Femtosecond transillumination tomography in thick tissues

Distortion matrix concept for deep optical imaging in scattering media

In optical imaging, light propagation is affected by the inhomogeneities of the medium. Sample-induced aberrations and multiple scattering can strongly degrade the image resolution and contrast. On the basis of a dynamic correction of the incident and/or reflected wavefronts, adaptive optics has been used to compensate for those aberrations. However, it only applies to spatially invariant aberrations or to thin aberrating layers. Here, we propose a global and noninvasive approach based on the distortion matrix concept. This matrix basically connects any focusing point of the image with the distorted part of its wavefront in reflection. A singular value decomposition of the distortion matrix allows to correct for high-order aberrations and forward multiple scattering over multiple isoplanatic modes. Proof-of-concept experiments are performed through biological tissues including a turbid cornea. We demonstrate a Strehl ratio enhancement up to 2500 and recover a diffraction-limited resolution until a depth of 10 scattering mean free paths.

The effect of temporal impulse response on experimental reduction of photon scatter in time-resolved diffuse optical tomography

New fast detector technology has driven significant renewed interest in time-resolved measurement of early photons in improving imaging resolution in diffuse optical tomography and fluorescence mediated tomography in recent years. In practice, selection of early photons results in significantly narrower instrument photon density sensitivity functions (PDSFs) than the continuous wave case, resulting in a better conditioned reconstruction problem. In this work, we studied the quantitative impact of the instrument temporal impulse response function (TIRF) on experimental PDSFs in tissue mimicking optical phantoms. We used a multimode fiber dispersion method to vary the system TIRF over a range of representative literature values. Substantial disagreement in PDSF width--by up to 40%--was observed between experimental measurements and Monte Carlo (MC) models of photon propagation over the range of TIRFs studied. On average, PDSFs were broadened by about 0.3 mm at the center plane of the 2 cm wide imaging chamber per 100 ps of the instrument TIRF at early times. Further, this broadening was comparable on both the source and detector sides. Results were confirmed by convolution of instrument TIRFs with MC simulations. These data also underscore the importance of correcting imaging PDSFs for the instrument TIRF when performing tomographic image reconstruction to ensure accurate data-model agreement.

High-resolution computed tomography of refractive index distribution by transillumination low-coherence interferometry

We present a method to image refractive index distribution within a sample across 8 mm dimension with high spatial resolution by a transmission low-coherence interferometer. The relative strong forward-scattering light is collected, from which the parallel projections of refractive indices within the sample are obtained. A convolution backprojection algorithm is used to transform the projection data set recorded at sufficient angular views into the spatial distribution of refractive indices within the sample. We experimentally demonstrate this method by imaging a phantom. We show that this method can achieve a precision of 0.01 in determining the refractive index and a spatial resolution of 40 microm.

Pass-through photon-based biomedical transillumination

We present the mathematical foundation and the experimental validation of a technique that utilizes pass-through (ballistic) photons in a partial coherence interferometric transillumination setup for biomedical analyses. We demonstrate that the implementation depends closely on tissue under test, incident power, spatial and spectral characteristics of the radiation source, and detection electronics. With the aid of the complex material coherence function concept, we foresee tissue characterization and diagnostic imaging as potential applications for the technique. We propose a normalization procedure for in vitro and in vivo measurements, where nontissue-related quantities are canceled out. The validation of the proposal is achieved by obtaining the sample coherence function of a tissue phantom. The expected exponential attenuation is confirmed, and the corresponding scattering coefficients are determined. A good agreement between theory and experiment, for the initial set of samples, serves to establish that pass-through photon-based transillumination is feasible for selected biomedical applications.

Fourier-domain holography in photorefractive quantum-well films

Fourier-domain holography (FDH) is investigated as a candidate for holographic optical coherence imaging to produce real-time images of structure inside living tissue and turbid media. The effects of spatial filtering, the background intensity distributions, and the role of background noise in determining dynamic range are evaluated for both FDH and image-domain holography (IDH). The grating washout effect in FDH (edge enhancement) is removed by use of a vibrating diffuser that consequently improves the image quality. By comparing holographic images and background images of FDH and IDH we show that FDH provides a higher dynamic range and a higher image quality than IDH for this specific application of imaging diffuse volumetric objects.

