Thinned-skull Cortical Window Technique for In Vivo Optical Coherence Tomography Imaging

Summary

We present a method of creating a thinned-skull cortical window (TSCW) in a mouse model for in vivo OCT imaging of the cerebral cortex.

Abstract

Optical coherence tomography (OCT) is a biomedical imaging technique with high spatial-temporal resolution. With its minimally invasive approach OCT has been used extensively in ophthalmology, dermatology, and gastroenterology^1-3. Using a thinned-skull cortical window (TSCW), we employ spectral-domain OCT (SD-OCT) modality as a tool to image the cortex in vivo. Commonly, an opened-skull has been used for neuro-imaging as it provides more versatility, however, a TSCW approach is less invasive and is an effective mean for long term imaging in neuropathology studies. Here, we present a method of creating a TSCW in a mouse model for in vivo OCT imaging of the cerebral cortex.

Introduction

Since its introduction in the early 1990's, OCT has been used extensively for biological imaging of tissue structure and function². OCT generates cross-sectional images by measuring echo time delay of backscattered light⁴ by implementing low coherence light source with a fiber-optic Michelson interferometer^2,4. SD-OCT, also known as Fourier domain OCT (FD-OCT), was first introduced in 1995⁵ and offers a superior imaging modality compared with traditional time domain OCT (TD-OCT). In SD-OCT, the reference arm is kept stationary resulting in a high speed and ultra high resolution image acquisition^6-9.

Protocol

1. Surgical Preparation

1. Female CD 1 mice between the ages of 6-8 weeks were used in our experiments.
2. Anesthetize the mouse with an intraperitoneal injection of a ketamine and xylazine combination (80 mg/kg ketamine/10 mg/kg xylazine). Place the mouse on a homeothermic pad to ensure optimal body temperature at ~37 °C. Continuously monitor level of anesthesia by testing animal's reflexes (e.g., pinching foot with blunt forceps) and inject more anesthesia when necessary.
3. Lubricate both eyes with an artificial tear ointment. Remove hairs on the scalp using a razor and remove residual hair using 70% alcohol prep pads. Apply a th....

Results

After creating a thinned window over the cerebral cortex the vasculature should now be more visually prominent (Figure 1) and will allow for a deeper imaging depth (up to 1 mm). The right cortex is thinned to approximately 55 μm as compared to a normal skull measured at 140 μm (Figure 1) and provides greater optical clarity. Further thinning to 10-15 μm is possible¹¹ however not necessary as the use of glass cover slips and skull plates are not implemented in our experiments (.......

Discussion

Imaging with OCT and a thinned-skull is a novel neuro-imaging technique that has only been recently investigated^{15, 16}. In our experiments, we demonstrated the feasibility of SD-OCT imaging through a TSCW in a mouse model in vivo. From our results, the skull is thinned to approximately 55 μm and the penetration depth is obtained at approximately 1 mm with image resolution of 8 μm and 20 μm in the axial and lateral direction, respectively. In the signal intensity profile, OCT imaging through a.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ketamine	Phoenix Pharmaceuticals	57319-542-02
Xylazine	Akorn, Inc.	139-236
Artificial Tears Ointment	Rugby	0536-6550-91
Nair	Church Dwight Co., Inc.	4010130
Sterile Alcohol Prep Pad	Kendall Healthcare	6818
Cotton Tipped Applicators	Fisherbrand	23-400-115
Betadine Solution Swabstick	Purdue Products	67618-153-01
Saline Solution, .9%	Phoenix Pharmaceuticals	57319-555-08
Stereotactic Frame	Stoelting
High Speed Surgical Hand Drill	Foredom		38,000 rpm
Carbide Round Bur	Stoelting		0.75 mm
Dura-Green Stones	Shofu		Shank: HP Shape: BA1
CompoMaster Coarse & CompoMaster Polisher	Shofu		Shape: Mini-Pt.
SpaceDrapes	Braintree Scientific, Inc.