Thermal dose optimization via temporal switching in ultrasound surgery

Temporal switching has been simulated and implemented in vivo experiments as a method to optimize thermal dose in ultrasound surgery. By optimizing the thermal dose over a tissue volume, the peak temperature is decreased, less average power is expended, and overall treatment time is shortened. To test this hypothesis, a 16 element, spherically sectioned array has been constructed for application in ultrasound surgery guided by magnetic resonance imaging. A simulation study for the array was performed to determine an optimal treatment from a set of multiple focus fields. These fields were generated using the mode scanning technique with power levels determined numerically using a direct weighted gradient search in the attempt to create an optimally uniform thermal dose over a 0.6x0.6x1.0 cm(3) tissue volume. Comparisons of the switched fields and a static multiple focus field indicate that the switching technique can lower power requirements and decrease treatment time by 20%. More importantly, the peak temperature of the sonication was lowered 13 degrees C, thus decreasing the possibility of cavitation. The simulated results of the 16 element array were then experimentally tested using MRI to noninvasively monitor temperature elevations and predict lesion size in rabbit thigh muscle in vivo. In addition, the results show that the switching technique can be less sensitive to tissue inhomogeneities than static field sonication while creating contiguous necrosis regions at equal average powers.     

Multiple-element crosseye

An extension to the traditional two-element array crosseye interferometric electronic jamming technique is investigated. Simulations are performed to obtain excitations for multiple-emitter linear arrays that produce distorted wavefronts over desired regions of space. This results in a greater number of degrees-of-freedom and a better ability to control the desired field pattern. The crosseye effect can be achieved over wider sectors, but only by increasing array power. The technique is extended to two-dimensional arrays and is applicable to multi-function radar antennas. Experimental measurements performed on a four-emitter array demonstrate the feasibility of the technique. Good agreement with predictions is shown     

Non-uniform sampling and polynomial interpolation for array synthesis

Two techniques are presented for array synthesis, based on the non-uniform sampling of the array factor, in order to control its shape. In the first technique, the ideal excitation distribution is truncated with an appropriated window function in order to obtain an initial approximation for the array factor. After that, certain points of the array factor are used for further control, usually the peaks of the ripple structure and of the sidelobes. The number of points used for this control is equal to the number of array elements and, imposing appropriated positions and values to these points, the polynomial interpolation allows the user to generate an array factor that approximates to the desired one. The second technique also uses polynomial interpolation, but the number of points is greater than the number of elements. In this case, the array factor matches some desired points and approximates to the other ones with the least minimum square error criterion. Both techniques consider the Fourier relation method, in order to realise the calculations efficiently. Among other applications, these techniques permit to define the level of each sidelobe in an array factor, to control the ripple structure of shaped patterns and to impose nulls in prescribed directions of any array factor, with minimum computation times, thereby permitting real-time applications.     

Two-step hybrid virtual array ray (VAR) technique for focusing through the rib cage

A new methodology for focusing ultrasonic beams noninvasively in the presence of the rib cage is investigated. This investigation is motivated by the need to employ high intensity focused ultrasound (HIFU) using phased array applicators for the treatment of liver tumors partially shadowed by the rib obstacles. This approach enables us to efficiently perform the ultrasound computational analysis and pattern synthesis in the interior region of the rib cage. The proposed technique consists of two main steps. First, a virtual array is introduced along the intercostal spacings between the solid ribs to generate the prespecified intensity levels at a set of control points within the target region. The second step involves the design of the actual feed array that induces the virtual sources between the intercostal spacings. This design optimization is carried out via the pseudo-inverse technique (minimum norm least squares solution) and by enforcing a constrained preconditioned pseudo-inverse method. The proposed procedure calculates the required primary sources (feed array) while maintaining minimal power deposition over the solid obstacles.     

