Speckle Signal Research Articles

Direct comparison of feature tracking and autocorrelation for velocity estimation

Feature tracking is an algorithm for estimating tissue motion and blood flow using pulse-echo ultrasound. It was proposed as a computationally simpler alternative to other techniques such as autocorrelation and time-domain cross correlation. The advantage of feature tracking is that it selectively extracts easily identifiable parts of the speckle signal (e.g., the local maxima), reducing the amount of information being processed. Studies on feature tracking to date have used stationary, speckle-generating targets to simulate blood flow. Also, feature tracking has not been compared with accepted commercial methods. This study directly compares feature tracking performance with the complex autocorrelation method, which is the most common color flow algorithm. Experiments were performed with both a rotating string phantom and a commercial flow phantom surrounded by tissue-mimicking material, using 2.25 MHz and 3.5 MHz transducers, under more realistic signal-to-clutter (-15 to -35 dB) and signal-to-noise ratios (SNR) (15 dB to 3 dB) than previous translating-phantom tests. The feature tracking approach is shown to produce mean estimates comparable to autocorrelation (R2 = 0.9954 and 0.9960 for 6-sample and 12-sample autocorrelation, respectively, and R2 = 0.9998 for both 6-sample and 12-sample feature tracking) for velocities ranging from 10 to 100 cm/s. The variance of feature-tracking estimates is shown to compare favorably to the complex autocorrelation approach using the same number of ensemble flow samples (19 to 28% lower standard deviation for 3.5 MHz, 36 to 55% lower standard deviation for 2.25 MHz). However, linear regression of the feature locations does not produce an appreciable improvement in estimation variance. Discussion of the need for further research, particularly in the areas of feature detection and feature correspondence, is given.

Variance analysis of arrival time and amplitude estimates from random speckle signal

An aberration estimation algorithm, previously developed in the frequency domain, is implemented in the time domain. The algorithm is used to estimate arrival time and amplitude fluctuations with signals from random scatterers. Simulations have been performed to investigate the variance of the estimates. Stability differences between the two implementations are also explored. Eight body wall models, emulating the human abdominal wall, were used for this purpose. The variance was investigated as a function of the number of independent scatterer realizations, used to obtain an estimate. Such signals may be acquired by imaging moving scatterers, e.g., blood or contrast agent. Alternatively they can be obtained by using different nonoverlapping beams from a sector/linear scan. The results show only minor differences between the two implementations with respect to stability. This means that the algorithm can be readily implemented in the time domain for real-time applications. The standard deviation of arrival time and amplitude fluctuation estimates decrease, when the number of independent signals increase. Using only one signal for estimation produces a relatively high standard deviation, but an iterative transmit-beam aberration correction scheme still converges to a properly corrected focus using between one and four transmit-beam iterations for the investigated aberrators.

The gated integration technique for the accurate measurement of the autocorrelation function of speckle intensities scattered from random phase screens

A new approach based on the gated integration technique is proposed for the accurate measurement of the autocorrelation function of speckle intensities scattered from a random phase screen. The Boxcar used for this technique in the acquisition of the speckle intensity data integrates the photoelectric signal during its sampling gate open, and it repeats the sampling by a preset number, m. The average analog of the m samplings output by the Boxcar enhances the signal-to-noise ratio by m , because the repeated sampling and the average make the useful speckle signals stable, while the randomly varied photoelectric noise is suppressed by 1 / m . In the experiment, we use an analog-to-digital converter module to synchronize all the actions such as the stepped movement of the phase screen, the repeated sampling, the readout of the averaged output of the Boxcar, etc. The experimental results show that speckle signals are better recovered from contaminated signals, and the autocorrelation function with the secondary maximum is obtained, indicating that the accuracy of the measurement of the autocorrelation function is greatly improved by the gated integration technique.

