Chrono-coherent imaging for medicine

Dynamic imaging through turbid media based on digital holography

Imaging through turbid media using visible or IR light instead of harmful x ray is still a challenging problem, especially in dynamic imaging. A method of dynamic imaging through turbid media using digital holography is presented. In order to match the coherence length between the dynamic object wave and the reference wave, a cw laser is used. To solve the problem of difficult focusing in imaging through turbid media, an autofocus technology is applied. To further enhance the image contrast, a spatial filtering technique is used. A description of digital holography and experiments of imaging the objects hidden in turbid media are presented. The experimental result shows that dynamic images of the objects can be achieved by the use of digital holography.

Single-shot holography for depth resolved three dimensional imaging

We introduce a method for depth-resolved photorefractive holographic imaging with potentially extremely short acquisition time for a complete three dimensional (3D) image. By combining the advantages of full-field frequency-domain optical coherence tomography with those of photorefractive holography our concept is capable of obtaining 3D information with only one single shot. We describe the operation principle of our concept and give a first experimental proof of principle.

Fourier-domain digital holographic optical coherence imaging of living tissue

Digital holographic optical coherence imaging is a full-frame coherence-gated imaging approach that uses a CCD camera to record and reconstruct digital holograms from living tissue. Recording digital holograms at the optical Fourier plane has advantages for diffuse targets compared with Fresnel off-axis digital holography. A digital hologram captured at the Fourier plane requires only a 2D fast Fourier transform for numerical reconstruction. We have applied this technique for the depth-resolved imaging of rat osteogenic tumor multicellular spheroids and acquired cross-section images of the anterior segment and the retinal region of a mouse eye. A penetration depth of 1.4 mm for the tumor spheroids was achieved.

Holographic optical coherence imaging of rat osteogenic sarcoma tumor spheroids

Holographic optical coherence imaging is a full-frame variant of coherence-domain imaging. An optoelectronic semiconductor holographic film functions as a coherence filter placed before a conventional digital video camera that passes coherent (structure-bearing) light to the camera during holographic readout while preferentially rejecting scattered light. The data are acquired as a succession of en face images at increasing depth inside the sample in a fly-through acquisition. The samples of living tissue were rat osteogenic sarcoma multicellular tumor spheroids that were grown from a single osteoblast cell line in a bioreactor. Tumor spheroids are nearly spherical and have radial symmetry, presenting a simple geometry for analysis. The tumors investigated ranged in diameter from several hundred micrometers to over 1 mm. Holographic features from the tumors were observed in reflection to depths of 500-600 microm with a total tissue path length of approximately 14 mean free paths. The volumetric data from the tumor spheroids reveal heterogeneous structure, presumably caused by necrosis and microcalcifications characteristic of some human avascular tumors.

Investigation of ultrafast time gating by spatial filtering

With a spatial-filtering method of gating, we explore image formation through scattering media using first-arriving light. Gating times of a few femtoseconds and less are produced, and the resolution at these extremely short gating times is investigated.

Ultrafast, cross-correlated harmonic imaging through scattering media

A simple upconversion scheme utilizing 40-fs pulses is shown to permit high-contrast imaging of objects obscured by a highly scattering medium when no ballistic component is evident in the scattered light and imaging is performed with any portion of the scattered light pulse. We present a time-gated, inherently low-pass spatially filtered imaging method that minimizes signal-averaging requirements and greatly facilitates imaging under severe scattering (turbid) conditions.

Polarization Effects in Optical Coherence Tomography of Various Biological Tissues

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain spatially resolved ex vivo images of polarization changes in skeletal muscle, bone, skin and brain. Through coherent detection of two orthogonal polarization states of the signal formed by interference of light reflected from the biological sample and a mirror in the reference arm of a Michelson interferometer, the depth resolved change in polarization was measured. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique investigating tissue structural properties. In addition, the effects of single polarization state detection on OCT image formation is demonstrated.

Electronic channel fringe holography for depth and delay measurements

Electronic spectral holography in the form developed by Shih [Ph.D. dissertation, University Microfilms, Ann Arbor, Mich. (1995)] is adapted to various applications, including optical coherence tomography in scattering media, contouring of surfaces, and optical fiber mode examination.

