磁纳米粒子介导的磁致振动超声成像研究现状及展望

超声成像作为临床上常用的影像检测方法,在疾病诊断、术中导航和术后评估等方面发挥重要作用.随着纳米技术的快速发展,不同的微纳米材料或成像探针的构建,为超声成像提供新的发展动力.其中,磁纳米粒子介导的磁致振动超声成像是近年来发展的一种新兴的成像技术.其主要原理是基于磁纳米粒子在变化磁场作用下产生磁致振动,利用超声波探测粒子...     

纳米磁粒子应用于非接触式温度测量的研究

借助纳米磁粒子,通过测量其在不同温度下一、三次谐波幅值,使用郎之万函数达到求解温度的目的。首先对郎之万函数进行Taylor展开,进行简单的化简,然后根据正弦磁场下一、三次谐波的求解方式,把郎之万函数代入其中,构建温度和一、三次谐波幅值的函数关系。在较弱磁场的作用下,进行了三组实验,分别在-5℃～0℃、-15℃～0℃以及-25℃～0℃上进行实验研究。通过与光纤温度计测试结果的对比,发现这种方法在-25℃～0℃这个温度区间内可以实现精确温度测量,其误差为0.02 K～0.2 K。     

基于金刚石量子传感的纳米磁成像及凝聚态物理应用

作为凝聚态物理的重要方向,磁性的研究不仅是发展自旋电子学器件的基础,也是突破已有材料和器件功能壁垒的关键之一。磁性材料的纳米分辨率成像对认识和理解物质微观性质至关重要。金刚石中的氮—空位(NV)色心是一种对磁信号敏感的原子缺陷,经过十余年的深入研究,其已经发展为兼具高灵敏度和高空间分辨率的磁量子传感器,能够以纳米分辨率对单层磁性材料进行成像。它作为一种广谱(DC-GHz)、高灵敏度(nT/Hz^1/2)、高空间分辨率(~10 nm,理论极限~1 nm)的磁成像技术,可以对包括二维磁性材料、电流分布、电导率分布乃至单个电子自旋,少数个核自旋进行纳米磁成像。文章从NV色心微观结构和性质出发,介绍其作为量子传感进行磁信号探测和成像的原理;进一步从技术层面介绍谱仪的构成和探针制备;最后选取有代表性的工作,简要介绍 NV扫描显微镜在各方面的应用。     

受力腓肠肌剪切波速度的超声检测

肌肉组织受力时的弹性特征变化,是其功能特性、健康状态的重要评估参量.该文利用在离体肌肉组织表面施加低频振动引起组织发生形变,在组织内部产生剪切波的方法,经过快速超声成像,通过剪切波图像估计剪切波的传播速度.研究结果表明顺肌纤维方向传播的剪切波速度快于垂直肌纤维方向传播的剪切波速度,剪切波的传播速度随受力的增加而变大,且...     

用于层析粒子图像测速的模拟粒子场成像

对层析粒子图像测速(PIV)技术中示踪粒子成像部分进行理论分析,并结合真实风洞的相应参数,通过搭建模拟粒子成像平台的方法来进行研究。设计了一套体积为80 mm×100 mm×100 mm的激光照明系统,以提供粒子场的入射光强。建立了示踪粒子的三维成像模型,从而得到层析PIV系统的模拟图像。分析了影响PIV系统成像质量的相关因素。在单像素粒子数为0.007 7的情况下,通过真实粒子图像和模拟粒子图像比较,验证了该方法的正确性。     

壳聚糖纳米粒子的制备

利用对离子复合凝聚法,以EDTA为对离子,戊二醛为交联剂,壳聚糖(CS)为原料制备了CS纳米粒子。用动力学光散射分析(DLS)、透射电子显微镜(TEM)、FTIR对合成的CS纳米粒子进行了表征。表征结果表明,采用该方法合成的CS纳米粒子约为70nm,CS和EDTA通过电荷吸引凝聚成纳米粒子。     

