Imaging of trace species distributions by degenerate four-wave mixing: diffraction effects, spatial resolution, and image referencing

We describe the theory of imaging by degenerate four-wave mixing (DFWM) using a standard diffraction theory of imaging by coherent light. We demonstrate that, even with the phase-conjugating geometry, no aberration correction can be achieved by DFWM imaging. We demonstrate the coherent nature of DFWM image formation using spatially modulated signals generated in flame OH in the phase-conjugating geometry. The intensity distribution in the Fourier plane of a telecentric lens system is shown to be the spatial Fourier transform of the object distribution characteristic of coherent imaging. The brightness of the DFWM signals exceeds that of similar laser-induced fluorescence signals that can be discriminated by restricting the aperture of the imaging system while still allowing a spatial resolution of approximately 70 ?m. DFWM imaging with the forward-folded boxcars geometry is demonstrated and used in a simple referencing scheme to compensate for structure on the images imposed by nonuniformity of the laser beams employed. Images formed in NO are used to illustrate that structure on a scale of less than 100 ?m arising from beam inhomogeneity can be removed by this referencing technique.     

X-ray reflection tomography: a new tool for surface imaging

We report here a novel technique of surface imaging by X-ray reflection tomography utilizing an ordinary laboratory X-ray source. The technique utilizes the line projection, at different rotation angles, of the reflected beam from a highly reflecting patterned sample at grazing incidence. Filtered back-projection algorithm is applied to the line projection data to reconstruct an image of the pattern on the sample surface. Spatial resolution currently obtained is ~1.6 mm. Nonetheless, we have achieved high correlation between the original image and the reconstructed image. This work is the first step in future efforts of nondestructive X-ray imaging for buried surfaces and interfaces.     

X-ray imaging using a consumer-grade digital camera

The recent advancements in consumer-grade digital camera technology and the introduction of high-resolution, high sensitivity CsBr:Eu²⁺ storage phosphor imaging plates make possible a new cost-effective technique for X-ray imaging. The imaging plate is bathed with red stimulating light by high-intensity light-emitting diodes, and the photostimulated image is captured with a digital single-lens reflex (SLR) camera. A blue band-pass optical filter blocks the stimulating red light but transmits the blue photostimulated luminescence. Using a Canon D5 Mk II camera and an f1.4 wide-angle lens, the optical image of a 240×180 mm² Konica CsBr:Eu²⁺ imaging plate from a position 230 mm in front of the camera lens can be focussed so as to laterally fill the 35×23.3 mm² camera sensor, and recorded in 2808×1872 pixel elements, corresponding to an equivalent pixel size on the plate of 88 μm. The analogue-to-digital conversion from the camera electronics is 13 bits, but the dynamic range of the imaging system as a whole is limited in practice by noise to about 2.5 orders of magnitude. The modulation transfer function falls to 0.2 at a spatial frequency of 2.2 line pairs/mm. The limiting factor of the spatial resolution is light scattering in the plate rather than the camera optics. The limiting factors for signal-to-noise ratio are shot noise in the light, and dark noise in the CMOS sensor. Good quality images of high-contrast objects can be recorded with doses of approximately 1 mGy. The CsBr:Eu²⁺ plate has approximately three times the readout sensitivity of a similar BaFBr:Eu²⁺ plate.     

Imaging of packaging-related problems in electronic components by scanning acoustic microscopy

Low-frequency (50 MHz) scanning acoustic microscopy has been applied to the problem of imaging defects in semiconductor packages. The results have been compared with the more established technique of X-ray shadow imaging. The scanning acoustic microscope has been found to be able to image a greater variety of defects than X-ray shadow imaging, although the acoustic image could often only be fully interpreted after comparison with the corresponding X-ray image. Scanning acoustic microscopy is now proving to be a valuable analysis technique for the detection and characterisation of packaging related problems.     

