Active microwave tomographic imaging of isolated, perfused animal organs

The relative transparency of biological materials to high-frequency electromagnetic waves has encouraged the development of new systems for imaging. This report describes experiments of microwave tomography conducted on a prototype. The object to be analyzed is submerged in water and is illuminated by a plane wave. The total electric field is analyzed by a microwave camera. The recorded data are then processed numerically in order to reconstruct the image that corresponds to the distribution of equivalent currents in a defined plane of a section. Experiments have been conducted on isolated kidneys with and without perfusion. The influence of the perfusing solution temperature has also been studied. These experiments show the potential of this system, especially through the correlation between microwave images and the biological structures. They also confirm previous results concerning spatial resolution and depth of exploration. Finally, the results demonstrate the influence of temperature and support the applicability of this imaging system in non-invasive thermometry, especially for clinical hyperthermia.     

Reconstruction of axial tomographic high resolution data from confocal fluorescence microscopy: a method for improving 3D FISH images.

Fluorescent confocal laser scanning microscopy allows an improved imaging of microscopic objects in three dimensions. However, the resolution along the axial direction is three times worse than the resolution in lateral directions. A method to overcome this axial limitation is tilting the object under the microscope, in a way that the direction of the optical axis points into different directions relative to the sample. A new technique for a simultaneous reconstruction from a number of such axial tomographic confocal data sets was developed and used for high resolution reconstruction of 3D-data both from experimental and virtual microscopic data sets. The reconstructed images have a highly improved 3D resolution, which is comparable to the lateral resolution of a single deconvolved data set. Axial tomographic imaging in combination with simultaneous data reconstruction also opens the possibility for a more precise quantification of 3D data. The color images of this publication can be accessed from http://www.esacp.org/acp/2000/20-1/heintzmann.++ +htm. At this web address an interactive 3D viewer is additionally provided for browsing the 3D data. This java applet displays three orthogonal slices of the data set which are dynamically updated by user mouse clicks or keystrokes.     

Time Multiplexing Super Resolving Technique for Imaging from a Moving Platform

We propose a method for increasing the resolution of an object and overcoming the diffraction limit of an optical system installed on top of a moving imaging system, such as an airborne platform or satellite. The resolution improvement is obtained in a two-step process. First, three low resolution differently defocused images are being captured and the optical phase is retrieved using an improved iterative Gerchberg-Saxton based algorithm. The phase retrieval allows to numerically back propagate the field to the aperture plane. Second, the imaging system is shifted and the first step is repeated. The obtained optical fields at the aperture plane are combined and a synthetically increased lens aperture is generated along the direction of movement, yielding higher imaging resolution. The method resembles a well-known approach from the microwave regime called the Synthetic Aperture Radar (SAR) in which the antenna size is synthetically increased along the platform propagation direction. The proposed method is demonstrated through laboratory experiment.     

声聚焦层析增强的三维微波热声成像

本文提出了一种提高微波热声断层成像层析能力的方法和装置.基于热声成像原理和声聚焦理论,搭建了由超短脉冲微波源、384阵元环形探测器、声聚焦透镜、384-64通道采集切换系统、精密扫描位移平台构成的微波热声三维成像系统,并实现了模拟样品的断层成像.实验结果表明该系统能够实现亚毫米级分辨率的热声成像,通过声聚焦方法成倍地提高了其层析分辨率.这对推动微波热声CT技术走向临床具有重要的意义.     

Extended depth-of-focus imaging of chlorophyll fluorescence from intact leaves

Imaging dynamic changes in chlorophyll a fluorescence provides a valuable means with which to examine localised changes in photosynthetic function. Microscope-based systems provide excellent spatial resolution which allows the response of individual cells to be measured. However, such systems have a restricted depth of focus and, as leaves are inherently uneven, only a small proportion of each image at any given focal plane is in focus. In this report we describe the development of algorithms, specifically adapted for imaging chlorophyll fluorescence and photosynthetic function in living plant cells, which allow extended-focus images to be reconstructed from images taken in different focal planes. We describe how these procedures can be used to reconstruct images of chlorophyll fluorescence and calculated photosynthetic parameters, as well as producing a map of leaf topology. The robustness of this procedure is demonstrated using leaves from a number of different plant species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Experimental nerve imaging at 1.5-T