基于粒子群算法的Pt-Pd合金纳米粒子的稳定结构研究

Pt-Pd合金纳米粒子相对于Pt及Pd单晶纳米粒子均具有更好的催化活性和选择性, 研究它的稳定结构对进一步了解催化性能具有重要意义. 本文采用粒子群算法和量子修正Sutton-Chen多体势对不同尺寸、 不同组成比例的二十四面体Pt-Pd合金纳米粒子的结构稳定性进行研究. 结果表明: Pt-Pd合金纳米粒子中Pt原子趋向于分布在纳米粒子内层, 而Pd原子趋向于分布在纳米粒子外层, 且Pt, Pd原子的分布越对称有序, 其能量越低, 结构越稳定; 随着Pt原子比例的增加, 三种不同尺寸的合金纳米粒子的Warren-Cowley化学短程有序值都逐渐升高, 即纳米粒子更趋向于偏聚分布状态; 在相同比例下, 小尺寸纳米粒子的偏聚程度比大尺 寸纳米粒子的大. 关键词： 合金纳米粒子 粒子群算法 稳态结构     

蜜蜂中磁铁矿纳米粒子的合成

磁铁矿在自然界中包括在生物体中分布广泛且极其重要,磁铁矿纳米粒子的制备是研究者关注的热点。生物合成磁铁矿纳米粒子具有人工合成磁铁矿纳米粒子所没有的特性和优势,本文就以蜜蜂为例,简要描述了蜜蜂体内超顺磁磁铁矿纳米颗粒的磁学特性及合成过程,以对磁铁矿纳米粒子在自然界中的合成提供参考和借鉴。     

不同表面结构的金纳米粒子荧光性质.

"研究了具有不同表面结构的金纳米粒子：裸金纳米粒子、三苯基膦修饰的金纳米粒子、巯基丙酸表面取代的金纳米粒子的荧光性质,及其对CdSe纳米粒子的荧光猝灭作用．发现不同的金纳米粒子荧光信号受粒子表面一价金离子与配体分子之间相互作用的影响,其荧光强度对其表面分子具有强烈的敏感性;具有不同表面结构的金纳米粒子对CdSe纳米粒子的荧光猝灭作用不同,与其吸收光谱和CdSe纳米粒子发射光谱的重叠程度相关."     

铂纳米粒子热稳定性的原子级模拟研究

本文采用分子动力学结合嵌入原子多体势,模拟了铂纳米粒子在升温过程中的热稳定性和熔化机制,并利用共近邻分析方法分析了它的微结构演化过程。模拟的结果表明：铂纳米粒子的熔点明显低于体材料的熔点;由于表面层原子的结合力较弱,在升温过程中表面会首先出现预熔;纳米粒子的熔化是从表面层开始的,并随着温度的升高,熔化的表面层会逐渐向内部扩展,最终导致纳米粒子整体转变为液态结构;当温度低于表面预熔温度时,纳米粒子保持良好的晶态结构。     

Diffraction-biased shear wave fields generated with longitudinal magnetic resonance elastography drivers

Magnetic resonance elastography (MRE) is a technique for quantifying the acoustic response of biological tissues to propagating waves applied at low frequencies in order to evaluate mechanical properties. Application-specific MRE drivers are typically required to effectively deliver shear waves within the tissue of interest. Surface MRE drivers with transversely oriented vibrations have often been used to directly generate shear waves. These drivers may have disadvantages in certain applications, such as poor penetration depth and inflexible orientation. Therefore, surface MRE drivers with longitudinally oriented vibrations are used in some situations. The purpose of this work was to investigate and optimize a longitudinal driver system for MRE applications. A cone-like hemispherical distribution of shear waves being generated by these drivers and the wave propagation being governed by diffraction in the near field are shown. Using MRE visualization of the vector displacement field, we studied the properties of the shear wave field created by longitudinal MRE drivers of various sizes to identify optimum shear wave imaging planes. The results offer insights and improvements in both experimental design and imaging plane selection for 2-D MRE data acquisition.     

