Barker码激励超声导波在长骨检测中的应用

单脉冲激励的超声导波在长骨中传播时,信号的衰减大,导致接收信号的幅度很小,且噪声严重。Barker码激励能有效增大接收信号幅度,提高信噪比(SNR)。将其应用到超声导波长骨检测中,进行仿真和长骨实验,得到的信号分别用加权匹配滤波器和有限冲激响应-最小均方误差(FIR-LS)逆滤波器进行压缩,并与单脉冲激励的结果进行了对比。结果表明,对于13位的Barker码,采用加权匹配滤波器进行解码时,压缩信号幅度是单脉冲激励接收信号的13倍;而FIR-LS逆滤波器则达到-63.59 dB的峰值旁瓣水平(PSL),更好地抑制噪声。说明可以将Barker码激励超声导波应用于长骨的检测中。      

Estimation of ultrasonic attenuation in a bone using coded excitation

This paper describes a novel approach to estimate broadband ultrasound attenuation (BUA) in a bone structure in human in vivo using coded excitation. BUA is an accepted indicator for assessment of osteoporosis. In the tested approach a coded acoustic signal is emitted and then the received echoes are compressed into brief, high amplitude pulses making use of matched filters and correlation receivers. In this way the acoustic peak pressure amplitude probing the tissue can be markedly decreased whereas the average transmitted intensity increases proportionally to the length of the code. This paper examines the properties of three different transmission schemes, based on Barker code, chirp and Golay code. The system designed is capable of generating 16 bits complementary Golay code (CGC), linear frequency modulated (LFM) chirp and 13-bit Barker code (BC) at 0.5 and 1 MHz center frequencies. Both in vivo data acquired from healthy heel bones and in vitro data obtained from human calcaneus were examined and the comparison between the results using coded excitation and two cycles sine burst is presented. It is shown that CGC system allows the effective range of frequencies employed in the measurement of broadband acoustic energy attenuation in the trabecular bone to be doubled in comparison to the standard 0.5 MHz pulse transmission. The algorithm used to calculate the pairs of Golay sequences of the different length, which provide the temporal side-lobe cancellation is also presented. Current efforts are focused on adapting the system developed for operation in pulse-echo mode; this would allow examination and diagnosis of bones with limited access such as hip bone.     

自纠错码在PWAM光纤传输系统中的应用

基于自纠错码的二级脉冲幅度调制与脉冲宽度调制(PWAM)光纤传输技术,对系统中的接收机灵敏度进行了分析.在模拟信号信噪比不小于50 dB,数字信号误码率不低于10－9情况下进行仿真.结果表明,在没有增加系统复杂性的前提下,使用自纠错码时二级脉冲幅度调制以及脉冲调制光纤接收机灵敏度将有1.1 dB的改善.     

Influence of the ultrasound transducer bandwidth on selection of the complementary Golay bit code length

In contrast to previously published papers [A. Nowicki, Z. Klimonda, M. Lewandowski, J. Litniewski, P.A. Lewin, I. Trots, Comparison of sound fields generated by different coded excitations – Experimental results, Ultrasonics 44 (1) (2006) 121–129; J. Litniewski, A. Nowicki, Z. Klimonda, M. Lewandowski, Sound fields for coded excitations in water and tissue: experimental approach, Ultrasound Med. Biol. 33 (4) (2007) 601–607], which examined the factors influencing the spatial resolution of coded complementary Golay sequences (CGS), this paper investigates the effect of ultrasound imaging transducer’s fractional bandwidth on the gain of the compressed echo signal for different spectral widths of the CGS. Two different bit lengths were considered, specifically one and two cycles. Three transducers having fractional bandwidth of 25%, 58% and 80% and operating at frequencies 6, 4.4 and 6 MHz, respectively were examined (one of the 6 MHz sources was focused and made of composite material). The experimental results have shown that by increasing the code length, i.e. decreasing the bandwidth, the compressed echo amplitude could be enhanced. The smaller the bandwidth was the larger was the gain; the pulse-echo sensitivity of the echo amplitude increased by 1.88, 1.62 and 1.47, for 25%, 58% and 80% bandwidths, respectively. These results indicate that two cycles bit length excitation is more suitable for use with bandwidth limited commercially available imaging transducers. Further, the time resolution is retained for transducers with two cycles excitation providing the fractional bandwidth is lower than approximately 90%. The results of this work also show that adjusting the code length allows signal-to-noise-ratio (SNR) to be enhanced while using limited (less that 80%) bandwidth imaging transducers. Also, for such bandwidth limited transducers two cycles excitation would not decrease the time resolution, obtained with “conventional” spike excitation. Hence, CGS excitation could be successfully implemented with the existing, relatively narrow band imaging transducers without the need to use usually more expensive wideband, composite ones.     

