Fluorescence tomography of targets in a turbid medium using non-negative matrix factorization.

A near-infrared optical tomography approach for detection, three-dimensional localization, and cross-section imaging of fluorescent targets in a turbid medium is introduced. The approach uses multisource probing of targets, multidetector acquisition of diffusely transmitted fluorescence signal, and a non-negative matrix factorization based blind source separation scheme to obtain three-dimensional location of the targets. A Fourier transform back-projection algorithm provides an estimate of target cross section. The efficacy of the approach is demonstrated in an experiment involving two laterally separated small fluorescent targets embedded in a human breast tissue-simulating sample of thickness 60 times the transport mean free path. The approach could locate the targets within ∼1 mm of their known positions, and provide estimates of their cross sections. The high spatial resolution, fast reconstruction speed, noise tolerance, and ability to detect small targets are indicative of the potential of the approach for detecting and locating fluorescence contrast-enhanced breast tumors in early growth stages, when they are more amenable to treatment.

Near-infrared center-of-intensity time gated imaging for detection of a target in a highly scattering turbid medium.

A near-infrared optical imaging approach for locating a target embedded in a turbid medium is introduced. The target localization is based on an analysis of the spatial variation of the transmitted-light intensity distribution for illumination at different positions on the sample boundary. The approach is used to detect, locate and generate images of absorbing targets embedded inside model scattering media of thickness approximately 50 times the transport mean free path of the medium, as well as, of ex vivo biological tissue specimens.

Time reversal optical tomography: locating targets in a highly scattering turbid medium.

A time reversal optical tomography (TROT) method for near-infrared (NIR) diffuse optical imaging of targets embedded in a highly scattering turbid medium is presented. TROT combines the basic symmetry of time reversal invariance and subspace-based signal processing for retrieval of target location. The efficacy of TROT is tested using simulated data and data obtained from NIR imaging experiments on absorptive and scattering targets embedded in Intralipid-20% suspension in water, as turbid medium. The results demonstrate the potential of TROT for detecting and locating small targets in a turbid medium, such as, breast tumors in early stages of growth.

Analytical cumulant solution of the vector radiative transfer equation investigates backscattering of circularly polarized light from turbid media.

The backscattering of circularly polarized light pulses from an infinite uniform scattering medium is studied as a function of helicity of the incident light and size of scatterers in the medium. The approach considers a polarized short pulse of light incident on the scattering medium, and uses an analytical cumulant solution of the vector radiative transfer equation with the phase matrix obtained from the Mie theory to calculate the temporal profile of scattered polarized photons for any position and any angle of detection. The general expression for the scattered photon distribution function is an expansion in spatial cumulants up to an arbitrary high order. Truncating the expansion at the second-order cumulant, a Gaussian analytical approximate expression for the temporal profile of scattered polarized photons is obtained, whose average center position and half width are always exact. The components of scattered light copolarized and cross polarized with that of the incident light can be calculated and used for determining the degree of polarization of the scattered light. The results show that circularly polarized light of the same helicity dominates the backscattered signal when scatterer size is larger than the wavelength of light. For the scatterers smaller than the wavelength, the light of opposite helicity makes the dominant contribution to the backscattered signal. The theoretical estimates are in good agreement with our experimental results.

Lasers in cancer detection and diagnosis research: enabling characteristics with illustrative examples.

The salient properties of laser light and the way light interacts with biological tissues and molecular constituents of tissues offer possibilities for detection and diagnosis of cancer. In particular, the wavelength selectivity of tunable lasers, narrow bandwidth around the selected wavelength, and spectral brightness enable probing of key molecular constituents of tissues, and endow laser-based techniques with much desired diagnostic potential. This article presents an overview of some recent developments in optical imaging and optical biopsy of different types of cancers, and illustrates the diagnostic role of the color of light.

Optical imaging of turbid media using independent component analysis: theory and simulation.

