Wave Imaging Applications Research Articles

Coherent enhancement of optical remission in diffusive media

Remitted waves are used for sensing and imaging in diverse diffusive media from the Earth's crust to the human brain. Separating the source and detector increases the penetration depth of light, but the signal strength decreases rapidly, leading to a poor signal-to-noise ratio. Here, we show, experimentally and numerically, that wavefront shaping a laser beam incident on a diffusive sample enables an enhancement of remission by an order of magnitude at depths of up to 10 transport mean free paths. We develop a theoretical model which predicts the maximal remission enhancement. Our analysis reveals a significant improvement in the sensitivity of remitted waves to local changes of absorption deep inside diffusive media. This work illustrates the potential of coherent wavefront control for noninvasive diffuse wave imaging applications, such as diffuse optical tomography and functional near-infrared spectroscopy.

Design of a V-band low noise amplifier for passive millimeter wave imaging application

This paper presents a design methodology for wideband low noise amplifier (LNA) with flat gain and low noise figure (NF). A parallel RC network loaded stub circuit is proposed to enhance simultaneous noise and impedance matching. Moreover, the same structure is applied to compensate for transistor gain roll-off with frequency. By properly selecting the stub lengths for different stages, flat gain and low NF are obtained in wideband. The method is verified by designing a V-band 4-stage LNA based on 0.15 μm GaAs pHEMT process. The LNA features 15.2 dB average gain within 45–61 GHz bandwidth. In the required band of 51–57 GHz, the LNA achieves flat gain of 14.9 ± 0.3 dB and low NF of 2.6–4.2 dB. It consumes 30 mA current from 3 V supply and occupies area of 2 mm × 1.3 mm. Although the LNA is inherently multipurpose design, it is particularly suited for passive millimeter wave imaging due to its characteristic of high sensitivity.

Tuning the working frequency of elastic metamaterials by heat

The limitation of passive elastic metamaterials is their fixed and narrow working frequency range. In this work, the manipulation of elastic waves using heat by an elastic metamaterial with resonant microstructures is presented for the first time. The effective density of the elastic metamaterial at different wave frequencies and temperatures is determined using effective continuum theory. We proved that the elastic metamaterial can have not only frequency - but also temperature-dependent negative effective density. By properly selecting materials for the building block, the effective mass density of the elastic metamaterial can be varied by increasing the temperature of the material. Elastic wave attenuation, waveguiding and wave focusing of the elastic metamaterial are then demonstrated by controlling the temperature of the elastic metamaterial. The developed methodology could significantly influence research areas including elastic wave attenuation, waveguiding, wave imaging and numerous other applications.

A flexible and ultra-broadband terahertz wave absorber based on graphene–vertically aligned carbon nanotube hybrids

A flexible THz wave absorber based on G-VACNT hybrids is demonstrated for ultra-broadband THz wave absorption and imaging applications.

Dimension-Factorized Range Migration Algorithm for Regularly Distributed Array Imaging.

The two-dimensional planar MIMO array is a popular approach for millimeter wave imaging applications. As a promising practical alternative, sparse MIMO arrays have been devised to reduce the number of antenna elements and transmitting/receiving channels with predictable and acceptable loss in image quality. In this paper, a high precision three-dimensional imaging algorithm is proposed for MIMO arrays of the regularly distributed type, especially the sparse varieties. Termed the Dimension-Factorized Range Migration Algorithm, the new imaging approach factorizes the conventional MIMO Range Migration Algorithm into multiple operations across the sparse dimensions. The thinner the sparse dimensions of the array, the more efficient the new algorithm will be. Advantages of the proposed approach are demonstrated by comparison with the conventional MIMO Range Migration Algorithm and its non-uniform fast Fourier transform based variant in terms of all the important characteristics of the approaches, especially the anti-noise capability. The computation cost is analyzed as well to evaluate the efficiency quantitatively.

