Computational high-resolution optical imaging of the living human retina

High-resolution in vivo imaging is of great importance for the fields of biology and medicine. The introduction of hardware-based adaptive optics (HAO) has pushed the limits of optical imaging, enabling high-resolution near diffraction-limited imaging of previously unresolvable structures^1,2. In ophthalmology, when combined with optical coherence tomography, HAO has enabled a detailed three-dimensional visualization of photoreceptor distributions^3,4 and individual nerve fibre bundles⁵ in the living human retina. However, the introduction of HAO hardware and supporting software adds considerable complexity and cost to an imaging system, limiting the number of researchers and medical professionals who could benefit from the technology. Here we demonstrate a fully automated computational approach that enables high-resolution in vivo ophthalmic imaging without the need for HAO. The results demonstrate that computational methods in coherent microscopy are applicable in highly dynamic living systems.

Three-dimensional motion correction using speckle and phase for in vivo computed optical interferometric tomography

Over the years, many computed optical interferometric techniques have been developed to perform high-resolution volumetric tomography. By utilizing the phase and amplitude information provided with interferometric detection, post-acquisition corrections for defocus and optical aberrations can be performed. The introduction of the phase, though, can dramatically increase the sensitivity to motion (most prominently along the optical axis). In this paper, we present two algorithms which, together, can correct for motion in all three dimensions with enough accuracy for defocus and aberration correction in computed optical interferometric tomography. The first algorithm utilizes phase differences within the acquired data to correct for motion along the optical axis. The second algorithm utilizes the addition of a speckle tracking system using temporally- and spatially-coherent illumination to measure motion orthogonal to the optical axis. The use of coherent illumination allows for high-contrast speckle patterns even when imaging apparently uniform samples or when highly aberrated beams cannot be avoided.

Synthetic optical holography with nonlinear-phase reference

Synthetic optical holography (SOH) provides efficient encoding of the complex optical signal, both amplitude and phase, for scanning imaging methods. Prior demonstrations have synthesized reference fields with a plane-wave-like linear variation of the phase with position. To record large images without probe-mirror synchronization, a long-travel, closed-loop reference mirror stage has been required. Here we present SOH with a synthetic reference wave with sinusoidal spatial variation of the phase. This allows the use of open loop, limited mirror travel range in SOH, and leads to a novel holographic inversion algorithm. We validate the theory with scans of graphene grain boundaries from a scanning near-field optical microscope, for which SOH has been shown to drastically increase scan speeds [Nat. Commun. 5, 3499 (2014)].

Computed optical interferometric tomography for high-speed volumetric cellular imaging

Three-dimensional high-resolution imaging methods are important for cellular-level research. Optical coherence microscopy (OCM) is a low-coherence-based interferometry technology for cellular imaging with both high axial and lateral resolution. Using a high-numerical-aperture objective, OCM normally has a shallow depth of field and requires scanning the focus through the entire region of interest to perform volumetric imaging. With a higher-numerical-aperture objective, the image quality of OCM is affected by and more sensitive to aberrations. Interferometric synthetic aperture microscopy (ISAM) and computational adaptive optics (CAO) are computed imaging techniques that overcome the depth-of-field limitation and the effect of optical aberrations in optical coherence tomography (OCT), respectively. In this work we combine OCM with ISAM and CAO to achieve high-speed volumetric cellular imaging. Experimental imaging results of ex vivo human breast tissue, ex vivo mouse brain tissue, in vitro fibroblast cells in 3D scaffolds, and in vivo human skin demonstrate the significant potential of this technique for high-speed volumetric cellular imaging.

Stability in computed optical interferometric tomography (part I): stability requirements

As imaging systems become more advanced and acquire data at faster rates, increasingly dynamic samples can be imaged without concern of motion artifacts. For optical interferometric techniques such as optical coherence tomography, it often follows that initially, only amplitude-based data are utilized due to unstable or unreliable phase measurements. As systems progress, stable phase maps can also be acquired, enabling more advanced, phase-dependent post-processing techniques. Here we report an investigation of the stability requirements for a class of phase-dependent post-processing techniques - numerical defocus and aberration correction with further extensions to techniques such as Doppler, phase-variance, and optical coherence elastography. Mathematical analyses and numerical simulations over a variety of instabilities are supported by experimental investigations.