不等厚复合构件相控阵超声脱粘检测技术仿真研究

鉴于不等厚复合构件结构的特殊性,采用传统超声检测技术检测此类构件的粘接质量是非常困难的。采用超声相控阵技术实现界面粘接质量的检测。分析研究了不等厚复合构件脱粘检测的特殊性。建立了相控阵超声换能器的辐射声场模型,基于仿真得到的声场,研究了采用超声相控阵技术检测不等厚复合构件界面脱粘的可行性。仿真结果表明,通过控制相控延时,可使检测声束在检测区域内聚焦到任意位置处,保证在厚度变化的界面处回波信号幅度的一致性,可用于此类构件的界面脱粘检测。     

NxN square-element ultrasound phased-array applicator: simulated temperature distributions associated with directly synthesized heating patterns

Computer simulations to demonstrate the possibility of heating small tumors by appropriately driving the electronic control circuitry of an nxn square-element ultrasonic phased array are conducted. The synthesis method consists of simultaneously focusing the ultrasonic beam at different points uniformly distributed along the tumor periphery and, hence, involves no scanning. It is demonstrated that by combining the multiple focusing feature with a new field phasing concept, typical undesired hot spots can be eliminated. The tissue thermal response to this heating modality is investigated by iteratively solving the three-dimensional steady-state bioheat equation. Temperature distributions associated with different directly synthesized power deposition patterns are simulated and discussed.     

A cylindrical-section ultrasound phased-array applicator for hyperthermia cancer therapy

A phased-array applicator geometry for deep localized hyperthermia is presented. The array consists of rectangular transducer elements forming a section of a cylinder that conforms to the body portals in the abdominal and pelvic regions. Focusing and scanning properties of the cylindrical-section array are investigated in homogeneous lossy media using appropriate computer simulations. The characteristic focus of this array is shown to be spatially limited in both transverse and longitudinal directions with intensity gain values suitable for deep hyperthermia applications. The ability of the cylindrical-section phased array to generate multiple foci using the field conjugation method is examined. The effect of the grating lobes on the power deposition pattern of the scanned field is shown to be minimal. Steady-state temperature distributions are simulated using a three-dimensional thermal model of the normal tissue layers surrounding a tumor of typical volume. The advantages and the limitations of this array configuration are discussed.     

A field conjugation method for direct synthesis of hyperthermia phases-array heating patterns

A phased-array field conjugation method is investigated as a means for synthesizing directly many ultrasound field patterns useful for tumor heating. For virtually any ultrasound phased array, the method permits the computation of element driving amplitude and phase distributions appropriate for synthesizing directly diffuse heating patterns without the need for mechanical or electrical scanning. Moreover, the proposed method offers the possibility of creating simultaneously, at different sites, more than one focus, which can then be scanned electronically. This attractive feature eliminates the need for operating at high spatial-peak temporal-peak focal intensities, a potential problem associated with conventional scanning, while achieving a desired heating pattern. The method is applied to two different applicator configurations: a concentric-ring and a square nxn array. Computer simulations of different heating patterns, synthesized using the field conjugation method, are presented. Important practical design parameters, such as the size and number of the array elements, are discussed.     

阵列永磁体产生旋转磁场的机理及实验

 诊疗微机器人外磁主动驱动是一种重要的可行的驱动方式,而如何产生合适的外部磁场是一个比较复杂的问题．相对于目前常用的通过电磁线圈组合产生驱动磁场方式,提出一种新的简单可行的永磁体组合产生旋转磁场方法,即圆周阵列永磁体并将其调整到对应的初始方位角后,同步转动在阵列中心区域产生旋转磁场,作为微机器人的主动驱动场．对于阵列中心区域的磁感应强度分布,作了理论分析和数值计算,并搭建了实验台．实验表明,采用边长为50 mm、高为18 mm的钕铁硼永磁体阵列,阵列数为6,阵列直径为275 mm时,可以在±50 mm×±50 mm×±20 mm阵列中心区域产生一个大小为0.012 T的均匀磁场,该磁场与永磁体同步旋转但是方向相反．这种新的磁场产生方法可以用于微机器人特别是诊疗微机器人的驱动,具有广阔的应用前景．     

基于凸优化方法的期望主瓣幅度响应波束图设计

将发射阵的期望主瓣幅度响应波束设计转化为凸优化问题,并利用cvx工具箱求解最优加权。首先,根据期望波束的主瓣范围,将空间区域分为主瓣区域和旁瓣区域,再在主瓣区域内将设计波束和期望波束之差的2-范数最小化,并将设计波束图旁瓣级控制在期望值之下。最后,利用cvx工具箱对该凸优化问题进行求解,获得满足要求的设计波束图。通过计算机仿真对所提波束图设计方法的有效性进行了验证。     

Phase and polarization control as a route to plasmonic nanodevices

We extend the concepts of phase, polarization, and feedback control of matter to develop a general approach for guiding light in the nanoscale via nanoparticle arrays. The phase and polarization of the excitation source are first introduced as tools for control over the pathway of light at array intersections. Genetic algorithms are next applied as a systematic design tool, wherein both the excitation field parameters and the structural parameters of the nanoparticle array are optimized to make devices with desired functionality. Implications to research fields such as single molecule spectroscopy, spatially confined chemistry, optical logic, and nanoscale sensing are envisioned.     

