Research ArticleAPPLIED PHYSICS

Smart optical coherence tomography for ultra-deep imaging through highly scattering media

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Science Advances  04 Nov 2016:
Vol. 2, no. 11, e1600370
DOI: 10.1126/sciadv.1600370
  • Fig. 1 Measuring the time-gated reflection matrix.

    (A) A femtosecond laser beam (center wavelength, 810 nm; bandwidth, 40 nm) is shaped by an SLM acting as a dynamic diffraction grating. A set of incident plane waves is thus emitted from the SLM and focused at a different position in the focal plane of an MO (NA, 0.25). The backscattered wave field is collected through the same MO and interferes with a reference beam on an CCD camera. The latter one is conjugated with the back focal plane of the MO. The amplitude and phase of the wave field are recorded by phase-shifting interferometry (5). The time of flight t is controlled by the length of the interferometric arm and is matched with the position of the focal plane. Experimental setup: L, laser; P, polarizer; MO, microscope objective; BS, beam splitter; PBS, polarized beam splitter; PZT, piezo phase shifter; M, mirror. (B) For each input-focusing point rin, the reflected wave field R(rin,kout) is recorded in the k space. (C) A two-dimensional Fourier transform yields the wave field in the real space R(rin,rout), where rout represents the output-focusing point in the focal plane. (D) For each incident-focusing point rin, the recorded wave field is stored along a column vector. Finally, the set of column vectors forms the reflection matrix R = [R(rin,rout)].

  • Fig. 2 Target detection in the deep MS regime.

    (A) The reference time-gated reflection matrix R0 measured for a ZnO bead deposited on a glass slide. (B) En face OCT image deduced from R0 (Eq. 1). (C) The time-gated reflection matrix R in the presence of a strongly scattering layer (L = 12.25ls). (D) En face OCT image deduced from R (Eq. 1). (E) The SS matrix RS deduced from R with lc = 5 μm (Eq. 3). (F) The eigenvalue histogram of RSRS compared to the random matrix prediction (red line): the largest eigenvalue emerges from the MS noise. (G) Smart-OCT image deduced from the first eigenstate of RS. Scale bars, 10 μm.

  • Fig. 3 Imaging in the deep MS regime.

    (A) The reflection matrix R associated to three 5-μm-diameter ZnO beads deposited on a glass slide. (B) Time-gated confocal image of the three beads in the absence of the scattering layer. (C) The reflection matrix R measured in the presence of a scattering layer (L = 6.2ls) placed before the three beads. (D) Time-gated confocal image in the presence of the scattering layer. (E) The SS matrix Rs built from R using Eq. 3. (F) The three first eigenstates of Rs, |UiVi|, are combined to yield the smart-OCT image of the three beads in the presence of the scattering layer. Scale bars, 10 μm.

  • Fig. 4 Imaging through thick biological tissues.

    (A) Schematic of the experimental configuration. A positive resolution target USAF 1951 is placed in the focal plane of an immersion MO, with an 800-μm-thick layer of rat intestine on top of it. The region of interest is surrounded by a green square in the bottom inset. DIC, differential interference contrast; WD, working distance. (B) En face OCT image of the resolution target. (C) SD of the smart-OCT image as a function of the number M of eigenstates of Rs considered to build the image. (D to F) Smart-OCT images of the resolution target obtained from the 20, 250, and 500 first eigenstates of RS.

  • Fig. 5 Imaging-depth limit in human soft tissues.

    This graph compares the SMRs expected for conventional microscopy (black line), confocal microscopy (green line), OCT (blue line), and smart-OCT (red line) as a function of the optical depth F/ls. The y axis is in log scale. These curves have been computed from the theoretical study developed in section SII, considering experimental parameters typical of full-field OCT (42). The detection threshold (SMR, ~1; black dashed horizontal line) yields the following imaging-depth limits: ~1ls for conventional microscopy, ~8ls for confocal microscopy, ~12ls for OCT, and ~22ls for smart-OCT.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/11/e1600370/DC1

    section SI. Optical characterization of the paper layers

    section SII. SMR for various imaging techniques

    section SIII. Eigenvalue distribution of the reflection matrix in the MS regime

    fig. S1. Measurement of the scattering mean free path in the paper.

    fig. S2. Measurement of the transport mean free path in the paper.

    fig. S3. Theoretical prediction of the SMR in the experimental conditions of the article.

    fig. S4. Iterative time reversal processing applied to the raw reflection and SS matrices measured through biological tissues.

    table. S1. Experimental parameters used for the theoretical prediction of the SMR in Fig. 5 and fig. S3.

    References (4459)

  • Supplementary Materials

    This PDF file includes:

    • section SI. Optical characterization of the paper layers
    • section SII. SMR for various imaging techniques
    • section SIII. Eigenvalue distribution of the reflection matrix in the MS regime
    • fig. S1. Measurement of the scattering mean free path in the paper.
    • fig. S2. Measurement of the transport mean free path in the paper.
    • fig. S3. Theoretical prediction of the SMR in the experimental conditions of the article.
    • fig. S4. Iterative time reversal processing applied to the raw reflection and SS matrices measured through biological tissues.
    • table S1. Experimental parameters used for the theoretical prediction of the SMR in Fig. 5 and fig. S3.
    • References (4459)

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