Large angle spin-echo imaging

A study was undertaken to assess the use of excitation flip angles greater than 90° for T₁ weighted spin-echo (SE) imaging with a single 180° refocusing pulse and short TR values. Theoretical predictions of signal intensity for SE images with excitation pulse angles of 90–180° were calculated based on the Bloch equations and then measured experimentally from MR images of MnCl₂ phantoms of various concentrations. Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110° excitation flip angles. The theoretical predictions showed significant improvements in SNR with excitation flip angles >90°, which were more pronounced at small TR values. The phantom studies showed reasonably good agreement with the theoretical predictions in correlating the excitation pulse angle with signal intensity. In the human imaging studies, the 110° excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CNR compared to the 90° pulse angle at TR = 275 ms. Increased signal intensity resulting from the use of large flip angle excitation pulses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans.     

1.5 T磁共振化学交换饱和转移成像的影响因素分析

本文探讨1.5 T磁共振化学交换饱和转移（Chemical Exchange Saturation Transfer,CEST）成像的影响因素.通过试管模型和临床病例,采用GE Signa HDe 1.5 T磁共振成像（Magnetic Resonance Imaging,MRI）扫描仪分别进行不同矩阵、激励次数、翻转角、磁化传递翻转角的CEST成像对比分析,以及不同激励次数、磁化传递翻转角的Z谱分析,并从成像组织、成像设备、成像技术等方面对原始图信号、酰胺质子转移（Amide Proton Transfer,APT）信号及Z谱进行分析研究.实验结果表明1.5 T MRI扫描仪的CEST图像信噪比相对较低,且磁场稳定性及均匀度影响了CEST成像的效果.在其他参数不变的情况下,降低采集矩阵和增加激励次数与翻转角可以增加原始图像信噪比.磁化传递翻转角为105°时,CEST成像效果最好.激励次数为2、磁化传递翻转角为105°时,所得数据符合组织Z谱情况.模型Z谱在磁化传递频率为-294~-194 Hz范围可显示30%谷氨酸（Glu）、碘剂（I₃₂₀）、纯水（H₂O）、肌酸（Cr）的信号差异,与H₂O差异最大处在-244~-214 Hz.原始图像信号30% I₃₂₀明显高于Glu、H₂O、Cr,Cr略低于Glu,APT图Cr略低于Glu.25例脑肿瘤的APT图呈高信号、12例脑梗塞的APT图呈低信号,CEST原始图像均可区分病变区域.有12例因采集时间、患者配合情况、环境及室温等影响导致CEST成像的失败.由此得出1.5 T场强下,CEST技术受到成像组织、设备、技术等因素的影响,需要进行多方面优化.在保证磁场稳定性及均匀度的情况下,优化参数的CEST成像和Z谱成像可以区分代谢物及其浓度.     

Comparison of sound fields generated by different coded excitations--experimental results

