Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.     

A new system for real-time synthetic aperture ultrasonic imaging

The authors devised a way to generate in real time a cross-sectional image of an object with uniformly high resolution based on the synthetic aperture focusing technique (SAFT). A computer simulation was conducted to study the effects of essential parameters on the resulting images. An imaging system was built that produces a cross-sectional image composed of an assembly of line images of depth direction, i.e. processed A-scan images, and displays a scroll picture on a CRT (cathode ray tube) with no interruption regardless of the object size. It takes only 3 ms from the start of transmission of the ultrasonic wave to the completion of a line image reconstruction, and the framed image on a CRT is updated at the TV rate of 1/30 s. Imaging experiments were conducted using the system, and its expected performance was demonstrated.     

Noise analysis of a combined optical coherence tomograph and a confocal scanning ophthalmoscope

A fiberized optical coherence tomographic (OCT) system is modified to produce a confocal image similar to that produced by a scanning laser ophthalmoscope. Two possible configurations are presented, one that can deliver either an OCT or a confocal image and another that is capable of producing both the OCT and the confocal images simultaneously. Using the later configuration, we demonstrate such images from the retina in the living eye. The penalty in terms of performance reduction introduced into the optical coherence tomograph when integrated with a confocal receiver and the signal-to-noise ratio analysis for the different confocal receiver configurations are presented.     

Nontranslational three-dimensional profilometry by chromatic confocal microscopy with dynamically configurable micromirror scanning

A confocal microscope profilometer, which incorporates chromatic depth scanning with a diffractive optical element and a digital micromirror device for configurable transverse scanning, provides three-dimensional (3D) quantitative measurements without mechanical translation of either the sample or the microscope. We used a microscope with various objective lenses (e.g., 40x, 60x, and 100x) to achieve different system characteristics. With a 100x objective, the microscope acquires stable measurements over a 320 mum x 240 mum surface area with a depth resolution of 0.39 mum at a 3-Hz scan rate. The total longitudinal field of view is 26.4 mum for a wavelength tuning range of 48.3 nm. The FWHM value of the longitudinal point-spread function is measured to be 0.99 mum. We present 3D measurements of a four-phase-level diffractive element and an integrated-circuit chip. The resolution and the accuracy are shown to be equivalent to those found with use of conventional mechanical scanning.     

Spatially resolved Raman spectroscopy of single- and few-layer graphene

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry.     

High-resolution full-field optical coherence microscopy using a Mirau interferometer for the quantitative imaging of biological cells

In this paper quantitative imaging of biological cells using high-resolution full-field optical coherence microscopy (FF-OCM) is reported. The FF-OCM was realized using a swept-source system, a Mirau interferometer, and a CCD camera (a two-dimensional detection unit). A Mirau-interferometric objective lens was used to generate the interferometric signal. The signal was analyzed by a Fourier analysis technique. Optically sectioned amplitude images and a quantitative phase map of biological cells such as onion skin and red blood cells (RBCs) are demonstrated. Further, the refractive index profile of the RBCs is also presented. For the 50× Mirau objective, the experimentally achieved axial and transverse resolution of the present system are 3.8 and 1.2 μm, respectively. The CCD provides parallel detection and measures enface images without X, Y, Z mechanical scanning.     

Design and imaging properties of a laser scanning microscope with a position-sensitive detector

An economical method of microscopic image formation that employs a raster-scanning laser beam focused on a sample, while a non-imaging detector receives the scattered light, is presented. The images produced by this method are analogous to the scanning electron microscopy with visible effects of shadowing and reflection. Compared to a conventional wide-field imaging system, the system allows for a greater flexibility, as the variety of optical detectors, such as PMT and position-sensitive quadrant photodiode can be used to acquire images. The system demonstrates a simple, low-cost method of achieving the resolution on the order of a micron. A further gain in terms of resolution and the depth of focus by using Bessel rather than Gaussian beams is discussed.     

High-speed path-length scanning with a multiple-pass cavity delay line

Techniques for high-speed delay scanning are important for low-coherence interferometry, optical coherence tomography, pump probe measurements, and other applications. We demonstrate a novel scanning delay line using a multiple-pass cavity. Differential delays are accumulated with each pass so that millimeter delays can be generated with tens of micrometer mirror displacements. With special design criteria, misalignment sensitivity can be dramatically reduced. The system is demonstrated to scan 6 m/s at 2-kHz repetition rates. Real-time optical coherence tomography imaging with 500 pixel images at four frames/s is performed. Using a Cr:forsterite laser source, we obtained axial image resolutions of 6 microm with 92-dB sensitivity.     

