Fossilized Teeth as a New Robust and Reproducible Standard for Polarization-Sensitive Optical Coherence Tomography

A clinical need exists for a cheap and efficient standard for polarization sensitive optical coherence tomography (PS-OCT). We utilize prehistoric fossilized teeth from the Megalodon shark and European horse as an unconventional, yet robust standard. Given their easy accessibility and the microstructural consistency conferred by the process of fossilization, they provide a means of calibration to reduce error from sources such as catheter bending and temperature changes. We tested the maximum difference in birefringence values in each tooth and found the fossilized teeth to be fast and repeatable. The results were compared to measurements from bovine meniscus, tendon, and destroyed tendon, which were verified with histology. 1. Introduction Polarization-sensitive optical coherence tomography (PS-OCT) is a micron-scale imaging/spectroscopic modality capable of assessing organized structure in biological tissues through birefringence [1]. Day-to-day variation in the imaging systems limits the precision of repeated measurements, requiring the need for standards. The sources for variation are numerous, including the movement of fiber optic components within the system or catheter/endoscope, and are prominent in producing error [2, 3]. Preventing this variation by checking and recalibrating the system using a cheap, widely accessible standard would greatly increase the robustness of imaging. Reproducible standards have been difficult to achieve. In this paper, we have found an unusual standard, fossilized prehistoric marine vertebrate (predominately megalodon) and dinosaur teeth. Being at least 10’s of millions of years old, their microstructure and birefringence have been stabilized with time [4]. We compare their results to the OCT imaging of birefringent tendon and cartilage, validating the results with histopathology. Optical coherence tomography is an imaging modality analogous to ultrasound, using low-coherence interferometry to measure the backreflection of infrared light rather than sound [5]. Already approved in both cardiology and ophthalmology for clinical imaging, OCT functions above video rate and at resolutions up to 25x higher than any other clinical imaging modality [6]. With respect to the single channel PS-OCT modality, as highly organized tissue is typically birefringent, it modifies the incident light polarization state, resulting in variations in the backreflection intensity. As early diseases progresses, birefringent materials such as collagen or enamel become disorganized, leading to a loss of birefringence. This occurs long before the