Stability in computed optical interferometric tomography (Part II): in vivo stability assessment

Stability is of utmost importance to a wide range of phase-sensitive processing techniques. In Doppler optical coherence tomography and optical coherence elastography, in addition to defocus and aberration correction techniques such as interferometric synthetic aperture microscopy and computational/digital adaptive optics, a precise understanding of the system and sample stability helps to guide the system design and choice of imaging parameters. This article focuses on methods to accurately and quantitatively measure the stability of an imaging configuration in vivo. These methods are capable of partially decoupling axial from transverse motion and are compared against the stability requirements for computed optical interferometric tomography laid out in the first part of this article.

Recovery of permittivity and depth from near-field data as a step toward infrared nanotomography

The increasing complexity of composite materials structured on the nanometer scale requires highly sensitive analytical tools for nanoscale chemical identification, ideally in three dimensions. While infrared near-field microscopy provides high chemical sensitivity and nanoscopic spatial resolution in two dimensions, the quantitative extraction of material properties of three-dimensionally structured samples has not been achieved yet. Here we introduce a method to perform rapid recovery of the thickness and permittivity of simple 3D structures (such as thin films and nanostructures) from near-field measurements, and provide its first experimental demonstration. This is accomplished via a novel nonlinear invertible model of the imaging process, taking advantage of the near-field data recorded at multiple harmonics of the oscillation frequency of the near-field probe. Our work enables quantitative nanoscale-resolved optical studies of thin films, coatings, and functionalization layers, as well as the structural analysis of multiphase materials, among others. It represents a major step toward the further goal of near-field nanotomography.

Multifocal interferometric synthetic aperture microscopy

There is an inherent trade-off between transverse resolution and depth of field (DOF) in optical coherence tomography (OCT) which becomes a limiting factor for certain applications. Multifocal OCT and interferometric synthetic aperture microscopy (ISAM) each provide a distinct solution to the trade-off through modification to the experiment or via post-processing, respectively. In this paper, we have solved the inverse problem of multifocal OCT and present a general algorithm for combining multiple ISAM datasets. Multifocal ISAM (MISAM) uses a regularized combination of the resampled datasets to bring advantages of both multifocal OCT and ISAM to achieve optimal transverse resolution, extended effective DOF and improved signal-to-noise ratio. We present theory, simulation and experimental results.

Quantitative confocal phase imaging by synthetic optical holography

We demonstrate quantitative phase mapping in confocal optical microscopy by applying synthetic optical holography (SOH), a recently introduced method for technically simple and fast phase imaging in scanning optical microscopy. SOH is implemented in a confocal microscope by simply adding a linearly moving reference mirror to the microscope setup, which generates a synthetic reference wave analogous to the plane reference wave of wide-field off-axis holography. We demonstrate that SOH confocal microscopy allows for non-contact surface profiling with sub-nanometer depth resolution. As an application for biological imaging, we apply SOH confocal microscopy to map the surface profile of an onion cell, revealing nanoscale-height features on the cell surface.

Abstract P2-03-11: In situ imaging of the tumor cavity during breast lumpectomy using optical coherence tomography

Re-operation rates for breast lumpectomy procedures are exceedingly high, often over 30%, depending on the institution and surgical technique. Because current standard-of-care relies on post-operative histopathology to provide a microscopic view and assessment of surgical margins, there has been great interest in developing new imaging solutions to visualize tissues intraoperatively with high-resolution, and provide real-time feedback on the margin status. While it is possible to use a variety of microscopic imaging methods in the operating suite, including frozen-section histology, touch-prep cytology, confocal or scattering-based microscopy, all these techniques are limited to visualizing margins on ex vivo resected specimens, and do not provide a means for visualizing the in situ tumor cavity for evidence of positive margins or residual disease.

