Showing papers on "Optical tomography published in 1998"

Nonuniqueness in diffusion-based optical tomography

Abstract: A condition on nonuniqueness in optical tomography is stated. The main result applies to steady-state (dc) diffusion-based optical tomography, wherein we demonstrate that simultaneous unique recovery of diffusion and absorption coefficients cannot be achieved. A specific example of two images that give identical dc data is presented. If the refractive index is considered an unknown, then nonuniqueness also occurs in frequency-domain and time-domain optical tomography, if the underlying model of the diffusion approximation is employed.

Transversal and longitudinal images from the retina of the living eye using low coherence reflectometry

Abstract: An optical coherence tomography (OCT) system to produce both longitudinal and transversal images of the in vivo human eye is presented. For the first time, OCT transversal images collected from the living eye at 50-µm depth steps show details unobtainable with the state-of-the-art scanning laser ophthalmoscope. Images of up to 3×3?mm are produced from the retina in less than a second. For images larger than 1.6×1.6?mm, a path modulation is introduced by the galvanometric scanning mirror and is used as an effective phase modulation method.

A gradient-based optimisation scheme for optical tomography

Abstract: Optical tomography schemes using non-linear optimisation are usually based on a Newton-like method involving the construction and inversion of a large Jacobian matrix. Although such matrices can be efficiently constructed using a reciprocity principle, their inversion is still computationally difficult. In this paper we demonstrate a simple means to obtain the gradient of the objective function directly, leading to straightforward application of gradient-based optimisation methods.

Optical CT reconstruction of 3D dose distributions using the ferrous-benzoic-xylenol (FBX) gel dosimeter.

Abstract: In recent years, magnetic-resonance imaging of gelatin doped with the Fricke solution has been applied to the direct measurement of three-dimensional (3D) radiation dose distributions. However, the 3D dose distribution can also be imaged more economically and efficiently using the method of optical absorptioncomputed tomography. This is accomplished by first preparing a gelatin matrix containing a radiochromic dye and mapping the radiation-induced local change in the optical absorption coefficient. Ferrous–Benzoic–Xylenol (FBX) was the dye of choice for this investigation. The complex formed by Fe 3+ and xylenol orange exhibits a linear change in optical attenuation ( cm −1 ) with radiation dose in the range between 0 and 1000 cGy, and the local concentration of this complex can be probed using a green laser light (λ=543.5 nm). An optical computed tomography(CT) scanner was constructed analogous to a first-generation x-rayCT scanner, using a He–Ne laser, photodiodes, and rotation–translation stages controlled by a personal computer. The optical CT scanner itself can reconstructattenuation coefficients to a baseline accuracy of 2% while yielding dose images accurate to within 5% when other uncertainties are taken into account. Optical tomography is complicated by the reflection and refraction of light rays in the phantom materials, producing a blind spot in the transmission profiles which, results in a significant dose artifact in the reconstructed images. In this report we develop corrections used to reduce this artifact and yield accurate dosimetric maps. We also report the chemical reaction kinetics, the dose sensitivity and spatial resolution (<1 mm3) obtained by optical absorptioncomputed tomography. The article concludes with sample dose distributions produced by “cross-field” 6 MV x-ray beams, including a radiosurgery example.

A transport-backtransport method for optical tomography

Abstract: Optical tomography is modelled by the inverse problem of the time-dependent linear transport equation in n spatial dimensions (n = 2,3). Based on the measurements which consist of some functionals of the outgoing density at the boundary for different sources , , two coefficients of the equation, the absorption coefficient and the scattering coefficient b(x), are reconstructed simultaneously inside . Starting out from some initial guess for these coefficients, the transport-backtransport (TBT) algorithm calculates the difference between the computed and the physically given measurements for a fixed source by solving a `direct' transport problem, and then transports these residuals back into the medium by solving a corresponding adjoint transport problem. The correction to the guess is calculated from the densities of the direct and the adjoint problem inside the medium. Doing this for all source positions , , one after the other yields one sweep of the algorithm. Numerical experiments are presented for the case when n = 2. They show that the TBT-method is able to reconstruct and to distinguish between scattering and absorbing objects in the case of large mean free path (which corresponds to x-ray tomography with scattering). In the case of very small mean free path (which corresponds to optical tomography), scattering and absorbing objects are located during the early sweeps, but phantoms are built up in the reconstructed scattering coefficient at positions where an absorber is situated and vice versa.