Scanning holographic microscopy with transverse resolution exceeding the Rayleigh limit and extended depth of focus

We demonstrate experimentally that the method of scanning holographic microscopy is capable of producing images reconstructed numerically from holograms recorded digitally in the time domain by scanning, with transverse and axial resolutions comparable to those of wide-field or scanning microscopy with the same objective. Furthermore, we show that it is possible to synthesize the point-spread function of scanning holographic microscopy to obtain, with the same objective, holographic reconstructions with a transverse resolution exceeding the Rayleigh limit of the objective up to a factor of 2 in the limit of low numerical aperture. These holographic reconstructions also exhibit an extended depth of focus, the extent of which is adjustable without compromising the transverse resolution.     

Shape connection by pattern recognition and laser metrology

Shape connection based on the pattern recognition of three-dimensional shapes is presented. In this technique, the object shape is reconstructed by laser scanning and image processing. The object is reconstructed from multiple views when an object occlusion appears. From this process, multiple parts of the object are reconstructed. Then, these parts are assembled to obtain the complete object shape. To perform the assembling, a matching procedure is applied to a transverse section of the multiple views by Hu moments. The depth of the transverse section is computed by an approximation network based on the behavior of the laser line and the camera position. Also, vision parameters are deduced by the network and image processing. In this manner, the shape connection is achieved automatically by computational algorithms. Therefore, errors of physical measurement are not passed to the reconstruction system. Thus, the performance and the accuracy of the reconstruction system are improved. This is elucidated by the comparison between the obtained results by the proposed technique and the obtained results by a contact method. Thus, a contribution in laser metrology for shape connection is achieved.     

Effect of lens distortion in optical step-and-scan lithography

A model is presented with which the effect of lens distortion on the optical transfer function is calculated for a step-and-scan lithographic system. The spatially varying lateral image shift and the relative loss in modulation depth are derived from the exposure pattern that is built up during the image scanning. Other important phenomena such as lens aberrations, the effect of finite image aperture, and polarization are deliberately left out of the model; from the simplified model, analytic expressions can be obtained that relate the distortion coefficients of the projection system to the local shift and the relative loss in modulation depth of the exposed pattern that is built up during scanning. As a scanning aperture, we have taken either part of an annulus or a rectangular section in the image field.     

Endoscopic optical coherence tomography with a modified microelectromechanical systems mirror for detection of bladder cancers

Experimental results of a modified micromachined microelectromechanical systems (MEMS) mirror for substantial enhancement of the transverse laser scanning performance of endoscopic optical coherence tomography (EOCT) are presented. Image distortion due to buckling of MEMS mirror in our previous designs was analyzed and found to be attributed to excessive internal stress of the transverse bimorph meshes. The modified MEMS mirror completely eliminates bimorph stress and the resultant buckling effect, which increases the wobbling-free angular optical actuation to greater than 37 degrees, exceeding the transverse laser scanning requirements for EOCT and confocal endoscopy. The new optical coherence tomography (OCT) endoscope allows for two-dimensional cross-sectional imaging that covers an area of 4.2 mm x 2.8 mm (limited by scope size) and at roughly 5 frames/s instead of the previous area size of 2.9 mm x 2.8 mm and is highly suitable for noninvasive and high-resolution imaging diagnosis of epithelial lesions in vivo. EOCT images of normal rat bladders and rat bladder cancers are compared with the same cross sections acquired with conventional bench-top OCT. The results clearly demonstrate the potential of EOCT for in vivo imaging diagnosis and precise guidance for excisional biopsy of early bladder cancers.     

Tunneling-stabilized magnetic force microscopy of bit tracks on ahard disk

A scanning tunneling microscope (STM) for surface magnetic force measurements on thin-film longitudinal magnetic storage media is described. The usual rigid PtIr tip of the STM was replaced by a flexible Fe-film tip and the tip position was stabilized near the surface of the sample using the STM feedback system as tunneling occurs between the tip and sample surface. Images of a CoCrTa thin-film hard disk showing 5 μm×3 μm bit tracks written by the ferrite head of a computer disk drive are presented. The images shown are comparable to images of the bit tracks on textured surfaces using either ferrofluid decoration or other magnetic force microscopy (MFM) imaging techniques. The sensitivity of the Fe-film tip was such that the influence on the image due to magnetic forces was larger than the influence due to sample surface topography     