A new imaging approach for 3-D localization and characterization of objects in a turbid medium using independent component analysis (ICA) from information theory is developed and demonstrated using simulated data. This approach uses a multisource and multidetector signal acquisition scheme. ICA of the perturbations in the spatial intensity distribution measured on the medium boundary sorts out the embedded objects. The locations and optical characteristics of the embedded objects are obtained from a Green's function analysis based on any appropriate model for light propagation in the background medium. This approach is shown to locate and characterize absorptive and scattering inhomogeneities within highly scattering medium to a high degree of accuracy. In particular, we show this approach can discriminate between absorptive and scattering inhomogeneities, and can locate and characterize complex inhomogeneities, which are both absorptive and scattering. The influence of noise and uncertainty in background absorption or scattering on the performance of this approach is investigated.

Spectral and temporal near-infrared imaging of ex vivo cancerous and normal human breast tissues.

Cancerous and normal ex vivo human breast tissues were investigated using spectroscopic and time-sliced two-dimensional (2-D) transillumination imaging methods in order to demonstrate the importance and potential of spectral and temporal measurements in breast cancer detection and diagnosis. The experimental arrangement for time-sliced optical imaging used 120 fs, 1 kHz repetition-rate, 800 nm light pulses from a Ti:sapphire laser system for sample illumination, and a 80 ps resolution ultrafast gated intensified camera system for recording 2-D time-sliced images. The spectroscopic imaging arrangement used 1225-1300 nm tunable output of a Cr: forsterite laser for sample illumination, a Fourier space gate to discriminate against multiple-scattered light, and a near-infrared area camera to record 2-D images. Images recorded with earlier temporal slices of transmitted light highlighted tumors, while those recorded with later slices accentuated normal tissues. When light was tuned closer to the 1203 nm absorption resonance of adipose tissues, a marked enhancement in contrast between the images of adipose and fibrous tissues was observed. A similar wavelength-dependent difference between normal and cancerous tissues was observed. These results correlate well with pathology and nuclear magnetic resonance based analyses of the samples.

Three-dimensional localization and optical imaging of objects in turbid media with independent component analysis.

A new approach for optical imaging and localization of objects in turbid media that makes use of the independent component analysis (ICA) from information theory is demonstrated. Experimental arrangement realizes a multisource illumination of a turbid medium with embedded objects and a multidetector acquisition of transmitted light on the medium boundary. The resulting spatial diversity and multiple angular observations provide robust data for three-dimensional localization and characterization of absorbing and scattering inhomogeneities embedded in a turbid medium. ICA of the perturbations in the spatial intensity distribution on the medium boundary sorts out the embedded objects, and their locations are obtained from Green's function analysis based on any appropriate light propagation model. Imaging experiments were carried out on two highly scattering samples of thickness approximately 50 times the transport mean-free path of the respective medium. One turbid medium had two embedded absorptive objects, and the other had four scattering objects. An independent component separation of the signal, in conjunction with diffusive photon migration theory, was used to locate the embedded inhomogeneities. In both cases, improved lateral and axial localizations of the objects over the result obtained by use of common photon migration reconstruction algorithms were achieved. The approach is applicable to different medium geometries, can be used with any suitable photon propagation model, and is amenable to near-real-time imaging applications.

Optical tomographic image reconstruction from ultrafast time-sliced transmission measurements.

Optical imaging and localization of objects inside a highly scattering medium, such as a tumor in the breast, is a challenging problem with many practical applications. Conventional imaging methods generally provide only two-dimensional (2-D) images of limited spatial resolution with little diagnostic ability. Here we present an inversion algorithm that uses time-resolved transillumination measurements in the form of a sequence of picosecond-duration intensity patterns of transmitted ultrashort light pulses to reconstruct three-dimensional (3-D) images of an absorbing object located inside a slab of a highly scattering medium. The experimental arrangement used a 3-mm-diameter collimated beam of 800-nm, 150-fs, 1-kHz repetition rate light pulses from a Ti:sapphire laser and amplifier system to illuminate one side of the slab sample. An ultrafast gated intensified camera system that provides a minimum FWHM gate width of 80 ps recorded the 2-D intensity patterns of the light transmitted through the opposite side of the slab. The gate position was varied in steps of 100 ps over a 5-ns range to obtain a sequence of 2-D transmitted light intensity patterns of both less-scattered and multiple-scattered light for image reconstruction. The inversion algorithm is based on the diffusion approximation of the radiative transfer theory for photon transport in a turbid medium. It uses a Green s function perturbative approach under the Rytov approximation and combines a 2-D matrix inversion with a one-dimensional Fourier-transform inversion to achieve speedy 3-D image reconstruction. In addition to the lateral position, the method provides information about the axial position of the object as well, whereas the 2-D reconstruction methods yield only lateral position.