On the Use of Ultrasound-Based Technology for Cargo Inspection

A new guided wave imaging application for fast, low-cost ultrasound-based cargo scanning system is proposed. The ultimate goal is the detection of high-atomic-number, shielding containers used to diminish the radiological signature of nuclear threats. This ultrasonic technology has the potential to complement currently deployed X-ray-based radiographic systems, thus enhancing the probability of detecting nuclear threats. An array of ultrasonic transceivers can be attached to the metallic structure of the cargo to create a guided Lamb wave. Guided medium thickness and composition variation creates reflections whose placement can be revealed by means of an imaging algorithm. The knowledge of the reflection position provides information about the shielding metallic container location inside the cargo. Moreover, due to the low coupling between metallic and nonmetallic surfaces, only the footprint of metallic containers shows up in the imaging results, thus avoiding false positives from plastic or wooden assets. As imaging capabilities are degraded if working with dispersive Lamb wave modes, the operating frequency is tuned to provide a tradeoff between low dispersion and real-time image resolution. Reflected waves in the guided domain bounds may limit the performance of imaging methods for guided media. This contribution proposes a solution based on real-time Fourier domain analysis, where plane wave components can be filtered out, thus removing nondesired contributions from bounds. Several realistic examples, scaled due to limited calculation capabilities of the available computational resources, are presented in this work, showing the feasibility of the proposed method.

Stochastic piecewise linear function fitting with application to ultrasound shear wave imaging.

Piecewise linear function fitting is ubiquitous in many signal processing applications. Inspired by an application to shear wave velocity imaging in ultrasound elastography, this paper presents a discrete state-space Markov model for noisy piecewise linear data and also proposes a tractable algorithm for maximum a posteriori estimation of the slope of each segment in the piecewise linear function. The number and locations of breaks is handled indirectly by the stochastics of the Markov model. In the ultrasound shear wave imaging application, these slope values have concrete physical interpretation as being the reciprocal of the shear wave velocities in the imaged medium. Data acquired on an ellipsoidal inclusion phantom shows that this algorithm can provide good contrast of around 6 dB and contrast to noise ratio of 25 dB between the stiff inclusion and surrounding soft background. The phantom validation study also shows that this algorithm can be used to preserve sharp boundary details, which would otherwise be blurred out if a sliding window least squares filter is applied.

Veselago lensing with ultracold atoms in an optical lattice

Veselago pointed out that electromagnetic wave theory allows for materials with a negative index of refraction, in which most known optical phenomena would be reversed. A slab of such a material can focus light by negative refraction, an imaging technique strikingly different from conventional positive refractive index optics, where curved surfaces bend the rays to form an image of an object. Here we demonstrate Veselago lensing for matter waves, using ultracold atoms in an optical lattice. A relativistic, that is, photon-like, dispersion relation for rubidium atoms is realized with a bichromatic optical lattice potential. We rely on a Raman π-pulse technique to transfer atoms between two different branches of the dispersion relation, resulting in a focusing that is completely analogous to the effect described by Veselago for light waves. Future prospects of the demonstrated effects include novel sub-de Broglie wavelength imaging applications.

Tapered labyrinthine acoustic metamaterials for broadband impedance matching

We present five kinds of labyrinthine or space-coiling acoustic metamaterials with tapered channels and apertures. These designs exhibit negative index behavior with modest dispersion, and also have substantially improved impedance matching compared to previously investigated labyrinthine cells. Experimentally measured effective material parameters are in good agreement with numerically computed results for the first two designs. Numerical results are presented for the other three unit cells. By virtue of their design tunability and small size, these tapered labyrinthine acoustic metamaterials show potential as building blocks for a wide range of acoustic wave manipulation and imaging applications.

Evaluating the feasibility of acoustic radiation force impulse shear wave elasticity imaging of the uterine cervix with an intracavity array: a simulation study

