecules and the concentration of MRI contrast agents

An intriguing phenomenon on enhancement of the relaxation rates and chemical shift of two typical magnetic resonance imaging (MRI) contrast agents based on gadolinium complex is observed. The relaxation enhancement or chemical shift change depends on the size of the molecule where the imaged nuclear species is located: the small molecules show a perfect linear relationship between the concentration and the relaxation enhancement or chemical shift change while for macromolecules pronounced nonlinearity is observed. The phenomenon is also confirmed with real images of a macromolecular sample. A quantitative theoretical interpretation of the phenomenon is proposed and the significance of this phenomenon to MRI of materials and biological systems is discussed.     

Modification of nitroxyl contrast agents with multiple spins and their proton T(1) relaxivity

The purpose of this study is to test the performance of multispin nitroxyl contrast agents in improving the sensitivity of MR detection for nitroxyl contrast agents. The relation between T(1) relaxivity and the number of paramagnetic centers in a molecule was investigated. Compound 1 is a single molecule of methoxycarbonyl-PROXYL (MC-PROXYL). Two and three MC-PROXYL molecules were chemically coupled to obtain Compounds 2 and 3, which have two and three nitroxyl spins in the molecule, respectively. A good linear relation, the slope of which increased depending on the number of nitroxyl spins in the molecule, was obtained between T(1)-weighted (fast low-angle shot) MR image contrast enhancement at 7 T and the concentration of nitroxyl contrast agents. T(1)-weighted MR image contrast enhancement and T(1) relaxivity levels of nitroxyl contrast agents were increased depending on the number of nitroxyl spins in the molecule. Multicoupling nitroxyl molecules can enhance the T(1)-weighted contrast effect while maintaining the quantitative behavior of the molecule for up to three spins.     

Combined ultrasound and germination treatment on the fine structure of highland barley starch

Highland barley is a grain crop grown in Tibet, China. This study investigated the structure of highland barley starch using ultrasound (40 kHz, 40 min, 165.5 W) and germination treatments (30℃ with 80% relative humidity). The macroscopic morphology and the barley's fine and molecular structure were evaluated. After sequential ultrasound pretreatment and germination, a significant difference in moisture content and surface roughness was noted between highland barley and the other groups. All test groups showed an increased particle size distribution range with increasing germination time. FTIR results also indicated that after sequential ultrasound pretreatment and germination, the absorption intensity of the intramolecular hydroxyl (–OH) group of starch increased, and hydrogen bonding was stronger compared to the untreated germinated sample. In addition, XRD analysis revealed that starch crystallinity increased following sequential ultrasound treatment and germination, but a-type of crystallinity remained after sonication. Further, the Mw of sequential ultrasound pretreatment and germination at any time is higher than that of sequential germination and ultrasound. As a result of sequential ultrasound pretreatment and germination, changes in the content of chain length of barley starch were consistent with germination alone. At the same time, the average degree of polymerisation (DP) fluctuated slightly. Lastly, the starch was modified during the sonication process, either prior to or following sonication. Pretreatment with ultrasound illustrated a more profound effect on barley starch than sequential germination and ultrasound treatment. In conclusion, these results indicate that sequential ultrasound pretreatment and germination improve the fine structure of highland barley starch.     

Effects of ultrasound pretreatment with different frequencies and working modes on the enzymolysis and the structure characterization of rice protein

The effects of ultrasound pretreatment with different frequencies and working modes, including mono-frequency ultrasound (MFU), dual-frequency ultrasound (DFU) and tri-frequency ultrasound (TFU), on the degree of hydrolysis (DH) of rice protein (RP) and angiotensin-I-converting enzyme (ACE) inhibitory activity of RP hydrolysate were investigated. Ultraviolet–visible (UV) spectroscopy, fourier transform infrared (FTIR) spectroscopy, surface hydrophobicity and scanning electron microscopy (SEM) of RP pretreated with ultrasound were measured. The results showed that ultrasound pretreatment did not increase DH of RP significantly (p > 0.05). However, all the ultrasound pretreatment increased the ACE inhibitory activity of RP hydrolysate significantly (p < 0.05). The MFU of 20 kHz showed higher ACE inhibitory activity compared to that of other MFU. The ACE inhibitory activity of sequential DFU was higher than that of simultaneous with the same frequency combination. Sequential TFU of 20/35/50 kHz produced the highest increase in ACE inhibitory activity in contrast with other ultrasound frequencies and working modes. All the results under ultrasound pretreatment showed that ultrasound frequencies and working modes were of great effect on the ACE inhibitory activity of RP. The changes in UV–Vis spectra and surface hydrophobicity indicated the unfolding of protein and exposure of hydrophobic groups by ultrasound. The FTIR analysis showed that all the ultrasound pretreatment with different frequencies and working modes decreased α-helix, β-turn content and increased β-sheet, random coil content of RP. The SEM results indicated that ultrasound pretreatment resulted in the deformation of RP. In conclusion, the frequency selection of ultrasound pretreatment of RP is essential for the preparation of ACE inhibitory peptide.     

Effect of high intensity ultrasound on the structure and solubility of soy protein isolate-pectin complex

In this study, a soy protein isolate (SPI)-pectin (PC) complex was prepared, and the effects of different high intensity ultrasound (HIU) powers on the structure and solubility of the complex were studied. Fourier transform infrared (FTIR) spectroscopy analysis exhibited that with increasing HIU power, the α-helix content of the SPI in the complex was significantly reduced, and the random coil content increased; however, an opposite trend appeared after higher power treatments. Fluorescence spectra showed that HIU treatment increased the fluorescence intensity of the complex, and the surface hydrophobicity was increased. The trend of the protein structure studied by Raman spectroscopy was similar to that of FTIR and fluorescence spectroscopy. When the HIU treatment was performed for 15 min and at 450 W power, the particle size of the complex was 451.85 ± 2.17 nm, and the solubility was 89.04 ± 0.19 %, indicating that the HIU treatment caused the spatial conformation of the protein to loosen and improved the functional properties of the complex. Confocal laser scanning microscopy (CLSM) revealed that the complex after HIU treatment exhibited improved dispersibility in water and smaller particle size. Gel electrophoresis results indicated that HIU treatment did not affect the protein subunits of the complex. Therefore, the selection of a suitable HIU treatment power can effectively improve the structural properties and solubility of SPI in the complex, and promote the application of the SPI-PC complex in food processing and industries.     

The effect of ultrasound on the structure and properties of the water-soluble corn hull heteroxylan

Ultrasonic irradiation of a water-soluble corn hull xylan fraction in neutral and alkaline aqueous medium has been found to produce significant changes in its molecular properties. Degradation is first manifested by a decrease in the large molar mass component under generation of polymer chains with about the same size as those of the main molar mass component. The latter is slightly shifted to the lower molar mass region only at stronger irradiation conditions. Ultrasonication of the xylan in neutral aqueous medium at high ultrasound power and/or long irradiation caused no significant changes in its sugar composition, primary structure and viscoelastic properties.