Time-resolved and nonlinear optical imaging for medical applications.

In this article, we have presented an overview of emerging novel techniques for early-light transillumination imaging as well as nonlinear optical tomography of body organs. The use of light for probing and imaging biomedical media offers the promise for development of safe, noninvasive, and inexpensive clinical imaging modalities with diagnostic ability. The strong scattering of light by biological tissues buries the shadowgram formed by forward-propatating image-bearing photons in the background noise of multiple-scattered light. Several methods for extraction of image-bearing light that capitalize on spatial, temporal and polarization characteristics of transmitted light are reviewed. More recently emerging nonlinear-optical histopathology methods for imaging subsurface structures of tissues in terms of its local spatial symmetry and molecular content are introduced. The progress made so far indicates that some of these techniques are apt to make a transition from laboratory to useful clinical modalities.

Two-dimensional near-infrared transillumination imaging of biomedical media with a chromium-doped forsterite laser.

Transillumination images of objects hidden in normal and cancerous human breast tissues and bovine, porcine, and gallinaceous (chicken) tissues as well as model-random-scattering media were recorded with 1250-nm light from a chromium-doped forsterite laser. A Fourier space gate and a polarization gate were used to sort out image-bearing photons and discriminate against multiply scattered image-blurring photons. Better contrast, higher spatial resolution, and deeper penetration of samples were achieved for imaging with 1250-nm light than those obtained at shorter wavelengths, such as 1064 nm from a neodymium-doped yttrium aluminum garnet (YAG) laser. Better contrast and higher resolution were also obtained when the object was imaged through normal human breast tissue than through cancerous breast tissue. Images with marked distinction between fatty and fibrous human breast tissues were obtained when the Cr:forsterite laser was tuned to 1225 nm, a wavelength that resonates with an optical absorption band of breast fat tissues. Imaging with linearly polarized light revealed that the image quality depends significantly on the orientation of the polarization of the incident light with respect to the fibers in the bovine tissue.

Advances in optical imaging of biomedical media.

In this article, we have presented an overview of fundamental issues involved in mediphotonic imaging, and reviewed some of the emerging techniques for early-light transillumination imaging of body organs. The results on human breast tissues presented here, together with the data accumulated and advances made by researchers around the globe, not only demonstrate the feasibility of optical imaging as a clinical procedure but indicate a road map to reach that goal. The milestones include evaluation of relative merits of available approaches for a particular imaging application; selection of diagnostic wavelengths, as well as sources to generate and detectors to monitor light at those wavelengths; accumulation of data on optical, spectroscopic, and transport properties of tissues and organs; in vivo testing; prototype instrumentation development; clinical trials; governmental approval; cost analysis and marketing; and finally system improvement based on feedback from end users. A new era of optical clinical imaging is at the door.

High-average-power kilohertz-repetition-rate sub-100-fs Ti:sapphire amplifier system.

We report on a femtosecond Ti:sapphire laser amplifier system that generates pulse energies >5 mJ at a 1-kHz repetition rate. The system consists of regenerative and multipass amplifiers and uses the technique of chirped-pulse amplification. When the system was seeded with 70-fs pulses at 800 nm from a self-mode-locked Ti:sapphire oscillator, amplified pulses of 94-fs duration at a repetition rate of 1 kHz and an average output power of 5.4 W were produced. The amplified pulse-repetition rate is variable from 250 Hz to 3 kHz. Pulse energies of >7.5 mJ were obtained at 500 and 250 Hz.

