Showing papers on "Optical tomography published in 1996"

Single mode fiber-optic catheter/endoscope for optical coherence tomography

Abstract: Summary form only given. In order to apply OCT for imaging of internal organ systems, a flexible, small diameter, catheter/endoscope, which is capable of delivering, focusing, scanning, and collecting a single-spatial-mode optical beam, must be constructed. In this summary, we describe the design and performance of a prototype single-mode fiber-optic scanning OCT catheter with a diameter of 1 mm. OCT imaging may be performed at 1.3-micron wavelengths using either a superluminescent laser diode source or a Kerr-lens mode-locked Cr:forsterite laser, which provides high powers for high-speed imaging. This device is an enabling technology for OCT and will permit micron scale, cross-sectional medical diagnostic imaging in tissues such as the vascular system, the gastrointestinal tract, the urinary tract, and the respiratory tract.

Optical coherence tomography and microscopy in gastrointestinal tissues

Abstract: Optical coherence tomography (OCT) and optical coherence microscopy (OCM) are novel techniques for noninvasive biomedical imaging based on low-coherence interferometry. OCT achieves high-spatial resolution ( 100 dB) in a fiber-optically integrated system which is suitable for application in minimally invasive diagnostics, including endoscopy. The technique of OCM combines the depth-ranging capability of OCT with the micron-scale resolution imaging capability of confocal microscopy to extend the available imaging depth of confocal microscopy up to several hundred micrometers deep in highly scattering tissues. The theoretical and technical bases for OCT and OCM imaging are described. Example OCT images are provided in gastrointestinal (GI) tissues to illustrate contrast between histological layers of the GI mucosa and differentiation of the mucosa from submucosa. Example OCM images revealing cellular-level microstructure up to several hundred micrometers deep in GI tissue are presented for the first time. The potential applications of OCT and OCM imaging in clinical diagnostic medicine are discussed.

Imaging Brain Injury Using Time-Resolved Near Infrared Light Scanning

Abstract: Conventional brain imaging modalities are limited in that they image only secondary physical manifestations of brain injury, which may occur well after the actual insult to the brain and represent irreversible structural changes. A real-time continuous bedside monitor that images functional changes in cerebral blood flow or oxygenation might allow for recognition of brain tissue ischemia or hypoxia before the development of irreversible injury. Visible and near infrared light pass through human bone and tissue in small amounts, and the emerging light can be used to form images of the interior structure of the tissue and measure tissue blood flow and oxygen utilization based on light absorbance and scattering. We developed a portable time-of-flight and absorbance system which emits pulses of near infrared light into tissue and measures the transit time of photons through the tissue. Images can then be reconstructed mathematically using either absorbance or scattering information. Pathologic brain specimens from adult sheep and human newborns were studied with this device using rotational optical tomography. Images generated from these optical scans show that neonatal brain injuries such as subependymal and intraventricular hemorrhages can be successfully identified and localized. Resolution of this system appears to be better than 1 cm at a tissue depth of 5 cm, which should be sufficient for imaging some brain lesions as well as for detection of regional changes in cerebral blood flow and oxygenation. We conclude that light-based imaging of cerebral structure and function is feasible and may permit identification of patients with impending brain injury as well as monitoring of the efficacy of intervention. Construction of real-time images of brain structure and function is now underway using a fiber optic headband and nonmechanical rotational scanner allowing comfortable, unintrusive monitoring over extended periods of time.

Optical coherence-gated imaging of biological tissues

Abstract: We present optical coherence-gated tomography (OCT) in turbid biological tissues. The fast OCT system that is used in this study is a single-mode fiber-optic interferometer with low-coherence light at 830 nm which can perform a cross-sectional image of 250-600 pixels in a few seconds. Preliminary results suggest that OCT can provide high-resolution imaging in low-scattering and high-scattering superficial tissues. In vitro imaging of porcine cornea after being coagulated with a laser shows that OCT is a promising tool for the evaluation of pathological structures in low-scattering tissue. This technique can also be used to diagnose disease in high-scattering tissues like bladder and living skin. In addition, 50 MHz ultrasound images and histological pictures are presented for comparison with OCT.