Experimental lesions of the peripheral nerve system can be visualized in vivo by magnetic resonance imaging (MRI). Many studies of the rat peripheral nervous systems were performed on dedicated animal MR scanners with a high magnetic field strength for good spatial resolution. Here, we present an MR protocol to study experimental lesions of the rat nervous system with clinical 1.5-T MR scanners and commercially available coils. Using a three-sequence approach (T1-weighted imaging, fat-saturated T2-weighted imaging and fat-saturated T1-weighted imaging with Gd-DTPA in the same plane), the relevant signal changes of the lesioned nerve can be visualized and separated from other structures, e.g., blood vessels. Furthermore, we give an overview on different types of contrast agents used for peripheral nerve MR imaging and MR findings in selected experimental models of rat peripheral nerve injury.     

Experimental investigation of an optimization approach to microwave tomography

Microwave imaging is an interesting and growing research field with a number of medical applications. This paper is based on the first series of experimental results using an iterative gradient algorithm based on the finite difference time domain (FDTD) method and synthetic pulses. Using our method, the permittivity and conductivity of an object are reconstructed layer by layer by minimizing a functional consisting of the difference between the measured and calculated electric field surrounding the object. This is done by surrounding the object with a number of antennas which are all in turn transmitting and receiving. The dielectric profiles used in the calculations are then iteratively updated until the functional is minimized. Results are presented demonstrating the ability to detect metallic and dielectric material in air and water.     

Near-Infrared Super Resolution Imaging with Metallic Nanoshell Particle Chain Array

We propose a near-infrared super resolution near field imaging system with an array of metallic nanoshell particle chain. The imaging array can plasmonically transfer the near field components of dipole sources and the super resolution images can be reconstructed in the output plane. By decreasing the metallic nanoshell’s thickness of the fixed size nanoparticle, the plasmon resonance wavelength of the isolate nanoshell particle is red-shifted to the near-infrared region. The operation wavelength of the imaging array is correspondingly red-shifted to the near-infrared region. In this paper, we study the incoherent and coherent super resolution imaging. The field intensity distributions at the different planes of imaging process are calculated using the finite element method. The simulation results demonstrate that the array has super resolution imaging capability at near-infrared wavelengths in the incoherent and coherent manners. The results also show that the image formation highly depends on the source coherence. In the same structural parameters, the reconstructed images under the illumination of incoherent light source reach to the higher image quality and spatial resolution than the images under the illumination of coherent light source of in phase. By reasonably designing parameters of the imaging array, the approximate spatial resolutions of λ/13 in incoherent case and λ/10 in coherent case are obtained at the near-infrared wavelength of 764 nm. Furthermore, the image–array distance and the chains’ spacing also affect the image reconstruction.     

Assessing the benefits of focal pair cryo-electron tomography

Cryo electron tomography provides nanometer-scale information on biological matter preserved in a close-to native state. The resolution of tomograms and structures resolved by sub-tomogram averaging is typically limited by the contrast transfer function of the electron microscope, which is especially critical for thick samples. Here, we report a method to increase the attainable resolution by recording tomographic 'focal pairs', which are pairs of tilt series of the same object acquired in complementary defocus conditions. Low defocus imaging provides high resolution at low contrast, while high defocus imaging yields high contrast at the price of limited resolution. Quantitative assessment of the quality of lipid bilayer reconstructions in the resulting tomograms demonstrates stable resolution preservation beyond 3 nm for cells thicker than 500 nm. Further, in computational simulations on synthetic datasets we show the applicability of the method to sub-tomogram averaging, demonstrating its potential for achieving higher resolution.     