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.

Noise and information in interferometric cross correlators

We consider optical interferometric cross correlators based on broadband light sources. We derive the signal-to-noise ratio from basic principles and supply experimental evidence that corroborates the theoretical analysis. Noise sources are discussed, and the signal-to-noise ratio of our experimental system is measured.

Time-gated ensemble-averaged imaging through highly scattering media

A previously described ensemble-averaged imaging method [Opt. Lett. 21, 1691 (1996)] is extended by its combination with holographically implemented time-gated imaging. This combined method is shown to extend the effectiveness of the ensemble-averaged method by permitting imaging through thicker diffusers. Experimental results are presented.

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.

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.

Coherent detection techniques in optical imaging of tissues

To form optical images from the transmitted or reflected light that is multiply scattered inside biological tissue, several detection techniques that extract the least-scattered photons or path-resolved photons have been developed. This paper reviews the coherent detection techniques. Emphasis is put on coherent detection imaging methods based on optical heterodyning, whose attractive features include quantum-noise-limited sensitivity, wide dynamic range, and excellent directionality and selectivity. Coherent detection methods have been implemented to achieve laser computed tomography and micrometre-resolution cross-sectional images in both in vivo and in vitro biological systems. Imaging works by ourselves and others are described, and an experimental study on coherent photon migration through highly scattering media is described to aid the understanding of the coherent detection method in selectively detecting the signal-carrying photons.

Ultrafast dark-field interferometric microscopic reflectometry

A new method of ultrafast dark-field correlation interferometry from reflective microscopic objects is described. A 120-fs single-shot registration is achieved with a dynamic range of >35 dB, a sensitivity of <-50 dB, and a resolution of 15 mm. To demonstrate the potential of the method, we measured the thickness of single-mode fiber cladding to be 19 microm.

Ensemble-averaged imaging through highly scattering media

A method for imaging through highly scattering media is described that consists of forming a multiplicity of holograms and performing an extensive averaging process. This process produces an estimate of the phase distribution across the exiting surface of the medium. This information is combined with the available magnitude data to form an ensemble-averaged wave front that can be backprojected to form an image of absorbers within or behind the scattering medium.

Motion-resolved imaging of moving objects embedded within scattering media by the use of time-gated speckle analysis

Spectral analysis of a time-evolving speckle pattern is used to provide motion-resolved detection of moving objects embedded within scattering media. Results show that the ability to detect small nonstationary scattering objects and to discriminate between objects moving at different rates is greatly enhanced.

Noise analysis for the holographic first-arriving-light technique

A noise analysis is performed on an electronic holography first-arriving-light system. Analytical expressions for the signal-to-noise ratio caused by the dominant noise terms are derived. The effect of various system parameters on the signal-to-noise ratio is explored; numerical and experimental examples are given.

Time-gated viewing studies on tissuelike phantoms

A time-gated technique to enhance viewing through highly scattering media such as tissue is discussed. Experiments have been performed on tissuelike plastic phantoms to determine the possibilities and limitations of the technique. The effects of the time-gate width and the localization, size, and optical properties of hidden objects have been studied. A computer model to simulate light propagation in tissue is also presented. The predictions of the model are compared with experimental results.

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.

Resolution-enhanced detection of moving objects embedded within scattering media using time-gated speckle methods

The holographic first-arriving-light method in combination with the speckle differencing method is used to provide resolution-enhanced detection of moving objects embedded in scattering media. Results show that the first-arriving-light technique provides significant resolution improvements over standard speckle differencing.

Observation of light diffraction by time-resolved femtosecond correlation interferometry

We demonstrate time development of the diffraction of light waves from objects for what is to our knowledge the f irst time by using a new femtosecond correlation interferometry. This new dynamical optics method allows for the conversion of temporal information to space information with femtosecond resolution and has the potential to produce a time-resolved femtosecond movie for the visualization of light-wave propagation in space for scientific, biological, and medical applications.

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.

Detection of moving objects embedded within scattering media by use of speckle methods

Speckle-pattern subtraction methods are used for the detection of moving objects embedded in scattering media. Results show that the ability to detect small nonstationary objects is greatly enhanced.