Time-resolved transillumination and optical tomography

Abstract: In response to an invitation by the editor-in-chief, I would like to present the current status of time domain imaging With exciting new photon diffusion techniques being developed in the frequency domain and promising optical coherence tomography, time-resolved transillumination is in constant evolution and the subject of passionate discussions during the numerous conferences dedicated to this subject The purpose of time-resolved optical tomography is to provide noninvasive, high-resolution imaging of the interior of living bodies by the use of nonionizing radiation Moreover, the use of visible to near-infrared wavelength yields metabolic information Breast cancer screening is the primary potential application for time-resolved imaging Neurology and tissue characterization are also possible fields of applications Time-resolvedtransillumination and optical tomography should not only improve diagnoses, but the welfare of the patient As no overview of this technique has yet been presented to my knowledge, this paper briefly describes the various methods enabling time-resolved transillumination and optical tomography The advantages and disadvantages of these methods, as well as the clinical challenges they face are discussed Although an analytic and computable model of light transport through tissues is essential for a meaningful interpretation of the transillumination process, this paper will not dwell on the mathematics of photon propagation

Polarized light transmission through skin using video reflectometry: toward optical tomography of superficial tissue layers

Abstract: The movement of polarized light through the superficial layers of the skin was visualized using a video camera with a polarizing filter. This study constitutes a description of the impulse response to a point source of incident collimated linearly polarized light. Polarization images reject unwanted diffusely backscattered light from deeper in the tissue and the specular reflectance from the air/tissue interface. Two experiments were conducted: (1) Video polarization reflectometry used a polarized HeNe laser (633 nm) pointing perpendicularly down onto a phantom medium (0.900-micrometer dia. polystyrene spheres in water). The video camera was oriented 10 degrees off the vertical axis and viewed the irradiation site where the laser beam met the phantom. Video images were acquired through a polarizing filter that was either parallel or perpendicular with the reference plane defined by the source, camera, and irradiation site on the phantom medium's surface. The source polarization was parallel to the reference plane. The two images (parallel and perpendicular) were used to calculate a polarization image which indicated the attenuation of polarization as a function of distance between the source and point of photon escape from the phantom. Results indicated a strong polarization pattern within approximately 0.35 cm (approximately 2.2 mfp') from source. [mfp' equals 1/(microna plus microns')]. (2) Optical fiber reflectometry using a polarized diode laser (792 nm) coupled to a polarization-maintaining single-mode fiber, and a multi-mode fiber collector to collect regardless of polarization. Reflectance as a function of fiber separation was measured for the source fiber oriented parallel and perpendicular with the reference plane. Results indicated that the strongest polarization propagated within approximately 0.43 cm (2.2 mfp') from source. The polarization survived approximately 2.2 mfp', which for skin at 630 - 800 nm (mfp' approximately equals 0.066 cm) corresponds to 1.5 mm (or 6.4 ps of travel at the speed of light). Using 6.4 ps as a maximum time of survival, classical paths of photon transport (Feynman paths) were calculated to illustrate the expected depth of interrogation by polarized imaging. The expected mean depth of photons is about 0.36 mm at these longer wavelengths. Shorter wavelengths would result in a shorter mfp' and therefore more superficial imaging of the skin. Polarization images offer an inexpensive approach toward 2-D acquisition of time- gated images based on the early light escaping the tissue. Polarization imaging is an opportunity for a new form of optical image especially useful for dermatology.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Hyperbolic damped-wave models for transient light-pulse propagation in scattering media

Abstract: Transient optical transport in highly scattering media such as tissues is usually modeled as a diffusion process in which the energy flux is assumed proportional to the fluence (intensity averaged over all solid angles) gradients. Such models exhibit an infinite speed of propagation of the optical signal, and finite transmission values are predicted even at times smaller than those associated with the propagation of light. If the hyperbolic, or wave, nature of the complete transient radiative transfer equation is retained, the resulting models do not exhibit such drawbacks. Additionally, the hyperbolic equations converge to the solution at a faster rate, which makes them very attractive for numerical applications in time-resolved optical tomography.

