基于Renyi熵滤波的光声图像重建算法设计与实现

针对光声图像重建过程中存在的原始光声信号信噪比差、重建图像对比度低、分辨率不足等问题,提出了基于Renyi熵的光声图像重建滤波算法.该算法首先根据原始光声信号的Renyi熵分布情况,确定分割阈值,并滤除杂波信号;再利用滤波后的光声数据进行延时叠加光声图像重建.利用该滤波算法分别处理铅笔芯横截面(零维)、头发丝(一维)以及小鼠大脑皮层血管(二维)等不同维度样本的光声信号,实验结果表明:相比Renyi熵处理之前,重建图像对比度平均增强了32.45%,分辨率平均提高了30.78%,信噪比提高了47.66%,均方误差降低了35.01%;相比典型的滤波处理算法(模极大值法和阈值去噪法),本研究中图像的对比度、分辨率和信噪比分别提高了25.94%/10.60%、27.90%/19.48%、35.21%/10.60%,均方误差减小了28.57%/16.66%.因此,选择利用Renyi熵滤波算法处理光声信号,从而使光声图像重建质量得到大幅改善.     

基于自适应尺度空间滤波的脑事件关联电位单次提取

提出一种基于自适应尺度空间滤波的脑事件关联电位单次提取方法。通过对背景噪声的分析,自适应地提取最大脉冲噪声宽度,将其作为尺度大小的基准,然后由此确定滤波窗口的大小,最后用尺度空间滤波算法对记录到的信号进行处理。实验结果表明,本文提出的方法不但具有良好的单次提取效果,而且具有很强的适应性和稳健性。     

钙火花研究进展与瞻望

钙离子是最广泛而又最重要的细胞内第二信使。自1993年以来,钙火花等一系列钙信号基本单元相继发现,揭示了细胞钙信号转导的数字-模拟二元特征：纳米-微米尺度上短暂的钙信号事件（数字系统）随机叠加于连续的全细胞钙信号（模拟系统）背景中。数字模式的微区域钙信号赋予细胞钙信号在时间、空间、幅度上多尺度多层次的精细结构。对钙火花激活机制、协同机制、终止机制等方面的研究,为钙释放通道阵列的门控及调节提出了新的见解和问题。钙火花等对于高域值钙依赖性过程（如肌细胞兴奋-收缩耦联、细胞兴奋性和神经细胞分泌）的激活和时空调控具有特别重要的生理和病理意义。钙信号“激-模二元性”的研究可望进一步揭示细胞钙信号的简单性与复杂性的统一。     

背景噪声对人感知声音时间信息的影响

对声音时间信息的分辨在人和动物感知声音信息的过程中至关重要.在自然声环境中,声音信息总处于一定的噪声背景下.文章以间隔探测阈值为指标测定了人对纯音和噪声的间隔探测阈值,以及持续噪声背景对间隔探测阈值的影响.声音信号采用1000～10000 Hz的纯音信号和白噪声信号,声音强度为70 dB SPL.背景噪声为持续白噪声,强度分别为45、55、65 dB SPL.结果表明,对纯音信号,随着背景噪声强度增加,间隔探测阈值有升高的趋势.对噪声信号来说,45、55 dB SPL的背景噪声对噪声信号的间隔探测阈值无显著影响,但65 dB SPL的背景噪声使间隔探测阈值显著升高.研究结果提示,背景噪声能够在一定程度上影响人对声音时间信息的感知,影响的程度与背景噪声的强度有关.     

基于双正交小波变换的宫缩信号消噪

小波分析的信号消噪方法是现代信号处理中的重要组成部分,小波基的不同选取将直接影响消噪的效果.本文在全局阈值的标准下,基于不同噪声水平,讨论了小波基的正交性和线性相位性对消噪结果的影响,提出了选取小波基的一般方法,最后利用双正交小波基在软阈值标准下实现了对宫缩信号的消噪处理,并取得了较好的效果.     

空间频率通道中噪声对体视匹配的影响

本工作希望了解噪声在各空间频率通道中对体视的影响.产生从5%至35%每隔5%的七个等级的随机点噪声图象,参照Wilson四通道模型的频率分别用两种不同滤波器对随机点立体图对(RDS)及噪声图象进行滤波,然后在各通道图对中加入不同通道噪声进行匹配观察.得到:1.高频通道的体视抗干扰能力高于低频通道.2.Gabor函数滤波后图对的体视抗干扰能力高于Butterworth滤波器滤波后的,但均低于未滤波的.3.当噪声大于15%时,低频噪声对低频图对及高频噪声对高、低频图对的体视匹配有抑制作用,而低频噪声对高频图对则影响很小.4.未滤波及各通道图对的最大噪声容限均不超过25%.     