Time reversal operator decomposition with focused transmission and robustness to speckle noise: Application to microcalcification detection

The decomposition of the time reversal operator (DORT) is a detection and focusing technique using an array of transmit-receive transducers. In the absence of noise and under certain conditions, the eigenvectors of the time reversal operator contain the focal laws to focus ideally on well-resolved scatterers even in the presence of strong aberration. This paper describes a new algorithm, FDORT, which uses focused transmission schemes to acquire the operator. It can be performed from medical scanner data. A mathematical derivation of this algorithm is given and it is compared with the conventional algorithm, both theoretically and with numerical experiments. In the presence of strong speckle signals, the DORT method usually fails. The influence of the speckle noise is explained and a solution based on FDORT is presented, that enables detection of targets in complex media. Finally, an algorithm for microcalcification detection is proposed. In-vivo results show the potential of these techniques.

Statistics of the self-mixing speckle interference in a laser diode and its application to the measurement of flow velocity

The statistical analysis of self-mixing speckles interference using Fabry–Perot cavity is described based on the theory of the speckle and the self-mixing interference in a laser diode. Speckle signal processing based on Fourier transform is presented in this paper. The linear dependence between the spectrum mean frequency and velocity of solution is given by simulation and proved by experiment. The experimental results show that this speckle signal processing can be used to measure velocity of target.

Apparatus for determining particle size

The invention provides a method and an apparatus for measuring the size, concentration and size distribution of particles in a fluid by determining the characteristics of an acoustic speckle signal of the particles and relating these characteristics to the size and concentration characteristics of the particles. The method and the apparatus are especially useful for the measurement of the size distribution of small particles in optically opaque liquids.

Signal analysis of total internal reflection fluorescent speckle microscopy (TIR‐FSM) and wide‐field epi‐fluorescence FSM of the actin cytoskeleton and focal adhesions in living cells

Fluorescent speckle microscopy (FSM) uses low levels of fluorescent proteins to create fluorescent speckles on cytoskeletal polymers in high-resolution fluorescence images of living cells. The dynamics of speckles over time encode subunit turnover and motion of the cytoskeletal polymers. We sought to improve on current FSM technology by first expanding it to study the dynamics of a non-polymeric macromolecular assembly, using focal adhesions as a test case, and second, to exploit for FSM the high contrast afforded by total internal reflection fluorescence microscopy (TIR-FM). Here, we first demonstrate that low levels of expression of a green fluorescent protein (GFP) conjugate of the focal adhesion protein, vinculin, results in clusters of fluorescent vinculin speckles on the ventral cell surface, which by immunofluorescence labelling of total vinculin correspond to sparse labelling of dense focal adhesion structures. This demonstrates that the FSM principle can be applied to study focal adhesions. We then use both GFP-vinculin expression and microinjected fluorescently labelled purified actin to compare quantitatively the speckle signal in FSM images of focal adhesions and the actin cytoskeleton in living cells by TIR-FM and wide-field epifluorescence microscopy. We use quantitative FSM image analysis software to define two new parameters for analysing FSM signal features that we can extract automatically: speckle modulation and speckle detectability. Our analysis shows that TIR-FSM affords major improvements in these parameters compared with wide-field epifluorescence FSM. Finally, we find that use of a crippled eukaryotic expression promoter for driving low-level GFP-fusion protein expression is a useful tool for FSM imaging. When used in time-lapse mode, TIR-FSM of actin and GFP-conjugated focal adhesion proteins will allow quantification of molecular dynamics within interesting macromolecular assemblies at the ventral surface of living cells.

High-frequency ultrasonic speckle velocimetry in sheared complex fluids

High-frequency ultrasonic pulses at 36 MHz are used to measure velocity profiles in a complex fluid sheared in the Couette geometry. Our technique is based on time-domain cross-correlation of ultrasonic speckle signals backscattered by the moving medium. Post-processing of acoustic data allows us to record a velocity profile in 0.02--2 s with a spatial resolution of 40 $\mu$m over 1 mm. After a careful calibration using a Newtonian suspension, the technique is applied to a sheared lyotropic lamellar phase seeded with polystyrene spheres of diameter 3--10 $\mu$m. Time-averaged velocity profiles reveal the existence of inhomogeneous flows, with both wall slip and shear bands, in the vicinity of a shear-induced ``layering'' transition. Slow transient regimes and/or temporal fluctuations can also be resolved and exhibit complex spatio-temporal flow behaviors with sometimes more than two shear bands.