Optical coherence tomography (OCT) is a high-resolution, real-time, optical biomedical imaging technology that is the optical analogue to ultrasound imaging, except images are based on backscattered near-infrared light. OCT is capable of performing optical biopsies of in situ tissue at resolutions that approach those in histopathology. With the use of an advanced computed imaging technique called ISAM (Interferometric Synthetic Aperture Microscopy), even higher imaging resolution over larger depths is possible, commensurate with the depths (1-2 mm) visualized by pathologists to determine negative, close, or positive margins. Past studies by our group and others have demonstrated the feasibility of intraoperative OCT for assessing tumor margin and lymph node status during breast cancer surgery, but to date, all studies have been performed on resected lumpectomy tissue.

In this study, we report the development of a novel handheld surgical imaging probe that enables 2-D and 3-D OCT/ISAM imaging of the in situ tumor cavity, in addition to the margins of excised specimens. To date, this handheld OCT/ISAM probe has been used in 10 breast cancer surgeries where both in situ and ex vivo imaging was performed. Four of these cases involved in situ imaging of the cavity margin after a suspicious area was visually and tactically identified, and was subsequently resected, followed by ex vivo imaging and validating post-operative histopathology. Representative cases included fibroadipose tissue, fibroadenomas, and high-grade ductal carcinoma in situ.

Distinct microstructural features identified on OCT/ISAM and confirmed with histopathology demonstrate that this technique can visualize the in situ tumor cavity, as well as the surgical margins on resected specimens, with micron-scale resolution. OCT/ISAM has the potential to determine margin status in real-time during the surgical procedure, when further surgical resection to establish clear margins and reduce re-operation rates is possible.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-03-11.

Real-time in vivo computed optical interferometric tomography

High-resolution real-time tomography of scattering tissues is important for many areas of medicine and biology^1-6. However, the compromise between transverse resolution and depth-of-field in addition to low sensitivity deep in tissue continue to impede progress towards cellular-level volumetric tomography. Computed imaging has the potential to solve these long-standing limitations. Interferometric synthetic aperture microscopy (ISAM)^7-9 is a computed imaging technique enabling high-resolution volumetric tomography with spatially invariant resolution. However, its potential for clinical diagnostics remains largely untapped since full volume reconstructions required lengthy postprocessing, and the phase-stability requirements have been difficult to satisfy in vivo. Here we demonstrate how 3-D Fourier-domain resampling, in combination with high-speed optical coherence tomography (OCT), can achieve high-resolution in vivo tomography. Enhanced depth sensitivity was achieved over a depth-of-field extended in real time by more than an order of magnitude. This work lays the foundation for high-speed volumetric cellular-level tomography.

Rapid spectral-domain localization

We present a method to dynamically image structures at nanometer spatial resolution with far-field instruments. We propose the use of engineered nanoprobes with distinguishable spectral responses and the measurement of coherent scattering, rather than fluorescence. Approaches such as PALM/STORM have relied on the rarity of emission events in time to distinguish signals from distinct probes. By distinguishing signals in the spectral domain, we enable the acquisition of data in a multiplex fashion and thus circumvent the fundamental problem of slow data acquisition of current techniques. The described method has the potential to image dynamic systems with a spatial resolution only limited to the size of the scattering probes.

Quantitative Measurement of Local Infrared Absorption and Dielectric Function with Tip-Enhanced Near-Field Microscopy

Scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy (nano-FTIR) are emerging tools for nanoscale chemical material identification. Here, we push s-SNOM and nano-FTIR one important step further by enabling them to quantitatively measure local dielectric constants and infrared absorption. Our technique is based on an analytical model, which allows for a simple inversion of the near-field scattering problem. It yields the dielectric permittivity and absorption of samples with 2 orders of magnitude improved spatial resolution compared to far-field measurements and is applicable to a large class of samples including polymers and biological matter. We verify the capabilities by determining the local dielectric permittivity of a PMMA film from nano-FTIR measurements, which is in excellent agreement with far-field ellipsometric data. We further obtain local infrared absorption spectra with unprecedented accuracy in peak position and shape, which is the key to quantitative chemometrics on the nanometer scale.