Ballistic attenuation of low-coherence optical fields

Using a novel experimental geometry, we measured the ballistic attenuation of low-coherence optical fields propagating in multiple-scattering media. The high dynamic range and the angular filtering capability permit detecting the ballistic component through random media thicker than 20 mean free paths. Owing to the accuracy of the technique, we observe deviations from the standard Lambert-Beer law that are induced by the broad incident optical spectrum. We discuss the significance of these observations, their dependence on the type of scattering, and several potential applications.

Methods for imaging the structure and function of living tissues and cells: 1. Optical coherence tomography

This is the first in a series of review articles which aim to present a concise and systematic overview of the principles, limitations, advantages, and uses of some of the more important recently developed techniques capable of imaging living histology. Optical coherence tomography (OCT) is now an established optical biopsy method, imaging 2-3 mm into opaque tissue. It is analogous to optical 'ultrasound' but has an outstanding resolution, being capable of imaging single cells in the intact animal via a surface, intravascular or endoscopic approach. Both two-dimensional (2D) and three-dimensional (3D) image datasets can be acquired and studied over time (4D imaging) in the live animal or human subject without the need to remove tissue or perform any tissue processing or staining. It has been used in ophthalmology, gastrointestinal tract (GI) studies, gynaecological tract investigation, and endovascular imaging, to name but a few areas. A degree of differential tissue contrast information can also be gleaned, since different tissue components give different OCT reflectivity signals such that adipose, muscle, collagen, and elastic components may all be resolved without staining. Continuing developments include faster data acquisition for real-time recording and Doppler OCT for more functional imaging.

Two-dimensional time-resolved direct imaging through thick biological tissues: a new step toward noninvasive medical imaging

We report an original two-dimensional time-resolved direct imaging method for transillumination optical tomography that combines the time-gating and forward phase-conjugation properties of type II degenerate parametric amplification. An object with subcentimeter resolution embedded in 4-cm-thick chicken breast tissue was imaged with a signal-to-noise ratio of 2.

Femtosecond single-shot correlation system: a time-domain approach

We introduce, analyze, and experimentally demonstrate what to the best of our knowledge is a new pulse correlation technique that is capable of real-time conversion of a femtosecond pulse sequence into its spatial image. Our technique uses a grating at the entrance of the system, thus introducing a transverse time delay (TTD) into the transform-limited reference pulse. The shaped signal pulses and the TTD reference pulse are mixed in a nonlinear optical crystal (LiB(3)O(5)), thus producing a second-harmonic field that carries the spatial image of the temporal shaped signal pulse. We show that the time scaling of the system is set by the magnification of the anamorphic imaging system as well as by the grating frequency and that the time window of the system is set by the size of the grating aperture. Our experimental results show a time window of approximately 20 ps. We also show that the chirp information of the shaped pulse can be recovered by measurement of the spectrum of the resulting second-harmonic field.

Coherence and polarization of light propagating through scattering media and biological tissues

The degree of polarization of light propagating through scattering media was measured as a function of the sample thickness in a Mach-Zehnder interferometer at a wavelength of lambda = 633 nm. For polystyrene microspheres of diameters 200, 430, and 940 nm, depolarization began to appear for thicknesses larger than 23, 19, and 15 scattering mean free paths (SMFP's), respectively, where the coherently detected scattered component dominates the ballistic component. For large particles (940 nm) the initial polarization survived partially in the scattering regime and progressively vanished up to the detection limit of our setup. This phenomenon was similarly observed in diluted blood from 12.5 to 280 SMFP's. Beyond this thickness the fluctuating parallel and crossed components of polarization became random. A dual-channel interferometer allowed us to detect simultaneously the low-frequency fluctuations of both polarized components through a few millimeters in liver tissue.

Accurate remote distance sensing by use of low-coherence interferometry: an industrial application

An apparatus for accurate remote distance sensing based on fiber-optic low-coherence light interferometry has been designed for molten glass level measurement. We demonstrate operation of the meter in an adverse industrial environment with <20-mum resolution (standard deviation) within a 20-mm range with the sensing head placed in an oven at ~800 degrees C. In laboratory conditions we were able to measure with 3-mum resolution, which could be improved to submicrometer level by optimization of a reference arm of the interferometer and detection electronics.

Coherence-based imaging through turbid media by use of degenerate four-wave mixing in thin liquid-crystal films and photorefractives

We describe a coherence-based imaging technique based on degenerate four-wave mixing in two materials. First, a 45 degrees -cut BaTiO(3) crystal was used as a self-pumped phase conjugator to obtain depth-resolved images through a 4 mean-free-path (mfp) scattering media. In addition, an HITC dye-doped K15 liquid-crystal layer was used to provide single-shot image acquisition through a 2 mfp scattering media. This technique can be used to provide instantaneous (single-pulse), depth-resolved, two-dimensional images of the internal structure of scattering materials. Potential applications of this technique include subsurface imaging of biomedical tissue and nondestructive evaluation of composite materials.

