HIFU治疗中媒质特性对声聚焦影响的研究

目的:研究声衰减系数、声速、温度等媒质参数对声聚焦的影响。方法:用瑞利积分对凹球面自聚焦超声换能器进行声场模拟。结果及结论:媒质的特性参数不同,聚焦表现不同的特点。声衰减系数增大,焦斑向声源靠近,且焦斑缩短;声速越大,焦距越小,但焦区最大声压升高,焦斑的面积增大;温度变化不大时对声聚焦的影响不明显。     

组织声学特性对高强度聚焦超声温度场的影响

目的 数值仿真组织声学特性对高强度聚焦超声（HIFU）焦域处温度场的影响,为HIFU治疗安全性和可靠性提供理论依据.方法 以实测新鲜离体猪肝组织不同温度下的声速和衰减系数为依据,利用时域有限差分（FDTD）法数值仿真研究HIFU治疗过程中组织内声速、衰减系数的变化和温度场的分布,分析讨论声速和衰减系数变化对60 ℃以上可治疗区域大小、位置的影响.结果 随着照射时间的延长,焦域处肝组织温升增大,声速下降,声衰减系数增大.随着声速的变化,形成的可治疗区域变大,焦点位置向远离换能器方向移动;随着声衰减系数的变化,焦域大小和焦点位置几乎不变.结论 猪肝组织内声速的变化对可治疗焦域的位置和大小影响较大;声衰减系数的变化对焦域的影响较小.     

高强度聚焦超声作用下的组织焦域能量分析及实验验证

为研究发生空化前高强度聚焦超声(HIFU)作用的最佳治疗模式,从声场和热场的仿真计算和新鲜离体牛肝实验验证出发,根据声能转化成热能的原理,分析了组织焦域温度上升造成组织损伤的变化过程;建立了与温度相关的组织损伤预测模型;计算了形成1 mm宽度的生物学焦域所吸收的能量;并在75、100、125、150 W这4种声功率下,按照350 J能量,共进行了24组新鲜离体牛肝组织的实验,验证该能量强度形成的生物学焦域尺寸。结果表明,经组织衰减后生物学焦域吸收的超声能量为350 J时,在新鲜离体牛肝组织中形成的生物学焦域宽度为(1.1±0.1)mm;当换能器物理学焦域的长短轴之比在4～10之间变动时,吸收350 J形成的生物学焦域宽度保持在(1.0±0.2) mm。在充分除气的均匀介质牛肝中,当换能器的物理学焦域长短轴比在4～10之间时,吸收350 J能量,声功率100～150 W是形成(1.0±0.2) mm宽度的生物学焦域的最佳治疗模式。     

一种消除浅表组织病灶的微型线聚焦换能器声场的数值计算与仿真方法

目的:本文涉及一种消除浅表组织病灶的微型线聚焦超声声场的数值计算与仿真,通过计算得出声场的三维分布,同时研究柱面自聚焦换能器各个参数对声场的影响,以指导微型自聚焦换能器的设计。方法:本文通过建立三维直角坐标系,应用Rayleigh积分得出微型线聚焦超声换能器聚焦区域内每一点的声强表达式,运用C语言和matlab采用直接数值积分的方法对柱面聚焦超声换能器的声场进行了数值计算和仿真,得出声场的三维分布。结果:从声场的三维分布可以看出:按照本文给定的条件设计,微型线聚焦超声换能器展现出良好的聚焦性能,形成明显的焦域。和普通的柱面超声换能器相比,小型的柱面超声换能器形成的声场在z轴上的声场分布更均匀,x,y方向上衰减更快。结论:通过不同的参数对声场三维分布影响可以看出:微型线聚焦超声换能器的厚度,半径,张角和长度对聚焦的效果有明显影响。与给定的条件相比,选择较厚、半径和张角较大的换能器,能提高聚焦性能,而换能器的长度对聚焦效果影响不大。     

