Spectral shaping to improve the point spread function in optical coherence tomography

We demonstrate inhibition of the sidelobes of the axial point spread function in optical coherence tomography by shaping the power spectrum of the light source with a remaining power of 4.54 mW. A broadband amplified spontaneous emission source radiating at 1565 +/- 40 nm is employed in a free-space optical coherence tomography system. The axial point spread functions before and after optical spectral shaping are presented. Results show that spectral shaping of the source can inhibit sidelobes of the point spread function up to 12.9 dB, with an associated small increase of 2.2 dB in noise floor in the far field. The effect of spectral shaping on axial resolution is demonstrated according to three metrics. Image quality improvement is also illustrated with optical coherence tomography images of an onion before and after spectral shaping.     

Spectral heterodyne tomography

A novel method of optical coherent tomography—spectral heterodyne tomography—is proposed. Spectral heterodyne tomography is based on parallel heterodyne detection of a multitude of frequencies of the light backscattered by an object under study. The result of this detection is a spectral distribution of the amplitude and phase of the scattered radiation. Subsequent numerical processing allows one to find the distribution of scattering centers over the depth corresponding to the point of entrance of the incident light. The proposed method is potentially characterized by a higher efficiency as compared with the most successful approach to optical coherence tomography, based on heterodyne scanning interferometry.     

Acousto-optic-assisted diffuse optical tomography

We introduce and experimentally demonstrate acousto-optic-assisted diffuse optical tomography (DOT) using a holography-based acousto-optic setup. The method is based on probing a scattering medium with a localized acoustical modulation of the phase of the scattered light. The optical properties of the scattering medium are recovered with ultrasound-limited resolution by applying DOT reconstruction methods on a set of the measured intensities of light, modulated at different locations throughout the medium.     

Multiple-gate time domain diffuse fluorescence tomography allows more sparse tissue sampling without compromising image quality

Diffuse fluorescence tomography systems that employ highly sensitive photo-multiplier tubes for single-photon detection are pushing the sensitivity limits of the field. However, each of these detectors only offers a single data projection to be collected, implying these imaging systems either require many detectors or long scan times to collect full data sets for image reconstruction. This study presents a method of utilizing the time-resolved collection capabilities of time-correlated single-photon counting techniques to increase spatial resolution and to reduce the number of data projections to produce reliable fluorescence reconstructions. Experimental tissue phantom results demonstrate that using data at 10 time gates in the fluorescence reconstructions for only 40 data projections provided superior image accuracy when compared to reconstructions on 320 continuous-wave data projections.     

Endoscopic, rapid near-infrared optical tomography

This is believed to be the first demonstration of near-infrared (NIR) optical tomography employed at the endoscope scale and at a rapid sampling speed that allows translation to in vivo use. A spread-spectral-encoding technique based on a broadband light source and linear-to-circular fiber bundling was used to provide endoscopic probing of many source-detector fibers for tomography as well as parallel sampling of all source-detector pairs for rapid imaging. Endoscopic NIR tomography at an 8 Hz frame rate was achieved in phantoms and tissue specimens with a 12 mm probe housing eight sources and eight detectors. This novel approach provides the key feasibility studies to allow this blood-based contrast imaging technology to be attempted in detection of cancer in internal organs via endoscopic interrogation.     

Superresolution in diffuse optical tomography

The possibility of applying image reconstruction methods used in optics to improve the resolution of the photon average trajectories method of diffuse optical tomography is demonstrated. The limit resolution determined by Nyquist’s consequence of the Kotel’nikov-Shannon theorem is achieved.     

The Carpet-3 experiment to search for diffuse gamma rays with energies of more than 100 TeV

An experiment for measuring the flux of cosmic diffuse gamma rays with energies above 100 TeV (the Carpet-3 air shower array) is now being prepared at the Baksan Neutrino Observatory, Institute for Nuclear Research, Russian Academy of Sciences. The preparations entail a substantial increase of the areas of both the muon detector and the surface air shower array. The experiment’s sensitivity to showers generated by primary gamma rays is estimated for different configurations of the array. In addition, preliminary estimates of the upper limit on the flux of diffuse gamma rays with energies higher than 1.3 PeV, derived using experimental data from the old Carpet-2 array, are presented for a net exposure time of 9.2 years.     

Spectral domain second-harmonic optical coherence tomography

Optical coherence tomography (OCT) provides micrometer-scale structural imaging by coherent detection of backscattered light. Molecular contrast in OCT has been demonstrated using transient absorption, coherent anti-Stokes Raman scattering, and second-harmonic (SH) generation. The sensitivity of molecular contrast signals can be enhanced by use of Fourier domain techniques. We have constructed a spectrometer-based Fourier domain SH-OCT system for simultaneous acquisition of the fundamental and SH signals. We report a >30 dB increase in SH sensitivity over a similar time domain SH-OCT system and demonstrate contrast between cartilage and bone using collagen as the contrast agent.     

Polarization-sensitive optoacoustic tomography of optically diffuse tissues

Polarization is indicative of material anisotropy, a property that reveals structural orientation information of molecules inside the material. Herein we investigate whether polarization can be detected optoacoustically in scattering and absorbing tissues. Using a laboratory prototype of polarization-sensitive optoacoustic tomography, we demonstrate high-resolution reconstructions of dichroism contrast deep in optically diffusive tissue-mimicking phantoms. The technique is expected to enable highly accurate imaging of polarization contrast in tissues, far beyond the current capabilities of pure optical polarization-imaging approaches.     