This work reports the results of measurements of spatial distributions of ultrasound fields obtained from five energizing schemes. Three different codes, namely, chirp signal and two sinusoidal sequences were investigated. The sequences were phase modulated with 13 bits Barker code and 16 bits Golay complementary codes. Moreover, two reference signals generated as two and sixteen cycle sine tone bursts were examined. Planar, 50% (fractional) bandwidth, 15 mm diameter source transducer operating at 2 MHz center frequency was used in all measurements. The experimental data were collected using computerized scanning system and recorded using wideband, PVDF membrane hydrophone (Sonora 804). The measured echoes were compressed, so the complete pressure field in the investigated location before and after compression could be compared. In addition to a priori anticipated increase in the signal to noise ratio (SNR) for the decoded pressure fields, the results indicated differences in the pressure amplitude levels, directivity patterns, and the axial distance at which the maximum pressure amplitude was recorded. It was found that the directivity patterns of non-compressed fields exhibited shapes similar to the patterns characteristic for sinusoidal excitation having relatively long time duration. In contrast, the patterns corresponding to compressed fields resembled those produced by brief, wideband pulses. This was particularly visible in the case of binary sequences. The location of the maximum pressure amplitude measured in the 2 MHz field shifted towards the source by 15 mm and 25 mm for Barker code and Golay code, respectively. The results of this work may be applicable in the development of new coded excitation schemes. They could also be helpful in optimizing the design of imaging transducers employed in ultrasound systems designed for coded excitation. Finally, they could shed additional light on the relationship between the spatial field distribution and achievable image quality and in this way facilitate optimization of the images obtained using coded systems.     

Quantification of guided mode propagation in fractured long bones

Guided modes propagation in intact, fractured and healing long bone has drawn significant research interests. However, mode quantifications for the direct comparison are still necessary to address. The aim of the study is to analyze the mode interaction with a notch-fracture in the long bone and find quantitative ultrasound parameters sensitive to depth and width variation of the fracture. We analyzed the impacts of the partially and completely diaphyseal osteotomy on fundamental guided modes propagation using the two-dimension finite-difference time-domain (2D-FDTD) simulations. The long bones were built as three layer models by a cortical plate embedded between overlying soft tissue and inner-coated marrow. Narrowband low-frequency sinusoids (100 kHz) were employed to only excite two fundamental guided modes. The mode amplitude variations were investigated as functions of the gap-breakage width and depth. It is found that the transverse fractures have strong influences on the anti-symmetric mode A0 transmission and reflection, whereas amplitudes of the symmetric mode S0 are not sensitive to the fracture degree. The quantitative results consistently indicate that reflection energy and transmission coefficients of the S0 and A0 modes can be used to quantify the mode interaction in the fractured long bone and further to evaluate long bone fracture status. Future study is needed to investigate the physical experiments on realistic fractured long bone and to insure that the proposed ultrasound parameters can be used to quantitatively evaluate the long bone fracture in clinical application.     

Parameter optimization of pulse compression in ultrasound imaging systems with coded excitation

A linear array imaging system with coded excitation is considered, where the proposed excitation/compression scheme maximizes the signal-to-noise ratio (SNR) and minimizes sidelobes at the output of the compression filter. A pulse with linear frequency modulation (LFM) is used for coded excitation. The excitation/compression scheme is based on the fast digital mismatched filtering. The parameter optimization of the excitation/compression scheme includes (i) choice of an optimal filtering function for the mismatched filtering; (ii) choice of an optimal window function for tapering of the chirp amplitude; (iii) optimization of a chirp-to-transducer bandwidth ratio; (iv) choice of an appropriate n-bit quantizer. The simulation results show that the excitation/compression scheme can be implemented as a Dolph–Chebyshev filter including amplitude tapering of the chirp with a Lanczos window. An example of such an optimized system is given where the chirp bandwidth is chosen to be 2.5 times the transducer bandwidth and equals 6 MHz: The sidelobes are suppressed to −80 dB, for a central frequency of 4 MHz, and to −94 dB, for a central frequency of 8 MHz. The corresponding improvement of the SNR is 18 and 21 dB, respectively, when compared to a conventional short pulse imaging system. Simulation of B-mode images demonstrates the advantage of coded excitation systems of detecting regions with low contrast.     

Maximizing MR signal for 2D UTE slice selection in the presence of rapid transverse relaxation

Ultrashort TE (UTE) sequences allow direct visualization of tissues with very short T2 relaxation times, such as tendons, ligaments, menisci, and cortical bone. In this work, theoretical calculations, simulations, and phantom studies, as well as in vivo imaging were performed to maximize signal-to-noise ratio (SNR) for slice selective RF excitation for 2D UTE sequences. The theoretical calculations and simulations were based on the Bloch equations, which lead to analytic expressions for the optimal RF pulse duration and amplitude to maximize magnetic resonance signal in the presence of rapid transverse relaxation. In steady state, it was found that the maximum signal amplitude was not obtained at the classical Ernst angle, but at an either lower or higher flip angle, depending on whether the RF pulse duration or amplitude was varied, respectively.     