A tomographic imaging system for pneumatic conveyors using optical fibres

Abstract: This thesis presents an investigation into the application of optical fibre sensors to a tomographic imaging system.Several sensing mechanisms for measurement using non-intrusive techniques are discussed and there relevance to pneumatic conveying discussed. Optical systems are shown to be worthy of investigation. The optical sensor is modelled to predict the expected sensor output voltage profiles arising from different, artificially produced flow regimes. These artificial flow regimes are created by placing a shaped obstruction inside a gravity drop conveyor in the path of the flowing solids. It is shown that for two arrays, each consisting of sixteen transducers, approximately 30% of the measurement volume is sampled.An image reconstruction method for optical tomography is described, based on the back projection between view lines algorithm.The design of the optical tomography system is described, with emphasis on preparation of the ends of the optical fibre, beam collimation and design of the transmitter and receiver circuits.The optical sensors are evaluated singly and as a tomographic array. Results relating to concentration measurement are presented for solids flow using sand with a mean of 300 micron and plastic beads of 2 mm nominal diameter. Measurements were made with a single optical sensor using the gravity flow rig. The results demonstrate the suitability of the optical sensor for concentration measurement for lightly loaded flows (up to approximately 2% solids by volume in the test). The test is extended to all thirty-two sensors using a range of solids mass flow rates from 40 to 320 gm/s with both dry sand and plastic beads over a range of artificially created flow regimes. The results obtained by comparing the measured and predicted flowrates show good general agreement. The statistical parameters for the error of the sand flow measurement have been calculated as having a mean of 6.76% and standard deviation of 3.94% and for plastic beads is 5.43% and standard deviation of 0.21%. The results also demonstrates that the system is reasonably independent of flow regime and so the optical fibre system is suitable as a concentration meter.Back projection is used to generate tomographic images as an alternative representation of the data on concentration measurement. This provides a visual representation of optical density (concentration) information which is not obvious from the concentration measurements.Results from experiments on particles with different sizes are presented. The results are analysed using frequency spectrum techniques and shown to be dependent upon the particle size for approximately spherical particles with diameters between 600 |im and 5 mm.Suggestions for further work on optical fibre sensors and optical fibre tomographic measurements are made.

Endoscopic optical coherence tomography

Abstract: We present advances in OCT technology that will enable in vivo OCT imaging of internal organ systems such as the cardiovascular system, the urinary tract, and the gastrointestinal tract. These advances include improvements in image acquisition speed to avoid motion artifacts, and the development of an OCT compatible catheter-endoscope for access to internal organ systems. A fast scanning OCT system has recently been constructed. This system employs a high power (200 mW) chromium doped forsterite laser as the low coherence source and a piezoelectric fiber stretcher to induce reference arm optical path length delay. The fast scanning system acquires OCT images with an acquisition rate of four images per second, an axial resolution of 15 /spl mu/m, and a signal to noise ratio of 112 dB.

Recent advances in coherent detection imaging (CDI) in biomedicine: laser tomography of human tissues in vivo and in vitro

Abstract: We review the advantages of the optical heterodyne detection-based coherent detection imaging system, for transillumination laser computed tomography (CT) in biomedicine using CW and single-frequency lasers as light sources. The unique properties of the coherent detection imaging system such as excellent directionality, selectivity, and high sensitivity are exploited to differentiate and detect the minimally deviated on-axis/near-axis photons emerging from a tissue enabling the reconstruction high-resolution laser CT images. Our recent progress on the applications of the coherent detection imaging system to laser CT of human fingers in vivo and calcified tissues in vitro at different wavelengths in the visible and near-infrared regions are described. The laser CT images are obtained with low incident power of a few milliwatts and are comparable to conventional imaging techniques. The internal layers of the imaged tissues could be clearly differentiated and identified with submillimeter resolution. We propose that further refinements in the coherent detection imaging system could lead to a novel and potential diagnostic tool in dentistry, osteology, and bone and joint related diseases and disorders.