Estimation of Local Displacement Vector in Random Medium Utilizing Synthetic Aperture Array Signal Processing

Real-time imaging of dynamical movement or stress distribution across living tissues by physiological internal or clinical external stress forces could be a promising tool for clinical diagnosis as visualized palpation. However, the transversal components of tissue local motion across an ultrasonic beam cannot be obtained utilizing conventional beam scanning. We proposed the combined algorithm of phase sensitive spatial cross-correlation between two sequential speckle echo frames and signal processing for a synthetic aperture of an array transducer which realizes a uniform spatial resolution through these echo frames. The two-dimensional local motion vector can be directly calculated from the sequentially estimated local phase changes by successive irradiation of different point sources. The accuracy and the statistical variance of the estimated two-dimensional displacement were investigated for the simulated random tissue and the received speckle echo frames from it. The obtained results showed the excellent performance of the proposed algorithm, both for local values and for the entire distribution of estimated displacement vectors throughout the measured medium.

Suppression of dynamic laser speckle signals in multimode fibers of various lengths.

The effects of fiber coupling and fiber length on photocurrent fluctuations are studied when the light of a laser diode transmitted to and from a dynamic turbid medium by a step-index multimode fiber is studied. When the laser light is coupled asymmetrically, filling only the higher-order modes, the photocurrent fluctuations are suppressed significantly when fiber lengths of as much as 16 m are added between the laser and the medium. Addition of as much as 16 m of detection fiber, or any fiber in the case of symmetric light coupling, leads to much less or no suppression of the photocurrent fluctuations.

Method and apparatus for visualization of motion in ultrasound flow imaging using packet data acquisition

A method and an apparatus for imaging blood motion by displaying an enhanced image of the fluctuating speckle pattern. A continuous stream of data frames, each the result of one scan, is available for processing. For each position in the scan plane, a respective time sequence of signal samples is available for processing. The first step in the blood motion image processing is high-pass filtering of this signal. Following the high-pass filter, a speckle signal is formed, e.g., by calculating the squared magnitude (i.e., power) of the high-pass-filtered signal (I/Q or RF). The resulting speckle signal can then undergo a nonlinear amplitude transformation to form a blood motion imaging signal for display.

Blood Flow Analyses of Skin Tissue Under the Sacrum Using Laser Speckle Flowgraphy

A new version of the Laser Speckle Flowgraphy was developed to study the dependence of the blood flow of the skin tissue under the sacrum and the supporting pressure against the sacrum. The measuring unit consists of the laser diode, imaging system, line sensor, scanning mirror, and one-board microcomputer. The speckle signal is analyzed and sent to a personal computer, where the blood flow in a tissue area of 19 cm × 25 cm is evaluated and displayed on a 2-D color map. It was found that the blood flow in the skin tissue sharply decreased under the sacrum when it was supported by a hard surface, while the decrease became negligible when the surface was replaced by a soft airbag.

Particle measurement by acoustic speckle

The invention provides a method and an apparatus for measuring the size, concentration and size distribution of particles in a fluid by determining the characteristics of an acoustic speckle signal of the particles and relating these characteristics to the size and concentration characteristics of the particles. The method and the apparatus are especially useful for the measurement of the size distribution of small particles in optically opaque liquids.

Phase conjugation of speckle-inhomogeneous radiation in a holographic Nd:YAG laser with a short thermal hologram

A model of the so-called short hologram, which does not exhibit in-depth diffraction deformation of the fine speckle pattern of the recording fields, is studied. The investigation is performed by the example of a thermal hologram recorded by two speckle waves, which is the output mirror of a ring laser produced as a result of this recording. It is shown that the ability of this short hologram to select a wave conjugated to a speckle signal in the mode of the holographic laser depends both on the degree of mutual mixing of the speckles of recording beams in the hologram volume and on the effects of its saturation by the beams. The maximum accuracy of phase conjugation of speckle radiation in the holographic Nd:YAG laser achieved upon the best selection of the conjugate wave by the short thermal hologram was 93%.