Diffractive electron imaging of nanoparticles on a substrate

The observation of the detailed atomic arrangement within nanostructures has previously required the use of an electron microscope for imaging. The development of diffractive (lensless) imaging in X-ray science and electron microscopy using ab initio phase retrieval provides a promising tool for nanostructural characterization. We show that it is possible experimentally to reconstruct the atomic-resolution complex image (exit-face wavefunction) of a small particle lying on a thin carbon substrate from its electron microdiffraction pattern alone. We use a modified iterative charge-flipping algorithm and an estimate of the complex substrate image is subtracted at each iteration. The diffraction pattern is recorded using a parallel beam with a diameter of approximately 50 nm, illuminating a gold nanoparticle of approximately 13.6 nm diameter. Prior knowledge of the boundary of the object is not required. The method has the advantage that the reconstructed exit-face wavefunction is free of the aberrations of the objective lens normally used in the microscope, whereas resolution is limited only by thermal vibration and noise.     

Fluorescence lidar imaging of historical monuments

What is believed to be the first fluorescence imaging of the facades of a historical building, which was accomplished with a scanning fluorescence lidar system, is reported. The mobile system was placed at a distance of ~60 m from the medieval Lund Cathedral (Sweden), and a 355-nm pulsed laser beam was swept over the stone facades row by row while spectrally resolved fluorescence signals of each measurement point were recorded. By multispectral image processing, either by formation of simple spectral-band ratios or by use of multivariate techniques, areas with different spectral signatures were classified. In particular, biological growth was observed and different stone types were distinguished. The technique can yield data for use in facade status assessment and restoration planning.     

Super‐Resolution Nonlinear Photothermal Microscopy

Super‐resolution fluorescence microscopy enables imaging of fluorescent structures beyond the diffraction limit. However, this technique cannot be applied to weakly fluorescent cellular components or labels. As an alternative, photothermal microscopy based on nonradiative transformation of absorbed energy into heat has demonstrated imaging of nonfluorescent structures including single molecules and ~1‐nm gold nanoparticles. However, previously photothermal imaging has been performed with a diffraction‐limited resolution only. Herein, super‐resolution, far‐field photothermal microscopy based on nonlinear signal dependence on the laser energy is introduced. Among various nonlinear phenomena, including absorption saturation, multiphoton absorption, and signal temperature dependence, signal amplification by laser‐induced nanobubbles around overheated nano‐objects is explored. A Gaussian laser beam profile is used to demonstrate the image spatial sharpening for calibrated 260‐nm metal strips, resolving of a plasmonic nanoassembly, visualization of 10‐nm gold nanoparticles in graphene, and hemoglobin nanoclusters in live erythrocytes with resolution down to 50 nm. These nonlinear phenomena can be used for 3D imaging with improved lateral and axial resolution in most photothermal methods, including photoacoustic microscopy.     

Milli X-ray fluorescence X-ray spectrum imaging for measuring potassium ion intrusion into concrete samples

Of particular interest when studying the effects of deicing solutions on concrete is the depth of penetration of ions from deicing salts. To determine the limits of positive ion infiltration, a method based on milli X-ray fluorescence (mXRF) has been developed. This method combines traditional energy dispersive spectrometry (EDS) with stage movement X-ray mapping to analyze comparatively large areas of concrete. The result is the ability to determine the depth of ion infiltration over a distance of tens of millimeters. The technique also requires minimal preparation of the sample, and due to the nature of the X-ray beam, concrete samples do not have to be coated to reduce charging. This paper describes in detail the method of mXRF with X-ray spectrum imaging (XSI) for concrete applications and shows two examples where potassium ion infiltration was measured with mXRF-XSI as part of broader studies in pavement durability.     