近场波束形成的约束最小平方和法

文章研究了近场情况下的波束形成问题。对于任意结构阵列,提出了近场波束形成的一种约束最小平方和法,用近场信号模型来设计近场波束。该方法借助于优化工具箱,在对波束旁瓣施加约束的条件下,使波束主瓣的误差平方和最小。该方法在主瓣区域和旁瓣区域都能得到满足设计要求的波束性能,可以方便地控制期望响应,简单易行。基于32元均匀离散圆阵的仿真设计结果表明了文中方法的有效性。     

E-plane sectoral horn power divider

E-plane sectoral horn power divider design method for high-power antenna array applications is presented. In this method, power division is proportional to the cross section areas of the waveguide channels at the aperture of an E-plane sectoral horn antenna. Two different power dividers are designed and produced to be used with an X-band antenna array having 30 dB Taylor distribution. The first one employs a simple E-plane sectoral horn antenna. This straightforward application of the method might be limited with production tolerances. The second one incorporates a corrugated E-plane sectoral horn antenna, whose design is relatively complex but less sensitive to production tolerances. The produced dividers are compared based on simulations and measurements and good agreement with the theoretical expectations is observed for both types.     

A point-matching method for array pattern synthesis

In this paper, a newly developed point-matching method is presented to obtain a set of excitation coefficients of a linear array that generates a desired radiation pattern with arbitrarily suppressed sidelobe levels. This method can be used for linear arrays with nonuniform spacing and nonisotropic elements. The design examples presented show that the point-matching method is both effective and efficient     

The combined concentric-ring and sector-vortex phased array for MRIguided ultrasound surgery

MRI guided ultrasound surgery requires small surgical equipment volumes to facilitate the treatment of larger patients in the limited space of a conventional MRI magnet. In addition, large focal volumes are required to reduce the treatment time of large tumors. The concentric-ring array is capable of moving the focus in one dimension, and previous studies have shown that a circular array composed of radial sectors is capable of producing enlarged focal volumes. These two array designs may be combined to create an array that is capable of both enlarging the focus and moving the focus along the axis of the array. Simulations were performed to predict the performance and capabilities of various combined array designs by using numerical routines to, calculate the acoustic power field, temperature distribution, and accumulated thermal dose. The results shown predict that the combined array can create necrosed tissue volumes over 30 times larger than the concentric-ring array while maintaining focal range. The simulation results were verified with an experimental array consisting of 13 rings and 4 sectors. In addition, simulations were performed where multiple focal patterns were cycled in the time domain to create an optimized heating pattern characterized by uniform thermal dose over the volume of the lesion. Such heating patterns resulted in a 40°C lower maximum temperature compared to single mode sonications while producing the same necrosed tissue volume, and yielded a rate of necrosis of 26.4 cm³ /h     

Design and evaluation of a feedback based phased array system for ultrasound surgery

A driving system has been designed for phased array ultrasound applicators. The system is designed to-operate in the bandwidth 1.2 to 1.8 MHz, with independent channel power control up to 60 W (8 bit resolution) for each array element. To reduce power variation between elements, the system utilizes switching regulators in a feedback loop to automatically adjust the DC supply of a class D/E power converter. This feedback reduces the RF electrical power variation from 20% to 1% into a 16 element array. DC-to-RF efficiencies close to 70% for all power levels eliminates the need for large heat sinks. In addition to power control, each channel may be phase shifted 360 degrees with a minimum of 8 bit resolution. To ensure proper operation while driving ultrasound arrays with varying element sizes, each RF driving channel implements phase feedback such that proper phase of the driving signal is produced either at the amplifier output before the matching circuitry or after the matching circuitry at the transducer face. This feedback has been experimentally shown to increase the focal intensities by 20 to 25% of two tested phased arrays without array calibration using a hydrophone.     