Optical imaging through turbid media with a degenerate four wave mixing correlation time gate

A novel method for detection of ballistic light and rejection of unwanted diffusive light to image structures inside highly scattering media is demonstrated. Degenerate four wave mixing (DFWM) of a doubled YAG laser in Rhodamine 6G is used to provide an ultrafast correlation time gate to discriminate against light that has undergone multiple scattering and therefore lost memory of the structures inside the scattering medium. We present preliminary results that determine the nature of the DFWM grating, confirm the coherence time of the laser, prove the phase-conjugate nature of the signal beam, and determine the dependence of the signal (reflectivity) on dye concentration and laser intensity. Finally, we have obtained images of a test cross-hair pattern through highly turbid suspensions of whole milk in water that are opaque to the naked eye. These imaging experiments demonstrate the utility of DFWM for imagingthrough turbid media. Based on our results, the use of DFWM as an ultrafast time gate for the detectionof ballistic light in optical mammography appears to hold great promise for improving the current state of the art.

Electronic holographic imaging through living human tissue

Electronic holography and a swept-frequency dye laser are used with the first-arriving-light method to image an absorbing object through the flesh of a human hand. Holography with living human tissue without the use of high-peak-power lasers is made possible by the high sensitivity of the CCD camera as well as its capability for making a large number of holograms in rapid succession, thus enabling the images to be combined to produce a resultant image with an improved signal-to-noise ratio.

Localization of absorbers in scattering media by use of frequency-domain measurements of time-dependent photon migration

Frequency-domain studies of time-dependent light propagation in tissuelike phantoms that contain optical heterogeneities are described. Specifically the phase shift and amplitude modulation of reemergent light were measured when illuminated by an amplitude-modulated light source. Changes in the phase angle and the extent of modulation revealed the presence of a light-absorbing object. Furthermore the magnitude and direction of these changes were sensitive to the absorber depth and the light modulation frequency in a manner that could be used to infer the location of the heterogeneity. These data suggest the feasibility of optical imaging by frequency-domain methods.

Characterization of the image resolution for the first-arriving-light method

A theoretical model is established for finding the resolution of imaging through random media with the first-arriving-light method. With this model analytical mathematical forms of the point-spread function are derived for transillumination and confocal scanning imaging modes combined with the first-arrivinglight method. Finally experiments were carried out with the holographic gating technique to demonstrate the validity of the theory. The experimental results show that the first-arriving-light method improves the image resolution by as much as a factor of 20 over the conventional transillumination mode.

Use of Fourier synthesis holography to image through inhomogeneities

A method for image formation through inhomogeneities is demonstrated. A broad spectral source is decomposed into its Fourier components, and a hologram is recorded at each wavelength through a diffusing medium. When the holograms are synthesized in a computer, a clear image can be formed of the obscured object.

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

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

Time-resolved Fourier spectrum and imaging in highly scattering media

Time and spatial-gated Fourier spectra and imaging were measured and analyzed. A picosecond Kerr-Fourier gate was used to image objects by selecting the spatial frequencies of objects illuminated by a laser pulse passing through a thick turbid medium. The earlier arriving ballistic/snake light and most of the later scattered light were spatially filtered and temporally separated to form an image. The image contrast and the signal-to-noise ratio of hidden objects in turbid media were greatly improved with the addition of Fourier spatial filtering.

Evaluation of holographic methods for imaging through biological tissue

Different holographic methods for imaging through biological tissue are evaluated and compared. The role of the source autocorrelation function is analyzed. A graphical plot for performance evaluation is introduced. Experimental results for the various methods are given, and possibilities for further development are indicated.

Nonlinear-optical field cross-correlation techniques for medical imaging with lasers

We show that certain resonant nonlinear-optical processes are capable of producing a cross-correlation function of the incident fields. This property can be used for time-gating purposes. Using a coherent anti-Stokes Raman scattering process as a time gate, we reproduce an image of an object obscured by a scattering medium.

