Combined ultrasound and optoacoustic system for real-time high-contrast vascular imaging in vivo

In optoacoustic imaging, short laser pulses irradiate highly scattering human tissue and adiabatically heat embedded absorbing structures, such as blood vessels, to generate ultrasound transients by means of the thermoelastic effect. We present an optoacoustic vascular imaging system that records these transients on the skin surface with an ultrasound transducer array and displays the images online. With a single laser pulse a complete optoacoustic B-mode image can be acquired. The optoacoustic system exploits the high intrinsic optical contrast of blood and provides high-contrast images without the need for contrast agents. The high spatial resolution of the system is determined by the acoustic propagation and is limited to the submillimeter range by our 7.5-MHz linear array transducer. A Q-switched alexandrite laser emitting short near-infrared laser pulses at a wavelength of 760 nm allows an imaging depth of a few centimeters. The system provides real-time images at frame-rates of 7.5 Hz and optionally displays the classically generated ultrasound image alongside the optoacoustic image. The functionality of the system was demonstrated in vivo on human finger, arm and leg. The proposed system combines the merits and most compelling features of optics and ultrasound in a single high-contrast vascular imaging device.     

Minimally Redundant 2-D Array Designs for 3-D Medical Ultrasound Imaging

In real-time ultrasonic 3-D imaging, in addition to difficulties in fabricating and interconnecting 2-D transducer arrays with hundreds of elements, there are also challenges in acquiring and processing data from a large number of ultrasound channels. The coarray (spatial convolution of the transmit and receive arrays) can be used to find efficient array designs that capture all of the spatial frequency content (a transmit–receive element combination corresponds to a spatial frequency) with a reduced number of active channels and firing events. Eliminating the redundancies in the transmit–receive element combinations and firing events reduces the overall system complexity and improves the frame rate. Here we explore four reduced redundancy 2-D array configurations for miniature 3-D ultrasonic imaging systems. Our approach is based on 1) coarray design with reduced redundancy using different subsets of linear arrays constituting the 2-D transducer array, and 2) 3-D scanning using fan-beams (narrow in one dimension and broad in the other dimension) generated by the transmit linear arrays. We form the overall array response through coherent summation of the individual responses of each transmit–receive array pairs. We present theoretical and simulated point spread functions of the array configurations along with quantitative comparison in terms of the front-end complexity and image quality.      

Designing and implementing the transition to a fully digitalhospital

The increase in the number of examinations performed in modern healthcare institutions in conjunction with the range of imaging modalities available today have resulted in a tremendous increase in the number of medical images generated and has made the need for a dedicated system able to acquire, distribute, and store medical image data very attractive. Within the framework of the Hellenic R&D program, we have designed and implemented a picture archiving and communication system for a high-tech cardiosurgery hospital in Greece. The system is able to handle in a digital form images produced from ultrasound, X-ray angiography, γ-camera, chest X-rays, as well as electrocardiogram signals. Based on the adoption of an open architecture highly relying on the DICOM standard, the system enables the smooth transition from the existing procedures to a fully digital operation mode and the integration of all existing medical equipment to the new central archiving system     

High resolution real-time medical imaging based on parallel beamforming technique

Improvement of frame-rate is very important for high quality ultrasound imaging of fast-moving structures. It is also one of the key technologies of Three-Dimension (3-D) real-time medical imaging. In this paper, we have demonstrated a beamforming method which gives imaging frame-rate increment without sacrificing the quality of medical images. By using wider and fewer transmit beams in combination with four narrower parallel receive beams, potentially increasing the imaging frame-rate by a factor four. Through employing full transmit aperture, controlling the mainlobe width, and suppressing sidelobes of angular responses, the inherent gain loss of normal parallel beamfomer can be compensated in the maximal degree. The noise and interference signals also can be suppressed effectively. Finally, we show comparable lateral resolution and contrast of ultrasound images to normal single widow weighting beamformer on simulated phantoms of point targets, cyst and fetus of 12th week. As the computational cost is linear with the number of array elements and the same with Delay And Sum (DAS) beamformers, this method has great advantages of possibility for high frame-rate real-time applications.     

Biological effects of ultrasound—A review

The history of ultrasound related to bioeffects studies begins in the 1920's and continues to the present. Because of the type of effects exhibited between high-frequency mechanical waves and biological systems, there emerged a low-intensity medical therapeutic regime using ultrasound in the 1930's. In the late 1940's and early 1950's, research began on the use of ultrasound in the pulse-echo mode to visualize soft tissues of the human body. The interaction of ultrasound and tissue in this mode underlies the present expanding capability of its medical diagnostic use. During the same time period, the use of intense, sometimes highly focused ultrasound was explored for its interactive effects on tissue. Out of these bioeffects studies emerge possibilities for therapy and some preliminary clinical trials have been conducted. Bioeffects studies involve assessment of thermal, cavitational, and other mechanical entities in their contribution to the interaction with ultrasound. Considerable progress has been made so that in some modes, it is possible to predict the magnitude of a given effect by computational means. In other modes the reliably, predictable effects have not yet become tractable by computational means alone.     