Time-correlated single photon counting imager for simultaneous magnetic resonance and near-infrared mammography

Abstract: We describe a near infrared (NIR) imager for mammography, designed to work simultaneously with a magnetic resonance (MR) scanner. The imager employs two pulsing laser diodes, with average power of 25 μW, at 780 and 830 nm. The two wavelengths are time multiplexed into 24 source fibers. The detection part consists of eight parallel time-correlated photon-counting channels with overall counting capacity of 106 photons/s. We use long optical fibers to avoid interference with the magnetic field. Specially designed coupling plates, for breast soft compression, bear both the MR radio-frequency coils and the optical source and detector fibers. Capillaries containing water and copper sulfate mark the position of the plates on the MR images for accurate coregistration of NIR and MR images. Instrument compatibility has been successfully tested with volunteers in the MR scanner. The use of gallium arsenide photomultiplier tubes has allowed penetration depths of 10 cm in the human breast. Imaging algorithms, based on the analytical modeling of photon propagation in inhomogeneous media, have been applied successfully to image 0.8-mm-diam absorbing and scattering cylindrical perturbations in transmittance geometry of breast-like phantoms.

Optical Tomography

Abstract: The knowledge of the optical properties of biological tissues in VIVO is of great importance for optical techniques presently being developed for medical diagnosis. Exfensive studies were performed in vitro, but in vivo only results obtained at single wavelengths or in a limited wavelength ranye are reported in the literature A system for time-resolved refleclance measurements' was used to estimate In vivo and non-invasively the absorption and transport scattering spectra of different human tissues. in the wavelength ranye from 610 nm to 1010 nm, every 5 nm. lilumlnation was performed with a synchronously pumped dye laser (610-700 nm) and an actively mode-iocked Ti:Sapphire laser (700 nm-1010 nm) A couple of I-mm diameter optical fibers was used for both light injection and coilection, setting the intediber distance to 2 cm Time-resolved data were collected with an electronic chain for time-correlated single photon counting with a response function of e150 ps FWHM. and interpreted with the diffusion theory for the evaluation of the absorption and transport scattering coefflcienls. Spectra were obtained from breast, arm muscle. head. and abdomen on healthy voiunteers. The scattering coefficient reduces upon increasing the wavelength, with minor changes amoig different tissue types. The absorption spectrum of 0 35 the various tissues reveals -0 30 the spectral features of oxy6 0 2 5 and deoxy-hemoglobin. iipid and water. A typical example for breast is shown In Fig 1. 0 20

Frequency-swept ultrasound-modulated optical tomography of scattering media

Abstract: A novel frequency-swept ultrasound-modulated optical tomography technique was developed to image scattering media. A frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. The modulated light was received by an optical detector and was heterodyned with a reference frequency sweep. The heterodyned signal was recorded in the time domain and was then analyzed in the frequency domain to yield a one-dimensional image along the ultrasonic axis. Multiple one-dimensional images obtained at various positions perpendicular to the ultrasonic axis were combined to yield a two-dimensional tomographic image of the medium.

Non-linear optical tomography of turbid media

Abstract: An apparatus utilizing non-linear optical signals for use in constructing a three-dimensional tomographic map of an in vivo biological tissue for medical disease detection purposes. In one embodiment, said apparatus comprises a stage for supporting the in vivo biological tissue; a laser for illuminating the in vivo biological tissue with a focused beam of laser light, the light emerging from the in vivo biological tissue comprising fundamental light, harmonic wave light, and fluorescence due to multi-photon excitation; a filter for selectively passing only at least one of the harmonic wave light and the fluorescence; one or more detectors for individually detecting each of the harmonic wave light and the fluorescence selectively passed; and a mechanism for moving the laser relative to the stage in x, y and z directions.

Comparison of two- and three-dimensional reconstruction methods in optical tomography.