Volumetric diffuse optical tomography of brain activity

We present three-dimensional diffuse optical tomography of the hemodynamic response to somatosensory stimulation in a rat. These images show the feasibility of volumetrically imaging the functional response to brain activity with diffuse light. A combination of positional optode calibration and contrast-to-noise ratio weighting was found to improve imaging performance.     

Depth-adaptive regularized reconstruction for reflection diffuse optical tomography

We study reflection diffuse optical tomography using two-dimensional (2D) continuous-wave source-detector arrays on the surface of semi-infinite medium, aiming at imaging the perfusion and the hemoglobin oxygen saturation variation of human cerebral cortex with brain activation. We had previously formulated the inverse problem with Moore-Penrose inversion. When we use simple regularization in this inverse problem, the reconstruction sensitivity decreases markedly with the depth so that the signal in the deep range may be masked by an unwanted signal in the shallow range. In this paper, we propose a depth-adaptive regularized reconstruction, in which we assign a smaller regularization parameter with the depth. We demonstrate improvement of the three-dimensional (3D) reconstruction uniformity using the proposed scheme.     

Krichever-Novikov algebras for more than two points

Krichever-Novikov algebras of meromorphic vector fields with more than two poles on higher genus Riemann surfaces are introduced. The structure of these algebras and their induced modules of forms of weight is studied.     

Massless spinors in more than four dimensions

We consider fermions in theories of higher dimensional gravity where the four-dimensional gauge group is embedded in the invariance group of d dimensional (d>4) Lorentz and general co-ordinate transformations. It is a necessary condition for obtaining massless chiral fermions from dimensional reduction that the d dimensional spinor does not admit a mass term consistent with Lorentz and general co-ordinate transformations. This is the case for a Weyl spinor for d = 6 8 mod 8, a Majorana spinor for d = 9 mod 8 or a Majorana-Weyl spinor for d = 2 mod 8.     

Video-rate near-infrared optical tomography using spectrally encoded parallel light delivery

A novel parallel source implementation approach to near-infrared tomography is demonstrated through spectral encoding of the light delivery. This new technique allows many sources to be input into the tissue at the same time, and a high-resolution spectrometer is used to spatially spread out the signals from each spectrally encoded source. The parallel sampling of all sources at all detection locations renders rapid imaging. Acquisition of complete tomographic data sets at a video rate of 35 frames/s is achieved for imaging of a 6.35 mm diameter inclusion with an absorption coefficient of 0.01 mm(-1) and a reduced scattering coefficient of 1.5 mm(-1) that is moving along a circular path inside a 1% Intralipid solution.     

Spectral calibration in spectral domain optical coherence tomography

A novel spectral calibration method is developed for spectral domain optical coherence tomography system. The method is based on two measurements of interference spectra from two reference mirror positions. It removes the influence of dispersion mismatch, and hence accurately determines the spectral distribution on the line-scan charge-coupled device (CCD) for sequent precise interpolation. High quality imaging can be realized with this method. Elimination of the degradation effect caused by dispersion mismatch is verified experimentally, and improved two-dimensional (2D) imaging of fresh orange pulp based on the proposed spectral calibration method is demonstrated.     

Method to improve spatial uniformity with lightpipes

A heuristic method to obtain lightpipes that provide both collimation and good spatial uniformity is proposed. The change in shape that is likely to improve spatial uniformity, with minimal efficiency loss, can be predicted. Several case studies where this technique has been used are presented.     

An experimental investigation on two-dimensional shape-based diffuse optical tomography

A two-dimensional （2D） shape-based approach of image reconstruction using a boundary element method is developed for diffuse optical tomography （DOT）. The experimental validation uses a four-channel time- correlated single photon counting （TCSPC） system for detection and an intensity data-type for image reconstruction. The optical and geometric parameters are simultaneously recovered using a difference imaging scheme. Results demonstrate that the proposed DOT modality is a promising methodology of in vivo reconstruction of the optical structures of tissues.     

快速多极边界元法用于扩散光学断层成像研究

在求解扩散光学断层成像中的正向问题时, 目前普遍采用有限元法, 但是随着实际模型规模的增大, 有限元法的计算量问题日益显著, 而边界元法则由于可以降低计算维度使计算量减少而备受关注. 本文以均匀的高散射介质为模型, 研究了将快速多极边界元法用于扩散光学断层成像的正向问题. 快速多极边界元法利用核函数的多极展开, 将常规边界元法中系数矩阵和迭代矢量的乘积项等价为相应四叉树结构的一次递归, 再结合广义最小残量法进行迭代求解. 将计算结果和蒙特卡罗法的模拟结果进行了比较, 表明利用快速多极边界元法的模拟结果和蒙特卡罗法的结果有很好的一致性. 研究结果验证了快速多极边界元法可以用于扩散光学断层成像, 为其大规模和实时成像带来可观的前景. 关键词： 扩散光学断层成像 边界元法 快速多极边界元法