医学超声成像中的编码激励技术及其应用

与传统的脉冲回波成像技术相比 ,编码激励技术能够提高超声图像的信噪比或相应提高声束的穿透深度。本文首先介绍了编码激励成像系统的原理 ,并对医学超声诊断系统中的发射声强进行了分析 ,得出了采用编码激励技术提高超声图像信噪比的上限。然后阐述了发射编码的选取原则 ,详细介绍了白噪编码、伪随机码、Golay码、Chirp和伪 Chirp码等编码技术。最后 ,介绍了编码激励技术在 B模式成像、Doppler血流测量、B- flow成像和谐波成像中的应用。     

Chirp编码激励磁声成像信号处理方法研究

目的磁声信号信噪比低限制了磁声成像的图像质量,本研究提出Chirp脉冲编码激励的磁声成像信号处理方法,以提高磁声信号信噪比,缩短信号处理时间。方法本研究通过仿真计算和磁声信号的实验测量,对不同脉冲宽度的Chirp信号编码激励的磁声信号进行了研究。结果对于体外实验猪肉与金属丝模型,编码激励明显提高磁声信号信噪比,10μs、50μs、100μs的Chirp激励,磁声信号信噪比相比于单脉冲激励分别提高7. 65倍、42倍和90. 1倍。同时处理时间明显缩短,100μs的Chirp激励下处理时间相比于单脉冲平均方法处理时间缩短为原来的1. 2%。结论本研究的脉冲编码处理方法对于提高磁声信号信噪比,改善成像质量,提高整体成像效率,具有重要意义。     

二维数字化超声软组织应变成像系统的研制

利用超声散射回波测量生物组织在静态压缩情况下弹性系数的超声弹性成像方法得到了很大的发展.利用国产高速数据采集卡以及商业线阵B型超声诊断仪,建立了一套二维数字化超声软组织应变成像系统.应用这一系统获得了数字化的射频组织超声散射回波信号,并对组织内的应变分布进行成像.实验结果表明,该应变成像系统在获得传统B型超声图像的同时,可以获得组织内部的应变分布图像,从而获取在B超图像上无法得到的组织弹性变化.这一系统的研制成功,不仅为超声弹性成像技术在医学临床上的应用研究打下了基础,同时也为扩宽普通商业B超的应用范围提供了途径.     

高频超声成像中的编码激励研究

本论文实现了基于编码激励的高频超声成像实验系统并对编码激励在实际应用中的性质进行了定量实验研究.以中心频率为10 MHz的眼科B超为实验平台,实现了一个适用于各种二进制编码激励的超声成像系统.系统使用FPGA实现了编码激励实时成像处理,并通过数据采集卡获得射频回波数据.实验表明,采用编码激励可以显著增加系统信噪比,降低发射电压.在单反射面情况下,15 V发射电压的编码激励回波的信噪比与70 V发射电压的单脉冲激励回波的信噪比相当,脉冲压缩的旁瓣水平在-30 dB以下.在超声体模实验中,20 V发射电压的编码激励回波的信噪比水平及穿透深度与60 V发射电压的单脉冲激励系统相当.编码激励实验结果为实现低发射电压的便携式超声诊断系统提供了实验基础.     

超声成像新技术及其临床应用

超声成像是临床上应用最广泛的医学成像模式之一.近年来,随着电子技术、计算机技术的发展,超声成像设备在成像方法和技术等层面上不断得到改进,临床诊断能力也得到进一步提高.本文主要介绍二维和三维超声成像的新技术及其临床应用,包括自适应图像处理、编码激励、大视野成像、空间复合成像、三维成像技术等.     

超声弹性成像用于高强度聚焦超声损伤的检测

采用超声弹性成像的方法检测高强度聚焦超声在动物组织内产生的损伤.结果表明,聚焦超声的损伤在应变分布图(弹性图像)上表现为应变较小的区域,即损伤部位的弹性模量较周围组织的大,组织切片的损伤范围和大小也与应变分布图的结果一致.     

医学超声关键技术研究和进展(二)超声弹性成像技术、超声图像处理与分析

超声弹性成像技术是近20年来在医学超声领域出现的新技术,它利用准静态压力、外加低频振动、声辐射力等形式激励组织振动,然后用超声方法检测组织的形变或者由振动引起的剪切波传播,最终得到组织的生物力学特性.超声弹性成像已经被广泛应用在乳腺、肝脏、前列腺等领域,显示了良好的临床应用前景.此外,超声图像处理与分析是超声成像系统的重要组成部分,主要包括超声图像的三维可视化、预处理、分割、配准及图像理解等,旨在为临床医生诊断提供有效的辅助手段.     