猫外膝体中继细胞低阈值钙通道的动力学特性

在猫外膝体脑片上用单电极电压钳位的方法研究了低阈值T-型钙离子通道的动力学特性.采用阻断各种钾和钠离子通道、减小钙离子电流和选择适当的维持电位等方法,改善了脑片实验的空间钳位.所得结果与大鼠外膝体急性分离神经元的结果一致.说明猫外膝体神经元T-型钙离子通道的动力学特性与大鼠的相同.     

基于平稳小波变换的EEG视觉的信号修正

眼球运动和眨眼会在眼球周围产生电信号,这种电信号的存在直接影响到对EEG信号的分析特征提取及EEG模式的分类等研究。本文提出了一种基于小波阈值滤噪方法来修正EEG信号中出现的视觉伪信号(OA)。这种用于EEG视觉伪信号处理的小波方法的实现过程如下：1)用平稳小波变换(SWT)对原始EEG信号进行处理;2)设置低频带信号的系数阈值;3)对滤噪后的信号进行重构。实验结果表明这种方法同时适用于眨眼和眼球运动产生的伪信号。最后,通过对采集的信号处理前后做了对比,说明其有效性。     

丹参注射液对顺铂致听力损伤的拮抗作用

目的:探讨丹参注射液对顺铂致小鼠耳毒性的保护作用,为顺铂耳毒性作用的防治提供实验依据.方法:应用听觉脑干反应(ABR)检测给药前后各组小鼠不同滤波click听力阈值的变化.结果:顺铂可引起小鼠体重明显下降和各滤波click听力阈值的升高.而丹参拮抗组小鼠给药前后体重和ABR听力阈值的改变明显低于单纯顺铂给药组,并呈剂量依赖关系.结论:丹参可明显减轻顺铂对小鼠的听力损伤.     

针对脑内源光学信号的自适应中值滤波处理

具有高空间分辨率的脑内源性信号光学成像术是研究大脑皮层功能构筑的有力工具.针对非常微弱的内源性光学信号的提取,本文首先讨论了其噪声来源和基于生理结构原因的信号特征:然后根据由标准化处理得到的功能图像的二阶统计量分析结果,提出了自适应滤波窗口设计:使用非线性中值空域滤波,对含有不同特征的图像区域,分别采取不同的滤波窗口进...     

Single-channel recording of ligand-gated ion channels

Electrophysiological recording of single-channel currents is the most direct method available for obtaining detailed and precise information about the kinetic behavior of ion channels. A wide variety of cell types can be used for single-channel recording, but to obtain the highest resolution of the briefest channel opening and closing events, low-noise recordings, coupled with a minimal filtering frequency, are required. Here, we present a protocol designed to help those with some electrophysiological expertise who wish to explore the properties of native and recombinant single ligand-gated ion channels. We have focused on the practical aspects of recording single GABA channels from cell-attached and outside-out patches and also introduced some of the preliminary considerations that are necessary for the analysis of single-channel data, including an introduction to single-channel analysis software.     

Measuring kinetics of complex single ion channel data using mean-variance histograms.

The measurement of single ion channel kinetics is difficult when those channels exhibit subconductance events. When the kinetics are fast, and when the current magnitudes are small, as is the case for Na+, Ca2+, and some K+ channels, these difficulties can lead to serious errors in the estimation of channel kinetics. I present here a method, based on the construction and analysis of mean-variance histograms, that can overcome these problems. A mean-variance histogram is constructed by calculating the mean current and the current variance within a brief "window" (a set of N consecutive data samples) superimposed on the digitized raw channel data. Systematic movement of this window over the data produces large numbers of mean-variance pairs which can be assembled into a two-dimensional histogram. Defined current levels (open, closed, or sublevel) appear in such plots as low variance regions. The total number of events in such low variance regions is estimated by curve fitting and plotted as a function of window width. This function decreases with the same time constants as the original dwell time probability distribution for each of the regions. The method can therefore be used: 1) to present a qualitative summary of the single channel data from which the signal-to-noise ratio, open channel noise, steadiness of the baseline, and number of conductance levels can be quickly determined; 2) to quantify the dwell time distribution in each of the levels exhibited. In this paper I present the analysis of a Na+ channel recording that had a number of complexities. The signal-to-noise ratio was only about 8 for the main open state, open channel noise, and fast flickers to other states were present, as were a substantial number of subconductance states. "Standard" half-amplitude threshold analysis of these data produce open and closed time histograms that were well fitted by the sum of two exponentials, but with apparently erroneous time constants, whereas the mean-variance histogram technique provided a more credible analysis of the open, closed, and subconductance times for the patch. I also show that the method produces accurate results on simulated data in a wide variety of conditions, whereas the half-amplitude method, when applied to complex simulated data shows the same errors as were apparent in the real data. The utility and the limitations of this new method are discussed.     

