Multiple scattering low coherence interferometry

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Multiple scattering low coherence interferometry (ms/LCI) is an imaging technique that relies on analyzing multiply scattered light in order to capture depth-resolved images from optical scattering media. With current applications primarily in medical imaging, has the advantage of a higher range since forward scattered light attenuates less with depth when compared to the specularly reflected light that is assessed in more conventional imaging methods such as optical coherence tomography. [1] This allows ms/LCI to image through up to 90 mean free scattering paths, compared to roughly 27 scattering MFPs in OCT and 1–2 scattering MFPs in confocal microscopy. [2]

Contents

Design

Time-domain implementation

Early implementations of ms/LCI were in the time domain using lock-in detection in order to take advantage of long scanning depths as well narrow detection bandwidths. [2] As in traditional OCT, the beam interference coherence gates the light in order to filter out photons that have not traveled a sufficient path length. The use of distinct, non-intersecting illumination and collection beams allows for time insensitive triangulation of the light that also considers physical depth penetration into the media in order to reject diffuse backscattering light such that only forward scattered photons are analyzed. Unlike traditional OCT, the measured forward scattered light is diverged by a lens in order to isolate the angular component to be compared with the reference arm. Due to the dominance of forward scattering photons at deeper penetration depths, this technique enjoys superior imaging depth and high detection throughput but suffers from long signal acquisition time and poor spatial resolution inherent to time-domain techniques.

Spectral-domain implementation

By adapting techniques used in spectroscopic OCT, ms/LCI can be done in the spectral domain in order to provide faster acquisition time and various multispectral capabilities, including the use of localized contrast agents. [1] As in OCT, enhanced depth imaging is used to place the zero-path delay point behind the focal volume in order to increase sensitivity. A broadband supercontinuum laser source as well as a customized spectrometer detector are implemented in order to access the spectral domain, and a short-time Fourier transform method is used to process the interferograms as in spectroscopic OCT.

Diagram of a hypothetical ms/LCI setup. Generic msLCI diagram.png
Diagram of a hypothetical ms/LCI setup.

Applications

Because of enhanced depth penetration, this technique's most promising applications lie in resolving features that exist deeper in tissue than other techniques such as OCT are capable of doing. Studies have determined ms/LCI's feasibility in a variety of clinical applications, including assessing burn injuries, as well as in vivo imaging of rat skin with superior depth results when compared to OCT. [3]

Considerations

Spatial resolution

Photons originating from the focal plane detected by ms/LCI will undergo multiple scattering events which causes extra travel time and resulting longer path lengths than the signal would otherwise indicate. This axial shift will make photons appear like they appear deeper from the sample than they actually are.

The presence of multiple scattering events causes a distribution of path lengths that intrinsically blurs the image, resulting in a maximum millimeter-scale resolution which is substantially poorer than OCT which operates at a micrometer-scale resolution.

Because of anisotropic propagation of light in tissue, the lateral profile will spread out slowly relative to axial profile, resulting in an elongated image.

Spectral-domain ms/LCI uniquely needs to account for the wavelength dependence of scattering when interpreting the reflectance depth profile as well as the transfer of information between layers; light emerging from deeper portions of tissue experience loss from absorption and scattering at all layers above it.

Signal-to-noise ratio

Predictably, ms/LCI requires high signal sensitivity in order to detect high-depth multiply scattered photons. Lock-in amplifiers help to increase signal-to-noise ratio (SNR) of the signals in both cases, and the use of apertures and precise illumination angles can minimize the amount of scattered light detected from superficial depths.

In time-domain ms/LCI, long integration times and depth scans provide improved contrast and sensitivity while sacrificing acquisition time. The use of a balanced photoreceiver is helpful to increase signal-to-noise ratio in time-domain ms/LCI but is difficult to do in spectral-domain ms/LCI because the increased depth range results in higher modulation frequencies which are difficult to calibrate between spectrometers [4]

Related Research Articles

In physics, coherence length is the propagation distance over which a coherent wave maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length. A wave with a longer coherence length is closer to a perfect sinusoidal wave. Coherence length is important in holography and telecommunications engineering.

<span class="mw-page-title-main">Interferometry</span> Measurement method using interference of waves

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<span class="mw-page-title-main">Optical coherence tomography</span> Imaging technique

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Terahertz tomography is a class of tomography where sectional imaging is done by terahertz radiation. Terahertz radiation is electromagnetic radiation with a frequency between 0.1 and 10 THz; it falls between radio waves and light waves on the spectrum; it encompasses portions of the millimeter waves and infrared wavelengths. Because of its high frequency and short wavelength, terahertz wave has a high signal-to-noise ratio in the time domain spectrum. Tomography using terahertz radiation can image samples that are opaque in the visible and near-infrared regions of the spectrum. Terahertz wave three-dimensional (3D) imaging technology has developed rapidly since its first successful application in 1997, and a series of new 3D imaging technologies have been proposed successively.