基于Golay-Chirp编码信号的松质骨BUA参数测量方法的研究

宽带超声衰减是反映松质骨中骨矿密度的一个重要参数,常用超声脉冲波进行测量.由于透过骨骼的超声波衰减严重,接收信号十分微弱并带有强噪声,从而限制了测量深度,并容易产生测量误差.编码信号发射技术是提高信噪比和增加超声透射深度的有效途径.本文提出一种新的编码方法,它用Chirp信号调制Golay互补码,产生称之为Golay-Chirp的编码信号.它相对于传统的Golay信号、Chirp信号等有更好的抗干扰能力,并可以进一步降低发射功率或增加超声透射深度.     

超声弹性成像中应变滤波器的理论及其发展

弹性成像能够提供组织弹性这一基本的力学属性,具有很广阔的应用前景。应变滤波器是超声弹性成像的重要理论框架,可广泛应用于超声弹性成像效果的评估、分析和预测。本文详细综述了应变滤波器的理论和发展,并对文献中的一些笔误作了修正,对实现的一些细节作了说明。通过对应变滤波器的理解,可以加深对超声弹性成像效果的理解,并对采用合适的方法和参数以获得最佳的成像效果以指导。     

超声弹性成像技术研究现状

超声弹性成像(ultrasound elastography,UE)是以软组织的弹性参量为对象的一种新的成像技术,它弥补了传统超声成像技术不能提供生态学特性的不足,拓宽了超声图像在肿瘤探测及扩散疾病成像方面的应用,具有非常重要的临床应用价值。本文详细介绍了超声弹性成像技术的实现原理、研究现状以及常用的临床衡量指标,如对比度传输率、应变滤波器、时间延时等,并对其发展做出展望。     

Application of laser pulse stretching scheme for efficiently delivering laser energy in photoacoustic imaging

High-energy and short-duration laser pulses are desirable to improve the photoacoustic image quality when imaging deeply seated lesions. In many clinical applications, the high-energy pulses are coupled to tissue using optical fibers. These pulses can damage fibers if the damage threshold is exceeded. While keeping the total energy under the Food and Drug Administration limit for avoiding tissue damage, it is necessary to reduce the peak intensity and increase the pulse duration for minimizing fiber damage and delivering sufficient light for imaging. We use laser-pulse-stretching to address this problem. An initial 17-ns pulse was stretched to 27 and 37 ns by a ring-cavity laser-pulse-stretching system. The peak power of the 37-ns stretched pulse reduced to 42% of the original, while the fiber damage threshold was increased by 1.5-fold. Three ultrasound transducers centered at 1.3-, 3.5-, and 6-MHz frequencies were simulated, and the results showed that the photoacoustic signal of a 0.5-mm-diameter target obtained with 37-ns pulse was about 98, 91, and 80%, respectively, using the same energy as the 17-ns pulse. Simulations were validated using a broadband hydrophone. Quantitative comparisons of photoacoustic images obtained with three corresponding transducers showed that the image quality was not affected by stretching the pulse.     

全数字超声成像中采用双脉冲激励时动态滤波器的设计

目的分析数字超声成像中采用双脉冲单极性激励时,超声回波的频谱及其随深度的变化趋势。方法根据在不同深度利用回波的不同频率的分量进行成像的滤波器匹配思想,设计了一组动态滤波器。既保证了近场图像分辨力又能兼顾远场图像的灵敏度。结果此滤波器在现场可编程门阵列（FPGA）芯片中实现。结论研究证明在全数字超声成像设备中,使用此滤波器得到的超声图像比使用常值滤波器得到的超声图像质量有所提高。     

Influence of x-ray pulse parameters on the image quality for moving objects in digital cardiac imaging