Single-channel recordings of two types of calcium channels in rat pancreatic beta-cells.

Using the cell-attached configuration of the patch clamp technique, we have identified two different types of Ca channels in rat pancreatic beta-cell membranes. The two channels differ in single channel conductance, voltage dependence, and inactivation properties. The single-channel conductance, measured with 100 mM Ba2+ in the pipette, was 21.8 pS for the large channel and 6.4 pS for the small channel. The large-conductance channel is similar to the fast deactivating or L-type Ca channel described in other preparations. It is voltage dependent, has a threshold for activation around -30 mV, and can be activated from a holding potential of -40 mV. On the other hand, the small-conductance Ca channel is similar to the SD or T type Ca channel; it has a lower activation threshold, around -50 mV, and it can be inactivated by holding the membrane potential at -40 mV.     

Mechanotransduction by Membrane Proteins: The speed of the hair cell mechanotransducer channel revealed by fluctuation analysis

Although mechanoelectrical transducer (MET) channels have been extensively studied, uncertainty persists about their molecular architecture and single-channel conductance. We made electrical measurements from mouse cochlear outer hair cells (OHCs) to reexamine the MET channel conductance comparing two different methods. Analysis of fluctuations in the macroscopic currents showed that the channel conductance in apical OHCs determined from nonstationary noise analysis was about half that of single-channel events recorded after tip link destruction. We hypothesized that this difference reflects a bandwidth limitation in the noise analysis, which we tested by simulations of stochastic fluctuations in modeled channels. Modeling indicated that the unitary conductance depended on the relative values of the channel activation time constant and the applied low-pass filter frequency. The modeling enabled the activation time constant of the channel to be estimated for the first time, yielding a value of only a few microseconds. We found that the channel conductance, assayed with both noise and recording of single-channel events, was reduced by a third in a new deafness mutant, Tmc1 p.D528N. Our results indicate that noise analysis is likely to underestimate MET channel amplitude, which is better characterized from recordings of single-channel events.     

Single-channel activity in sea urchin sperm revealed by the patch-clamp technique

Ionic fluxes are deeply involved in the response of spermatozoa to the egg. Using the patch-clamp technique, we show for the first time single ion channel activity in sea urchin spermatozoa and spermatozoa heads. Due to their small size gigaseals were obtained in suspended cells by applying suction through the pipette. The rate of gigaseal formation was very low and improved to 6% (n = 1145) when flagella were detached from sperm. Current-voltage curves created from single-channel events showed conductances of approx. 65 and 170 pS, suggesting the presence of two types of channels. At least one appears to be a K⁺ channel.     

Correction for missed events based on a realistic model of a detector.

Quantitative patch-clamp analysis based on dwell-time histograms has to deal with the problem of missed events. The correction of the evaluated time constants has to take into account the characteristics of the detector used for the reconstruction of the time series. In previous approaches a simple model of the detector has been used, which is based on the assumption that all events shorter than the temporal resolution tres were missed, irrespective of the preceding events. Rather than the standard assumption of a fixed dead time, we introduce a more realistic model of a detector by a continuous-time version of the Hinkley detector. The combined state of the channel and the detector obeys a Markov model, which is governed by a Fokker-Planck-Kolmogorov partial differential equation. The steady-state solution leads to the determination of the apparent time constants tau o and tau c depending on the true rate constants koc and kco and the temporal resolution tres of the detector. Simulations with different kinds of detectors, including the Bessel filter with half-amplitude threshold detection, are performed. They show that our new equation predicts the dependence of tau c and tau o on koc, kco, and tres better than the standard equation used until now.     

