共查询到20条相似文献,搜索用时 15 毫秒
1.
《Magnetic resonance imaging》1998,16(8):907-916
The purpose of this study was to investigate how flow affects slice-selective excitation, particularly for radiofrequency (rf) pulses optimized for slice-selective excitation of stationary material. Simulation methods were used to calculate the slice profiles for material flowing at different velocities, using optimal flow compensation when appropriate. Four rf pulses of very different shapes were used in the simulation study: a 90° linear-phase Shinnar-LeRoux pulse; a 90° self-refocusing pulse; a minimum-phase Shinnar-LeRoux inversion pulse; and a SPINCALC inversion pulse. Slice profiles from simulations with a laminar flow model were compared with experimental studies for two different rf pulses using a clinical magnetic resonance imaging (MRI) system. We found that, for a given rf pulse, the effect of flow on slice-selective excitation depends on the product of the selection gradient amplitude, the component of velocity in the slice selection direction, and the square of the rf pulse duration. The shapes of the slice profiles from the Shinnar-LeRoux pulses were relatively insensitive to velocity. However, the slice profiles from the self-refocusing pulse and the SPINCALC pulse were significantly degraded by velocity. Experimental slice profiles showed excellent agreement with simulation. In conclusion, our study demonstrates that slice-selective excitation can be significantly degraded by flow depending on the velocity, the gradient amplitude, and characteristics of the rf excitation pulse used. The results can aid in the design of rf pulses for slice-selective excitation of flowing material. 相似文献
2.
Bernhard G.Zogar 《中国光学快报(英文版)》2011,(7):45-48
Applying laser-speckle techniques in material sciences as well as in methods to characterize surface conditions of specimen has become the method of choice, especially if a non-contacting principle is sought. This is almost always the case for specimen that are small in at least one dimension as for example in the material testing of foils, fibres, or micromaterials and certainly also if elevated test-temperatures are preventing standard gauges. This letter discusses in some detail sources of error that are quite often overlooked or not even considered as significant at all, but still carry the potential to introduce uncertainties well above the system design specifications. 相似文献
3.
The effect of speckle pattern on laser Doppler velocimeters is studied theoretically and experimentally. We have found that dynamic speckle patterns can cause error in velocity measurement. Increasing sampling time and using proper signal processing circuits can eliminate this bad effect caused by speckle pattern.The accuracy of velocity measurement is better than 2.1% in a wide velocity range(4.66-468.86 mm/s)when speckle effects are taken into account. 相似文献
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Laser Doppler velocimetry (LDV) has become established as an important technique for the measurement of velocities of macroscopic objects and fluids: the dynamic range is large (~10-6-105 ms-1) and the measurement is absolute and non-invasive. However, the size and cost of LDV has restricted its use in some areas. This paper presents two separate approaches to reduce these problems: we describe a compact LDV system incorporating a solid state laser diode and also an investigation of the feasibility of a fibre optic LDV system in which the conventional optical components are replaced by fibre optics.The experimental arrangement used for the solid state LDV system was of the Doppler difference type; i.e. a system of parallel interference fringes is focused in the measurement volume, so that a particle passing through this volume produces a scattered light signal which is intensity modulated. In its simplest form, the technique cannot determine the direction of motion of the particle, but this difficulty may be overcome by causing the fringes to ‘move’ within the measurement volume with known velocity. In the present experiments, the laser output frequency was modulated by modulating its drive current; since the path lengths of the two beams interfering in the measurement volume were unequal, fringe motion was achieved.The fibre optic LDV experiment was also of the Doppler difference type, and it was demonstrated that the necessary stabilised interference fringe system could be projected using a fibre optic system. An electronic servo was devised to compensate for the random differential thermal drifts in the fibres which would otherwise have produced unacceptable drifts in the fringe pattern. 相似文献
6.
In this paper, temperature measurement of axisymmetric dilute hydrogen flame is described using digital laser-speckle technique,
which allows the measurement of refractive index field of the flame without any tracers. The measurement system consists of
laser and CCD camera, which are placed in-line configuration. The temperature field of the flame is derived from the refractive
index field using the cross-correlation analysis of the speckle pattern with and without flame. The influence of the species
concentration on the temperature measurement is evaluated from the gas composition in the flame using gas chromatography.
It is found that the temperature field measured by the present method with concentration correction agrees with that of the
thermocouple measurement within an experimental accuracy, suggesting the usefulness of this measurement technique. 相似文献
7.
Basic forms of the optical systems of laser Doppler velocimeters with supression of the additive noise component in the output signal are investigated by the Jones matrix method. The expressions for the resulting Doppler signal, taking into account the parameters and orientation of phase elements and the receiving polarization analyser, have been obtained and analysed. Experimental results are given. 相似文献
8.
《Infrared physics》1985,25(1-2):457-466
A review of principles and design features emphasizes how the choice of transmitter waveform is affected both by the intended application and by the ever-advancing capabilities of lasers, modulators and receiver signal-processing systems. Achievements in application areas are surveyed including the work at RSRE on rangefinders/velocimeters and on Doppler anemometers. 相似文献
9.
A differential arrangement using a laser for the measurement of the velocity of moving surfaces is discussed. Configurations of optical velocimeters with diffraction beam-splitters are shown not to be critical on the wavelength stability of a semiconductor laser. Laser meters measuring the velocity and length of rolled stock have been built on the basis of the devices considered. 相似文献
10.