Fast-Fourier-domain delay line for in vivo optical coherence tomography with a polygonal scanner

We demonstrate in vivo optical coherence tomography using a Fourier-domain optical delay line constructed with a commercially available polygonal scanner. The 20-faceted polygonal mirror array, capable of scanning at rates up to 15 kHz, is implemented at 4 kHz to acquire 500 x 500 pixel images at 8 frames/s with a signal-to-noise ratio of 80 dB. Features of this delay line include scalability to high repetition rates, 98.6% linearity in group delay over 2 mm, and bandwidth support exceeding 150 nm. Images are obtained in an animal model (Xenopus laevis), and limitations due to phase-delay nonlinearity and polygon asymmetry are discussed.     

Temperature Dependence of the Delay in Pulse Radiation of an Injection Laser

A theoretical analysis of the temperature dependence of the delay in stimulated radiation of an injection laser is made with allowance for the temperature dependence of the lifetime of the current carriers and the threshold current at different excitation levels. In admissible regimes of excitation the dependence of the delay on the temperature is investigated experimentally. The experimental results agree satisfactorily with those predicted. A method of stabilization of radiation delay in an injection laser based on priority discrimination of pulse signals is described, and its efficiency is investigated experimentally.     

A Prism Line-scanner for High-speed Thermography

Thermographic instruments using single-element line scanning usually have very low line frequencies in comparison with television. To meet the requirements for higher frame rates in thermography and infra-red viewing a line scanner with an indium antimonide detector and a rotating germanium prism has been designed. It produces heat images with 100 lines per frame on a television-type display tube at a flicker-free frame rate. The sweep efficiency of the scanning system is almost 100 per cent. The optical aberrations introduced by the scanning prism are analysed and found to be at a minimum when the refractive index lies between 3 and 4. Examples of heat pictures taken with different modes of operation are given.     

Photoacoustic holographic imaging of absorbers embedded in silicone

Light absorbing objects embedded in silicone have been imaged using photoacoustic digital holography. The photoacoustic waves were generated using a pulsed Nd:YAG laser, λ=1064 nm, and pulse length=12 ns. When the waves reached the silicone surface, they were measured optically along a line using a scanning laser vibrometer. The acoustic waves were then digitally reconstructed using a holographic algorithm. The laser vibrometer is proven to be sensitive enough to measure the surface velocity due to photoacoustic waves generated from laser pulses with a fluence allowed for human tissue. It is also shown that combining digital holographic reconstructions for different acoustic wavelengths provides images with suppressed noise and improved depth resolution. The objects are imaged at a depth of 16.5 mm with a depth resolution of 0.5 mm.     

Decoding of depth and motion in ambiguous binocular perception

We have shown previously that random dots with an interocular time delay (ITD), the time difference of the onset of dots between the two eyes, yield both apparent depth and motion, although depth and velocity are covariant and, thus, ITD is inherently ambiguous. The depth of random dots with ITD was proportional to ITD, suggesting that the visual system assumes a constant velocity of the dots and determines depth on the basis of this constant velocity. We performed psychophysical experiments to investigate whether subjects perceive a constant velocity with a variety of ITDs in random dots aligned along a single vertical line that ensures neither apparent motion nor accidental disparity between the dots. The results showed that subjects perceive a constant velocity for a variety of ITDs with simultaneous perception of depth in proportion to ITD, indicating the priority of depth over velocity in ambiguous binocular perception derived from ITD.     

Reconstruction of three-dimensional chemiluminescence images with a maximum entropy deconvolution algorithm

Three-dimensional (3D) images of flame emission are reported using a single direction of optical access. A Cassegrain system was designed with narrow depth of field. Images from this system are dominated by emission from the focused object plane with defocused contributions from out-of-plane structures. Translation of one mirror in the system allows for scanning the object plane through the flame. Images were taken at various depths to create a family of images. Reconstruction of the 3D flame structure was accomplished using a maximum entropy algorithm adapted for use with 3D imaging. Spatial resolution in the direction of imaging is examined using laminar flames with variable offset.     

Scanning holographic microscopy of three-dimensional fluorescent specimens

We demonstrate experimentally the three-dimensional reconstructions of fluorescent biological specimens using scanning holographic microscopy. Three-dimensional reconstructions with transverse resolution below about 1 microm of transmission and fluorescence emission images are presented and analyzed. The limitations of the method are discussed.