Optical coherence propagation and imaging in a multiple scattering medium

Abstract: We present a formal, microscopic, solution of the wave propagation problem for an inhomogeneity embedded in an isotropically disordered, multiple scattering, homogeneous background. The inhomogeneity is described by a local change in the complex, dielectric autocorrelation function B(r,r8) [v4/c4^e*(r)e(r8)&ensemble for a wave of frequency w and velocity c. For the homogeneous background, we consider a dielectric autocorrelation function Bh(r−r8) arising from a colloidal suspension of small dielectric spheres. This autocorrelation function can be determined using a newly developed technique called phase space tomography for optical phase retrieval. This technique measures the optical Wigner distribution function I(R,k) defined as the Fourier transform, with respect to r, of the electric field mutual coherence function ^E*(R+r/2)E(R −r/2)&ensemble. The Wigner distribution function is the wave analog of the specific light intensity, Ic(R,k ˆ ), in radiative transfer theory which describes the number of photons in the vicinity of R propagating in direction k ˆ. The Wigner function describes coherence properties of the electromagnetic field which can propagate much longer than the transport mean-free-path l* and which are not included in radiative transfer theory. Given the nature of the homogeneous background, repeated light intensity measurements, which determine the optical phase structure at different points along the tissue surface, may be used to determine the size, shape, and internal structure of the inhomogeneity. In principle, this method improves the resolution of optical tomography to the scale of several optical wavelengths in contrast to methods based on diffusion approximation which have a resolution on the scale of several transport mean-free-paths.

Optical tomography using a genetic algorithm.

Abstract: A new tomographic image reconstruction method is proposed that uses a genetic algorithm (GA), a robust optimization algorithm based on the genetic principle of natural selection. For the purpose of description, a simple axisymmetric reference density field is reconstructed from its interferometric projection by the developed GA-based tomography. This preliminary investigation shows a promising potential of the GA-based tomography to overcome the problems associated with other existing tomographic methods, particularly for limited projections.

Symplectic tomography as classical approach to quantum systems

Abstract: By using a generalization of the optical tomography technique we describe the dynamics of a quantum system in terms of equations for a purely classical probability distribution which contains complete information about the system.

Optical tomography by the temporally extrapolated absorbance method

Abstract: The concept of the temporally extrapolated absorbance method (TEAM) for optical tomography of turbid media has been verified by fundamental experiments and image reconstruction. The TEAM uses the time-resolved spectroscopic data of the reference and object to provide projection data that are processed by conventional backprojection. Optical tomography images of a phantom consisting of axisymmetric double cylinders were experimentally obtained with the TEAM and time-gating and continuous-wave (CW) methods. The reconstructed TEAM images are compared with those obtained with the time-gating and CW methods and are found to have better spatial resolution.

Optical tomography at the microscopic scale by means of a numerical low-coherence holographic technique

Abstract: We describe here a numerical application of the optical low- coherence holographic microscopy (OLCHM) technique. A low coherent source illuminates an object and the backscattered light interfere with a reference wave which optical path's length can be precisely adjusted. Holograms are recorded by a CCD camera and numerically reconstructed. With this technique optical tomography can be performed with a single scan along the optical axis. The transverse resolution of the reconstructed images mainly depends on the optics components used and approaches the diffraction limit. Using a Ti:Sapphire laser a depth resolution of about 30 micrometers has been achieved.

Spectroscopic optical coherence tomography

Abstract: Summary form only given. Implementations of OCT, which take advantage of the spectral bandwidth of low coherence sources for tissue spectroscopy, have not yet been reported. We describe a novel technique for depth-resolved coherent backscatter spectroscopy, which is an extension of OCT technology. Our system incorporates an OCT scanner illuminated by a superluminescent diode (SLD) at 1.3-/spl mu/m center wavelength. In the low-coherence interferometer, a scanning reference mirror generates the temporal cross-correlation function of light reflected from the reference mirror, and that backscattered from the sample arm target. A separate helium-neon interferometer is utilized for digital correction of artifacts in the low-coherence interferometer output due to nonlinearities in the reference arm retroreflector stage velocity. The low-coherence interferometric signal is digitally demodulated and filtered to obtain the complete complex envelope of the interferometric signal, which is required for the spectroscopic technique. Using this system, low-coherence interferograms with accurate sampling intervals and high dynamic range (>98 dB) are acquired.

Adoption of a genetic algorithm (GA) for tomographic reconstruction of line-of-sight optical images

Abstract: A new tomographic reconstruction scheme is proposed that uses a genetic algorithm (GA), a robust and combinatorial function optimization based on the mechanics of the genetic principle. The paper first discusses the implicitly parallel and scaled random nature of the GA optimization using an illustrative example. An introduction of the elementary distribution function (EDF) to constitute the cross-sectional field shows a successful adoption of GA for optical tomography. The GA-based tomography was examined for interferometric projections of computer-synthesized phantom density fields. The GA-based tomography shows accurate and stable image reconstruction, particularly for limited projections.