Time-domain optimized near-field estimator for ultrasound imaging: initial development and results

For nearly four decades, adaptive beamforming (ABF) algorithms have been applied in RADAR and SONAR signal processing. These algorithms reduce the contribution of undesired off-axis signals while maintaining a desired response along a specific look direction. Typically, higher resolution and contrast is attainable using adaptive beamforming at the price of an increased computational load. In this paper, we describe a novel ABF designed for medical ultrasound, named the Time-domain Optimized Near-field Estimator (TONE). We performed a series of simulations using synthetic ultrasound data to test the performance of this algorithm and compared it to conventional, data independent, delay and sum beamforming (CBF) method. We also performed experiments using a Philips SONOS 5500 phased array imaging system. CBF was applied using the default parameters of the Philips scanner, whereas TONE was applied on per channel, unfocused data using an unfocused transmit beam. TONE images were reconstructed at a sampling of 67 microm laterally and 19 microm axially. The results obtained for a series of five 20-microm wires in a water tank show a significant improvement in spatial resolution when compared to CBF. We also analyzed the performance of TONE as a function of speed of sound errors and array sparsity, finding it robust to both.     

相干宽带线性调频信号的波达方向估计新方法

提出了一种相干宽带线性调频(LFM)信号的波达方向(DOA)估计新方法。该方法利用LFM信号在分数阶Fourier域上的解线调特性,构造出新的解线调域阵列数据模型,然后结合传统的矩阵重构解相干以及MUSIC算法实现相干LFM信号的DOA估计。若同时存在多组相干LFM信号入射,则首先在不同的能量聚集域上将各信号组分离,然后逐一进行各组内相干信号的DOA估计。该方法充分地挖掘了观测信号所包含的时频信息,增加了可检测的DOA数目,提高了分辨性能和抗噪声性能。此外,该方法无冗余阵元与孔径损失,且适用于任意流型阵列。仿真结果显示,在DOA估计的均方根误差(RMSE)相同时,与传统方法相比,本方法可获得8dB左右的信噪比增益。     

Volume rendering of 3D medical ultrasound data using direct feature mapping

The authors explore the application of volume rendering in medical ultrasonic imaging. Several volume rendering methods have been developed for X-ray computed tomography (X-CT), magnetic resonance imaging (MRI) and positron emission tomography (PET). Limited research has been done on applications of volume rendering techniques in medical ultrasound imaging because of a general lack of adequate equipment for 3D acquisitions. Severe noise sources and other limitations in the imaging system make volume rendering of ultrasonic data a challenge compared to rendering of MRI and X-CT data. Rendering algorithms that rely on an initial classification of the data into different tissue categories have been developed for high quality X-CT and MR-data. So far, there is a lack of general and reliable methods for tissue classification in ultrasonic imaging. The authors focus on volume rendering methods which are not dependent on any classification into different tissue categories. Instead, features are extracted from the original 3D data-set, and projected onto the view plane. The authors found that some of these methods may give clinically useful information which is very difficult to get from ordinary 2D ultrasonic images, and in some cases renderings with very fine structural details. The authors have applied the methods to 3D ultrasound images from fetal examinations. The methods are now in use as clinical tools at the National Center of Fetal Medicine in Trondheim, Norway.     

Linear Superposition Electrical Impedance Tomography Imaging With Multiple Electrical/Biopsy Probes

In medical diagnostics, tissue is often examined with multiple discrete biopsies taken under ultrasound placement. In a previous theoretical study, we have suggested that the linear nature of the equations used in electrical impedance tomography (EIT) can be employed with the conventional practice of biopsy sampling to produce an image of the tissue between the biopsy samplings. Specifically, the biopsy probes can be used to record EIT-type electrical data during the discrete tissue sampling. The location of the discrete biopsy needle insertions available from the ultrasound placement of the probes can be combined with the electrical measurement data and used with linear superposition to produce a complete EIT image of the tissue between the sampled sites. In this study, we explore the concept experimentally using gel phantoms to simulate tissue and heterogeneities in the tissue. The experiments are performed in 2-D and 3-D configurations, and data are taken discretely, one at a time, through single electrical probe insertions. In the 2-D configuration, we were able to produce images of reasonable quality for heterogeneities with a diameter larger than 3 mm (conductivity ratio 1:5) and with relative conductivity differences above 50% (diameter 5 mm).      