Abstract: We present a three-dimensional (3D) image reconstruction scheme for optical near-infrared imaging of highly scattering material The algorithm reconstructs the spatial distribution of the optical parameters of a volume Omega from transillumination measurements on the boundary of Omega We test the performance of the method for a cylindrical object with embedded absorbing perturbation for a number of different source and detector arrangements Furthermore, we investigate the effect of a mismatched reconstruction, using a two-dimensional (2D) reconstruction model to image a single plane of the object from 3D tomographic data obtained in a single plane The motivation for the application of 2D models is their advantage in speed and memory requirements We found that the difference in the measurement data between 2D and 3D models depends greatly on the measurement type used, giving a much better agreement for mean time-of-flight data than for dc intensity data Image artifacts that are due to data model mismatches can therefore be significantly reduced by use of mean time data

Stationary Headband for Clinical Time‐of‐Flight Optical Imaging at the Bedside

Abstract: Conventional brain-imaging modalities may be limited by high cost, difficulty of bedside use, noncontinuous operation, invasiveness or an inability to obtain measurements of tissue function, such as oxygenation during stroke. Our goal was to develop a bedside clinical device able to generate continuous, noninvasive, tomographic images of the brain using low-power nonionizing optical radiation. We modified an existing stage-based time-of-flight optical tomography system to allow imaging of patients under clinical conditions. First, a stationary head-band consisting of thin, flexible optical fibers was constructed. The headband was then calibrated and tested, including an assessment of fiber lengths, the existing system software was modified to collect headband data and to perform simultaneous collection of data and image reconstruction, and the existing hardware was modified to scan optically using this headband. The headband was tested on resin models and allowed for the generation of tomographic images in vitro; the headband was tested on critically ill infants and allowed for optical tomographic images of the neonatal brain to be obtained in vivo.

Optical Tomography of Real Three-Dimensional Foams

Abstract: Optical tomography has been implemented as a nondestructive method for investigating the structure of real three-dimensional polyhedral soap foams. Images of successive slices of the transparent foam are obtained by means of a CCD camera equipped with a very thin depth-of-field objective, and the coordinates of the foam vertices are determined. The 3D foam is then numerically reconstructed using surface energy minimization software to calculate the volume, the area, and the isoperimetric quotient of each bubble. This optical tomography has been used on a four bubbles thick layer of foam at two different capillary pressures in order to get a dry foam and a wet foam with similar topology. The evolution of the foam morphology upon drainage has been investigated. The network of the Plateau borders of the wet foam was also reconstructed, assuming that the Plateau borders are like prisms and the vertices are like octahedra. If the liquid volume is neglected in the films, the liquid fraction of the wet foam can be directly evaluated.

Two- and three-dimensional optical tomography of finger joints for diagnostics of rheumatoid arthritis

Abstract: Rheumatoid arthritis (RA) is one of the most common diseases of human joints. This progressive disease is characterized by an inflammation process that originates in the inner membrane (synovalis) of the capsule and spreads to other parts of the joint. In early stages the synovalis thickness and the permeability of this membrane changes. This leads to changes in the optical parameters of the synovalis and the synovial fluid (synovia), which occupies the space between the bones. The synovia changes from a clear yellowish fluid to a turbid grayish substance. In this work we present 2 and 3-dimensional reconstruction schemes for optical tomography of the finger joints. Our reconstruction algorithm is based on the diffusion approximation and employs adjoint differentiation techniques for the gradient calculation of the objective function with respect to the spatial distribution of optical properties. In this way, the spatial distribution of optical properties within the joints is reconstructed with high efficiency and precision. Volume information concerning the synovial space and the capsula are provided. Furthermore, it is shown that small changes of the scattering coefficients can be monitored. Therefore, optical tomography has the potential of becoming a useful tool for the early diagnosis and monitoring of disease progression in RA.

Improved Reconstruction Algorithm for Luminescence Optical Tomography when Background Lumiphore is Present.

Abstract: We examine the impact of background lumiphore on image quality in luminescence optical tomography A modification of a previously described algorithm [ J Chang H L Graber R L Barbour , J Opt Soc Am A14, 288–299 (1997); J Chang H L Graber R L Barbour , IEEE Trans Biomed Eng44, 810–822 (1997)] that estimates the background luminescence directly from the detector readings is developed Numerical simulations were performed to calculate the diffusion-regime limiting form of forward-problem solutions for a specific test medium We performed image reconstructions with and without white noise added to the detector readings, using both the original and the improved versions of the algorithm The results indicate that the original version produces unsatisfactory reconstructions when background lumiphore is present, whereas the improved algorithm yields qualitatively better images, especially for small target-to-background luminescence yield ratios

Fast image reconstruction for optical absorption tomography in media with radially symmetric boundaries.