Dual modality tomography for the monitoring of constituent volumes in multi-component flows.

Abstract: This thesis describes an investigation into the use of dual modality tomography to monitor multi-component flows. The concept of combining two modalities for this purpose evolved from a desire by the Water Research Council to determine volume flow rates of the major components of sewage. No single sensing method is capable of detecting all suspended solids in sewage flows therefore a decision was taken to combine the technologies of electrical resistance and optical tomography to produce a single measurement system. Sensors for both were positioned around the periphery of a static, circular phantom to allow comparisons between dual and single modalities. Modelling was carried out to determine the behaviour of the electrical impedance and optical tomography technologies in a three dimensional situation, where a variety of flow components exist. This provided a greater understanding of the problems involved in combining these technologies and an appreciation of the potential benefits. The remainder of the work can be divided into three areas. Firstly, hardware was constructed to make voltage measurements for both modalities. Secondly, software was written to perform data acquisition, data manipulation and image reconstruction using a simple back projection algorithm developed for this purpose. Finally, an integration of the individual hardware and software components was performed to produce a dual modality system on which tests were carried out to determine the resulting benefits over single modality alternatives.

Moire tomography by ART

Abstract: Moire tomography based on algebraic reconstruction technique (ART) is presented in this paper. The maximum entropy type ART-QMART is modified to apply to moire tomography. A new kind of ART based on Rytov approximation is also presented. Numerical results show that ART type moire tomography can be used to reconstructed 3D refractive index distribution.

Optical tomography of the in vivo human lens: three-dimensional visualization of cataracts

Abstract: An in vivo human lens containing a cataract has been visualized by a series of orthogonal slices made through a three-dimensional volume reconstruction. Data acquisition was made with a transformed series of 60 rotated Scheimpflug digital images. Each digital image represents the light scatter from the lens in a plane that contains the optic axis. At each angular position of the camera, a digital image of the in vivo ocular lens was acquired. The set of 60 Scheimpflug digital images was mathematically transformed into a new data set in which the images were oriented perpendicular to the optic axis of the eye. The transformed set of optical sections was aligned to correct for small eye movements during the data collection process. In order to visualize the volume of the in vivo human lens, slices were projected through the lens volumetric data set. The use of orthogonal slices to visualize lenticular light scatter represents a new technique to visualize human cataracts in vivo in three dimensions.

Compact fast-scanning OCT device for in vivo biotissue imaging

Abstract: Summary form only given. Applications of optical coherence tomography (OCT) as a new method for bioimaging are definitely related to the creation of compact, portable devices suitable for clinical experiments. We report on the development of a compact fast-scanning OCT device comprising a movable fiber probe and on the use of this device in clinics to monitor pathological and postoperative states of human skin in vivo, including inflammation processes, tumors, sutures, and burns. The OCT device is based on a polarisation-preserving optical fiber Michelson interferometer with an integrated unit for fast electrooptic scanning in the depth of the investigated tissue.

Optical tomography improves mammography

Abstract: Near-infrared laser illumination and novel scanner design yield a potentially practical optical tomography system to breast-cancer screening.

New treatment of the focusing method and tomography of the refractive index distribution of inhomogeneous optical components

Abstract: A new mathematical formulation of the focusing method connecting the refractive index distribution of inhomogeneous optical components to the transversally transmitted light distribution is presented. This formulation has the following advantages: the characteristic singularity occurring in the focusing method is avoided; a single numerical integration is used instead of a double one, ensuring a faster data processing; the refractionless approximation is introduced only in the final step of calculations, leading to smaller errors; and the avoidance of a residual logarithmic singularity. An inverse functional transform connecting the intensity distribution in the near field to the refractive index distribution inside the object is deduced and used to obtain analytical test functions. The theoretical results are supported by computer simulations and experiments with gradient-index rods and optical preforms, which have shown improvements in the accuracy, stability, and speed of the method. Moreover, this mathematical formulation for the axisymmetric objects is generalized to a tomographic formulation for nonsymmetric objects using the inverse Radon transform. An efficient algorithm and suitable computer simulations are presented for this case.