高冗余光纤光栅传感网络的设计与实验研究

结合无源光学器件,提出并设计了一种新型的高冗余光纤布拉格光栅(FBG)传感模块,并将设计的FBG传感模块与波分复用技术相结合,构建了高冗余FBG传感网络.以长方形铝合金板为研究对象,对高冗余FBG传感网络的可靠性进行研究,理论比较并实验分析了高冗余FBG传感阵列的适用性与可靠性.研究结果表明,利用光开关在传感阵列支路之间的切换,使得FBG传感网络更具有冗余性.这一方面能够解决使用过程中多个部位出现故障导致的某些FBG传感模块无法被计算机检测到的问题,有效提高了传感系统的可靠性、容错性;另一方面为工程应用中结构健康监测以及特殊部位监测提供了一种有效可行的监测手段.     

阵列探测器在成像光谱偏振探测技术中的应用

为了在成像光谱偏振仪中应用近红外焦平面阵列探测器,得到高质量图像信息,结合新型弹光调制型成像光谱偏振探测技术(PEM-ISP),提出了一种基于近红外焦平面阵列探测器的成像光谱偏振探测技术。系统采用FPA-640512 InGaAs焦平面阵列探测器作为光学探测接收元件,采用高速现场可编程门阵列(FPGA)作为信号处理单元,做到对光学信号的快速采集与并行处理,满足高速、实时的信号传输与处理技术等要求。将经高速A/D采集得到的数据存储到FPGA外扩的静态随机存储器中,以保证数据的完整性。数据最终通过通用串行总线传至上位机,上位机通过LabVIEW实现图像还原。结果表明,该系统可以应用于PEM-ISP中,实现对测量信号的精确探测与采集,并得到完整的图像信息。     

微波致热超声信号时频特征及其影响因素

微波致热超声成像是一种兼顾了微波和超声成像两方面长处的新型生物医学成像方法。本文首先给出了一个微波致热超声（Microwave-Induced Thermo-Acoustic,MITA）实验系统,然后结合信号理论和MITA机制,讨论微波脉冲激励源和样品尺寸这两方面因素对MITA信号的时域波形和频域分布的影响。由实验数据和理论分析得出MITA信号时域波形强度与入射微波脉冲宽度和样品截面大小成正比;MITA信号的频率分布除了与激励微波脉冲频谱有关外,更主要的特性是其频谱分布中心与样品厚度所确定的超声波本振频率存在对应关系。     

Optical phase conjugation (OPC) for focusing light through/inside biological tissue

Optical phase conjugation(OPC) is a technique that generates a light field with reversed wavefront and identical amplitude distribution as the incident light. It has a unique feature of suppressing the aberration of incident beam induced by inhomogeneous or disturbing medium. Although this technique has been extensively studied since the 1970s, it has become more attractive because of unprecedented achievements and prospective potentials in biomedical applications. OPC-based techniques have been successfully utilized to form a focus through/inside highly scattered biological samples. It opens a new avenue by significantly enhancing the light delivery in biological tissue for high-resolution imaging, diagnosis and treatment of medical diseases. In order to provide insight into its further development, recent progress of OPC techniques for focusing light through/inside biological tissue was summarized.     

Constrained least squares filtering algorithm for ultrasound image deconvolution

A new medical ultrasound tissue model is considered in this paper, which incorporates random fluctuations of the tissue response and provides more realistic interpretation of the received pulse-echo ultrasound signal. Using this new model, we propose an algorithm for restoration of the degraded ultrasound image. The proposed deconvolution is a modification of the classical regularization technique which combines Wiener filter and the constrained least squares (LS) algorithm for restoration of the ultrasound image. The performance of the algorithm is evaluated based on both the simulated phantom images and real ultrasound radio frequency (RF) data. The results show that the algorithm can provide improved ultrasound imaging performance in terms of the resolution gain. The deconvolved images visually show better resolved tissue structures and reduce speckle, which are confirmed by a medical expert.     

Reconstruction from NMR data acquired with imaging gradients having arbitrary time dependence

An algebraic reconstruction method for NMR imaging from data acquired with imaging gradients having essentially arbitrary time dependence is presented. With reasonable discretization, an overdetermined linear system is obtained for which there exist simple factorized 1-inverses of the coefficient matrices; a computationally efficient reconstruction scheme then follows. Some numerical simulations are also described.     

A distributed differentially encoded OFDM scheme for asynchronous cooperative systems with low probability of interception

Recently, Li, Hwu and Ratazzi have proposed a physical-layer security design to guarantee low probability of interception (LPI) for MIMO systems without relying on upperlayer data encryption. The proposed scheme utilizes antenna array redundancy to deliberately randomize the transmitted signals to prevent eavesdropping. Motivated by their idea, in this paper we design a physical-layer transmission scheme to achieve LPI in cooperative systems. There are two major differences in cooperative systems: 1) each relay node may have only one antenna that can not provide antenna array redundancy for signal randomization; 2) there may exist timing errors due to the asynchronous nature of cooperative systems. Considering the two differences, we propose a distributed differentially encoded OFDMtransmission scheme with deliberate signal randomization to prevent eavesdropping and exploit the available spatial and frequency diversities in asynchronous cooperative systems. We use diagonal unitary codes to perform the differential encoding in the frequency domain over subcarriers within each OFDM block, or we use general (not necessarily diagonal) unitary codes to perform the differential encoding in the frequency domain across several OFDM blocks. By some deliberate signal randomization, the eavesdropper can not detect the transmitted symbols, while the authorized receiver can perform differential decoding successfully without the knowledge of the channels or the timing errors.     

