Subnanosecond-resolution phase-resolved fluorescence imaging technique for biomedical applications

Characterization of fluorescence emissions from cells often leads to conclusive results in the early detection of cellular abnormalities. Cellular abnormalities can be characterized by their difference in the fluorescence lifetime, which may be less than nanoseconds. A sensitive frequency domain technique, also called a phase-resolved fluorescence imaging technique, is proposed in which fluorescence emissions at the same wavelengths can more effectively be separated with subnanosecond resolution in their lifetime difference. The system configuration is optimized by incorporating even-step phase shifting in the homodyne-assisted signal-processing concept along with the phase-resolved fluorescence technique to eliminate the dc offsets of emission. Experiments are carried out with simulated samples composed of two fluorescence emissions of the same wavelength but with different lifetime values. Suppression of either of the fluorescence emissions by selective imaging of the other validates the superiority of the proposed technique. Hence, this technique can potentially be applied in the early detection of cellular abnormalities.     

Imaging strongly scattering media using a multiple frequency distorted Born iterative method

The distorted Born iterative (DBI) method is a powerful approach for solving the inverse scattering problem for ultrasound tomographic imaging. This method alternates between solving the inverse scattering problem for the scattering function and the forward scattering problem for the total field and the inhomogeneous Green's function. The algorithm is initialized using the basic Born inverse solution. One fundamental problem is the algorithm diverges for strongly scattering media. This is caused by the limitation of the Born assumption in estimating the initial step of the algorithm. We present a multiple frequency DBI approach to alleviate this problem, thus extending the applicability of the DBI method to the level of dealing with biological tissue. In this multiple frequency approach, a low frequency DBI-based solution, is used to initialize the algorithm at higher frequencies. The low frequency allows convergence of the algorithm to a contrast level that is close to the true level, however, with a poor spatial resolution. The high frequency improves the spatial resolution while preserving convergence because the difference between the true contrast and the initial contrast is relatively small. We present numerical simulations that demonstrate the ability of this method to reconstruct strongly scattering regions.     

Phase aberration correction using near-field signal redundancy. I. Principles [Ultrasound medical imaging

The signal redundancy principle in the near field is analyzed quantitatively. It is found that common midpoint signals are not identical (or redundant) for echoes coming from arbitrary target distributions in the near field. A dynamic near-field correction is proposed to reduce the difference between common midpoint signals for echoes coming from the region of interest. When phase aberrations are present, it is shown that the dynamic correction can generally be done assuming no phase aberration, and the relative time-shift between common midpoint signals can be used to measure phase-aberration profiles. A phase-aberration correction algorithm based on that principle is proposed. In this algorithm, after common midpoint signals are collected they are dynamically corrected for near-field effects and cross-correlated with one another. In a related way, the phase errors are measured from peak positions of these cross-correlation functions. The phase-aberration profile across the array is derived from these measurements. The relationship between the errors in the derived phase aberration profile and the errors in the measured relative time-shift between common midpoint signals is derived. A method for treating the situation of different transmission and reception phase-aberration profiles is also proposed. This algorithm works for general target distributions, iteration is not required, and it can be used in other near-field, pulse-echo, imaging systems.     

Nonlinear model structure identification of complex biomedical data using a genetic-programming-based technique

In this contribution, a genetic programming (GP)-based technique, which combines the ability of GP to explore both automatically and effectively, the whole set of candidate model structures and the robustness of evolutionary multimodel partitioning filters, is presented. The method is applied to the nonlinear system identification problem of complex biomedical data. Simulation results show that the algorithm identifies the true model and the true values of the unknown parameters for each different model structure, thus assisting the GP technique to converge more quickly to the (near) optimal model structure. The method has all the known advantages of the evolutionary multimodel partitioning filters, that is, it is not restricted to the Gaussian case; it is applicable to on-line/adaptive operation and is computationally efficient. Furthermore, it can be realized in a parallel processing fashion, a fact which makes it amenable to very large scale integration implementation.     

Description and simulation of an active imaging technique utilizing two speckle fields: iterative reconstructors

Quasi-monochromatic light will form laser speckle upon reflection from a rough object. This laser speckle provides information about the shape of the illuminated object. In a prior paper [J. Opt. Soc. Am. A 19, 444 (2002)], it was shown that two intensities of two speckle patterns and their interference are sufficient to produce an unambiguous (except for object translation) band-limited image of the object, based on a root-matching technique described therein, in the absence of measurement error and in the case of distinct roots of the field polynomials and their complex conjugates. On the other hand, algorithms based on the root-matching technique are found to be slow and sensitive to noise. So motivated, several other techniques are applied to the problem, including phase retrieval, expectation maximization, and statistical maximization. The phase-retrieval and expectation-maximization techniques proved to be most effective for reconstructions of complex objects larger than 10 pixels across, and high-quality images were formed by using three independent sets of two-field data (three frames of two-wavelength data), each comprising two speckle intensity patterns and their interference. Two additional results of note are reported. First, the expectation-maximization algorithm produced relatively good images when three or more frames each of only one speckle intensity pattern (data at just one wavelength) were used and second, the phase-retrieval algorithm when only the object autocorrelation was used also produced relatively good images for the chosen test object.     