Recovery of absorption spectra from Fourier transform infrared (FT-IR) microspectroscopic measurements of intact spheres

An infrared spectrum recorded from a microscopic sample depends on spectral properties of the constituent material as well as on morphology. Many samples or domains within heterogeneous materials can be idealized as spheres, in which both scattering and absorption from the three-dimensional shape affect the recorded spectrum. Spectra recorded from such objects may be altered to such an extent that they bear little resemblance to spectra recorded from the bulk material; there are no methods, however, to reconcile the two from first principles. Here we provide the mathematical description of the optical physics underlying light-spherical sample interaction within an instrument. We use the developed analytical expressions to predict recorded data from spheres using Fourier transform infrared (FT-IR) spectroscopic imaging. Recorded spectra are shown to depend strongly on the size of the sphere as well as the optical arrangement of the instrument. Next, we present theory and experiments demonstrating the recovery of the complex refractive index of the material using data recorded from a sphere. The effects of the sample morphology on the measured spectra can be removed, and using the imaginary part of the index, the shape-independent IR absorption spectrum of the material is recovered.

Competition Between Extinction and Enhancement in Surface Enhanced Raman Spectroscopy

Conjugated metallic nanoparticles are a promising means to achieve ultrasensitive and multiplexed sensing in intact three-dimensional samples, especially for biological applications, via surface enhanced Raman scattering (SERS). We show that enhancement and extinction are linked and compete in a collection of metallic nanoparticles. Counterintuitively, the Raman signal vanishes when nanoparticles are excited at their plasmon resonance, while increasing nanoparticle concentrations at off-resonance excitation sometimes leads to decreased signal. We develop an effective medium theory that explains both phenomena. Optimal choices of excitation wavelength, individual particle enhancement factor and concentrations are indicated. The same processes which give rise to enhancement also lead to increased extinction of both the illumination and the Raman scattered light. Nanoparticles attenuate the incident field (blue) and at the same time provide local enhancement for SERS. Likewise the radiation of the Raman-scattered field (green) is enhanced by the near-by sphere but extinguished by the rest of the spheres in the suspension on propagation.

Off-resonance surface-enhanced Raman spectroscopy from gold nanorod suspensions as a function of aspect ratio: not what we thought

Design of nanoparticles for surface-enhanced Raman scattering (SERS) within suspensions is more involved than simply maximizing the local field enhancement. The enhancement at the nanoparticle surface and the extinction of both the incident and scattered light during propagation act in concert to determine the observed signal intensity. Here we explore these critical aspects of signal generation and propagation through experiment and theory. We synthesized gold nanorods of six different aspect ratios in order to obtain longitudinal surface plasmon resonances that incrementally spanned 600-800 nm. The Raman reporter molecule methylene blue was trap-coated near the surface of each nanorod sample, generating SERS spectra, which were used to compare Raman signals. The average number of reporter molecules per nanorod was quantified against known standards using electrospray ionization liquid chromatography mass spectrometry. The magnitude of the observed Raman signal is reported for each aspect ratio along with the attenuation due to extinction in suspension. The highest Raman signal was obtained from the nanorod suspension with a plasmon resonance blue-shifted from the laser excitation wavelength. This finding is in contrast to SERS measurements obtained from molecules dried onto the surface of roughened or patterned metal substrates where the maximum observed signal is near or red-shifted from the laser excitation wavelength. We explain these results as a competition between SERS enhancement and extinction, at the excitation and scattered wavelengths, on propagation through the sample.

High-definition infrared spectroscopic imaging

The quality of images from an infrared (IR) microscope has traditionally been limited by considerations of throughput and signal-to-noise ratio (SNR). An understanding of the achievable quality as a function of instrument parameters, from first principals is needed for improved instrument design. Here, we first present a model for light propagation through an IR spectroscopic imaging system based on scalar wave theory. The model analytically describes the propagation of light along the entire beam path from the source to the detector. The effect of various optical elements and the sample in the microscope is understood in terms of the accessible spatial frequencies by using a Fourier optics approach and simulations are conducted to gain insights into spectroscopic image formation. The optimal pixel size at the sample plane is calculated and shown much smaller than that in current mid-IR microscopy systems. A commercial imaging system is modified, and experimental data are presented to demonstrate the validity of the developed model. Building on this validated theoretical foundation, an optimal sampling configuration is set up. Acquired data were of high spatial quality but, as expected, of poorer SNR. Signal processing approaches were implemented to improve the spectral SNR. The resulting data demonstrated the ability to perform high-definition IR imaging in the laboratory by using minimally-modified commercial instruments.

