Showing papers on "Optical tomography published in 1993"

Application of the finite-element method for the forward and inverse models in optical tomography

Abstract: The development of an optical tomographic imaging system for biological tissue based on time-resolved near-infrared transillumination has received considerable interest recently The reconstruction problem is ill posed because of scatter-dominated photon propagation, and hence it requires both an accurate and fast transport model and a robust solution convergence scheme The iterative image recovery algorithm described in this paper uses a numerical finite-element solution to the diffusion equation as the photon propagation model The model itself is used to compare the influence of absorbing and scattering inhomogeneities embedded in a homogeneous tissue sample on boundary measurements to estimate the possibility of separating absorption and scattering images Images of absorbers and scatterers reconstructed from both mean-time-of-flight and logarithmic intensity data are presented It is found that mean-time-of-flight data offer increased resolution for reconstructing the scattering coefficient, whereas intensity data are favorable for reconstructing absorption

Time-resolved optical tomography

Abstract: We have obtained transaxial slice images of a cylindrical object containing a highly scattering, low-absorbing solution by using picosecond pulses of visible light. Embedded in the solution was an assortment of six glass tubes containing different concentrations of absorbing dye. We reconstructed images tomographically from projections generated by using light transmitted through the object with the shortest flight times.

Inverse Methods for Optical Tomography

Abstract: We describe the main theoretical principles behind Time-resolved Optical Absorption and Scattering Tomography (TOAST). The problem is viewed as the optimisation of an error-norm derived from correlated statistics of the time-dependent photon intensity at the surface of an object. The field is compared with Electrical Impedance Tomography (EIT). Some inverse algorithms are suggested and one implemented in detail: a modified Newton-Raphson approach. Several regularisation schemes are described. Results are given for these schemes applied to several different data sets.

Multiplicative algebraic computed tomographic algorithms for the reconstruction of multidirectional interferometric data

Abstract: There has been much recent interest in the application of optical tomography to the study of transport phenomena and chemical reactions in transparent fluid flows. An example is the use of multidirectional holographic interferometry and computed tomography for the study of crystal growth from solution under microgravity conditions. A critical part of any such measurement system is the computed tomography program used to convert the measured interferometric data to refractive index distributions in the object under study. Several of the most promising computed tomography algorithms for this application are presented and compared. Because of the practical difficulty of making multidirectional interferometric measurements, these measurements generally provide only limited amounts of data. Recent studies have indicated that of the several classes of reconstruction algorithms applicable in the limited-data situation, those based on the multiplicative algebraic reconstruction technique (MART) are the fastest, most flexible, and most accurate. Several MART-type algorithms have been proposed in the literature. The performance of state-of-the-art implementations of four such algorithms under conditions of interest to those reconstructing multidirectional interferometric data are compared. The algorithms are tested using numerically generated data from two phantom objects, with two levels of added noise and with two different imaging geometries. A reconstruction of real data from a multidirectional holographic interferometer using the best of the algorithms is shown.

Method and apparatus for obtaining an image indicating metabolism in a body

Abstract: An X-ray tomography scanner and an optical tomography scanner are combined to obtain an optical image indicting metabolism in the body. Optical projection data indicating light pulse transmissions in the body are compensated by using a light scattering coefficient distribution determined by an X-ray tomogram image discriminated into individual internal organs or tissues. The compensated projection are back-projected.

Use of multiple data types in time-resolved optical absorption and scattering tomography

Abstract: In Time-resolved Optical Absorption and Scattering Tomography (TOAST) the imaging problem is to reconstruct the coefficients of absorption (mu)a and scattering (mu)s of light in tissue given the time-dependent photon flux at the surface of the subject, resulting from ultrafast laser input pulses. This inverse problem is mathematically similar to the Electrical Impedance problem (EIT) but presents some unique features. In particular the necessity of searching in two solution spaces requires the use of multiple data types that are maximally uncorrelated with respect to the solution spaces. We developed an algorithm for TOAST that uses an iterative non-linear gradient descent method to minimize an appropriate error norm. The algorithm can work on multiple types of data and an important topic is the choice of the best data format to use. Usually the choice is integrated intensity and mean time- of-flight for the temporal domain data. In this paper we compare these data types with the use of higher order moments of the temporal distribution (variance, skew, kurtosis). We show that reliable results must take detailed account of the confidence limits on each data point. We demonstrate how the probability distribution function for photon propagation can be calculated so that the variance of any given measurement type can be derived.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Three-dimensional temporal image reconstruction of an object hidden in highly scattering media by time-gated optical tomography.

Abstract: Three-dimensional images of an object hidden in a thick highly scattering medium were reconstructed from sets of two-dimensional images obtained from a time-gated optical imaging system. CCD images were combined by use of the backprojection algorithm to render a three-dimensional picture on a personal computer monitor. The image quality varied with the delay of the Kerr gate system. When the reconstructions were produced by using the early light, submillimeter resolution was achieved with the optical time-gating tomographic technique.

Beam-deflection optical tomography of the refractive-index distribution based on the Rytov approximation.

Abstract: A method of beam-deflection optical tomography based on the Rytov approximation of the wave equation is proposed. A linear relation between the deflection angle and the refractive-index distribution from the Rytov approximation and an equation of geometrical optics is derived. The tomographic reconstruction in this method is reduced to the solution of a system of linear equations. Experimental results show that this method can reconstruct strongly refracting fields from projection data of six view angles.

