Speckle variance optical coherence tomography of the rodent spinal cord: in vivo feasibility

Abstract

Optical coherence tomography (OCT) has the combined advantage of high temporal (µsec) and spatial (<10µm) resolution. These features make it an attractive tool to study the dynamic relationship between neural activity and the surrounding blood vessels in the spinal cord, a topic that is poorly understood. Here we present work that aims to optimize an in vivo OCT imaging model of the rodent spinal cord. In this study we image the microvascular networks of both rats and mice using speckle variance OCT. This is the first report of depth resolved imaging of the in vivo spinal cord using an entirely endogenous contrast mechanism.

Keywords: (110.4500) Optical coherence tomography; (170.2655) Functional monitoring and imaging; (170.3880) Medical and biological imaging.

Fig. 1

Schematic diagram of the swept-source OCT system used in this set of experiments. A 54 kHz polygon filter based swept source laser was used with grating that enables buffering to 108kHz and post amplified by a semiconductor optical amplifier (SOA). The coherence of the laser source is approximately 1.5 mm. PC: polarization controller; CIR: optical circulator; C: collimator; L: lenses; FBG: fiber Bragg gratings; D: photodetector; BD: balanced detector.

Fig. 2

Experimental setup of rat SV-OCT. Panel A shows an overview of the imaging station with the male Wistar rat pinned to the imaging breadboard. Panel B shows a close up of the pin fixation. The animal is resting on a heated gel pack to maintain 37°C body temperature (panel B). The animal underwent tracheotomy and is receiving inhalational anesthetic. Breath-hold was conducted to reduce bulk motion from >40μm in the anterior-posterior direction to <8μm (panel C). A close up of the exposed spinal cord is shown in panel D. Panel E demonstrates structural OCT data. Panel F shows an en face projection of the SV-OCT data. A histology specimen is provided in panel G for comparison, with the dorsal vein (1) marked.

Fig. 3

Experimental setup of mouse SV-OCT imaging. Bulk motion correction of the spinal column was carried out with a homemade jig as shown in panel A. Briefly, we used two pairs of forceps to gently grasp the spinal column one vertebral body level above and below the exposed spinal cord. A histology specimen is provided in panel B that demonstrates the dorsal vein (1), dorsal white matter (2) and dorsal gray matter (3). A structural OCT image is shown in panel C that demonstrates the dorsal vein, dorsal white and gray matter. SV-OCT images are shown in panel D demonstrating the microvascular network of the mouse spinal cord resolving vessels with a diameter of approximately 10-20μm. Panel E illustrates a depth dependent false color map of the mouse spinal cord. [Table: see text]

Fig. 4

A three-dimensional structural OCT volume (3mm(x) × 3mm (y) × 2mm (z)) of the mouse spinal cord (Media 1). [Table: see text]

Fig. 5

A three-dimensional SV-OCT volume (3mm(x) × 3mm (y) × 2mm (z)) of the mouse spinal cord (Media 2).