The acoustic lens design and in vivo use of a multifunctional catheter combining intracardiac ultrasound imaging and electrophysiology sensing

A multifunctional 9F intracardiac imaging and electrophysiology mapping catheter was developed and tested to help guide diagnostic and therapeutic intracardiac electrophysiology (EP) procedures. The catheter tip includes a 7.25-MHz, 64-element, side-looking phased array for high resolution sector scanning. Multiple electrophysiology mapping sensors were mounted as ring electrodes near the array for electrocardiographic synchronization of ultrasound images. The catheter array elevation beam performance in particular was investigated. An acoustic lens for the distal tip array designed with a round cross section can produce an acceptable elevation beam shape; however, the velocity of sound in the lens material should be approximately 155 m/s slower than in tissue for the best beam shape and wide bandwidth performance. To help establish the catheter's unique ability for integration with electrophysiology interventional procedures, it was used in vivo in a porcine animal model, and demonstrated both useful intracardiac echocardiographic visualization and simultaneous 3-D positional information using integrated electroanatomical mapping techniques. The catheter also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures.     

Elimination of conjugate image for holograms using a resolution redistribution optical system

A technique to alter the ratio of the horizontal and vertical resolution of a spatial light modulator has been proposed. This technique increases the horizontal resolution by a factor of K and decreases the vertical resolution by a factor of 1/K. The proposed technique increases the horizontal viewing angle by a factor of approximately K, although a conjugate image appeared. In the present study, the resolution redistribution technique is modified to eliminate the conjugate image. The height of a horizontal slit placed on the Fourier plane of a 4 f imaging system used for the resolution redistribution system is reduced by half. The horizontal resolution becomes K times larger, and the vertical resolution becomes 1/2K times smaller. The improved technique generates only the object wave. We demonstrated fourfold enlargement of the horizontal resolution to increase the horizontal viewing angle by approximately four times without generating the conjugate image.     

A framework for optimising the radiographic technique in digital X-ray imaging

The transition to digital radiology has provided new opportunities for improved image quality, made possible by the superior detective quantum efficiency and post-processing capabilities of new imaging systems, and advanced imaging applications, made possible by rapid digital image acquisition. However, this transition has taken place largely without optimising the radiographic technique used to acquire the images. This paper proposes a framework for optimising the acquisition of digital X-ray images. The proposed approach is based on the signal and noise characteristics of the digital images and the applied exposure. Signal is defined, based on the clinical task involved in an imaging application, as the difference between the detector signal with and without a target present against a representative background. Noise is determined from the noise properties of uniformly acquired images of the background, taking into consideration the absorption properties of the detector. Incident exposure is estimated or otherwise measured free in air, and converted to dose. The main figure of merit (FOM) for optimisation is defined as the signal-difference-to-noise ratio (SdNR) squared per unit exposure or (more preferably) dose. This paper highlights three specific technique optimisation studies that used this approach to optimise the radiographic technique for digital chest and breast applications. In the first study, which was focused on chest radiography with a CsI flat-panel detector, a range of kV(p) (50-150) and filtration (Z = 13-82) were examined in terms of their associated FOM as well as soft tissue to bone contrast, a factor of importance in digital chest radiography. The results indicated that additive Cu filtration can improve image quality. A second study in digital mammography using a selenium direct flat-panel detector indicated improved SdNR per unit exposure with the use of a tungsten target and a rhodium filter than conventional molybdenum target/molybdenum filter techniques. Finally, a third study focusing on cone-beam computed tomography of the breast using a CsI flat-panel detector indicated that high Z filtration of a tungsten target X-ray beam can notably improve the signal and noise characteristics of the image. The general findings highlight the fact that the techniques that are conventionally assumed to be optimum may need to be revisited for digital radiography.     

A review of X-ray imaging phosphors

The important methods of X-ray imaging use various phosphors. The phosphors give light proportional to the amount of radiation. The light is emitted either as X-ray excited fluorescence, phosphorescence, or due to stimulated, radiative recombinations of defects generated by X-ray exposures. The role of phosphors in improving image quality and reducing exposures to patients is important. Properties of various phosphors which can be used for X-ray imaging applications are reviewed here.     