一种基于白噪声响应的随机载荷谱识别方法

基于频域内随机振动响应与载荷的关系,根据对结构控制点随机振动响应谱的预设,提出一种新的随机载荷谱简便识别方法。首先计算结构在白噪声载荷谱下,结构控制点处的响应功率谱,即系统的频响函数;然后将控制点的期望输出响应谱与控制点在白噪声载荷谱下的响应谱相比,进而得到相应的随机载荷谱。应用上述方法,借助于有限元仿真软件在设定控制点输出加速度响应谱为梯形谱的条件下,对某典型舱段结构进行基础加速度载荷谱逆向识别与正向检验,证明方法的有效性与准确性。     

A three-dimensional BEM solution for plasticity using regression interpolation within the plastic field

This paper presents an improved solution of three-dimensional plasticity problems using the boundary element method (BEM). The BEM formulation for plasticity requires volume as well as boundary discretizations. An initial stress formulation is used to satisfy the material non-linearity. Conventionally, the plastic field in the volume element (or cell) is interpolated based on the value of plastic stress at the nodes of the cell. In this paper, the distribution of the plastic field in the cell is based on a number of points interior to the cell. The plastic field is described using regression interpolation polynomials through these interior points. The constitutive relation is satisfied at each interior point. The number of points can be varied in each cell, thus allowing for adaptive volume cells. The plastic stresses are computed at the interior points only, therefore, the need for surface stress computation (which uses numerical derivatives at the surface) is completely eliminated. Three-dimensional applications are used to compare the present regression interpolation procedure with the conventional method for elasto-plasticity problems. In all variations of the applications studied regression interpolation based on interior points provided superior results to those determined via the conventional nodal interpolation method.     

Ultrasound therapy transducers with space-filling non-periodic arrays

Ultrasound transducers designed for therapeutic purposes such as tissue ablation, histotripsy, or drug delivery require large apertures for adequate spatial localization while providing sufficient power and steerability without the presence of secondary grating lobes. In addition, it is highly preferred to minimize the total number of channels and to maintain simplicity in electrical matching network design. To this end, we propose array designs that are both space-filling and non-periodic in the placement of the elements. Such array designs can be generated using the mathematical concept of non-periodic or aperiodic tiling (tessellation) and can lead to reduced grating lobes while maintaining full surface area coverage to deliver maximum power. For illustration, we designed two 2-D space-filling therapeutic arrays with 128 elements arranged on a spherical shell. One was based on the two-shape Penrose rhombus tiling, and the other was based on a single rectangular shape arranged non-periodically. The steerability performance of these arrays was studied using acoustic field simulations. For comparison, we also studied two other arrays, one with circular elements distributed randomly, and the other a periodic array with square elements. Results showed that the two space-filling non-periodic arrays were able to steer to treat a volume of 16 x 16 x 20 mm while ensuring that the grating lobes were under -10 dB compared with the main lobe. The rectangular non-periodic array was able to generate two and half times higher power than the random circles array. The rectangular array was then fabricated by patterning the array using laser scribing methods and its steerability performance was validated using hydrophone measurements. This work demonstrates that the concept of space-filling aperiodic/non-periodic tiling can be used to generate therapy arrays that are able to provide higher power for the same total transducer area compared with random arrays while maintaining acceptable grating lobe levels.     

Improved resolution three-dimensional integral imaging using optimized irregular lens-array structure

A rigorous approach is proposed to improve the resolution of integral imaging (InI) by finding the appropriate form of irregularity in the arrangement of the InI lenslets. The improvement of the resolution is achieved through redistribution of the sampling points in a uniform manner. The optimization process for finding the optimum pattern of the lens-array irregularity is carried out by minimizing a cost function, whose mathematical closed-form expression is provided. The minimization of the proposed cost function ensures the uniform distribution of sampling points and thus improves the resolution within the desired depth of field (DOF) and field of view (FOV). A set of standard resolution charts is used to demonstrate the improvement of the quality of the three-dimensional (3D) images obtained by using the optimized irregular lens array. It is shown that the overall level of the lateral and depth resolutions is improved at the same time.