The image quality of a single frame in a modern cardiac imaging x-ray facility can be improved by adjusting the automatic pulse exposure parameters. The effects of acquisition rate on patient dose and the detectability of moving objects have been fully described in scientific literature. However, the influence of automatic pulse exposure parameters is still to be determined. Images of a moving wheel (with lead wires) were acquired using an H5000 Philips Integris cardiac x-ray system. Poly(methylmethacrylate) plastic samples 20 and 30 cm thick were employed as the build-up phantom to simulate a patient. The images were obtained using preset clinical parameters for cardiac imaging procedures. The signal detectability and motion blur of a contrast bar at a transversal speed in the range of 100-150 mm/s were evaluated with a cine pulse width of 3, 5, 7, and 10 ms under automatic mA kV regulation. Two levels of exposure at the image intensifier entrance were included in this study. Signal detectability was analyzed in terms of the signal-to-noise ratio (SNR) and the value of SNR2/entrance surface dose. The blurring was modeled as a Gaussian-shaped blurring function, and the motion blur was expressed in terms of the peak full width at half maximum and amplitude (apparent contrast) of the resolution functions. A contrast bar simulating a vessel in motion at the maximum velocities of typical cardiac structures was exposed. Severe loss of image quality occurred at pulse widths > or =7 ms. It is also shown that below 5 ms static nonlinearities, likely caused by the need to use a large focus for cine acquisition, dominate the blurring process.     

Retrospective motion gating in cardiac MRI using a simultaneously acquired navigator

A simultaneous acquisition technique of image and navigator signals (simultaneously acquired navigator, SIMNAV) is proposed for cardiac magnetic resonance imaging (CMRI) in Cartesian coordinates. To simultaneously acquire both image and navigator signals, a conventional balanced steady‐state free precession (bSSFP) pulse sequence is modified by adding a radiofrequency (RF) pulse, which excites a supplementary slice for the navigator signal. Alternating phases of the RF pulses make it easy to separate the simultaneously acquired magnetic resonance data into image and navigator signals. The navigator signals of the proposed SIMNAV were compared with those of current gating devices and self‐gating techniques for seven healthy subjects. In vivo experiments demonstrated that SIMNAV could provide cardiac cine images with sufficient image quality, similar to those from electrocardiogram (ECG) gating with breath‐hold. SIMNAV can be used to acquire a cardiac cine image without requiring an ECG device and breath‐hold, whilst maintaining feasible imaging time efficiency.     

快速动态自动匀场技术在永磁磁共振成像系统中的实现

目的：为满足临床的需求，在万东医疗i_Open 0．36T永磁开放式磁共振成像MRI系统上发展实现一种快速动态自动匀场技术。方法：利用梯度回波成像技术得到恰当的匀场电流提高丰磁场B13均匀度。结果：（1）视野（FOV）内的磁场均匀度显著提高。（2）匀场后MRI图像质罱得到显著改善。结论：该方法能够满足临床动态实时匀场的需要。     

Assessing image quality in effective Poisson's ratio elastography and poroelastography: II

Poroelastography is a novel elastographic technique for imaging the time variation of the mechanical behaviour of poroelastic materials. Poroelastograms are generated as a series of time-sequenced effective Poisson's ratio (EPR) elastograms, obtained from the imaged material under sustained compression. In the companion report (Righetti et al 2007 Phys. Med. Biol. 52 1303), we investigated image quality of EPR elastography starting from a theoretical analysis of the performance limitations of axial strain elastography and lateral strain elastography. In this report, we extend this analysis to poroelastography. The theoretical analysis reported in these two companion papers allows understanding the performance limitations of these novel techniques and identifying the fundamental parameters that control their signal-to-noise ratio, contrast-to-noise ratio and resolution. The results of these studies also indicate that EPR elastograms and poroelastograms of reasonable image quality can be generated in practical applications that may be of clinical interest provided that advanced elastographic techniques in combination with other commonly employed imaging methods to increase signal-to-noise and contrast-to-noise ratios are used.     

Rapid imaging of free radicals in vivo using field cycled PEDRI

Imaging of free radicals in vivo using an interleaved field-cycled proton-electron double-resonance imaging (FC-PEDRI) pulse sequence has recently been investigated. In this work, in order to reduce the EPR (electron paramagnetic resonance) irradiation power required and the imaging time, a centric reordered snapshot FC-PEDRI pulse sequence has been implemented. This is based on the FLASH pulse sequence with a very short repetition time and the use of centric reordering of the phase-encoding gradient, allowing the most significant free induction decay (FID) signals to be collected before the signal enhancement decays significantly. A new technique of signal phaseshift correction was required to eliminate ghost artefacts caused by the instability of the main magnetic field after field cycling. An FID amplitude correction scheme has also been implemented to reduce edge enhancement artefacts caused by the rapid change of magnetization population before reaching the steady state. Using the rapid pulse sequence, the time required for acquisition of a 64 x 64 pixel FC-PEDRI image was reduced to 6 s per image compared with about 2.5 min with the conventional pulse sequence. The EPR irradiation power applied to the sample was reduced by a factor of approximately 64. Although the resulting images obtained by the rapid pulse sequence have a lower signal to noise than those obtained by a normal interleaved FC-PEDRI pulse sequence, the results show that rapid imaging of free radicals in vivo using snapshot FC-PEDRI is possible.     