Amplitude modulation control of escape from a potential well

We demonstrate the effectiveness of periodic amplitude modulations in controlling (suppressing and enhancing) escape from a potential well through the universal model of a damped Helmholtz oscillator subjected to an external periodic excitation (the escape-inducing excitation) whose amplitude is periodically modulated (the escape-controlling excitation). Analytical and numerical results show that this multiplicative control works reliably for different subharmonic resonances between the two periodic excitations involved, and that its effectiveness is comparable to those of different methods of additive control. Additionally, we demonstrate the robustness of the multiplicative control against the presence of low-intensity Gaussian noise.     

Performance evaluation of Fi-Wi network based on SCM–optical code division multiple access architecture

This paper investigates the performance of a spectral amplitude coding (SAC)–OCDMA system design utilizing subcarrier multiplexing techniques for use in the popular fiber-wireless (Fi-Wi) technology. Zero cross correlation code is employed because of its ability to eliminate phase induced intensity noise (PIIN) and the simplicity of using a direct detection technique at the receiving end. The performances are evaluated theoretically to derive the signal-to-noise ratio and results are presented in terms of bit error rates (BER). Results are analyzed to investigate the effect of code weight and variable data rates on the system performance. To validate the results, simulative analysis is also done using Optisys version 6.0. We demonstrated that the ZCC code performed better compared to other code for a BER floor of 10⁻⁹ with larger cardinality of subscribers. Results obviously indicate that system performance can be improved with larger code weight. In addition, the choice of lower bit rate is observed to perform better and more suitable for application in this hybrid Fi-Wi network.     

Thiol-Capped CdTe Quantum Dots with Two-Photon Excitation for Imaging High Autofluorescence Background Living Cells

To effectively image living cells with quantum dots (QDs), particularly for those cells containing high content of native fluorophores, the two-photon excitation (TPE) with a femto-second 800 nm laser was employed and compared with the single-photon excitations (SPE) of 405 nm and 488 nm in BY-2 Tobacco (BY-2-T) and human hepatocellular carcinoma (QGY) cells, respectively. The 405 nm SPE produced the bright photoluminescence (PL) signals of cellular QDs but also induced a strong autofluorescence(AF) from the native fluorophores like flavins in cells. The AF occupied about 30% and 13% of the total signals detected in QD imaging channel in the BY-2-T and QGY cells, respectively. With the excitation of 488 nm SPE, the PL signals were lower than those excited with the 405 nm SPE, although the AF signals were also reduced. The 800 nm TPE generated the best PL images of intracellular QDs with the highest signal ratio of PL to AF, because the two-photon absorption cross section of QDs is much higher than that of the native fluorophores. By means of the TPE, the reliable cellular imaging with QDs, even for the cells having the high AF background, can be achieved.     

Generation of high-power ultrawideband electromagnetic pulses in a system with a coaxial tem horn

A coaxial TEM horn was designed on the basis of results from nonstationary computer modeling using code KARAT. With its high dielectric strength, this antenna is capable of radiating high-power ultrawideband nanosecond pulses. The pulse source used was a compact generator built around a coaxial forming line with a built-in Tesla transformer, which shapes pulses up to 1 GW high at repetition frequencies up to 1 kHz. The amplitude of the pulses on a matched load was 20 kV at a duration of 4 nsec. Returns of ultrawideband signals from objects with simple geometric shapes were studied in laboratory experiments using this radiator. Institute of High-Current Electronics, Siberian Division, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 110–118, December, 1996.     

A Noise-Robust Pulse for Excitation Transfer in a Multi-Mode Quantum Memory

Multi-mode quantum memory is a basic element required for long-distance quantum communication,as well as scalable quantum computation.For on-demand readout and long storage times,control pulses are crucial in order to transfer atomic excitations back and forth into spin excitations.Here,we introduce noise-robust composite pulse sequences for high-fidelity excitation transfer in multi-mode quantum memory.These pulses are robust to the deviations in amplitude and the detuning parameters of realistic conditions.We show the efficiency of these composite pulses with a typical rare-earth ion-doped system.This approach could be applied to a variety of quantum memory schemes.     