Abstract: In this paper we present a reconstruction algorithm to invert the linearized problem in optical absorptiontomography for objects with radially symmetric boundaries. This is a relevant geometry for functional volume imaging of body regions that are sensitive to ionizing radiation, e.g., breast and testis. From the principles of diffuse light propagation in scattering media we derive the governing integral equations describing the effects of absorption variations on changes in the measurement data. Expansion of these equations into a Neumann series and truncation of higher-order terms yields the linearized forward imaging operator. For the proposed geometry we utilize an invariance property of this operator, which greatly reduces the problem dimensionality. This allows us to compute the inverse by singular value decomposition and consequently to apply regularization techniques based on the knowledge of the singular value spectrum. The inversion algorithm is highly efficient computing slice images as fast as convolution-backprojection algorithms in computed tomography(CT). To demonstrate the capacity of the inversion scheme we present reconstruction results for synthetic and phantom measurement data.

Investigations of RA-Diagnostics applying Optical Tomography in frequency-domain

Abstract: Our aim is to reconstruct the optical parameters in a slice of a finger joint phantom for further investigations about rheumatoid arthritis (RA). Therefore, we have developed a flexible NIR scanning system in order to collect amplitude and phase delay of photon density waves in frequency-domain. A cylindrical finger joint phantom was embedded in a container of Intralipid® solution due to the application of an inverse method for infinite geometry. The joint phantom was investigated by a laser beam obtaining several projections. The average optical parameters of each projection was calculated. Using different reconstruction techniques, e.g. ART and SIRT with a special projection operator, we reconstructed the optical parameters in a slice. The projection operator can be heuristically described by a photon path density function of a homogeneous media with infinite geometry. Applied to an object with an unknown distribution of optical parameters, it calculates the expectation value of the investigated object. The potentials and limits of these fast reconstruction methods will be presented.

Advances in optical biopsy and optical mammography

Abstract: Part 1: optical imaging in tissue. Part 2: optical tomography. Part 3: fluorescence and spectroscopy. Part 4: pathology and correlation studies. Part 5: panel discussion and closing remarks.

Imaging and characterization of dental structure using optical coherence tomography

Abstract: Optical probing of dental structure has thus far been primarily limited to visible and ultraviolet wavelengths where scattering dominates the propagation of light. Optical coherence tomography (OCT) techniques, however, have demonstrated that light in the near-infrared region of the spectrum can often provide higher resolution interior imaging of biological tissues. Here, we use OCT to obtain high-resolution interior images and investigate the optical properties of normal and diseased dentin and enamel.

High-speed measurements in optical low-coherence reflectometry

Abstract: A high-speed, depth optical scanner with a longitudinal scan speed of up to and repetition rate of 28.5 kHz was designed. Optical low-coherence reflectometry is used in this non-contacting depth scanning method. A Michelson interferometer uses a glass cube which rotates at a speed of 427 000 rpm as an optical path length variation. Details of the adopted design, the precision glass cube positioning system (tripod), the experimental set-up, and some experimental results are discussed. This technique is especially well suited for biomedical applications in the field of optical tomography of tissues, e.g. skin, retina, etc where motion artefacts of the patients limit the imaging performance.

Resolving the Structure of Cellular Foams by the Use of Optical Tomography

Abstract: We present several methods which may be used to resolve the structure of liquid and solid foams by optical means only. One system is based on confocal imaging, and two others use variants of computerized axial tomography. In each case we show how the reconstruction accuracy depends on the details of the scanning system and show how much is possible with simple digital video technology housed in a personal computer.

Optical tomography of the aurora and EISCAT

Abstract: . Tomographic reconstruction of the three-dimensional auroral arc emission is used to obtain vertical and horizontal distributions of the optical auroral emission. Under the given experimental conditions with a very limited angular range and a small number of observers, algebraic reconstruction methods generally yield better results than transform techniques. Different algebraic reconstruction methods are tested with an auroral arc model and the best results are obtained with an iterative least-square method adapted from emission-computed tomography. The observation geometry used during a campaign in Norway in 1995 is tested with the arc model and root-mean-square errors, to be expected under the given geometrical conditions, are calculated. Although optimum geometry was not used, root-mean-square errors of less than 2% for the images and of the order of 30% for the distribution could be obtained. The method is applied to images from real observations. The correspondence of original pictures and projections of the reconstructed volume is discussed, and emission profiles along magnetic field lines through the three-dimensionally reconstructed arc are calibrated into electron density profiles with additional EISCAT measurements. Including a background profile and the temporal changes of the electron density due to recombination, good agreement can be obtained between measured profiles and the time-sequence of calculated profiles. These profiles are used to estimate the conductivity distribution in the vicinity of the EISCAT site. While the radar can only probe the ionosphere along the radar beam, the three-dimensional tomography enables conductivity estimates in a large area around the radar site. Key words. Tomography · Aurora · EISCAT · Ionosphere · Conductivity