Abstract OT2-1-04: Intraoperative assessment of tumor margins with a new optical imaging technology: A multi-center, randomized, blinded clinical trial

Background: Partial mastectomy is the most commonly performed procedure for invasive breast cancer and is associated with a reexcision rate commonly ranging from 20% to 40% in the literature. This high rate of reexcision is associated with significant additional cost (estimated over $4,000 per reexcision) and lower quality outcomes.

Optical coherence tomography (OCT) is a high-resolution imaging technology that images tissue structure with micron-scale resolution – on the same scale as histopathology. It is similar to ultrasound except it uses near infra-red light waves instead of sound waves to create detailed images several millimeters deep into tissue. Although widely used in ophthalmology with growing use in cardiovascular imaging, high-resolution OCT imaging has a narrow depth of focus and requires instrumentation that is not well suited for intraoperative use. Drawing from OCT technology, interferometric synthetic aperture microscopy (ISAM) is a computational imaging technique that creates high-resolution, always in-focus images in software with basic optical instrumentation. A high-resolution ISAM probe and imaging system has been developed for intraoperative imaging of tissue structure and has the potential to broadly impact intraoperative assessment of tumor margins. Intraoperative ISAM imaging of the excised breast cancer specimen margins and in vivo imaging within the surgical cavity may reduce the high rate of reexcision associated with partial mastectomy.

Trial Design: The trial design is a prospective, multi-center, randomized, double arm study comparing the reexcision rate of standard of care partial mastectomy versus the reexcision rate of standard of care partial mastectomy plus intraoperative ISAM imaging.

Inclusion Criteria: Women histologically diagnosed with invasive carcinoma of the breast (invasive ductal or lobular)Undergoing partial mastectomy (lumpectomy) procedureAge 18 years or more

Exclusion Criteria Multicentric diseaseBilateral diseaseNeoadjuvant systemic therapyAll T4 tumorsPrevious radiation in the operated breastPrior surgical procedure in the same quadrantImplants in the operated breastPregnancyLactationParticipating in any other investigational study which can influence collection of valid data

Primary Endpoints Measure of surgical reexcision rateRate of tumor at final surgical marginsSecondary EndpointsVolume of tissue excisedClinical and economic measures of addressing asymmetry

Statistical Methods: The trial is designed to show superiority of the ISAM imaging arm to the standard of care. Statistical design is two group, continuity corrected chi-squared test of equal proportions with 90% power and alpha=0.05. The trial design assumes a baseline reoperation rate in the standard of care arm of 24% with at least a 50% reduction in the ISAM imaging arm.

Present Accrual and Target Accrual

Not yet recruiting. Target accrual is 230 patients in the partial mastectomy + imaging arm and 230 patients in the standard of care partial mastectomy arm.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr OT2-1-04.

Guide-star-based computational adaptive optics for broadband interferometric tomography

We present a method for the numerical correction of optical aberrations based on indirect sensing of the scattered wavefront from point-like scatterers ("guide stars") within a three-dimensional broadband interferometric tomogram. This method enables the correction of high-order monochromatic and chromatic aberrations utilizing guide stars that are revealed after numerical compensation of defocus and low-order aberrations of the optical system. Guide-star-based aberration correction in a silicone phantom with sparse sub-resolution-sized scatterers demonstrates improvement of resolution and signal-to-noise ratio over a large isotome. Results in highly scattering muscle tissue showed improved resolution of fine structure over an extended volume. Guide-star-based computational adaptive optics expands upon the use of image metrics for numerically optimizing the aberration correction in broadband interferometric tomography, and is analogous to phase-conjugation and time-reversal methods for focusing in turbid media.