A medical perspective at the threshold of clinical optical tomography

Abstract: Optical imaging and spectroscopy use light emitted into opaque media such as human tissue to determine interior structure and chemical content, respectively, and have broad application to the field of medicine [1]. Few developments have improved medical diagnos- tics as much as the ability to noninvasively peer inside the body, and it is expected that newly developing optical imaging techniques will continue this trend. Optical imaging and spectroscopy, key components of optical tomography, center around the simple idea that light passes through the body in small amounts, emerging bearing clues about tissues through which it passed. Rapid progress over the past decade, made possible by the collective output of multiple laboratories and advancements in the opto-electronics field, have brought optical imaging to the brink of clinical usefulness. In recognition of this impending transition from lab curiosity to medical tool, it was felt that a comprehensive review was in order. This book, written in large part by members of the key laboratories responsible for recent optical advances, aims to fill that need by exploring state-of-the-art methods, hardware, and applications of optical tomography. It is intended to serve as both an introduction for those unacquainted with the field, as well as a reference for those actively involved. It is hoped that the reader will gain an appreciation of the power and breadth of optical tomography, as well as an understanding of the fundamental limitations and unsolved problems that need study before optical imaging becomes clinically viable. Lastly, we hope to communicate the excitement and enthusiasm felt by those studying optical imaging and spectroscopy, and demonstrate why the field is undergoing both an explosion of interest and rapid progress.

Frequency-domain methods in optical tomography - detection of localized absorbers and a backscattering reconstruction scheme

Abstract: We studied the ability to detect small absorbing objects embedded in a highly scattering medium. Absorbing spheres of varying size, from 0.8 mm to 6.8 mm radius, submerged in a solution of highly scattering, low absorbing liquid: skim milk, were studied in a trans- illumination geometry. Groups of more than one sphere and a single circular disk, with radius identical to that of one of the spheres, were also studied. Single linear raster scans in the plane of the sphere, with the spheres centered between the source and detector, were made. Data was taken in the frequency-domain, yielding profiles of the objects in each of the three measurable quantities: dc intensity, phase, and modulation. The diffraction pattern form the sphere differed from that of the disk, demonstrating a volume effect associated with photon diffusion. The diffraction pattern of multiple spheres differed from that of single spheres.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Application of Computer Optical Tomography to the Study of Methane-Air Turbulent Flame

Abstract: Computer optical tomography has been used as a diagnostic technique in the study of a premixed methane-air diffusion turbulent flame of equivalence aidfuel ratio of 0.5. Visible light from a tungsten filament lamp at a wavelength of 0.65pm was used. The test plane, of 12 mm diameter, was scanned using eleven equi-spaced parallel optical paths which were rotated through 180 deg and a complete set of readings were taken at 30 cleg intervals. The optical thickness and emission ratio wen: measured along each path and at each 30 deg interval. A computer program based on the convolution method combined with the Shepp-Logan filter [ 11, was used to reconstruct a two-dimensional image of the absorption coefficient, emission function, temperature distribution and soot concentration at a chosen instant.

Time-dependent photon migration imaging in two dimensions: a method for detection and localization of absorbers in tissuelike media

Abstract: Measurements of time-dependent photon migration appear to provide more information for biomedical optical image reconstruction than continuous wave measurements. Yet the ultimate success of photon migration imaging (PMI) for biomedical optical tomography depends upon developing a method which can rapidly measure `time-of-flight' information, and, in near-real time, extract important information required for image reconstruction. Image reconstruction requires information to (1) detect, (2) locate the position and volume, and (3) characterize the optical properties of an optical heterogeneity that would otherwise be obscured by tissue-like scattering. In this presentation, we report PMI `images' of an obscured absorber obtained from two-dimensional time-dependent photon migration measurements which arise from single point source illumination of a scattering medium with modulated light. These PMI `images' along with a theoretical basis for PMI, suggest the potential to rapidly detect and locate the three- dimensional position of an absorber from two-dimensional frequency-domain measurements of phase, (Theta) ((rho) ,f), and modulation, M((rho) ,f). Independent single-pixel measurements and Monte Carlo simulations of (Theta) ((rho) ,f) and M((rho) ,f) confirm the PMI `images' and the hypothesis for PMI.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Interferometric tomography in a three-dimensional differentially heated enclosure

Abstract: Optical tomography using interferometric data is applied in this study to investigate the flow and heat transfer phenomena a 3D differentially-heated cubic enclosure. The interferometric recording and reconstruction system, associated processing and reconstruction procedures, and results from this analysis are described. The experimental results are verified by comparison with independent temperature measurements. The comparisons indicate that the experimental technique can yield 3D perspectives of complex flows having sufficient spatial resolutions to verify detailed 3D direct numerical simulations.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Algorithms for image reconstruction from projections in optical tomography

Abstract: It is well known that the determination ofthe temperature field by holographic interferometry is a successful method in the measurement of thermophysics. In this paper some practical algorithms for image reconstruction from projections are presented to produce the temperature field. The algorithms developed consists in that the Radon transform integral equation is directly solved by grid method and that the Radon inversion formula is numerically evaluated by twodimensional Fourier transform technique. Some examples are given to verify the validity of the above methods in practice.