Improved cine displacement-encoded MRI using balanced steady-state free precession and time-adaptive sensitivity encoding parallel imaging at 3 T

Cine displacement-encoded MRI is a promising modality for quantifying regional myocardial function. However, it has two major limitations: low signal-to-noise ratio (SNR) and data acquisition efficiency. The purpose of this study was to incrementally improve the SNR and the data acquisition efficiency of cine displacement-encoded MRI through the combined use of balanced steady-state free precession (b-SSFP) imaging, 3T imaging, echo-combination image reconstruction, and time-adaptive sensitivity encoding (TSENSE) parallel imaging. Phantom experiments were performed to empirically determine the optimal excitation angle (alpha) and to estimate the measurement errors in the presence of 130 Hz peak-to-peak static magnetic field (B0) variation. The optimal alpha was determined to be 20 degrees . The intrinsic phase correction in the echo-combination effectively reduced the phase error, which produced small displacement errors (0.11 versus 0.11 mm) and negligible strain errors (-0.001 versus -0.002). Six healthy volunteers were imaged in three short-axis levels of the heart to evaluate the SNR and the relative accuracy of strain calculations. Compared with the 24-heartbeat cine echo-planar imaging acquisition, the 24-heartbeat non-accelerated b-SSFP acquisition yielded approximately 65% higher SNR, and the 12-heartbeat twofold accelerated b-SSFP acquisition yielded approximately 28% higher SNR. The 12-heartbeat twofold accelerated b-SSFP acquisition yielded functional maps with spatial resolution of 3.6 x 3.6 mm, temporal resolution of 35 ms, and relatively high SNR (31.2 +/- 5.4 at end diastole; 19.9 +/- 3.6 at end systole; 10.3 +/- 1.1 at late diastole; mean +/- SD). The left ventricular strain values between the non-accelerated and twofold accelerated b-SSFP acquisitions correlated strongly (slope = 0.99; bias = 0.00; R2 = 0.91) and were in excellent agreement. The combined implementation of b-SSFP imaging, 3T imaging, echo-combination image reconstruction, and TSENSE parallel imaging can be used to incrementally improve the cine displacement-encoded MRI pulse sequence.     

Free of speckle ultrasonic imaging of soft tissue with account of second harmonic signal

The Born approximation deconvolved inverse scattering imaging technique is an alternative to the conventional pulse-echo method. This novel technique deconvolves the incident pulse from the reflected pulse, and uses the resulting impulse response to produce an image of the acoustic impedance distribution. It is applicable mainly to structures that resemble a layered medium. The images captured by this method prove to have improved resolution and are free of speckle. With this method one can use ultrasound of lower frequencies than would be required by the pulse-echo method to achieve the same resolution. To provide further improvement of images the second harmonic signals can be employed. Here we describe the deconvolved inverse scattering imaging technique with account of the second harmonic signal. For this purpose the hybrid transducer by Krautkramer Branson Co., which consists of a cylindrical 5 MHz transducer wrapped in an annulus-shaped 2.5 MHz transducer, has been used. The phantom and soft tissue were imaged and in both cases the account of the second harmonic reflection data provides an improvement of the image quality.     

The quest for high spatial resolution diffusion‐weighted imaging of the human brain in vivo

Diffusion‐weighted imaging, a contrast unique to MRI, is used for assessment of tissue microstructure in vivo. However, this exquisite sensitivity to finer scales far above imaging resolution comes at the cost of vulnerability to errors caused by sources of motion other than diffusion motion. Addressing the issue of motion has traditionally limited diffusion‐weighted imaging to a few acquisition techniques and, as a consequence, to poorer spatial resolution than other MRI applications. Advances in MRI imaging methodology have allowed diffusion‐weighted MRI to push to ever higher spatial resolution. In this review we focus on the pulse sequences and associated techniques under development that have pushed the limits of image quality and spatial resolution in diffusion‐weighted MRI.