Phase in nanooptics

Quantitative phase measurements in imaging, microscopy, and nanooptics provide information not carried in amplitude measurements alone. In this issue of ACS Nano, Honigstein et al. present a new method in phase measurement. In this Perspective, we comment on this work and more broadly on the emerging role of phase and phase measurements in nanooptics.

Enhanced resolution in subsurface near-field optical microscopy

We report an experimental analysis of the capabilities of scattering-type scanning near-field optical microscopy for mapping sub-surface features at varying depths. For the first time, we demonstrate experimentally that both the spatial resolution and depth contrast can be improved in subsurface microscopy by demodulating the measured near-field signal at higher harmonics of the probe's tapping frequency and by operating at smaller tapping amplitudes. Our findings are qualitatively supported by a simple dipole model.

Theory of infrared microspectroscopy for intact fibers

Infrared microspectroscopy is widely used for the chemical analysis of small samples. In particular, spectral properties of small cylindrical samples are important in forensic analysis, understanding relationships between microstructure and mechanical properties in fibers or fiber composites, and development of cosmetics and drugs for hair. The diameters of the constituent cylinders are typically of the order of the central wavelength of light used to probe the sample. Hence, structure and material spectral response are coupled and recorded spectra are usually distorted to the extent of becoming useless for molecular identification. In this paper, we apply rigorous optical theory to predict the spectral distortions observed in IR microspectroscopic data of fibers. The theory is used, first, to compute the changes that are observed for cylinders of various dimensions under different instrument configurations when compared to the bulk spectrum from the same material. We provide a method to recover intrinsic material spectral response from fibers by correcting for distortion introduced by the cylindrical structure. The theory reported here should enable the routine use of IR microspectroscopy and imaging for the molecular analysis of cylindrical domains in complex materials.

Information content of the near field: three-dimensional samples

We present an analysis of the accuracy and information content of three-dimensional reconstructions of the dielectric susceptibility of a sample from noisy, near-field holographic measurements, such as those made in scanning probe microscopy. Holographic measurements are related to the dielectric susceptibility via a linear operator within the accuracy of the first Born approximation. The maximum-likelihood reconstruction of the dielectric susceptibility is expressed as a linear combination of basis functions determined by singular value decomposition of the weighted measurement operator. Maximum a posteriori estimates based on prior information are also discussed. Semianalytical expressions are given for the likely error due to measurement noise in the basis function coefficients, resulting in effective resolution limits in all three dimensions. These results are illustrated by numerical examples.

Theory of midinfrared absorption microspectroscopy: I. Homogeneous samples

Midinfrared (IR) microspectroscopy is widely employed for spatially localized spectral analyses. A comprehensive theoretical model for the technique, however, has not been previously proposed. In this paper, rigorous theory is presented for IR absorption microspectroscopy by using Maxwell's equations to model beam propagation. Focusing effects, material dispersion, and the geometry of the sample are accounted to predict spectral response for homogeneous samples. Predictions are validated experimentally using Fourier transform IR (FT-IR) microspectroscopic examination of a photoresist. The results emphasize that meaningful interpretation of IR microspectroscopic data must involve an understanding of the coupled optical effects associated with the sample, substrate properties, and microscopy configuration. Simulations provide guidance for developing experimental methods and future instrument design by quantifying distortions in the recorded data. Distortions are especially severe for transflection mode and for samples mounted on certain substrates. Last, the model generalizes to rigorously consider the effects of focusing. While spectral analyses range from examining gross spectral features to assessing subtle features using advanced chemometrics, the limitations imposed by these effects in the data acquisition on the information available are less clear. The distorting effects are shown to be larger than noise levels seen in modern spectrometers. Hence, the model provides a framework to quantify spectral distortions that may limit the accuracy of information or present confounding effects in microspectroscopy.

Transport of intensity and spectrum for partially coherent fields

Transport-of-intensity and transport-of-spectrum equations are derived using the coherent mode decomposition for paraxial fields having an arbitrary state of coherence. We give a simple example that demonstrates the difference between a partially coherent and a fully coherent transport of intensity or spectrum. The results presented here may be used to estimate the intensity response in a variety of phase-contrast imaging modalities and may form the basis for improved phase-retrieval techniques.