Frequency decomposition and compounding of ultrasound medical images with wavelet packets

Ultrasound beams propagating in biological tissues undergo distortions due to local inhomogeneities of the acoustic parameters and the nonlinearity of the medium. The spectral analysis of the radio-frequency (RF) backscattered signals may yield important clinical information in the field of tissue characterization, as well as enhancing the detectability of tissue parenchymal diseases. In this paper, we propose a new tissue spectral imaging technique based on the wavelet packets (WP) decomposition. In a conventional ultrasound imaging system, the received echo-signals are generally decimated to generate a medical image, with a loss of information. With the proposed approach, all the RF data are processed to generate a set of frequency subband images. The ultrasound echo signals are simultaneously frequency decomposed and decimated, by using two quadrature mirror filters, followed by a dyadic subsampling. In addition, to enhance the lesion detectability and the image quality, we apply a nonlinear filter to reduce noise in each subband image. The proposed method requires simple additional signal processing and it can be implemented on any real-time imaging system. The frequency subband images, which are available simultaneously, can be either used in a multispectral display or summed up together to reduce speckle noise. To localize the different frequency response in the tissues, we propose a multifrequency display method where three different subband images, chosen among those available, are encoded as red, green, and blue intensities (RGB) to create a false-colored RGB image. According to the clinical application, different choices can evidence different spectral proprieties in the biological tissue under investigation. To enhance the lesion contrast in a grey-level image, one of the possible methods is the summation of the images obtained from narrow frequency subbands, according to the frequency compounding technique. We show that by adding the denoised subband images created with the WP decomposition, the contrast-to-noise ratio in two phantom images is largely increased.     

Array redundancy for active line arrays

Active imaging arrays are used to image scenes composed of reflectors of transmitted radiation, and in many such applications, line arrays are employed. In this paper, we discuss scanned active line arrays for imaging based on image synthesis. We define the novel concept of array redundancy for active arrays, analogous to the well-known concept of redundancy applied to passive arrays, and we define and give examples of minimum redundancy and reduced redundancy line arrays composed of transmit/receive elements. Such arrays differ from their passive imaging counterparts both in geometry and in element count.     

Thin linear thermometer arrays for use in localized cancer hyperthermia

A thin linear array of silicon diodes has been developed to measure one-dimensional temperature profiles in tissue during treatment of cancer by localized hyperthermia. The array is composed of six discrete diodes on three flexible stainless steel wires, with a maximum cross-sectional dimension of 0.5 mm, so that it can be introduced into a tumor area via a small puncture wound. Temperature data are extracted using an external electronics system under microprocessor control; the present overall accuracy of the system is 0.2°C over the range of 37-45°C. The array has been tested in ultrasound, RF, and microwave heating fields. Computer simulation shows this array to be nonperturbing of the thermal field in tissue, so that it can provide accurate temperature data. Development of a batch-fabricated array of twenty diodes on five leads is under way. A monolithic silicon diode array is shown to produce large temperature perturbations because of its high thermal conductivity, while an alternative technology using silicon micromachining and adaptations of existing techniques for lead fabrication should produce an array of low thermal conductivity which can obtain accurate measurements. The present and future arrays should also be suitable for data collection in many in-vivo situations other than hyperthermia.     

基于数学形态学与自适应的超声医学图像滤波方法的研究

超声医学成像作为主要的医学影像技术之一,因其对人体无伤害、实时、价格便宜和使用方便等优点已广泛应用于临床.然而,在成像过程中形成的特有的图像斑点,使得对比度弱的人体软组织中的正常组织和病变组织不易区分,给临床诊断和医学研究带来不便.针对医学超声图像的特点,在研究了几种常用滤波方法后,提出一种自适应中值滤波和形态滤波结合的新方法,并做了实验验证.实验方法是:首先对所选择的医学超声图像施加瑞利噪声,然后采用中值滤波、自适应中值滤波的方法对被污染的图像进行去噪处理,接下来先采用自适应中值滤波对图像进行预处理,抑制斑点噪声,保留必要细节,再采用数学形态学方法进行二次滤波和增强对比度,进一步改善图像质量.最后从去噪图像和评价指标上对三种滤波去噪方法进行了比较.实验证明,新方法优于其他方法.