HIFU在牛肝组织中的传播衰减研究

运用辐射压力法研究高强度聚焦超声在牛肝组织中的传播衰减。在室温 2 0℃下 ,利用 4种治疗超声换能器 ,在不同的换能器表面输出声功率下 ,分别测量 2 0、4 0、6 0、80 m m不同厚度新鲜离体牛肝在放入脱气水前后的声辐射力 F、F′,然后计算出声衰减。使用治疗超声换能器 4 ,对于不同辐照深度 ,以声强 ISATA=2 2 .0× 10 3W/cm2、辐照时间 5 s的超声剂量定点辐照新鲜离体牛肝 ,以断层方式剖开后测量凝固性坏死组织体积。研究结果表明 ,对于一个具体的治疗超声换能器 ,在牛肝厚度一定时 ,辐射力穿透率 F′/F法与换能器表面声强及样品近治疗超声换能器表面声窗面积无关。采用最小 F′/F法拟合得出辐射力穿透率随牛肝厚度增加呈指数规律递减 ,且与凝固性坏死组织体积随牛肝厚度 (辐照深度 )增加呈指数规律递减趋势一致。采用最小二乘法拟合得出高度聚集超声在牛肝组织中传播的声衰随频率近似线性增加且呈幂函数关系。这为进一步研究高强度聚焦超声治疗剂量学提供了实验依据     

基于声换能器特性的三维磁感应磁声成像重建算法

为了完善磁感应磁声成像算法,使其在真实声检测系统下都适用,研究声换能器特性对磁感应磁声成像重建效果的影响。依据换能器声场分布测量数据,采用插值方法建立真实声换能器三维声场分布模型,利用格林函数法及离散化方法建立磁感应磁声成像正逆问题算法。为了验证该算法,分别采用球扫描和柱扫描方式进行仿真。基于S L CT仿真模型建立三维仿体电导率模型,用有限元方法对瞬态电磁场进行分析,根据计算的涡流分布结果,使用Matlab进行数值计算。仿真结果表明：该算法可基本重建出原始力源的分布,重建图像的相关系数分别达到98.49%和94.96%,对一般声换能器比较适用。该研究为进一步的实验研究和实验重建的精确性提供理论依据。     

基于超声医学应用的声传播的研究

目的;近年来。含气泡液体的声衰减和色散广泛应用于生物医学领域,利用这一特性可以对某些生物组织进行超声成像,也对血流进行超声多普勒成像。基于含气泡液体的声学特性对生物医学应用的影响,研究了声波在含有气泡粘性液体内的传播。方法：当液体中入射一列声波时,气泡的存在将改变液体内的传声特性,依据球贝塞尔和汉克尔函数,利用分界面处声波传播引起的质点振动速度和应力为边界条件研究其传播。结果：求解了声波在气．液混合介质内传播的声学参数．均与气泡含量、声波频率及粘滞系数等因素有关;并得到不同频率下声波的传播速度及衰减系数随气泡含量的变化曲线。结论：气泡和声波的传播频率都会影响声波在混合介质内的传播;具体表现在相同频率下,气泡含量增加使声波的传播速度不断下降,而其衰减系数却不断增大;气泡含量一定时,随着声波频率的升高,声波的传播速度下降越来越明显。衰减系数不断增大。即声波的传播能量损失越来越大。所得结论将对超声在医学上的研究和广泛应用有重要的指导意义。     

高强度聚焦超声"切除"组织的剂量学研究

运用高强度聚焦超声（High intensity focused ultrasound．HIFU）技术完整切除组织块并确立相应的能效关系，从而进行HIFU剂量学研究。按照由生物学焦域（Biological focal region，BFR）→束损伤→片损伤→块损伤的治疗原则，使用ISATA为0～27700W／cm^2，扫描速度1～4mm／s，束损伤的空间间距5～10mm，片损伤的空间间距10～20mm，在离体牛肝组织中形成不同治疗深度的束损伤、片损伤和块损伤，从而实现完整切除组织块。并把形成单位体积凝固性坏死所需的HIFU超声能量叫做HIFU治疗的能效因子（Energy—efficiency factor．EEF），用EEF量化HIFU在组织内的能量存积。研究结果表明，在不同治疗深度处形成的束损伤的EEF随治疗深度的增加而增大。形成片损伤、块损伤的EEF远远小于在不同治疗深度处形成束损伤的EEF，且形成块损伤的EEF小于形成片损伤的EEF。形成片损伤、块损伤的EEF并不是不同治疗深度的束损伤的EEF、不同治疗层面的片损伤的EEF的简单叠加，它与一个已存在的损伤改变了组织声环境有关，提示可通过改变组织声环境来改变EEF。因此，用EEF来进行HIFU的剂量学研究是一个新思路。对一个固定的聚焦超声换能器，EEF除了与声功率、辐照时间、治疗深度、组织结构和功能状态有关外，另一个重要的影响因素是H1FU治疗过程中组织声环境的改变。     