Characterization of reinforced syntactic foams using ultrasonic imaging technique

Non-destructive evaluation of defects like voids in syntactic foam reinforced with epoxy compatible chopped strand glass fibres, employing ultrasonic C-scan immersion through transmission method, was under-taken. The results showed that in four of the five similarly processed foam samples, the voids were uniformly spread while in the fifth, which was processed by a different route, a large spread of low dense area was noticed emphasizing the influence that processing technique has on the amount of voids present in the composites.     

High-spatial-resolution sub-surface imaging using a laser-based acoustic microscopy technique

Scanning acoustic microscopy techniques operating at frequencies in the gigahertz range are suitable for the elastic characterization and interior imaging of solid media with micrometer-scale spatial resolution. Acoustic wave propagation at these frequencies is strongly limited by energy losses, particularly from attenuation in the coupling media used to transmit ultrasound to a specimen, leading to a decrease in the depth in a specimen that can be interrogated. In this work, a laser-based acoustic microscopy technique is presented that uses a pulsed laser source for the generation of broadband acoustic waves and an optical interferometer for detection. The use of a 900-ps microchip pulsed laser facilitates the generation of acoustic waves with frequencies extending up to 1 GHz which allows for the resolution of micrometer-scale features in a specimen. Furthermore, the combination of optical generation and detection approaches eliminates the use of an ultrasonic coupling medium, and allows for elastic characterization and interior imaging at penetration depths on the order of several hundred micrometers. Experimental results illustrating the use of the laser-based acoustic microscopy technique for imaging micrometer-scale subsurface geometrical features in a 70-μm-thick single-crystal silicon wafer with a (100) orientation are presented.     

Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique

A frequency-domain method for implementing the synthetic aperture focusing technique is developed and demonstrated using computer simulation. As presented, the method is well suited to reconstructing ultrasonic reflectivity over a volumetric region of space using measurements made over an adjacent two-dimensional aperture. Extensive use is made of both one- and two-dimensional Fourier transformations to perform the temporal and spatial correlation required by the technique, making the method well suited to general-purpose computing hardware. Results are presented demonstrating both the lateral and axial resolution achieved by the method. The effect of limiting the reconstruction bandwidth is also demonstrated.     

A 3-GHz microwave imaging system based on a modulated scattering technique and on a modified Born approximation

This article deals with short-range active microwave imaging. In particular, a 3-GHz imaging system based on a modulated scattering technique is described. By using a probe made of an array of (passive) dipoles rotating around a test area, it is possible to measure the field scattered by a long cylinder under approximately transverse-magnetic illumination conditions. Moreover, some considerations are made concerning the relationship for the modulated scattering techniques. Finally, some preliminary experimental results are presented. They were obtained by using a numerical approach to the electromagnetic inverse scattering problem which is developed in the spatial domain, and by making use of a modified Born approximation. © 1998 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 9, 395–403, 1998     

Non-destructive evaluation of composite materials using a capacitive imaging technique

This paper describes the application of the capacitive imaging technique to the inspection of composite materials. The fundamental theory of the capacitive imaging technique was briefly described. Experiments using prototype capacitive imaging probes were also carried out. The proof-of-concept results indicated that the capacitive imaging technique could be used to detect cracks and delaminations in the glass fibre composite, defects in the aluminium core through the glass fibre face as well as surface features on the carbon fibre specimens.     

Lateral RF image synthesis using a synthetic aperture imaging technique

The oscillating profile naturally present in ultrasound images has been shown to be extremely valuable in different applications, particularly in motion estimation. Recent studies have shown that it is possible to produce images with transverse oscillations (TOs) based on a specific type of beamforming. However, there is still a great difference between the nature of the lateral oscillations produced with current methods and the axial profile of ultrasound images. In this study, we propose to combine synthetic aperture imaging (synthetic transmit aperture, STA) using a specific beamformer in both transmit mode and receive mode combined with a heterodyning demodulation method to produce lateral radiofrequency signals (LRFs). The aim was to produce lateral signals as close as possible to conventional axial signals, which would make it possible to estimate lateral displacements with the same accuracy as in the axial direction. The feasibility of this approach was validated in simulation and experimentally on an ultrasound research platform, the Ultrasonix RP system. We show that the combination of STA and the heterodyning demodulation can divide the wavelength of the LRF signals by 4 and divide the width of the lateral envelope of the point spread function (PSF) by 2 compared with the previous approaches using beamforming in receive mode only. Finally, we also illustrate the potential of our beamforming for motion estimation compared with previous TO methods.     