Coded excitation using biphase-coded pulse with mismatched filters for high-frequency ultrasound imaging

A scheme of using phase-coded excitation and mismatched filter compression for high-frequency ultrasound imaging is presented in this paper. Biphase-coded pulses were constructed to excite the transducer. Received signals were compressed with mismatched filters optimized by minimizing peak-sidelobe-level (PSL). Both simulation and experiments were carried out to demonstrate the advantage of this technique. The simulation results demonstrated a possible sidelobe reduction (<-90 dB) with a slightly decrease of the signal-to-noise ratio of less than 1 dB compared with the compression using matched filters alone. The experimental results showed about 14 dB SNR improvement as well as -40 dB sidelobe level when the Barker-13 code excitation with 3-cycle sinusoidal wave carrier was used.     

EHD size distribution in pure Ge at pulse optical excitation

The cyclotron resonance (CR) kinetics of free carriers surrounding electron-hole drops (EHD) was investigated in pure Ge (N_D = N_A = 10¹¹cm^?3). We observed the fast decay of the CR amplitude at the long delay time after the excitation pulse [1], that may be connected with intense evaporation of the drops, when the EHD radius has achieved the critical value. The method for investigation of EHD size distribution was proposed for case of pulse excitation. The Gaussian EHD size distribution has been found.     

Fourier-domain methodology for depth-selective photothermoacoustic imaging of tissue chromophores

Development of a novel photothermoacoustic (PTA) imaging technique utilizing a frequency-modulated (chirped) optical excitation and Fourier-domain methodology for depth-selective imaging of tissue chromophores is presented. Use of frequency-domain signal detection rather than short-pulse time-resolved measurements of pressure transients give an advantage of higher SNR typical to coherent detection techniques. Additionally, we introduce chirped optical excitation to generate linear frequency modulated PTA response which enables unambiguous and precise depth measurements of tissue chromophores. In order to obtain depth profilometric information from the frequency-domain PTA (FD-PTA) measurements, we describe implementation of two signal processing algorithms: matched filter compression and heterodyne mixing with coherent detection. We show that direct relationship between chromophore depth and spectrum of photothermo acoustic signals can be established to enable depth-selective tissue imaging. Application of amplitude and phase FD-PTA imaging is demonstrated in experiments with light-scattering phantoms and chicken breast tissues containing subsurface inclusions. The potential of the FD-PTA method for noninvasive tissue tomography and molecular imaging is discussed.     

用于同时多层MRI的选择性射频脉冲的优化设计

近年来,为提高磁共振成像（MRI）信号信噪比（SNR）、缩短成像时间,同时多层成像技术受到了极大的关注.为了实现同时多层的选择性激发,现有的多层成像序列大多使用组合射频（RF）脉冲,该脉冲可包含多个独立的幅值相同相位不同的简单脉冲,由于其采用简单的线性叠加方法,该类脉冲射频功率随脉冲数量呈现平方增长,因而应用受限.针对这一问题,基于自旋动力学和优化控制原理,本文提出了一种针对同时多层MRI的选择性射频脉冲的数值优化方法,该方法充分运用射频脉冲的调控机制,获得优化脉冲,并配合层选梯度,可实现任意层厚、层间距、层数的同时高效选择性激发.最后,通过数字模体的同时多层模拟成像实验验证了优化脉冲的有效性.     

长骨裂纹宽度和角度对低阶超声导波的影响

超声导波检测长骨骨折和监测骨折愈合已成为一种极具前景的课题,但高频厚积下导波多模式混叠带来的模式识别和分离问题一直是一个难点。为避免上述问题,在低频下仅激励两个低阶兰姆波SO和AO模式,采用二维时域有限差分(2DFDTD)法定量分析裂纹宽度和骨折角度对SO和AO幅度的影响。结果表明,SO与AO模式的幅度均随裂纹宽度的增加而下降;AO的幅度随骨折角度增大而上升,SO幅度随骨折角度增大先下降后上升,转折点约为45°。SO与AO的幅度比值在不同骨折角度下均能较好地表征裂纹宽度的变化,可为横断型和斜切型长骨骨折状况的超声评价及骨折愈合监测提供依据。