03.11.2021

STOC imaging

Conventional scanning OCT combines time- with confocal gating enabling high-speed, high-resolution cross-sectional imaging of the human retina. Classic OCT, however, does not provide high-resolution en face images of the outer retinal layers due to eye aberrations and the fundamental tradeoff between imaging depth and transverse resolution.

This tradeoff is reduced by a full-field OCT (FF-OCT) method that uses a two-dimensional camera instead of a single-element photodiode. However, an attempt to boost the FF-OCT imaging speed by Fourier-domain (FD) detection resulted in another severe limitation – spatial coherence of the laser generates coherent artifacts, which reduces the spatial resolution.

To solve this problem, we developed the new way of controlling the optical phase called STOC (Spatio-Temporal Optical Coherence). Application of STOC to Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is called STOC tomography (STOC-T) or STOC imaging, and enabled obtainining in vivo high-resolution, volumetric images of skin [1], the retina [2], and cornea [3].

Fig. 1. STOC imaging enables high-resolution imaging of the retina by spatial phase modulation (a). Computational aberration correction enables to correct the data in post-processing to remove aberrations (b). By repeating measurements at different locations, and stitching together the resulting images we obtain high fidelity wide area retinal images (c).

FF-OCT is significantly different from the classic OCT. FF-OCT is closer to multi-color (or multi-wavelength) digital holography than scanning microscopy, especially with the Fourier detection (FD) with the tunable laser. FD-FF-OCT uses a tunable laser and an ultra-fast area scan camera. The tunable laser encodes depth information about the sample. We extended FD-FF-OCT by the spatial phase modulator (SPM) [Fig. 1(a)]. The SPM dynamically modulates the phase of incident light. The resulting signals are processed and averaged to produce noise-free volumetric images of the sample. Phase modulation works here as an additional gating mechanism that rejects the multiply scattered light effectively [last column in Fig. 1(b)]. However, as shown in Fig. 1(b), the en face images are distorted by eye aberrations. We overcome them computationally using the computational aberration correction (CAC) [4].

CAC proceeds, as sketched in Fig. 1(b). The complex data (amplitude and phase) representing the layer at a depth z_l, U(x,y,z=z_l) of the sample is 2D Fourier transformed to achieve the spatial spectrum U ̃(k_x,k_y,z=z_l). The latter is multiplied by the variable phase mask, M(k_x,k_y) and the resulting modified spatial spectrum U ̃(k_x,k_y,z)M(k_x,k_y) is inverse Fourier-transformed to achieve the corrected field U_c (x,y,z=z_l). We then evaluate the image quality metric on |U_c (x,y,z=z_l )|^2. Here, for the metric, we use the kurtosis. The phase mask is M(k_x,k_y,z_l )=exp[i∑_(n=1)^N▒〖α_n (z_l ) 〗 Z_n (k_x,k_y)], where α_n (z_l) are the depth-dependent adjustable parameters, and Z_n (k_x,k_y) are the Zernike polynomials. The representative estimated real part of the resulting phase mask is shown in the second column of Fig. 2(b). The last column of Fig. 2(b) demonstrates that CAC enables to depict otherwise invisible photoreceptor cone mosaic, the primary sensing element of the human eye.

To achieve wide-field retina images we perform measurements at different locations, and then stitch resulting volumes together to render high resolution, high fidelity retinal images at different depths [Fig. 1(c)]. Specifically, we render the choroid, which was not possible with conventional Fourier-domain FF-OCT (without phase modulation).

Text: Dawid Borycki, PhD, e-mail: dborycki@ichf.edu.pl

References:

  1. Borycki, D., et al., Spatiotemporal optical coherence (STOC) manipulation suppresses coherent cross-talk in full-field swept-source optical coherence tomography. Biomed Opt Express, 2019. 10(4): p. 2032-2054.
  2. Stremplewski, P., et al., In vivo volumetric imaging by crosstalk-free full-field OCT. Optica, 2019. 6(5): p. 608-617.
  3. Auksorius, E., D. Borycki, and M. Wojtkowski, Crosstalk-free volumetric in vivo imaging of a human retina with Fourier-domain full-field optical coherence tomography. Biomed Opt Express, 2019. 10(12): p. 6390-6407.
  4. Auksorius, E., et al., In vivo imaging of the human cornea with high-speed and high-resolution Fourier-domain full-field optical coherence tomography. Biomed Opt Express, 2020. 11(5): p. 2849-2865.
  5. Borycki, D., et al., Computational aberration correction in spatiotemporal optical coherence (STOC) imaging. Opt Lett, 2020. 45(6): p. 1293-1296.