Effect of off-frequency sampling in magnetic resonance elastography

In magnetic resonance elastography (MRE), shear waves at a certain frequency are encoded through bipolar gradients that switch polarity at a controlled encoding frequency and are offset in time to capture wave propagation using a controlled sampling frequency. In brain MRE, there is a possibility that the mechanical actuation frequency is different from the vibration frequency, leading to a mismatch with encoding and sampling frequencies. This mismatch can occur in brain MRE from causes both extrinsic and intrinsic to the brain, such as scanner bed vibrations or active damping in the head. The purpose of this work was to investigate how frequency mismatch can affect MRE shear stiffness measurements. Experiments were performed on a dual-medium agarose gel phantom, and the results were compared with numerical simulations to quantify these effects. It is known that off-frequency encoding alone results in a scaling of wave amplitude, and it is shown here that off-frequency sampling can result in two main effects: (1) errors in the overall shear stiffness estimate of the material on the global scale and (2) local variations appearing as stiffer and softer structures in the material. For small differences in frequency, it was found that measured global stiffness of the brain could theoretically vary by up to 12.5% relative to actual stiffness with local variations of up to 3.7% of the mean stiffness. It was demonstrated that performing MRE experiments at a frequency other than that of tissue vibration can lead to artifacts in the MRE stiffness images, and this mismatch could explain some of the large-scale scatter of stiffness data or lack of repeatability reported in the brain MRE literature.     

Modeling shear modulus distribution in magnetic resonance elastography with piecewise constant level sets

Magnetic resonance elastography (MRE) is designed for imaging the mechanical properties of soft tissues. However, the interpretation of shear modulus distribution is often confusing and cumbersome. For reliable evaluation, a common practice is to specify the regions of interest and consider regional elasticity. Such an experience-dependent protocol is susceptible to intrapersonal and interpersonal variability. In this study we propose to remodel shear modulus distribution with piecewise constant level sets by referring to the corresponding magnitude image. Optimal segmentation and registration are achieved by a new hybrid level set model comprised of alternating global and local region competitions. Experimental results on the simulated MRE data sets show that the mean error of elasticity reconstruction is 11.33% for local frequency estimation and 18.87% for algebraic inversion of differential equation. Piecewise constant level set modeling is effective to improve the quality of shear modulus distribution, and facilitates MRE analysis and interpretation.     

Synthesis and self-assembly of magnetic nanoparticles

We report the synthesis and self-assembly of different shapes and sizes of FePt nanoparticles. Our study shows that surfactants and solvent play an important role in the synthesis of different shapes and sizes of FePt nanoparticles. Higher boiling point solvents lead to the formation of spherical nanoparticles and low boiling point solvents form cubic nanoparticles. Our studies also indicate that self-assembly of FePt nanoparticles on substrates is a complex process that is sensitive to the concentration of excess surfactant in the nanoparticle solution.     

Magnetic resonance elastography using an air ball-actuator

The purpose of this study was to develop a new technique for a powerful compact MR elastography (MRE) actuator based on a pneumatic ball-vibrator. This is a compact actuator that generates powerful centrifugal force vibrations via high speed revolutions of an internal ball using compressed air. This equipment is easy to handle due to its simple principles and structure. Vibration frequency and centrifugal force are freely adjustable via air pressure changes (air flow volume), and replacement of the internal ball. In order to achieve MRI compatibility, all parts were constructed from non-ferromagnetic materials. Vibration amplitudes (displacements) were measured optically by a laser displacement sensor. From a bench test of displacement, even though the vibration frequency increased, the amount of displacement did not decrease. An essential step in MRE is the generation of mechanical waves within tissue via an actuator, and MRE sequences are synchronized to several phase offsets of vibration. In this system, the phase offset was detected by a four-channel optical-fiber sensor, and it was used as an MRI trigger signal. In an agarose gel phantom experiment, this actuator was used to make an MR elastogram. This study shows that the use of a ball actuator for MRE is feasible.     

