Polarization sensitive optical coherence microscopy for brain imaging

Abstract

Optical coherence tomography (OCT) and optical coherence microscopy (OCM) have demonstrated the ability to investigate cyto- and myelo-architecture in the brain. Polarization-sensitive OCT provides sensitivity to additional contrast mechanisms, specifically the birefringence of myelination and, therefore, is advantageous for investigating white matter fiber tracts. In this Letter, we developed a polarization-sensitive optical coherence microscope (PS-OCM) with a 3.5 μm axial and 1.3 μm transverse resolution to investigate fiber organization and orientation at a finer scale than previously demonstrated with PS-OCT. In a reconstructed mouse brain section, we showed that at the focal depths of 20-70 μm, the PS-OCM reliably identifies the neuronal fibers and quantifies the in-plane orientation.

Fig. 1

System schematic of the PS-OCM. C, collimator; D, dispersion compensation block; CM, concave mirror; FC, fiber coupler; G, grating; LSC, line scan camera; P, polarizer; PC, polarization controller; PMC, polarization-maintaining fiber coupler; PS, polarization splitter; QWP, quarter-wave plate; SLD, superluminescent diode.

Fig. 2

Characterization of retardance (a) and optic axis orientation (b).

Fig. 3

Stitched en-face images of reflectivity (a), retardance (b) and optic axis orientation (c). In (c), the orientation value is indicated by the color wheel on the top right corner. The brightness is controlled by the retardance value. The numbers in (a) indicate the ROIs where the depth profiles of reflectivity (d), retardance (e) and optic axis orientation (f) were conducted at 10 imaging depths. ROIs: 1 – corpus callosum; 2 – cingulum; 3 – fimbria of hippocampus; 4 – fornix; 5 – anterior commissure; 6 – lateral septal nucleus. Scale bar: 400 μm.

Fig. 4

Fiber orientation maps in seven representative tiles (a – g) at the focal depth of 30 – 50 μm. Top: reflectivity; middle: optic axis orientation measured by PS-OCM; bottom: computed orientation using a structure tensor approach. The color wheel at the bottom right indicates the orientation values. Scale bars: 50 μm.