Femtosecond transillumination optical coherence tomography

We describe a new technique, femtosecond transillumination optical coherence tomography, for time-gated imaging of objects embedded in scattering media. Time gating is performed with a fiber-optic interferometer with femtosecond pulses and coherent heterodyne detection to achieve a 130-dB dynamic range. A confocal imaging arrangement provides additional spatial discrimination against multiply scattered light. By time gating ballistic photons, we achieve 125-microm-resolution images of absorbing objects in media 27 scattering mean free paths thick. We derive a fundamental limit on ballistic imaging thickness based on quantum noise considerations.

Highly sensitive object location in tissue models with linear in-phase and anti-phase multi-element optical arrays in one and two dimensions

Based upon previous observations of low-frequency photon diffusion waves within highly scattering tissue, this paper explores the "near-field" phenomena of such waves of approximately 10-cm wavelength with 200-MHz phase modulation equipment. Multiple-element source arrays consist of laser diode sources modulated at 180 degrees out of phase with respect to the other sources. The diffusing waves originating from the out-of-phase sources give, in the midplane, an amplitude null and a sharp phase transition. These may be observed in a highly scattering intralipid medium simulating the breast or brain (0.5% intralipid), 3-5 cm from the transmitting laser diodes. In the plane containing the array, there is a high sensitivity for a small volume of a hidden absorber (indocyanine green) deep within a highly scattering medium; 20 pmol in a volume of 70 microliters can be detected. Two-dimensional arrays consisting of four or more elements in two orthogonal planes give sensitivity on both axes similar to the one-dimensional array. Measurements show that in the presence of a light-absorbing object, the amplitude null and the interference plane becomes a curved surface which is deflected toward the heterogeneity. The degree of deflection is related to the volume and the absorption characteristics of the heterogeneity and provides detection of the heterogeneity, and thereby may provide localization information for the detection of small tumors within the human breast, or stroke volumes, aneurysms, and tumors in the human brain.

Imaging through random scattering media by using cw broadband interferometry

Imaging of an object hidden in random scattering media is achieved with cw broadband (white-light) interferometry. The light source is an inexpensive, easy-to-operate, superluminescent diode laser. An efficient image-enhancing algorithm is developed to eliminate the effect of phase noise in the interferometer and enhance the recovered image. Submillimeter spatial resolution is achieved.

Numerical method for studying the detectability of inclusions hidden in optically turbid tissue

We introduce an efficient numerical method for studying the detectability of absorptive inclusions in a multiple-scattering optical medium. Use of the method is demonstrated by the forward calculation of integrated and time-gated photon intensities. Schemes for positioning the light source above an inclusion and otherwise determining the location of a hidden object, involving either reflected or transmitted reemissions, are discussed as examples. They are investigated for several illustrative models, and images are calculated as a function of the size and shape of the inclusion.

Double-stage picosecond Kerr gate for ballistic time-gated optical imaging in turbid media

We demonstrate that use of a picosecond double-stage Kerr gate system results in a 3-orders-of magnitude improvement in signal-to-noise ratio and a threefold improvement in shutter speed comparedwith those of a single-stage Kerr gate.

Medical transillumination imaging using short-pulse diode lasers

The recently introduced time-resolved technique for enhanced medical transillumination imaging has been demonstrated for the important case of a diode laser transmitter. This type of gated-viewing technique utilizes early received light only to reject multiply scattered, delayed light, normally blurring the image. Human breast-cancer detection is demonstrated in vitro, and the observations are explained by using theoretical modeling and tissue phantom experiments.

Effects of optical constants on time-gated transillumination of tissue and tissue-like media

Light transillumination was used to study structures inside turbid media. Time-gated viewing was performed to suppress multiply-scattered light and thus improve spatial resolution. We demonstrate that, for the case of scattering-dominated attenuation (scattering coefficient much greater than the absorption coefficient), the detection of early transmitted light will be practically insensitive to variations in the absorption coefficient. This is an important observation for the development of time-gated optical mammography, since optical mammography using continuous-wave light is based on increased light absorption in the tumour region caused by the neovascularization surrounding a tumour. In order to detect tumours in time-gated viewing it is the scattering coefficient of the tumour that must be characteristic. The scattering coefficient is measured to be lower in the tumour region than in the surrounding breast tissue for one resected breast specimen.