Application of a novel diffraction-based tomography method for imaging biological samples
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A novel system to image and reconstruct a 3-dimensional map of the refractive index based on the diffraction of light through a transparent sample is presented. This method is tested and validated on computer-generated data sets. The proposed system is an advanced variation of an imaging technique used in engineering for the study of aerodynamics. This method, which is termed Reference Image Topography, is used to reconstruct the water/air interface of the free surface in fluid dynamics studies. This surface profile is reconstructed by comparing an image of a random pattern viewed through the transparent free surface against a reference image, to determine the change in the refractive index caused by changes in the height. The proposed system is highly sensitive and capable of imaging intricate features in the transparent sample that are of low contrast when imaged with other imaging methods. For each projection, the change in direction of the light passing through the sample when placed in between the light source and the imaging system, can be related to the line integral for the change in refractive index across the sample. Utilizing multiple projections, a 3- dimensional map of the refractive index of the sample is reconstructed with computed tomography.Keywords:
Sample (material)
Refractive index provides fundamental insights into the electronic structure of materials. At high pressure, however, the determination of refractive index and its wavelength dispersion is challenging, which limits our understanding of how physical properties of even simple materials, such as MgO, evolve with pressure. Here, we report on the measurement of room-temperature refractive index of MgO up to ∼140 GPa. The refractive index of MgO at 600 nm decreases by ∼2.4% from ∼1.737 at 1 atm to ∼1.696 (±0.017) at ∼140 GPa. Despite the index at 600 nm is essentially pressure independent, the absolute wavelength dispersion of the refractive index at 550–870 nm decreases by ∼28% from ∼0.015 at 1 atm to ∼0.011 (±8.04 × 10−4) at ∼103 GPa. Single-effective-oscillator analysis of our refractive index data suggests that the bandgap of MgO increases by ∼1.1 eV from 7.4 eV at 1 atm to ∼8.5 (±0.6) eV at ∼103 GPa.
Step-index profile
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Objective:To study the theory and the value of direct projection tomography technology.Methods:Made a cylinder lens and a direct back projection tomography machine.Check a model by this machine.Results:The direct projection tomography machine can display tomography image of the model.Conclusion:It is reasonable and valuable that the direct projection tomography technology.
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In quantitative phase imaging, a priori knowledge of either refractive index or physical thickness is used to estimate the change in one of these parameters. Here, we report a method for decoupling geometric thickness from refractive index in quantitative phase microscopy.
Decoupling (probability)
Interference microscopy
Phase imaging
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We acquire the first experimental 3-D tomographic images with magnetic particle imaging (MPI) using projection reconstruction methodology, which is similar to algorithms employed in X-ray computed tomography. The primary advantage of projection reconstruction methods is an order of magnitude increase in signal-to-noise ratio (SNR) due to averaging. We first derive the point spread function, resolution, number of projections required, and the SNR gain in projection reconstruction MPI. We then design and construct the first scanner capable of gathering the necessary data for nonaliased projection reconstruction and experimentally verify our mathematical predictions. We demonstrate that filtered backprojection in MPI is experimentally feasible and illustrate the SNR and resolution improvements with projection reconstruction. Finally, we show that MPI is capable of producing three dimensional imaging volumes in both phantoms and postmortem mice.
Tomographic reconstruction
Magnetic Particle Imaging
Point spread function
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Two-photon microscopy has been successfully applied in biological and material sciences since 1990. However, it is known that the resolution of two-photon imaging is adversely affected by the index mismatch induced spherical aberrations. Spherical aberration increases the focal volume and causes degradation of image resolution. The problem becomes worse as one image deeper into the specimen. In this work, we propose to use this intrinsic artifact to measure the refractive index in specimens of uniform refractive indices. Using the intensity profiles of standard refractive liquids as reference, we can compare the intensity profile of an unknown specimen of uniform refractive index with that of the reference to determine the refractive index of the unknown specimen.
Artifact (error)
X-ray optics
Step-index profile
Refractometry
Intensity
Normalized frequency (unit)
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In the situations where the classic refractometry methods cannot be applied, the immersion methods are used. Phase contrast microscopy is one of the methods that make evident the difference between the refractive index of an immersed sample and the refractive index of the immersion medium. The measurement of the refractive index of the sample is performed by changing under control the refractive index of the immersion medium, until this becomes equal to the index of the sample. The present work is aiming to answer to a series of questions concerning the performances that a system composed of a phase contrast microscope equipped with a CCD camera and a temperature control device should achieve in order to measure the refractive index with a prescribed measurement error. The possibilities of measurement of the refractive index by means of the controlled heating of the sample-immersion system assembly and by means of changing the concentrations of the components of the immersion medium are examined.
Immersion
Refractometry
Oil immersion
X-ray optics
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Most of the materials in nature have complex refractive indexes, especially for biological tissues. The real and imaginary components of the complex refractive index are respectively responsible for the phase delay and absorption of light traveled in the tissues. Taking the complex refractive index into consideration, we modified the imaging theory of phase contrast microscopy. A relationship between the distributions of complex refractive index in measured samples and the intensity distributions of phase contrast micrographs is obtained. Based on this relationship, the influences of the refractive index and thickness of biological slices on the image contrast of phase contrast micrographs are analyzed in detail.
Image contrast
Intensity
Refractive index contrast
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Step-index profile
X-ray optics
Normalized frequency (unit)
Prism
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Quantitative in-line X-ray phase-contrast tomography methods seek to reconstruct separate images that depict an object's absorption and real-valued refractive index distributions. They hold great promise for biomedical applications due to their ability to distinguish soft tissue structures based on their complex X-ray refractive index values. In this work, we investigate the second-order statistical properties of images in phase-contrast tomography and describe how they are distinct from those associated with conventional absorption-based tomography.
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