An Application of X-ray Fluorescence Spectrometry to the Study of Thin-Layer Chromatography

The combination of thin-layer chromatography (TLC) with X-ray fluorescence spectrometry (XRF) has been accomplished without any interfaces. It enables the direct, nondestructive visualization of elements developed on a TLC plate. In addition to inorganic compounds, organic compounds including electronegative elements can be detected simultaneously that cannot always be measured satisfactorily by competitive techniques; e.g., phenolic compounds containing chlorine, bromine, or iodine were distinctly detected by individual elemental imaging. The background of commercially available TLC plates with various thicknesses of the stationary phase and an outline of element detection limits were also investigated. TLC/XRF was found suitable for in situ TLC imaging of elements.     

Two-point separation in far-field super-resolution fluorescence microscopy based on two-color fluorescence dip spectroscopy, Part I: Experimental evaluation

The two-point resolution of a novel two-color far-field super-resolution fluorescence microscopy was evaluated by measuring fluorescent beads 100 nm in diameter. This microscopy is based on a combination of two-color fluorescence dip spectroscopy and a phase-modulation technique for a laser beam. By simply introducing two-color laser light, the size of the fluorescent image of a bead was shrunk down to a diameter of 250 nm from the diffraction-limited image with a diameter of 360 nm. For two closely adjacent fluorescent beads with a separation distance of 350 nm, the two-color microscope clearly gave separated fluorescence images, while the conventional one-color fluorescence microscope could not resolve them. It has been proved that our technique breaks Rayleigh's diffraction limit.     

Carbon contamination analysis and its effect on extreme ultra violet mask imaging performance using coherent scattering microscopy/in-situ accelerated contamination system

The coherent scattering microscopy/in-situ accelerated contamination system (CSM/ICS) is a developmental metrology tool designed to analyze the impact of carbon contamination on the imaging performance. It was installed at 11B EUVL beam-line of the Pohang Accelerator Laboratory (PAL). Monochromatized 13.5 nm wavelength beam with Mo/Si multilayer mirrors and zirconium filters was used. The CSM/ICS is composed of the CSM for measuring imaging properties and the ICS for implementing acceleration of carbon contamination. The CSM has been proposed as an actinic inspection technique that records the coherent diffraction pattern from the EUV mask and reconstructs its aerial image using a phase retrieval algorithm. To improve the CSM measurement accuracy, optical and electrical noises of main chamber were minimized. The background noise level measured by CCD camera was approximately 8.5 counts (3 sigma) when the EUV beam was off. Actinic CD measurement repeatability was <1 A (3 sigma) at 17.5 nm line and space pattern. The influence of carbon contamination on the imaging properties can be analyzed by transferring EUV mask to CSM imaging center position after executing carbon contamination without a fine alignment system. We also installed photodiode and ellipsometry for in-situ reflectivity and thickness measurement. This paper describes optical design and system performance observed during the first phase of integration, including CSM imaging performance and carbon contamination analysis results.     

Submicrometer hyperspectral X-ray imaging of heterogeneous rocks and geomaterials: applications at the Fe k-edge

Because of their complex genesis, rocks and geomaterials are commonly polycrystalline heterogeneous systems, with various scale-level chemical and structural heterogeneities. Like most other μ-analytical techniques relying on scanning instruments with pencil-beam, the X-ray absorption near edge structure (XANES) technique allows elemental oxidation states to be probed with high spatial resolution but suffers from long acquisition times, imposing practical limits on the field of view. Now, regions of interest of sample are generally several orders of magnitude larger than the beam size. Here, we show the potential of coupling XANES and full-field absorption radiographies with a large hard X-ray beam. Thanks to a new setup, which allows both the acquisition of a XANES image stack and the execution of polarization contrast imaging, 1 to 4 mega-pixel crystallographic orientations and Fe oxidation state mapping corrected from polarization effects are obtained in a couple of hours on polycrystalline materials with submicrometric resolution. The demonstration is first carried out on complex metamorphic rocks, where Fe(3+)/Fe(total) images reveal subtle redox variations within single mineralogical phases. A second application concerns a bentonite analogue considered for nuclear waste and CO(2) storage. Proportion mappings of finely mixed phases are extracted from hyperspectral data, imaging the spatial progress of reaction processes essential for the safety of such storage systems.     

