Numerical Reconstruction Method Research Articles

L'échotomographie analogique. Etude de la reconstruction acousto-optique à partir de données échographiques

Tomography has been a highly active research field for the last 20 years and great progress has been made in numerical reconstruction methods. This paper deals with a new method of tomographic reconstruction based on optical processing. We first present a system of acquisition by ultrasound and then the backprojection procedure which is performed by optical methods. The projections are coded by an opto-acoustic modulator and deflector into a laser beam and the image is obtained by integrating the resulting pattern over one revolution by a CCD camera. This procedure, which is entirely analogical, allows the real time reconstruction of images, the only delay being the time required for the aquisition (approximatively 1 sec.). Echotomographies obtained by this method are presented

SPINR: two-dimensional spectral imaging through tomographic reconstruction

We describe a novel technique that enables us to conduct 2-D imaging spectroscopy with a 1-D imaging spectrometer. A typical imaging spectrometer obtains a series of 1-D monochromatic images that contain the entire field of view, but the spatial information in the dispersion direction is lost. By rotating the instrument (hence, rotating the field of view), we compile a series of time-dependent profiles. We use a numerical tomographic reconstruction method to recover the second spatial dimension and thereby obtain 2-D monochromatic images of the field of view. This technique is more sensitive and, hence, collects a full 2-D image more quickly than a conventional push-broom scan. We present the results of numerical simulations and discuss the prospects of this method for magnetospheric imaging applications.

Tokamak edge plasma densities measured by means of active lithium beam diagnostics

The injection of fast (1–100 keV) Li 0 diagnostic beams into magnetically confined fusion plasmas caused collisionally induced LiI emission at 670.8 nm, the intensity of which is related to the plasma (electron) density. A numerical method for quantitative reconstruction of abolute radial plasma densities from raw LiI 670.8 nm diagnostic data has been developed, which takes care of all relevant collisional interactions of the Li atmos with plasma consitutents. The applicability of this method is illustrated using experimental results from the TEXTOR and ASDEX tokamak plasma discharges.

Reconstruction of plasma edge density profiles from Li I (2s-2p) emission profiles

The injection of 10-100 keV Li0 diagnostic beams into magnetically confined fusion plasmas causes collisionally induced Li I emission at 670.8 nm, in close relation to the edge plasma electron density. A numerical method for quantitative reconstruction of the plasma density exclusively from relative Li I 670.8 nm emission profiles as measured along the diagnostic beam has been developed, involving all relevant collisional interactions of the Li atoms with plasma constituents. The applicability of the described algorithm is illustrated by experimental results obtained for the TEXTOR Tokamak edge plasma at KFA Julich.

Perioperative neutron brachytherapy with Californium-252

Between 1973 and 1988, 495 patients were treated with Cf-252 neutron brachytherapy. Cf-252 neutron therapy sources developed in the USSR has been used in the trial. A numerical reconstruction method for localization of Cf-252 cell coordinates by projections on orthogonal radiographs has been designed and used for treatment planning. Eight (1.6%) patients with recurrent and persistent head and neck tumors and ages from 32 to 48 years (mean age 43 years) were treated with Cf-252 perioperative neutron brachytherapy. There were three patients with oral cavity, one with oropharynx, three with parotid gland cancers, and one with a skin tumor. The dose rate ranged from 3.2 cGy/h to 11.1 cG/h, the minimal peripheral dose ranged from 3 Gy to 8 Gy. Initial local control was achieved in all patients. Local recurrence developed in two cases. Three patients died in first year after therapy. Three patients died during the second year. Two patients are long term cures, one patient more than nine years and one eight years, that is 25% of the treated patients.

A New FFT Method for Numerical Reconstruction in Electron Holography

A new numerical reconstruction method for electron holography is proposed. This method based on the FFT subfringe interferometry needs two images, coherent intensity distribution (hologram) which has interference fringes and incoherent intensity distribution which loses those fringes. Subtraction between these two intensity distributions allows the reconstructed images to consist of spatial frequency as high as carrier spatial frequency of the hologram

Modified Fourier reconstruction method using shifted transform samples

Numerical methods for image reconstruction from projections are essential to the success of computed tomography in diagnostic radiology, and are also needed for some types of magnetic resonance imaging. The authors describe a modified Fourier reconstruction method using shifted transform samples, and they point out the relationship between this method and other reported implementations. They then present and discuss the results of computational experiments with this previously untried method.