Image Alignment for Tomography Reconstruction from Synchrotron X-Ray Microscopic Images

A synchrotron X-ray microscope is a powerful imaging apparatus for taking high-resolution and high-contrast X-ray images of nanoscale objects. A sufficient number of X-ray projection images from different angles is required for constructing 3D volume images of an object. Because a synchrotron light source is immobile, a rotational object holder is required for tomography. At a resolution of 10 nm per pixel, the vibration of the holder caused by rotating the object cannot be disregarded if tomographic images are to be reconstructed accurately. This paper presents a computer method to compensate for the vibration of the rotational holder by aligning neighboring X-ray images. This alignment process involves two steps. The first step is to match the “projected feature points” in the sequence of images. The matched projected feature points in the - plane should form a set of sine-shaped loci. The second step is to fit the loci to a set of sine waves to compute the parameters required for alignment. The experimental results show that the proposed method outperforms two previously proposed methods, Xradia and SPIDER. The developed software system can be downloaded from the URL, http://www.cs.nctu.edu.tw/~chengchc/SCTA or http://goo.gl/s4AMx.     

Molecular dynamics simulations for the prediction of the dielectric spectra of alcohols,glycols and monoethanolamine

The response of molecular systems to electromagnetic radiation in the microwave region (0.3–300 GHz) has been principally studied experimentally, using broadband dielectric spectroscopy. However, relaxation times corresponding to reorganisation of molecular dipoles due to their interaction with electromagnetic radiation at microwave frequencies are within the scope of modern molecular simulations. In this work, fluctuations of the total dipole moment of a molecular system, obtained through molecular dynamics simulations, are used to determine the dielectric spectra of water, a series of alcohols and glycols, and monoethanolamine. Although the force fields employed in this study have principally been developed to describe thermodynamic properties, most them give fairly good predictions of this dynamical property for these systems. However, the inaccuracy of some models and the long simulation times required for the accurate estimation of the static dielectric constant can sometimes be problematic. We show that the use of the experimental value for the static dielectric constant in the calculations, instead of the one predicted by the different models, yields satisfactory results for the dielectric spectra, and hence the heat absorbed from microwaves, avoiding the need for extraordinarily long simulations or re-calibration of molecular models.     

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.     

X-ray scatter data for flat-panel detector CT

In modern X-ray computed tomography (CT) a trend to increased volume coverage by using multi-row detectors is apparent. Flat-panel detector CT (FPD-CT) systems provide an even larger field of measurement which, however, results in an increased scatter fraction. We investigated the scatter intensities registered in the case of FPD-CT. A hybrid model for the simulation of scatter combining deterministic and Monte Carlo methods was used for the scatter calculations. The influence of imaging parameters on the registered scatter intensity was examined both in single projections and reconstructed images. Scatter-to-primary ratios (SPRs) are given for various values of object thickness, field size, object-to-detector distance, incident energy and projection angle. For the simulations, homogeneous water phantoms and realistic patient data sets were used to produce scatter data representative for clinical situations. The SPR increases with object size, collimation and z-extent resulting in SPR ≫ 1 and respective scatter artifacts in the reconstructed images. In contrary, the scatter intensity decreases non-linearly with the object-to-detector distance. The angular and spatial distributions of scatter form a flat function as compared to the distribution of the primary signal. Single scatter appears to determine the distribution and magnitude of the total-scatter intensity at the detector.     

Optofluidic rotation of living cells for single‐cell tomography

Flow cytometry provides a high throughput, multi‐dimensional analysis of cells flowing in suspension. In order to combine this feature with the ability to resolve detailed structures in 3D, we developed an optofluidic device that combines a microfluidic system with a dual beam trap. This allows for the rotation of single cells in a continuous flow, around an axis perpendicular to the imaging plane. The combination of both techniques enables the tomographic reconstruction of the 3D structure of the cell. In addition this method is capable to provide detailed 3D structural data for flow cytometry, as it improves the reconstructed z‐resolution of a standard microscopy system to produce images with isotropic resolution in all three axes.

     

Ultrasonographic contrast-agent imaging of sub-millimeter vessel structures with spatial compounding: in vitro analyses.

In clinical diagnostics, ultrasonographic contrast-agent imaging gives access to medical parameters such as perfusion and vascularization. In addition to the artifacts that are typical for ultrasonic imaging, e.g., speckle noise and depth-dependent sensitivity and resolution, contrast-agent imaging shows more pronounced depth dependence and may suffer from shadowing artifacts that arise from high attenuation of the ultrasound waves by the contrast agent at high concentrations. By imaging an object from different viewing angles in one 2D image plane and summing the images obtained (spatial compounding), image quality can be increased and artifacts can be suppressed. In the present study, we combined both techniques to overcome the limitations of contrast-agent imaging. We used a commercially available ultrasound scanner and a custom-made high-precision mechanical system to rotate the ultrasound transducer fully around the object under investigation. Using this set-up, ultrasound data were acquired in reflection mode to generate a 360 degrees compound scan of a flow-mimicking phantom supplied with contrast agent.     