Experimental demonstration of an excited-state Faraday filter operating at 532 nm.

We outline what is to our knowledge the first experimental demonstration of an excited-state Faraday filter. The filter consists of potassium vapor between crossed polarizers in a dc magnetic field and operates on the 4P((1/2)) ? 8S((1/2)) transition in potassium. The 4P((1/2)) state is populated by a linearly polarized, 10-ns light pulse from a dye laser operating at 769.9 nm. Another linearly polarized, 10-nsec pulse at 532.33 nm traverses the pumped volume of the K cell and is absorbed from the 4P((1/2)) state to the 8S((1/2)) state. The transmission of the filter is approximately 3.5% at 532.33 nm with a bandwidth of less than 10 GHz.

Induced-dichroism-excited atomic line filter at 532 nm.

The operation of a narrow-linewidth optical f ilter based on the 4P((1/2)) ? 8S((1/2)) excited-state transition in potassium vapor is reported. The 4P((1/2)) state is excited by a circularly polarized, 769.9-nm, 10-ns pulse from a dye laser. A linearly polarized, time-sequenced, and spatially overlapped probe pulse at 532.33 nm completes the transition to the 8S((1/2)) state. The peak filter transmission is ~40% with a bandwidth of less than 4 GHz. Corroborative experimental results suggest that the rotation of probe-pulse polarization by an induced circular birefringence is the dominant mechanism behind the filter operation.

Site-selective electronic Raman excitation spectroscopy of the lowest 4f ? 5d transitions in Ce(3+):Y(2)O(3).

The intensities of electronic Raman-scattering transitions between the ground state and the first excited state of the (2)F(7/2) manifold of Ce(3+) in both the C(2) and C(3i) sites of Y(2)O(3) are measured as a function of excitation frequency in the region of the lowest 4f ? 5d intermediate-state resonance with a wavelength-tunable pulsed dye laser. As the dye laser is tuned through the center of the resonance, we observe a 2-order-of-magnitude enhancement in electronic Raman-transition intensity over that for nonresonant excitation. The electronic Raman excitation profiles (resonance enhancement as a function of frequency) in the 15 949- to 23 810-cm(-1) (627-420-nm) range provide spectrally resolved information about the lowest 4f ? 5d transitions of both the C(2) and C(3i) centers that could not be obtained from the strongly overlapping absorption spectra.

Continuous-wave laser operation of chromium-doped forsterite.

Room-temperature continuous-wave laser action in chromium-activated forsterite (Cr:Mg(2)SiO(4)) has been achieved for longitudinal pumping in a nearly concentric cavity by the 1064-nm radiation from a cw Nd:YAG laser. The laser emission is centered at 1244 nm and has a spectral bandwidth of 12 nm. An output-power slope efficiency of 6.8% is measured. The effective emission cross section is estimated to be ~1.1 x 10(-19) cm(2).

Near infrared tunable operation of chromium doped forsterite laser.

Tunable, room temperature pulsed laser operation of a chromium-doped forsterite laser for 1064-nm pumping is reported. Using different sets of mirrors and a single birefringent plate as the intracavity wavelength selecting element, tunability over the 1167-1345-nm spectral range has been demonstrated.

The crystal structure of tortoise egg-white lysozyme at 6 A resolution.

Lysozyme extracted from the egg-white of tortoise, the first example of a reptilian lysozyme to have been purified, has been crystallized and its tertiary structure determined at low resolution by X-ray analysis. This structure is shown to be closely homologous to that of hen egg-white lysozyme. The crystals of tortoise egg-white lysozyme contain a large proportion of liquid and the X-ray map shows that this forms large channels through the crystals into which the active sites of the enzyme molecules open. This indicates that tortois lysozyme crystals may be suitable for low-temperature studies of true enzyme substrate complexes.