The Estimation of Rapid Rate Constants from Current-Amplitude Frequency Distributions of Single-Channel Recordings

A method is described for estimating rapid rate constants from the distributions of current amplitude observed in single-channel electrical recordings. It has the advantages over previous, similar approaches that it can accommodate both multistate kinetic models and adjustable filtering of the data using an 8-pole Bessel filter. The method is conceptually straightforward: the observed distributions of current amplitude are compared with theoretical distributions derived by combining several simplifying assumptions about the underlying stochastic process with a model of the filter and electrical noise. Parameters are estimated by approximate maximum likelihood. The method was used successfully to estimate rate constants for both a simple two-state kinetic model (the transitions between open and closed states during the rapid gating of an outward-rectifying K⁺-selective channel in the plasma membrane of Acetabularia) and a complex multistate kinetic model (the blockade of the maxi cation channel in the plasma membrane of rye roots by verapamil). For the two-state model, parameters were estimated well, provided that they were not too fast or too slow in relation to the sampling rate. In the three-state model the precision of estimates depended in a complex way on the values of all rate parameters in the model. Received: 4 October 1996/Revised: 2 September 1997     

Threshold fluctuations in an N sodium channel model of the node of Ranvier.

Computer simulations of stochastic single-channel open-close kinetics are applied to an N sodium channel model of a node of Ranvier. Up to 32,000 voltage-gated sodium channels have been simulated with modified amphibian sodium channel kinetics. Poststimulus time histograms are obtained with 1000 monophasic pulse stimuli, and measurements are made of changes in the relative spread of threshold (RS) with changes in the model parameters. RS is found to be invariant with pulse durations from 100 microseconds to 3 ms. RS is approximately of inverse proportion to square-root of N. It decreases with increasing temperature and is dependent on passive electrical properties of the membrane as well as the single-channel conductance. The simulated RS and its independence of pulse duration is consistent with experimental results from the literature. Thus, the microscopic fluctuations of single, voltage-sensitive sodium channels in the amphibian peripheral node of Ranvier are sufficient to account for the macroscopic fluctuation if threshold to electrical stimulation.     

Cav1.4 encodes a calcium channel with low open probability and unitary conductance

When transiently expressed in tsA-201 cells, Ca(v)1.4 calcium channels support only modest whole-cell currents with unusually slow voltage-dependent inactivation kinetics. To examine the basis for this unique behavior we used cell-attached patch single-channel recordings using 100 mM external barium as the charge carrier to determine the single-channel properties of Ca(v)1.4 and to compare them to those of the Ca(v)1.2. Ca(v)1.4 channel openings occurred infrequently and were of brief duration. Moreover, openings occurred throughout the duration of the test depolarization, indicating that the slow inactivation kinetics observed at the whole-cell level are caused by sustained channel activity. Ca(v)1.4 and Ca(v)1.2 channels displayed similar latencies to first opening. Because of the rare occurrence of events, the probability of opening could not be precisely determined but was estimated to be <0.015 over a voltage range of -20 to +20 mV. The single-channel conductance of Ca(v)1.4 channels was approximately 4 pS compared with approximately 20 pS for Ca(v)1.2 under the same experimental conditions. Additionally, in the absence of divalent cations, Ca(v)1.4 channels pass cesium ions with a single-channel conductance of approximately 21 pS. Although Ca(v)1.2 opening events were best described kinetically with two open time constants, Ca(v)1.4 open times were best described by a single time constant. BayK8644 slightly enhanced the single-channel conductance in addition to increasing the open time constant for Ca(v)1.4 channels by approximately 45% without, however, causing the appearance of an additional slower gating mode. Overall, our data indicate that single Ca(v)1.4 channels support only minute amounts of calcium entry, suggesting that large numbers of these channels are needed to allow for significant whole-cell current activity, and providing a mechanism to reduce noise in the visual system.     

Open channel noise. IV. Estimation of rapid kinetics of formamide block in gramicidin A channels.

Blocking events in currents through biological ion channels occur over a wide range of characteristic times. The interruptions in single-channel currents from blocking events may be characterized by the direct measurement of gap durations or by analyzing open-channel current histograms, provided that the events are not much shorter than the time resolution of single-channel recordings (approximately 10 microseconds). Here we present a method for the characterization of channel block on a much faster time scale by combining open-channel noise measurements with subsequent model fits according to a theoretical approach (Frehland, E. 1978. Biophysical Chemistry. 8:255-265). Although the bandwidth limitations in open-channel noise experiments are the same as in conventional single-channel experiments, from the dependence of the mean current and the spectral density of the noise on the concentration of the blocking agent, kinetics of very brief blocking events can be estimated. As an example we have analyzed the open-channel noise of K+ currents through the gramicidin A channel in the presence of various concentrations of formamide, a weak blocker, at neutral pH. We estimate the blocking and unblocking rates to be approximately 10(7)s-1 at 1 M formamide and discuss possible mechanisms for the blocking process.