Fine elasticity imaging utilizing the iterative RF-echo phase matching method

To non-invasively quantify elasticity of soft tissue, we previously developed the iterative two-dimensional (2-D) rf-echo phase matching method for accurately measuring a 2-D displacement vector field generated in vivo in soft tissue during acquisition of two successive rf-echo data frames. We also developed a stable method for uniquely reconstructing a shear modulus distribution using strains derived from the measurement data. However, as in our measurement method a displacement is determined by using the phase characteristics of the finite local echo data as the index to iteratively search for the corresponding local data, change of the local phase characteristics due to tissue deformation deteriorates the accuracy of the determination. Thus, we improve the previous method such that, in principle, the displacement can be determined using an infinitesimal phase characteristics. That is, we incorporate an effective mechanism into the previous iterative phase matching scheme: the local size is made suitably smaller during the iterative phase matching. The demonstrated ability of measurement and reconstruction in simulation, and experiments on in vitro in pork rib and in vivo in breast tissue, shows this refinement allows not only better spatial resolution of the shear modulus image but also improved accuracy, and indicates that the improved method has a high potential to be applied for various soft tissues.     

Practical implementation of the phase-quantization technique in an iterative Fourier-transform algorithm

Implementation of the phase-quantization method in the design of computer-generated holograms (CGHs), especially binary CGHs, is a major influence on the diffraction efficiency of the CGHs. We describe the use of various quantization methods in the various steps of an iterative Fourier-transform algorithm and compare the root-mean-square (RMS) errors of the CGHs. As a result of the practical quantization technique, the most effective RMS value of a binary CGH should be decreased by at least 13% to retain maximum diffraction efficiency.     

漫话数字影像技术

本文阐述了有关数字影像技术的一些基本概念、其特征、优越性和发展的成果,以及所采用的装备和材料。在数字影像技术的基础上,已开发出新的一代影像设备系列,为当代各行各业的影像体系开创了新的前景,包括:电影、电视、印刷、通讯等等。     

Analysis of cracked plates using an iterative hybrid technique of boundary element method and distributed dislocation method

An iterative hybrid technique of boundary element method (BEM) and distributed dislocation method (DDM) is introduced for solving two dimensional crack problems. The technique decomposes the problem into (n + 1) subsidiary problems where n is the number of crack branches. The required solution will be the sum of these (n + 1) solutions. The first subsidiary problem is to find the stress distribution induced in the plate in the absence of the crack using BEM. All of the remaining subsidiary problems, are stress disturbance ones that will be solved using DDM. The results will be added and compared with the boundary conditions of the original problem. Iteration will be performed between the plate boundaries and crack faces until all of the boundary conditions are satisfied.     

Validity criterion for the Born approximation convergence in microscopy imaging: comment

A recent paper [J. Opt. Soc. Am. A26, 1147 (2009)] proposed a new criterion to identify the validity of the Born approximation. In this paper, we present our doubts on the mathematical basis and the applicability of this criterion.     

Evaluation of aerosol sizing characteristic of an impactor using imaging plate technique

The activity-size distribution of radon decay products are normally determined using two approaches: direct and indirect. The present study utilises the direct approach to evaluate sizing information of a low pressure cascade impactor using imaging plate (IP) technique for radon decay products. The experiment verified the use of the collection media as suggested by the manufacturer of impactor and proposed a few improvements toward sizing characteristics of impactor. The obtained relative activity-size distribution of radon decay products presents a sharp unimodal log-normal distribution of the particle characterised by activity median aerodynamic diameter (AMAD) of 268 nm and geometric standard deviation (sigma(g)) of 1.66. The obtained data with all the suggested improvements were evaluated by the data obtained from a scanning mobility particle sizer (SMPS, Model 3934, TSI Inc), as reference data. The verification lead to a derivative area ratio of 0.803 between the reference and experimental data.     

Evidence for kinetic inhomogeneity in the curing of epoxy using the near-infrared multispectral imaging technique

The kinetics of curing of an epoxy resin by amine was studied using a near-infrared (NIR) multispectral imaging spectrometer. This imaging spectrometer is capable of sensitively and rapidly recording NIR spectral images of a sample because it was constructed with an acousto-optic tunable filter and an InGaAs focal plane array NIR camera. The high sensitivity and fast scanning ability of the spectrometer make it suitable for kinetic determination of fast reactions. Additionally, it has features that conventional NIR spectrometers cannot offer, namely, its ability to provide kinetic information at different positions within a sample. Furthermore, the high spatial resolution and sensitivity of the InGaAs camera make it possible to determine the kinetics from data collected by a single pixel in the camera. The kinetics of curing of epoxy by amine, determined by this multispectral imaging instrument, show that the reaction rates within the sample are very inhomogeneous. Because of this kinetic inhomogeneity, differences in the degrees of cure at different positions within the sample can be as high as 37% when data from only a single pixel were used for calculation. The inhomogeneity was not be observed if an average of a large number of pixels were used.