Investigating a stretched vortex with ultrafast two-dimensional ultrasonic speckle velocimetry

Ultrasonic speckle velocimetry (USV) was recently introduced as a new ultrafast tool for measuring fluid velocities in two dimensions by analyzing the speckle signal backscattered from particles moving with the flow [Sandrin et al., Appl. Phys. Lett. 78, 1155 (2001)]. An experimental study of a stationary stretched vortex is presented, which demonstrates the efficiency of ultrafast 2D USV in terms of quantitative assessment of vortical flows. The full velocity field is recovered and compared to the Burgers model for two different fluids: water and milk. The dependence of the vortex characteristics on the main control parameter is investigated.

Polarisation capture of the wave frontof depolarised speckle radiationby a holographic laser mode

The polarisation properties of a reference-free hologram inside which the diffraction of the recording depolarised radiation from the speckle inhomogeneities is insignificant, are studied. It is shown that such a hologram, used asthe output mirror of a ring laser, has selective properties forthe specklon waves comprising the lasing mode. The behaviour of the fundamental mode of a holographic laser with aninhomogeneous birefringent medium inside the cavity is analysed. Conditions for attaining amplitude-phase relations between mode specklons, which are required for vector phaseconjugation of depolarised speckle signal, are determined.

Ultrafast two-dimensional ultrasonic speckle velocimetry: A tool in flow imaging

A technique for imaging fluid flows is proposed, based on the ultrafast analysis of the ultrasonic speckle signal backscattered by particles following the flow. Such an “ultrasonic speckle velocimetry” (USV) provides two-dimensional measurements of one component of the fluid velocity field at about 5000 frames per second. USV is applied to three different flows and future improvements of the technique are described.

A speckle velocimeter using a semiconductor laser with external opticalfeedback from a moving surface: effects of system parameters on thereproducibility and accuracy of measurements

A speckle velocimeter exploiting the effects of external opticalfeedback in a semiconductor laser is analysed. The laser diode is used both asthe light source and as the detector making the system self-detecting andself-aligning with a single optical axis. The velocimeter employs anautocorrelation method to analyse the detected self-mixing speckle signal. Thelinear relation between the velocity and the reciprocal of the autocorrelationtime of fluctuations in the amplitude of the speckle signal allows thedetermination of the velocity. Effects of various parameters of themeasurement system are analysed and the dependences of the reproducibility andaccuracy of the measurements on these parameters are discussed in order toachieve optimization of the system. The optimized system has a measurementerror of less than 1.4% within the range of calibration velocities and lessthan 2.7% outside this range.

Noninvasive blood flow measurement using speckle signals from a self-mixing laser diode: <italic>in vitro</italic> and <italic>in vivo</italic> experiments

A semiconductor laser speckle velocimeter that uses the self- mixing effect is studied for noninvasive relative blood flow measure- ments. The random modulation of intensity and spectra of the laser di- ode caused by the backcoupling of the scattered light from the red blood cells into the laser cavity is detected as a speckle signal with a photodi- ode inside the laser package. The autocorrelation of this self-mixing speckle signal gives information on the flow velocity of the blood. The proposed method is elucidated with in vitro and in vivo experiments. The results of these measurements are given together with a discussion of dependence of speckle signal of a self-mixing laser diode (SMLD) on various parameters such as velocity, hematocrit level of blood, and back- ground reflectance of blood suspension. © 2000 Society of Photo-Optical Instru- mentation Engineers. (S0091-3286(00)01009-6)

Self-mixing laser speckle velocimeter for blood flow measurement

A velocimeter using speckle phenomena in self-mixing laser diodes (SM-LDs) is used to evaluate the blood flow noninvasively. The mean frequency of the speckle signal obtained from the self-mixing laser diode reflects the activity of the blood flow in a certain probing area. The experimental results show that this new type of speckle velocimeter can be useful for the relative evaluation of blood flow in human tissues.