声致荧光成像的初步研究

目的：本文对一种改进型快速的声致荧光成像方法进行了初步研究，即探索在体荧光增强剂和灵敏的ICCD阵列结合成像的可行性。方法：该方法利用聚焦超声对样品内的一层进行逐点扫描，超声焦区将发出声致荧光，借助用于灵敏的弱光接受ICCD阵列,大范围快速收集该超声焦区的声致荧光。结果：实验结果给出聚焦超声场的分布，并利用扫描各点的差异导致的声荧光强度的不同进行模拟样品的图像重建。结论：该研究证明了该技术实现声致荧光成像的可行性．而且其图像的横向分辨率主要受超声聚焦区的形状影响，本实验中接近2mm。     

双频共焦高强度超声增强仿组织体模毁损效果的初步研究

目的：比较双频共焦模式和单频模式高强度聚焦超声（HIFU）单次辐照毁损灶的大小。方法：分别采用相同声强度的双频共焦HIFU与传统单频HIFU对透明仿组织体模进行不同时间长度的辐照，直接在透明的仿组织体模上测量HIFU毁损灶的长度和直径。结果：双频和单频模式下，随辐照时间的延长毁损灶逐渐增大；相同声强相同辐照时间下，双频HIFU产生的毁损灶显著大于单频模式所致毁损灶。结论：双频共焦模式可以增大HIFU单次辐照毁损灶的体积．为HIFU治疗效率的提高提供了一种新技术。     

Acoustical properties of selected tissue phantom materials for ultrasound imaging

This note summarizes the characterization of the acoustic properties of four materials intended for the development of tissue, and especially breast tissue, phantoms for the use in photoacoustic and ultrasound imaging. The materials are agar, silicone, polyvinyl alcohol gel (PVA) and polyacrylamide gel (PAA). The acoustical properties, i.e., the speed of sound, impedance and acoustic attenuation, are determined by transmission measurements of sound waves at room temperature under controlled conditions. Although the materials are tested for application such as photoacoustic phantoms, we focus here on the acoustic properties, while the optical properties will be discussed elsewhere. To obtain the acoustic attenuation in a frequency range from 4 MHz to 14 MHz, two ultrasound sources of 5 MHz and 10 MHz core frequencies are used. For preparation, each sample is cast into blocks of three different thicknesses. Agar, PVA and PAA show similar acoustic properties as water. Within silicone polymer, a significantly lower speed of sound and higher acoustical attenuation than in water and human tissue were found. All materials can be cast into arbitrary shapes and are suitable for tissue-mimicking phantoms. Due to its lower speed of sound, silicone is generally less suitable than the other presented materials.     

高强度聚焦超声换能器声场的数值仿真与实验测量

目的 高强度聚焦超声(HIFU)肿瘤治疗过程中,HIFU换能器形成的声场分布是决定其治疗效果的关键因素之一,为了提高HIFU肿瘤治疗的安全性和可靠性,需要对HIFU换能器形成的声场进行预测.方法 采用时域有限差分(FDTD)法数值仿真水体内形成的声压分布与实验测量结果对比的研究方法,分析讨论了HIFU换能器在不同激励功...     

An investigation of the use of transmission ultrasound to measure acoustic attenuation changes in thermal therapy

