Endoscopic imaging of white matter fiber tracts using polarization-sensitive optical coherence tomography

Abstract

Polarization sensitive optical coherence tomography (PSOCT) has been shown to image and delineate white matter fibers in a label-free manner by revealing optical birefringence within the myelin sheath using a microscope setup. In this proof-of-concept study, we adapt recent advancements in endoscopic PSOCT to perform depth-resolved imaging of white matter structures deep inside intact porcine brain tissue ex-vivo, through a small, rotational fiber probe. The probe geometry is comparable to microelectrodes currently used in neurosurgical interventions. The presented imaging system is mobile, robust, and uses biologically safe levels of optical radiation making it well suited for clinical translation. In neurosurgery, where accuracy is imperative, endoscopic PSOCT through a narrow-gauge fiber probe could provide intra-operative feedback on the location of critical white matter structures.

Keywords: Birefringence; Intracranial imaging; Optical anisotropy; Optical coherence tomography; Polarized light imaging.

Fig. 1.

System description and imaging geometry. (A) Photograph of commercial neurosurgical tools, PSOCT endoscope and capillary tube for size comparison. BP: Biopsy probe (Medtronic), ME: microelectrode (Alpha Omega), PE: PSOCT endoscope, CT: capillary tube cannula. (B) Schematic of the PSOCT system used herein. (C) Representative illustration of the benchtop imaging configuration, wherein 3D information is derived from a fast scanning galvanometer mirror and a slower translation stage. The schematic also depicts the round-trip nature of the measured cumulative Jones matrix J(z), (white arrow) consisting of sequential transmission ΔJ_m through individual tissue layers of one pixel thickness δz, and indicates the color-coded optic axis orientations in the image coordinates (colored double-arrows). (D) Representative illustration of the endoscopic imaging configuration, wherein 3D information is derived from a continuous rotation of the side-looking imaging probe combined with a translational “pullback” motion. (E) Birefringence and its optic axis orientation are derived through iterative reconstruction from cumulative round-trip matrices and displayed using a combined color hue and brightness map. The brightness maps the birefringence range of 0–1.3 × 10^–3.

Fig. 2.

Large FOV benchtop imaging of thick, fresh porcine brain tissue using PSOCT. (A) Reference picture of imaged brain slice with magnified inset, corresponding to PSOCT-imaged area, with labelled anatomical areas. CGM: Cortical Gray Matter. CWM: Cortical White Matter. Cd: Caudate Nucleus. IC: Internal Capsule. Put: Putamen. Th: Thalamus. (B) En-face PSOCT images. Order from left to right: logarithm of scattering intensity, scalar birefringence, optic axis orientation with birefringence brightness overlay. The white arrows highlight the different strands of the WMT structure. Colored dashed lines represent the position of the corresponding cross-sectional slices in the other panels. (C) Cross-sectional slice along the slow scan direction. (D) Cross-sectional slices along the fast scan direction. The white dashed line indicates the depth position of the en-face view in B. (E) Cross-sectional slices highlighting depth-resolved features. Scale bars are 1 mm in general, and 500μm for panel E. Color maps for various contrasts are located as inserts in C and show the range of 0–40 dB for the intensity signal and 0–1.3 × 10^–3 for birefringence. Images present mean projections computed along the respective out-of-plane directions over 30 μm for en-face and 70 μm for cross-sectional views, centered on the indicated locations.

Fig. 3.

Comparing PSOCT contrast between benchtop microscope and small form-factor rotational endoscope. (A) Camera image of brain slice with probe inside capillary in place, before initiating imaging pullback. White arrow highlights probe-tip within capillary. (B) Preceding benchtop en-face image from Fig. 2 for reference. White dashed lines correspond to endoscopic imaging area. Gray bars indicate anatomical areas: CGM: Cortical Gray Matter. CWM: Cortical White Matter. Cd: Caudate Nucleus. IC: Internal Capsule. Th: Thalamus. (C) Unfolded en-face view of PSOCT contrasts imaged with endoscopic probe. Order from left to right: logarithm of scattering intensity, scalar birefringence, optic axis orientation with birefringence brightness overlay. Colored dashed lines represent the position of the corresponding cross-sectional slices in D and E. Gray bars highlight same anatomical areas as in B. (D) Cross-sectional slice along the pullback direction. (E) Cross-sectional slices along the rotational direction. White dashed circle indicates position of unfolded en-face image in C. All scale bars are 1 mm. Color maps for various contrasts are located as inserts in D and show the range of 0–40 dB for the intensity signal and 0–1.3 × 10^–3 for birefringence. Images present mean projections computed along the respective out-of-plane directions over 30 μm for en-face, 2.5° for cross-sectional pullbacks, and 140 μm for cross-sectional rotational views, respectively, centered on the indicated locations.

Fig. 4.

Deep brain PSOCT in intact, fresh, porcine brain. (A) Reference image of capillary tube cannula inserted into the right hemisphere of the fresh brain tissue, used to guide imaging probe descent. (B) Schematic legend for corresponding 2D slice locations within 3D imaging volume. Black lines represent different depths of cross-sectional rotational slices in C, and red circle and colored lines indicate locations of unwrapped en-face view in D and of tangential cross-sections in E, respectively. (C) Rotational cross-sections at various positions along pullback. Top image is deepest in the brain. Order from left to right: logarithm of scattering intensity, scalar birefringence, optic axis orientation with birefringence brightness overlay. (D) Unwrapped en-face view at constant distance from the capillary surface. Position of en-face image is indicated by dashed white circle in first intensity cross-section of C. (E) Tangential cross-sections within imaging volume. All scale bars are 1 mm. Color maps for various contrasts are located as inserts in E and show the range of 0–40 dB for the intensity signal and 0–1.3 × 10^–3 for birefringence. Images present mean projections computed along the respective out-of-plane directions over 60 μm for cross-sectional rotational views, and 30 μm for en-face and tangential cross-sectional views, respectively, centered on the indicated locations.