Aberration Correction by Electron Holography Using Numerical Reconstruction Method

Electron holography has been performed to correct the aberrations of an objective lens in electron microscope. The aberrations, particularly defocusing aberration, are corrected at the stage of numerical reconstruction according to the wave aberration function. The influence of defocusing aberration on both the amplitude and the phase distributions is eliminated in the reconstruction images. Since spherical aberration is represented as the similar function of defocusing aberration, the results confirm the possibility of correcting the spherical aberration of the objective lens by the high resolution electron holography

The Fringe Scanning Method as Numerical Reconstruction for Electron Holography

A novel numerical reconstruction method for electron holography is proposed. This method based on both the fringe scanning technique and the Moire topography has the advantages of being independent of the number of interference fringes in the electron hologram and of being processed in short real time. The experimental result has confirmed that the object wave, including both the amplitude and the phase distributions, is successfully reconstructed without any help of an advanced image processing system

Holographic Interference Electron Microscopy by Numerical Reconstruction Method

Holographic interference electron microscopy has recently been put to practical use with a field-emission electron microscope. In this microscopy, the optical image reconstruction method are usually used, but many practical difficulties such as optical adjustment are always accompanied. On the other hand, the numerical image reconstruction method has many advantages. In this paper, therefore, holographic interference electron microscopy are tried by using a numerically reconstructed image.In the first step, an off-axis Fresnel electron hologram (Fig.1) was formed in a 100 kV thermionic-emission electron microscope equipped with an electron biprism. MgO smoke particles were used as a specimen. A reconstructed image is given by the convolution of amplitude transmittance of an electron hologram with propagation function from a hologram plane to an image reconstruction plane. In the second step for the numerical reconstruction, the hologram was enlarged and printed on a photographic paper.

Reconstruction of high-energy bremsstrahlung spectra by numerical analysis of depth-dose data

The determination of the spectral distribution of a photon beam is important for exact dosimetry and for the calculation of dose distributions. Direct measurements are not possible because of physical reasons. The approximation described uses a numerical reconstruction method based upon depth-dose data measured in water. The basic idea of the method is that a measured depth-dose curve can be thought of as a weighted superposition of monoenergetic depth-dose curves. The monoenergetic data are obtained by convolution of Monte Carlo generated energy deposition kernels. They were fitted to the measured data by means of a least squares method. Several constraints were applied to the superposition coefficients. The optimization algorithm uses the Wolfe reduced gradient method. The results were compared to Monte Carlo generated thick target spectra.

Analysis of time series signals of wave fields using state vectors

AbstractThis paper proposes a method of analysis of time seriesignals of wave fields by state vectors with regard to complex variablez and discrete time variable. The scalar product of these state vectors, the condition of completeness and operators acting on states are defined and basic equations arqderived. The z‐transformation and the convolution theorem are discussed. The present method is applied to the signal analysis by digital filters wherein the signals in the spatial coordinate of wave fields are converted to the time series signals. Two digital filters are introduced to perform Fresnel transform to express the wave propagation in the Fresnel zone. In this analysis, the filter must have characteristics of propagator transform. Further, amethod of reconstructing images as filter outputs is discussed; the hologram signal is converted to the time s eries signals and input to the filter which performs the Fresnel transformation. The result of image reconstruction by this method is compared with the result obtained by the conventional numerical image reconstruction method.

Determination of axially symmetrical refractive index distribution from directions of emerging rays

A successful verification of a method for numerical reconstruction of axially symmetric refractive index distribution is reported. Directions of rays emerging from a test object are used as input data for reconstruction. They are obtained by numerically calculating the spatial frequency of the fringes in an interferogram. It is also shown that the same data can be obtained directly by counting moiré fringes arising from two slightly displaced interferograms. An acrylic resin immersed in liquid is used as a test object. The reconstruction is carried out with errors of a few percent.

A Method for Measuring Axially Symmetrical Refractive Index Distribution Using Eikonal Approximation

A method for numerical reconstruction of an axially symmetrical refractive index distribution is proposed on the basis of the eikonal approximation. Although the procedure of numerical reconstruction is the same in principle as the corresponding theory used in geophysics, practical systems for acquiring data for reconstruction arc quite different. Possible optical systems are discussed and computational simulations are performed based on one of the systems. The results of simulations with the use of a hypothetical object confirm the possibility of practical reconstruction even when the light ray is curved in the object.