Synthesis and magnetic properties of size-selected CoPt nanoparticles

CoPt nanoparticles are widely studied, in particular for their potentially very high magnetic anisotropy. However, their magnetic properties can differ from the bulk ones and they are expected to vary with the particle size. In this paper, we report the synthesis and characterization of well-defined CoPt nanoparticle samples produced in ultrahigh vacuum conditions following a physical route: the mass-selected low energy cluster beam deposition technique. This approach relies on an electrostatic deviation of ionized clusters which allows us to easily adjust the particle size, independently from the deposited equivalent thickness (i.e. the surface or volume particle density in a sample). Diluted samples made of CoPt particles, with different diameters, embedded in amorphous carbon are studied by transmission electron microscopy and superconducting interference device magnetometry, which gives access to the magnetic anisotropy energy distribution. We then compare the magnetic properties of two different particle sizes. The results are found to be consistent with an anisotropy constant (including its distribution) which does not evolve with the particle size in the range considered.     

Annealing effects on magnetic properties of acicular hematite nanoparticles

The effect of transformation on the structure and magnetic properties of -Fe₂O₃ acicular nanoparticles (major axis, 330 nm; minor axis, 70 nm) has been investigated. The particles were prepared by hydrolysis and polymerization in an aqueous solution of FeCl₃. Particles in the as-prepared sample are constituted by sub-units of 3–5 nm. The results indicate that the changes in the magnetic properties of the samples under thermal treatments (e.g. Morin transition, superparamagnetic behaviour) are mainly caused by the coalesce of the sub-units and by the variation of the crystallinity of nanoparticles.     

Effects of gadoxetic acid on liver elasticity measurement by using magnetic resonance elastography

The purpose of this study was to evaluate the effect of gadoxetic acid (Gd-EOB-DTPA) on measurements of liver stiffness by using magnetic resonance elastography (MRE). In this study, 104 consecutive patients (mean age, 67.7±9.4 years) underwent MRE using a 1.5-T MR scanner equipped with a cylindrical passive driver that was placed across the right chest wall for delivering vibrations. Axial gradient-echo images, which were automatically converted to elastograms that represented stiffness (kPa), were acquired using a continuous sinusoidal vibration of 60 Hz. Two raters independently placed a region of interest on the right lobe of the liver on the elastograms obtained before and after Gd-EOB-DTPA was administered. Liver stiffness was measured using these two elastograms and compared using a paired t test and correlation analysis. No significant difference was observed in liver stiffness before and after Gd-EOB-DTPA was administered (Rater 1, P=.1200; Rater 2, P=.3585). The correlation coefficients were 0.986 (Rater 1) and 0.984 (Rater 2), indicating excellent correlation between the stiffness values before and after Gd-EOB-DTPA was administered. Liver stiffness measured by MRE did not differ before and after Gd-EOB-DTPA was administered.     

Scaling laws and approximate expressions for the dynamic magnetic susceptibility of Brownian nanoparticles

We present simplified expressions for the out-of-phase component of the dynamic susceptibility χ″ of lognormal-sized magnetic nanoparticles under Brownian rotation. These expressions are based on transforming the general integral functions used for χ″ in the convolution of gaussian functions. χ″ can thus be expressed as a sum of gaussians with parameters directly related to those of the size distribution and to the saturation magnetization. The gaussian fit of χ″(ω) (where ω is the ac field frequency) is a simpler way to determine these structural and magnetic parameters as it avoids fitting χ″(ω) to an integral function. The expressions derived for χ″ suggest that χ″T data collapses in a ωη(T)/T scale (where T is the temperature and η the fluids viscosity), which is confirmed by numerical calculations. We also discuss the limits of validity of these approximations in real systems where both Néel and Brownian relaxation mechanisms coexist and we present further approximations for the relation of ω_χ with the average volume (being ω_χ the frequency at which χ″ is maximum). The ωη(T)/T scale can be used to qualitatively evaluate the dominance of the Brownian relaxation mechanism.     

MnZnFe nanoparticles for self-controlled magnetic hyperthermia

Manganese zinc iron magnetic nanoparticles were synthesized by a co-precipitation method for application as hyperthermia inducing agents. The structure, morphology and magnetic properties of the nanoparticles are characterized using scanning electron microscopy, X-ray diffraction, and a superconducting quantum interference device. The magnetic properties being investigated include Curie temperature, saturation magnetization, remnant magnetization, coercive field, and hysteresis. The study showed that adjusting the Mn contribution to the particles contributed to the adjustment of all magnetic properties of the complex.