Frequency‐domain spectroscopy and imaging of tissue and tissue‐simulating media

Abstract: The goal of this work was to develop and study the use of a diagnosticin vivo tissue spectroscopy system based upon frequency‐domain light measurements. Intensity‐modulated light which is incident upon a scattering sample creates waves of light intensity which propagate through the medium in a manner which is dependent upon the scattering and absorption characteristics of the tissue. These waves can be used to recover these optical interaction parameters using a diffusion model of light propagation in tissue. This method can be used to quantify chromophores for dosimetry in therapeutic laser treatments or for diagnosticmedical applications. The theoretical modeling for diffuse fluorescence signals was also developed and experimentally tested in a tissue‐simulating phantom with excellent agreement, suggesting that fluorescence lifetime or quantum yield can be made from tissue. Preliminary work was done on an optical tomography algorithm using measurements of phase and intensity at multiple points on a tissue surface to reconstructimages of the optical properties of the interior. The development of a tomographic imaging system was examined in the final section of this thesis with data from a tissue simulating phantom.

Diffuse transmittance measurements of homogeneous and inhomogeneous cylindrical phantoms: comparison with FEM calculations

Abstract: Time-resolved and continuous wave diffuse transmittances of near IR light through cylindrical phantoms were measured for the purpose of inhomogeneity detection and image reconstruction. Performed model experiment for homogeneous and inhomogeneous cylindrical phantoms provides a complete experimental data set for optical tomography. Some of the acquired data are presented. Possibility to use them for validation of analytical and numerical models of photon propagation is demonstrated. Good agreement with an analytical solution and 2D FEM simulated data was reached. A new index for inhomogeneity detection is introduced and its effectiveness is demonstrated.

Spectrally shaped rare-earth-doped fiber ASE sources for use in optical coherence tomography

Abstract: Summary form only given. Optical coherence tomography (OCT) has proven over the past few years to be a powerful new optical imaging modality. OCT uses interferometric detection and a short-coherence-length light source to achieve high sensitivity, high-dynamic range, and high resolution (1-10 /spl mu/m). Biomedical applications include ophthalmology, microscopy, endoscopy, laproscopy, dermatology, developmental biology, dentistry etc. One of the key components in OCT systems is the optical source. This paper presents preliminary results on the use of spectrally flattened rare-earth-doped fiber ASE source in OCT systems. Rare-earth-doped fibers are a very promising candidate for future OCT systems. The authors expect spectrally shaped rare earth-doped fiber ASE sources to play an important role in future OCT systems. In the near future, source powers in excess of 100 mW and coherence lengths under 10 cm should be available at a variety of wavelengths. The authors review some of the design details on this Nd source, quantify its optical parameters power, bandwidth, blindness, etc.), and present its application in tomographic biomedical imaging (e.g. skin and bone structure). The authors also compare its performance with other types of sources such as: semiconductor sources (light-emitting diodes, edge-emitting diodes, superluminescent diodes), model-lock lasers, and supercontinuum sources.

Optical tomography of rat brain

Abstract: An experimental set-up for the optical tomography using photon density waves is described. The intensity of a NIR laser diode (825 nm) is modulated with a frequency of 110 MHz (modulating degree equal to 80%). The line scan of a rat brain at 64 linear steps and 64 angular steps contains the ac attenuation and the phase shift separately. Due to the impact of scattering in the medium these data are not the basis for a backprojection and deconvolution for the tomographic reconstruction itself. The first approach for a preprocessing consists in the determination of the geometry dependent modulation transfer function (MTF). The line scan is then corrected by the filter which varies with the position. An optical tomogram of a rat brain is presented. This procedure is suitable for biological objects up to a diameter of 30 mm approximately, for example small finger joints.

Application of optical coherence tomography in nondestructive evaluation of material microstructure

Abstract: Summary form only given. Techniques for nondestructive evaluation (NDE), testing, and inspection of parts during manufacturing and materials processing, in situ, or during design, is a key technology. The authors use optical coherence tomography (OCT) is a high-resolution, high-sensitivity imaging technology that is based on the coherence properties of light.