Relativistic effects of laser Doppler velocimeters (LDV's) are discussed and novel LDV systems are proposed. If the direction of the scattered light makes a right angle with the flow direction, relativistic effects completely disappear no matter how high the velocity of a moving particle becomes. The proposed LDV's involve that the velocity can be measured from one scattered light beam with two different single frequencies. It is also predicted that the usual optical heterodyne-detection techniques can be made applicable to measure even ultra-high velocities up to the region where relativistic effects should be taken into account. 相似文献
11.
Analogue and digital techniques are proposed for the simulation of signals arising in laser Doppler velocimeters. Their merits are evaluated.Of special interest is the estimation of the instantaneous frequency of the Doppler signal from the zero crossing rate. An analysis is presented whereby the biases introduced in this estimation method are derived for two optical configurations of the velocimeter. This analysis is extended to the case of additive Gaussian noise on the signal, and the cumulative effect of additive noise on the zero crossing rate is determined.The so called drop-out problem which pervades the frequency estimation of the instantaneous Doppler signal is next considered, and expressions are derived for mean drop-out rate and the mean interval between drop-outs in terms of an arbitrary signal threshold. These should facilitate choice of threshold detection for given rms Doppler signal levels, and in achieving relative compromises in terms of signal bandwidth. Finally a novel form of frequency tracking system is discussed and its performance critically assessed.List of symbols
A
1,2
Weighting constant for Doppler signal arising from different velocimeter configurations
-
E
Expectation operator
- {itH(t)}
Envelope of Doppler signal
-
H
0
Doppler signal threshold level
-
K
1,2
Bandwidth ratio
-
N
i
Total input additive noise power
-
R()
Autocorrelation function for lag
-
W(t)
Weighting function associated with velocimeter set-up
-
p(.)
Probability density function of random variable (.)
-
x(t),y(t)
Bandpass filtered Doppler signal
- (f)
Power spectral density function at frequencyf
-
i
Total input signal power
-
i
Input signal-to-noise ratio
-
Zero crossing rate
-
()
Normalized autocorrelation function for lag ()
-
Normalized threshold level
- (
0)f
0
(Angular) central frequency of Doppler signal
- (
1)f
1
(Angular) central frequency of bandpass filter. 相似文献
12.
The possibility of using an electro-optical frequency modulator with a rotating electric field (MREF) in LDV is discussed. Prototype systems measuring one component of the velocity vector are described. Photographs of the experimental signals obtained are given. 相似文献
13.
研究了反常色散介质中脉冲形变对超光速群速度的影响,发现即使光脉冲完全不产生形变群速度仍会超过真空中的光速。但波包的群速度并不等同于信号的传输速度,采用信息论方法,定义了信号的有效传输速度,并用于解释WKD(Wang,Kuzmich,Dogariu)实验。通过计算入射光与出射光信号所携带的信息量,发现由于光的波动衍射及光子散粒噪声的影响,出射光所携带的信息量会损失,使得光信号的有效传播速度不会超过真空中的光速。 相似文献
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I-Kun Chen Hitoshi Funagane Tai-Ping Liu Shigeru Takata 《Communications in Mathematical Physics》2016,341(1):105-134
We study the boundary singularity of the solutions to the Boltzmann equation in the kinetic theory. The solution has a jump discontinuity in the microscopic velocity \({\zeta}\) on the boundary and a secondary singularity of logarithmic type around the velocity tangential to the boundary, \({\zeta_{n} \sim 0_{-}}\), where \({\zeta_{n}}\) is the component of molecular velocity normal to the boundary, pointing to the gas. We demonstrate this secondary singularity by obtaining an asymptotic formula for the derivative of the solution on the boundary with respect to \({\zeta_{n}}\) that diverges logarithmically when \({\zeta_{n} \sim 0_{-}}\). Our study is for the thermal transpiration problem between two plates for the hard sphere gases with sufficiently large Knudsen number and with the diffuse reflection boundary condition. The solution is constructed and its singularity is studied by an iteration procedure. 相似文献
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提出了一种改进的速度加角速度传递对准的方法,该方法用主惯导输出的姿态角速度信息计算角速度,利用速度和角速度在导航系下的投影作为观测信息,直接估计子惯导姿态误差。仿真结果表明,在低海况情况下该方法比传统的速度加角速度匹配具有更好的估计效果。 相似文献
18.
We study the velocity of bubble walls in the electroweak phase transition. For several extensions of the Standard Model, we estimate the friction and calculate the wall velocity, taking into account the hydrodynamics. We find that deflagrations are generally more likely than detonations. Nevertheless, for models with extra bosons, which give a strongly first-order phase transition, the deflagration velocity is in general quite high, 0.1vw0.6. Therefore, such phase transitions may produce an important signal of gravitational waves. On the other hand, models with extra fermions which are strongly coupled to the Higgs boson may provide a strongly first-order phase transition and small velocities, 10−2vw10−1, as required by electroweak baryogenesis. 相似文献
19.
Gnecco E Bennewitz R Gyalog T Loppacher C Bammerlin M Meyer E Guntherodt H 《Physical review letters》2000,84(6):1172-1175
Sliding friction between the tip of a friction force microscope and NaCl(100) was studied to deduce the velocity dependence of friction forces on the atomic scale. A logarithmic dependence of the mean friction force is revealed at low velocities. The experimental data are interpreted in terms of a modified Tomlinson model which is based on reaction rate theory. 相似文献
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