Investigation of dispersion effects in ocular media by multiple wavelength partial coherence interferometry

We report on quantitative measurements of group refractive indices and group dispersion in water and in human ocular media such as the cornea, the aqueous humor, the lens, artificial intraocular lenses, as well as a total value averaged over the media along the axial eye length of normal subjects and pseudophakic patients in vivo using dual beam partial coherence interferometry. Different optical thickness values due to the dispersion of the cornea are demonstrated using two spectrally displaced light sources. The displacement can be used to indirectly calculate the group dispersion of the human cornea in the spectral region between 810 nm and 860 nm. If the object under investigation is dispersive, resolution is limited due to a broadening of the detected signals. This broadening increases with group dispersion, i.e., the extent to which the group refractive index of the medium varies with wavelength and thickness of the tissue under investigation as well as with the spectral bandwidth of the light source. Measurements of the group dispersion in the cornea, lens and vitreous of pseudophakic and normal human eyes, show that the cornea and the lens are more dispersive than water-by a factor of about 5 and 2, respectively-in the investigated spectral region. The cornea is approximately threefold more dispersive than the human crystalline lens, the aqueous humor is less dispersive than water and the group dispersion of all ocular components together, averaged over the axial length of normal and pseudophakic eyes, was only slightly higher compared to that of water. Since the highly dispersive cornea and lens together have only a thickness of about one sixth of that of the axial eye length, it seems that their contribution to the group dispersive effect along the whole axial eye length is only small.

Optical imaging in medicine: I. Experimental techniques

The overwhelming scatter which occurs when optical radiation propagates through tissue severely limits the ability to image internal structure using measurements of transmitted intensity. A broad range of methods has been proposed during the past decade or so in order to improve imaging performance. Direct methods involve isolating an unscattered or least-scattered component of transmitted scattered light. Indirect methods generally involve measuring some characteristic of the temporal distribution of transmitted light, or an equivalent in the frequency domain, and obtaining a computational solution to the inverse problem. In this paper, we review the experimental techniques which have been proposed in order to explore both direct and indirect imaging. The relative merits and limitations of the various experimental methods are discussed, and we consider the future directions and likelihood of success of optical imaging in medicine.

Coherence imaging by use of a Newton rings sampling function

We show that, with suitable optics in the arm of a Michelson interferometer, orthogonal galvo-scanning mirrors build a sampling function in the form of Newton rings when the two interferometer arms are matched. Using a low-coherence source, one can obtain transversal depth-resolved images. A fast display procedure using a storage oscilloscope was devised based on this method.

Rotating polarization imaging in turbid media

It is shown experimentally that one can image an object embedded in a turbid medium by probing the medium with light with a rotating linear polarization. This method permits the ballistic photons to be isolated from the large background of photons that have been multiply scattered by optically dense anisotropic scatterers. This technique achieves a good signal-to-noise ratio even with a low-power continuous laser, leading to images with a diffraction-limited resolution comparable with that obtained in optically homogeneous media.

Time-resolved imaging on a realistic tissue phantom: μ(s)' and μ(a) images versus time-integrated images

A method is proposed by which we construct images through turbid media, plotting directly either the transport-scattering coefficient μ(s) ' or the absorption coefficient μ(a). These optical parameters are obtained from the best fit of the time-resolved transmittance curves with a diffusion model. Measurements were performed with a time-correlated single-photon counting system on realistic tissue phantoms simulating a tumor mass within a breast. Images were obtained with an incident power of <1 mW and an acquisition time of 1 s/point. Comparison of μ(s) ' and μ(a) images with time-integrated images constructed from the same experimental data shows that the fitting method discriminates between scattering and absorption inhomogeneities and improves image quality for scattering but not for absorption inhomogeneities.

Improved imagery through scattering materials by quasi-Fourier-synthesis holography

An improvement on the technique of Fourier-synthesis holography is proposed and demonstrated. Artifacts produced during the process of sampling are eliminated when the laser is swept over a continuous bandwidth between samples. The advantages of Fourier-synthesis holography, such as the ability to select the gating time delay and to shape the autocorrelation function after data acquisition, are retained.