The uterine cervix softens, shortens, and dilates throughout pregnancy in response to progressive disorganization of its layered collagen microstructure. This process is an essential part of normal pregnancy, but premature changes are associated with preterm birth. Clinically, there are no reliable noninvasive methods to objectively measure cervical softening or assess cervical microstructure. The goal of these preliminary studies was to evaluate the feasibility of using an intracavity ultrasound array to generate acoustic radiation force impulse (ARFI) excitations in the uterine cervix through simulation, and to optimize the acoustic radiation force (ARF) excitation for shear wave elasticity imaging (SWEI) of the tissue stiffness. The cervix is a unique soft tissue target for SWEI because it has significantly greater acoustic attenuation (α = 1.3 to 2.0 dB·cm(-1)·MHz(-)1) than other soft tissues, and the pathology being studied tends to lead to an increase in tissue compliance, with healthy cervix being relatively stiff compared with other soft tissues (E ≈ 25 kPa). Additionally, the cervix can only be accessed in vivo using a transvaginal or catheter-based array, which places additional constraints on the excitation focal characteristics that can be used during SWEI. Finite element method (FEM) models of SWEI show that larger-aperture, catheter-based arrays can utilize excitation frequencies up to 7 MHz to generate adequate focal gain up to focal depths 10 to 15 mm deep, with higher frequencies suffering from excessive amounts of near-field acoustic attenuation. Using full-aperture excitations can yield ~40% increases in ARFI-induced displacements, but also restricts the depth of field of the excitation to ~0.5 mm, compared with 2 to 6 mm, which limits the range that can be used for shear wave characterization of the tissue. The center-frequency content of the shear wave particle velocity profiles ranges from 1.5 to 2.5 kHz, depending on the focal configuration and the stiffness of the material being imaged. Overall, SWEI is possible using catheter-based imaging arrays to generate adequate displacements in cervical tissue for shear wave imaging, although specific considerations must be made when optimizing these arrays for this shear wave imaging application.

OPTICAL PERFORMANCE ANALYSIS USING A POINT SPREAD FUNCTION AND MODULATION TRANSFER FUNCTION FOR W-BAND PMMW IMAGING SYSTEM BASED ON QUASI OPTICS FOCUSED ARRAY OF DIELECTRIC ROD WAVEGUIDE ANTENNA

The appropriateness of dielectric loaded antenna for the passive millimeter wave imaging application has recently been demonstrated. In this paper, we analyze the optical performance of the passive millimeter wave (PMMW) imaging system based on a 1D focal plane array (FPA) of dielectric rod waveguide (DRW) antennas. A flrst step in the design process is to analyze the image quality potential of 1D FPA-based imaging system in terms of the point spread function (PSF) and the modulation transfer function (MTF). We consider the efiect

Application of laser ultrasound for surface acoustic wave microscopy

This paper discusses the approach we have taken to developing laser ultrasonics so that it can fufil many of the functions normaly associated with the scanning acoustic microscope. In particular, we discuss applications involving accurate measurement of surface acoustic wave velocity and imaging applications involving the scattering of surface acoustic waves (SAWs). We present some novel techniques for detection ofSAWs and show how computer-generated holography should be a valuable solution to improve the laser generation efficiency of SAWs.

Characteristics of Millimeter-Wave Imaging Optics Using a Single Offset Paraboloidal Reflector

ABSTRACT All-reflective systems with focal–plane arrays of monolithic elements promise major improvements in millimeter wave imaging applications. In this work high resolution, diffraction limited, wide–field imaging performance is investigated for a single offset paraboloidal reflector. Using accurate electromagnetic vector diffraction analysis, off-axis image degradation is computed for progressively larger field-of-view. Comparisons validate the estimates obtained from the closed-form expressions used to initiate the design process. As a practical example of the design process, a system with an unobstructed 500 wavelengths entrance aperture diameter is designed and its imaging performance, at various locations within a 6048 elements image plane, is computed. The offset reflector dimensions result in a less than 0.2 degree half-power beamwidth and about 50 percent directivity loss at the edges of a rectangular 13 × 6.5 degrees image field of view. Review of these results demonstrate the high–performance...

Aperture-domain signal processing

An elementary reconsideration of first principles in aperture-domain processing of wave phenomena for reception (and, by reciprocity, for transmission) can yield revealing, and in some cases novel, insights into what can or cannot be achieved. The subjects covered here include direction-selective reception, superdirectivity, direction finding, focused near-field reception, circular arrays and circular modes in such arrays, and the new concepts of arrays composed of ‘random symmetrical pairs’, and of real gain in omnidirectional receiving antennas. The ideas are basic to all wave directional analysis and imaging applications, be they electromagnetic or acoustic, in radar or sonar, communications, navigation, surveillance or medical imaging.