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Angle-resolved low-coherence interferometry (a/LCI) is an emerging biomedical imaging technology which uses the properties of scattered light to measure the average size of cell structures, including cell nuclei. The technology shows promise as a clinical tool for in situ detection of dysplastic, or precancerous tissue.

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<span class="mw-page-title-main">White light scanner</span>

A white light scanner (WLS) is a device for performing surface height measurements of an object using coherence scanning interferometry (CSI) with spectrally-broadband, "white light" illumination. Different configurations of scanning interferometer may be used to measure macroscopic objects with surface profiles measuring in the centimeter range, to microscopic objects with surface profiles measuring in the micrometer range. For large-scale non-interferometric measurement systems, see structured-light 3D scanner.

<span class="mw-page-title-main">White light interferometry</span> Measurement technique

As described here, white light interferometry is a non-contact optical method for surface height measurement on 3D structures with surface profiles varying between tens of nanometers and a few centimeters. It is often used as an alternative name for coherence scanning interferometry in the context of areal surface topography instrumentation that relies on spectrally-broadband, visible-wavelength light.

Multi-spectral optoacoustic tomography (MSOT), also known as functional photoacoustic tomography (fPAT), is an imaging technology that generates high-resolution optical images in scattering media, including biological tissues. MSOT illuminates tissue with light of transient energy, typically light pulses lasting 1-100 nanoseconds. The tissue absorbs the light pulses, and as a result undergoes thermo-elastic expansion, a phenomenon known as the optoacoustic or photoacoustic effect. This expansion gives rise to ultrasound waves (photoechoes) that are detected and formed into an image. Image formation can be done by means of hardware or computed tomography. Unlike other types of optoacoustic imaging, MSOT involves illuminating the sample with multiple wavelengths, allowing it to detect ultrasound waves emitted by different photoabsorbing molecules in the tissue, whether endogenous or exogenous. Computational techniques such as spectral unmixing deconvolute the ultrasound waves emitted by these different absorbers, allowing each emitter to be visualized separately in the target tissue. In this way, MSOT can allow visualization of hemoglobin concentration and tissue oxygenation or hypoxia. Unlike other optical imaging methods, MSOT is unaffected by photon scattering and thus can provide high-resolution optical images deep inside biological tissues.

<span class="mw-page-title-main">Light-in-flight imaging</span>

Light-in-flight imaging — a set of techniques to visualize propagation of light through different media.

<span class="mw-page-title-main">Intracoronary optical coherence tomography</span>

Intracoronary optical coherence tomography (OCT) is a catheter-based imaging application of optical coherence tomography. Currently prospective trials demonstrate OCT alters morbidity and/or mortality in coronary stenting and cervical cancer screening as discussed below.

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Quantum optical coherence tomography (Q-OCT) is an imaging technique that uses nonclassical (quantum) light sources to generate high-resolution images based on the Hong-Ou-Mandel effect (HOM). Q-OCT is similar to conventional OCT but uses a fourth-order interferometer that incorporates two photodetectors rather than a second-order interferometer with a single photodetector. The primary advantage of Q-OCT over OCT is insensitivity to even-order dispersion in multi-layered and scattering media.

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References

  1. 1 2 Matthews, Thomas E.; Giacomelli, Michael G.; Brown, William J.; Wax, Adam (1 December 2013). "Fourier domain multispectral multiple scattering low coherence interferometry". Applied Optics. 52 (34): 8220–8228. Bibcode:2013ApOpt..52.8220M. doi:10.1364/AO.52.008220. PMID   24513821 . Retrieved 26 November 2018.
  2. 1 2 Giacomelli, Michael G.; Wax, Adam (17 February 2011). "Imaging beyond the ballistic limit in coherence imaging using multiply scattered light". Optics Express. 19 (5): 4268–4279. Bibcode:2011OExpr..19.4268G. doi:10.1364/OE.19.004268. PMC   3368313 . PMID   21369257 . Retrieved 26 November 2018.
  3. Zhao, Yang; Maher, Jason R.; Ibrahim, Mohamed M.; Chien, Jennifer S.; Levinson, Howard; Wax, Adam (1 Oct 2016). "Deep imaging of absorption and scattering features by multispectral multiple scattering low coherence interferometry". Biomedical Optics Express. 7 (10): 3916–3926. doi:10.1364/BOE.7.003916. PMC   5102527 . PMID   27867703 . Retrieved 26 November 2018.
  4. Kuo, Wen-Chuan; Lai, Chih-Ming; Huang, Yi-Shiang; Chang, Cheng-Yi; Kuo, Yue-Ming (7 August 2013). "Balanced detection for spectral domain optical coherence tomography". Optics Express. 21 (16): 19280–19291. Bibcode:2013OExpr..2119280K. doi: 10.1364/OE.21.019280 . PMID   23938845 . Retrieved 26 November 2018.