RA diagnostics applying optical tomography in frequency domain

Abstract: Our aim is to reconstruct the optical parameters in a slice of a finger joint phantom for further investigations about rheumatoid arthritis (RA). Therefore, we have developed a flexible NIR scanning system in order to collect amplitude and phase delay of photon density waves in frequency-domain. A cylindrical finger joint phantom was embedded in a container of Intralipid solution due to the application of an inverse method for infinite geometry. The joint phantom was investigated by a laser beam obtaining several projections. The average optical parameters of each projection was calculated. Using different reconstruction techniques, e.g. ART and SIRT with a special projection operator, we reconstructed the optical parameters in a slice. The projection operator can be heuristically described by a photon path density function of a homogeneous media with infinite geometry. Applied to an object with an unknown distribution of optical parameters it calculates the expectation value of the investigated object. The potentials and limits of these fast reconstruction methods will be presented.

Investigation of alternative data types for time-resolved optical tomography

Abstract: The technique known as optical tomography involves reconstructing a two- or three-dimensional map of the internal distribution of optical properties of an object from a series of measurements made of the light transmitted between pairs of points on the surface. In this paper we consider the form in which time-resolved measurements should be provided in order to achieve optimum simultaneous reconstruction of scatter and absorption. Various data types are proposed, and an examination is reported of the effect of stochastic noise on their intrinsic uncertainty, and on the ability to uniquely specify the global internal optical properties using pairs of different data types.

Optical tomography by means of a numerical low-coherence holographic technique

Abstract: Standard Optical Coherence Tomography (OCT) setups involves scanning mechanisms with focused beams and point detectors.

Optical tomography for biomedical applications

Abstract: Nonionizing optical tomography is a new and active research field although projection-light imaging was investigated as early as 1929. The optical properties of normal and diseased tissues are usually different despite the large variation of values in optical properties of the normal tissues alone. Therefore, it is possible to detect some breast cancers based on measurements of optical properties. In summary, optical techniques have the following advantages: (1) the use of nonionizing radiation, (2) the capability of measuring functional (physiological) parameters, (3) the potential high sensitivity to pathologic state of biological tissues, and (4) low cost. However, biomedical optics is a challenging research field requiring the participation of many diverse, talented scientists and engineers.

Biomedical imaging using optical coherence tomography

Abstract: Optical coherence tomography (OCT) is a new and powerful technology that can overcome many of the limitations of excisional biopsy. OCT is a recently developed optical imaging technique for performing high-resolution cross-sectional imaging tomography of microstructures in biological systems. OCT performs imaging by using low-coherence interferometry to measure the optical backscattering of tissue as a function of echo delay and transverse position. The resulting two-dimensional data can be displayed as a gray scale or false color image. OCT functions as a type of "optical biopsy" to provide cross-sectional images of tissue structure on the micron scale. OCT is a powerful imaging technology because it can provide images of tissue in situ and in real time, without the need for tissue excision and processing. This presentation will review recent advances in OCT technology and possible future clinical applications such as arterial imaging, the detection of early neoplastic changes, and guiding surgical intervention.

Method and device for measuring the optical properties of transparent and/or diffusive objects

Abstract: The inventive method for measuring the optical properties of transparent and/or diffusive objects (18), especially their reflection profile, uses two coupled interferometers. Said coupled interferometers are operated with two beams of different coherence lengths. The object to be measured (18) is exposed to the beam with the short coherence length. A path length variation unit (15) is located in the common branch of the two interferometers. This unit impresses a double frequency on both beams, and this double frequency is then evaluated. A device for carrying out the method can be used especially as a reflectometer, preferably for applications in the area of surface diagnosing, optical tomography and photometry, as a high-dynamic means of determining optical properties with a high signal-noise ratio.

Hemispherical-rod microlens as a variant fractional fourier transformer

Abstract: It is shown that a hemispherical-rod microlens can act as a fractional Fourier transform device with a continuously varying degree of fractionality. It is believed to be the first proposed device to perform such a task; its most immediate applications include optical tomography.