High Resolution 3D Imaging of Ex-Vivo Biological Samples by Micro CT

Non-destructive volume visualization can be achieved only by tomographic techniques, of which the most efficient is the x-ray micro computerized tomography (μCT).High resolution μCT is a very versatile yet accurate (1-2 microns of resolution) technique for 3D examination of ex-vivo biological samples^{1, 2}. As opposed to electron tomography, the μCT allows the examination of up to 4 cm thick samples. This technique requires only few hours of measurement as compared to weeks in histology. In addition, μCT does not rely on 2D stereologic models, thus it may complement and in some cases can even replace histological methods^{3, 4}, which are both time consuming and destructive. Sample conditioning and positioning in μCT is straightforward and does not require high vacuum or low temperatures, which may adversely affect the structure. The sample is positioned and rotated 180° or 360°between a microfocused x-ray source and a detector, which includes a scintillator and an accurate CCD camera, For each angle a 2D image is taken, and then the entire volume is reconstructed using one of the different available algorithms^5-7. The 3D resolution increases with the decrease of the rotation step. The present video protocol shows the main steps in preparation, immobilization and positioning of the sample followed by imaging at high resolution.     

Giga-Pixel Lensfree Holographic Microscopy and Tomography Using Color Image Sensors

We report Giga-pixel lensfree holographic microscopy and tomography using color sensor-arrays such as CMOS imagers that exhibit Bayer color filter patterns. Without physically removing these color filters coated on the sensor chip, we synthesize pixel super-resolved lensfree holograms, which are then reconstructed to achieve ∼350 nm lateral resolution, corresponding to a numerical aperture of ∼0.8, across a field-of-view of ∼20.5 mm². This constitutes a digital image with ∼0.7 Billion effective pixels in both amplitude and phase channels (i.e., ∼1.4 Giga-pixels total). Furthermore, by changing the illumination angle (e.g., ±50°) and scanning a partially-coherent light source across two orthogonal axes, super-resolved images of the same specimen from different viewing angles are created, which are then digitally combined to synthesize tomographic images of the object. Using this dual-axis lensfree tomographic imager running on a color sensor-chip, we achieve a 3D spatial resolution of ∼0.35 µm×0.35 µm×∼2 µm, in x, y and z, respectively, creating an effective voxel size of ∼0.03 µm³ across a sample volume of ∼5 mm³, which is equivalent to >150 Billion voxels. We demonstrate the proof-of-concept of this lensfree optical tomographic microscopy platform on a color CMOS image sensor by creating tomograms of micro-particles as well as a wild-type C. elegans nematode.     

Micro-computed tomography: Introducing?new?dimensions to taxonomy

Continuous improvements in the resolution of three-dimensional imaging have led to an increased application of these techniques in conventional taxonomic research in recent years. Coupled with an ever increasing research effort in cybertaxonomy, three-dimensional imaging could give a boost to the development of virtual specimen collections, allowing rapid and simultaneous access to accurate virtual representations of type material. This paper explores the potential of micro-computed tomography (X-ray micro-tomography), a non-destructive three-dimensional imaging technique based on mapping X-ray attenuation in the scanned object, for supporting research in systematics and taxonomy. The subsequent use of these data as virtual type material, so-called “cybertypes”, and the creation of virtual collections lie at the core of this potential. Sample preparation, image acquisition, data processing and presentation of results are demonstrated using polychaetes (bristle worms), a representative taxon of macro-invertebrates, as a study object. Effects of the technique on the morphological, anatomical and molecular identity of the specimens are investigated. The paper evaluates the results and discusses the potential and the limitations of the technique for creating cybertypes. It also discusses the challenges that the community might face to establish virtual collections. Potential future applications of three-dimensional information in taxonomic research are outlined, including an outlook to new ways of producing, disseminating and publishing taxonomic information.