The potential of using a commercial ultrasound transmission imaging system to quantitatively monitor tissue attenuation changes after thermal therapy was investigated. The ultrasound transmission imaging system used, the AcoustoCam (Imperium Inc., MD) allows ultrasonic images to be captured using principles similar to that of a CCD-type camera that collects light. Ultrasound energy is focused onto a piezoelectric array by an acoustic lens system, creating a gray scale acoustic image. In this work, the pixel values from the acoustic images were assigned acoustic attenuation values by imaging polyacrylamide phantoms of varying known attenuation. After the calibration procedure, data from heated polyacrylamide/bovine serum albumin (BSA) based tissue-mimicking (TM) phantoms and porcine livers were acquired. Samples were heated in water at temperatures of 35, 45, 55, 65, and 75°C for 1 h. Regions of interest were chosen in the images and acoustic attenuation values before and after heating were compared. An increase in ultrasound attenuation was found in phantoms containing BSA and in porcine liver. In the presence of BSA, attenuation in the TM phantom increased by a factor of 1.5, while without BSA no significant changes were observed. The attenuation of the porcine liver increased by up to a factor of 2.4, consistent with previously reported studies. The study demonstrates the feasibility of using a quantitative ultrasound transmission imaging system for monitoring thermal therapy.     

Magnetic resonance acoustic radiation force imaging

Acoustic radiation force impulse imaging is an elastography method developed for ultrasound imaging that maps displacements produced by focused ultrasound pulses systematically applied to different locations. The resulting images are "stiffness weighted" and yield information about local mechanical tissue properties. Here, the feasibility of magnetic resonance acoustic radiation force imaging (MR-ARFI) was tested. Quasistatic MR elastography was used to measure focal displacements using a one-dimensional MRI pulse sequence. A 1.63 or 1.5 MHz transducer supplied ultrasound pulses which were triggered by the magnetic resonance imaging hardware to occur before a displacement-encoding gradient. Displacements in and around the focus were mapped in a tissue-mimicking phantom and in an ex vivo bovine kidney. They were readily observed and increased linearly with acoustic power in the phantom (R2=0.99). At higher acoustic power levels, the displacement substantially increased and was associated with irreversible changes in the phantom. At these levels, transverse displacement components could also be detected. Displacements in the kidney were also observed and increased after thermal ablation. While the measurements need validation, the authors have demonstrated the feasibility of detecting small displacements induced by low-power ultrasound pulses using an efficient magnetic resonance imaging pulse sequence that is compatible with tracking of a dynamically steered ultrasound focal spot, and that the displacement increases with acoustic power. MR-ARFI has potential for elastography or to guide ultrasound therapies that use low-power pulsed ultrasound exposures, such as drug delivery.     

Model-based ultrasound tomography: tissue phantom experiments

We present a detailed experimental study to evaluate our finite element based nonlinear reconstruction algorithm for recovery of acoustic properties in heterogeneous scattering media. Using a circularly scanning ultrasound system at 500 KHz, tissue phantom experiments were performed to study spatial resolution and contrast issues in model-based ultrasound tomography. Our results show that both acoustic attenuation and speed images can be quantitatively reconstructed in terms of the location, size, shape, and acoustic property value of the target when different contrast levels between the target and background were used. We also demonstrate that a high contrast target as small as 3 mm in diameter can be quantitatively resolved with our acoustic speed and attenuation images.     

A unified model for the speed of sound in cranial bone based on genetic algorithm optimization

The density and structure of bone is highly heterogeneous, causing wide variations in the reported speed of sound for ultrasound propagation. Current research on the propagation of high intensity focused ultrasound through an intact human skull for non-invasive therapeutic action on brain tissue requires a detailed model for the acoustic velocity in cranial bone. Such models have been difficult to derive empirically due to the aforementioned heterogeneity of bone itself. We propose a single unified model for the speed of sound in cranial bone based upon the apparent density of bone by CT scan. This model is based upon the coupling of empirical measurement, theoretical acoustic simulation and genetic algorithm optimization. The phase distortion caused by the presence of skull in an acoustic path is empirically measured. The ability of a theoretical acoustic simulation coupled with a particular speed-of-sound model to predict this phase distortion is compared against the empirical data, thus providing the fitness function needed to perform genetic algorithm optimization. By performing genetic algorithm optimization over an initial population of candidate speed-of-sound models, an ultimate single unified model for the speed of sound in both the cortical and trabecular regions of cranial bone is produced. The final model produced by genetic algorithm optimization has a nonlinear dependency of speed of sound upon local bone density. This model is shown by statistical significance to be a suitable model of the speed of sound in bone. Furthermore, using a skull that was not part of the optimization process, this model is also tested against a published homogeneous speed-of-sound model and shown to return an improved prediction of transcranial ultrasound propagation.