Effect of an annular pupil on confocal imaging through highly scattering media

We report on confocal scanning imaging through highly scattering media. Various practical effects including those of the annular pupils and the size of the confocal pinhole as well as of the numerical aperture of objectives on the image quality are examined xperimentally. The combination of an annular objective with a finitesized detector may prove advantageous for improving image quality.

Fermat photons in turbid media: an exact analytic solution for most favorable paths-a step toward optical tomography

Photon propagation in a turbid medium characterized by highly forward-directed light scattering can be described by a generalized Fermat principle, a counterpart of the conventional Fermat principle for transparent media. An exact analytic solution for the most favorable photon paths (Fermat paths) has been found and is investigated. An analysis intended to encourage and facilitate experimental observation of Fermat photons is presented. The feasibility of using the theory in time-resolved optical tomography for the recognition of objects hidden in turbid media is discussed.

Extinction measurements in diffusing mammalian tissue with heterodyne detection and a titanium:sapphire laser

Total optical absorption in mammalian tissues is measured in the near infrared by the use of heterodyne detection and a Ti:sapphire laser. Because of the high sensitivity, directivity, and signal-to-noise ratio of the setup, we were able to detect coherent photons after attenuation by more than 9 optical densities. This method allows us to detect unscattered photons that are passing through more than 7 mm of various tissues such as brain, muscle, liver, skin, and fat selectively.

Analysis of Fourier synthesis holography for imaging through scattering materials

The technique of Fourier synthesis holography to image through scattering materials is analyzed in detail. A broad spectral source is decomposed into its Fourier components, and a hologram is formed at each wavelength and stored in the computer. Upon synthesis in the computer, a clear image can be formed of the obscured object. Post-data-acquisition processing such as selection of the gating time delay and autocorrelation shaping are also demonstrated.

Continuous-wave ultrasonic modulation of scattered laser light to image objects in turbid media

Continuous-wave ultrasonic modulation of scattered laser light has been used to image objects in tissue-simulating turbid media for what is to our knowledge the first time. The ultrasound wave focused into the turbid media modulates the laser light passing through the ultrasonic focal zone. The modulated laser light collected by a photomultiplier tube reflects the local mechanical and optical properties in the focal zone. Buried objects are located with millimeter resolution by scanning and detecting alterations of the modulated optical signal. This technique has the potential to provide a noninvasive, nonionizing, inexpensive diagnostic tool for diseases such as breast cancer.

Laser picosecond acoustics in double-layer transparent films

By the optical pump-and-probe technique, picosecond acoustic pulses are excited and detected in thin transparent double-layer films of silica and silicon nitride upon opaque chromium substrates. By taking both acoustic and optical multiple reflections into account, one can successfully model the main features of the reflectance variation. The film thicknesses, sound velocities, and photoelastic constants are derived by use of known values of the refractive indices.

Concentration measurements of a light absorber localized in a scattering medium

Using a time-resolved technique, we have studied two methods of quantifying the concentration of a light absorber localized in a scattering medium. Because some of the transmitted photons pass through the absorber region many times because of complicated photon propagation in the scattering medium, we used the following methods. First, early-arriving photons passing through the absorber approximately once were selected. Using these photons, we quantified the concentration of the absorber by an ordinary spectroscopic method based on the Beer-Lambert law. Second, for later-arriving photons, after empirically determining the distribution of the number of passes through the absorber as a function of detection time, we obtained the concentration by applying this function to the Beer-Lambert law.

Transmitted photon intensity through biological tissues within various time windows

The intensity of the early-arriving snake portion of 100-fs ultrashort laser pulses transmitted through biological tissue of increasing thickness was measured by a streak camera. The snake photon intensity within the first arrival time interval Deltat was found to decrease exponentially with tissue thickness (z) as I(Deltat) = I(0)A exp[-b(Deltat)z/l(t)], where I(0) is the incident laser pulse intensity, l(t) is the transport mean free path of the medium, and the parameters b and A depend on the time interval Deltat. This result shows that the intensity of snake photons decays significantly more slowly than that of ballistic photons as tissue thickness increases.

Image resolution by use of multiply scattered light

We present calculations based on the Markov-chain technique of the intensity and direction of light traversing a multiply scattering medium in the isotropic scattering case for pulsed point-source illumination. From these calculations we derive expressions for the trade-off between spatial image resolution and detector integration time when the diffusion approximation is not valid. We show that image resolution better than the diffusion limit is theoretically possible use of multiply scattered light for samples thinner than ~35 scattering lengths. Monte Carlo simulations are used to extend these results to the anisotropic scattering case.