Ground-to-Satellite Narrow-Laser-Beam Pointing by Use of a Backscattered Image

A technique for transmitting a narrow laser beam from a ground station to a satellite has been developed. The principle of pointing a laser beam to a distant target in a scattering medium by use of a backscattered laser beam image is described. We calculated the intensity distribution of the image by using a typical model of atmospheric coefficients. The method was applied to transmit a laser beam from a ground station to Engineering Test Satellite-VI. The accuracy of pointing the laser beam to the satellite was approximately 10 murad in this experiment.     

Quantitative characterization of multiple delaminations in laminated composites using the compton backscatter technique

A Compton X-ray backscatter technique was used to supplement ultrasonic pulse-echo C-scan imaging to quantitatively assess the impact damage in quasi-isotropic laminated composites which were impacted by a drop-weight tester. A Compton backscatter imaging system with a slit-type camera was developed to obtain a cross-sectional profile of impact-damaged laminated composites from the density variation of the cross-section. A nonlinear reconstruction model is introduced to overcome distortion of the Compton backscatter image due to attenuation effects, beam hardening, and irregular distributions of the fibers and the matrix in composites. An adaptive filter is used to reduce noise from many sources including quantum noise, especially when the SNR (signal-to-noise ratio) of the image is relatively low. Delaminations masked or distorted by the first few delaminations in an ultrasonic C-scan image are detected and characterized by the Compton back-scatter technique, both in width and location.     

Excitation-and-collection geometry insensitive fluorescence imaging of tissue-simulating turbid media

We present an imaging technique for the correction of geometrical effects in fluorescence measurement of optically thick, turbid media such as human tissue. Specifically, we use the cross-polarization method to reject specular reflection and enhance the diffusive backscattering of polarized fluorescence excitation light from the turbid media. We correct the nonuniformity of the image field caused by the excitation-and-collection geometry of a fluorescence imaging system by normalizing the fluorescence image to the cross-polarized reflection image. The ratio image provides a map of relative fluorescence yield, defined as the ratio of emerging fluorescence power to incident excitation, over the surface of an imaged homogeneous turbid medium when fluorescence excitation-and-collection geometries vary in a wide range. We investigate the mechanism of ratio imaging by using Monte Carlo modeling. Our findings show that this technique could have a potential use in the detection of early cancer, which usually starts from a superficial layer of tissue, based on the contrast in the tissue fluorescence of an early lesion and of the surrounding normal tissue.     

Light profile microscopy based on Raman and wavelength resolved luminescence contrast

Light profile microscopy based on contrast from wavelength resolved Raman and luminescence measurements is demonstrated experimentally for the first time. A Raman/multispectral light profile microscope (RMSLPM) has been constructed based on a line profiling geometry in which the sample is irradiated with a tightly focused laser beam (of ten micrometers radius or less) behind a polished view surface and the resulting line image is dispersed over the wavelength using an imaging spectrograph. The instrumentation developed in this laboratory has a spectral resolution approaching 10 cm(-1) and an (actual) depth independent spatial resolution of 6-8 times the Rayleigh diffraction limit, limited at present by optical aberrations and alignment. The technique has the potential to image at approximately twice the Rayleigh diffraction limit. The spectral signatures reconstructed from a variety of common industrial polymers show excellent agreement with reference spectra from the literature, and may be used to identify individual layers in depth images of unknown materials. RMS-LPM image data based on luminescence contrast have also been used to provide concentration depth profiles of additives and degradation products in injection molded samples of high-density poly(ethylene) (HDPE).