UNC-CH Physics and Astronomy PhD Dissertation Defense

Jessica Barrick

“Development of a line-field magneto-motive optical coherence tomography system”

The mechanism by which certain species of animals are able to detect the Earth’s magnetic field has remained a mystery for as long as we have known that they exhibit geomagnetic navigation. No one has yet been able to identify the magnetoreceptors within any animal which convert the magnetic field to a signal interpretable by the animal’s brain. Certain species of bacteria are known to contain single chains of magnetite crystals, each with a diameter of ~50 nm, that are used to orient the bacteria. Searching for similar magnetoreceptors in larger animals requires a high-speed, high-resolution imaging system with the ability to detect single magnetic nanoparticles. Current imaging modalities are unsuitable for the detection of potentially-isolated magnetic nanoparticles located within large volumes of animal tissue because they are either too slow or do not have single-particle sensitivity. Optical coherence tomography (OCT) is a biomedical imaging modality that produces 2D, cross-sectional images of optically turbid media with a resolution on the order of 1-10 µm. Magneto-motive OCT (MMOCT) is a functional form of OCT that can detect the sub-resolution displacement of magnetic nano- or micro-particles embedded in weakly diamagnetic, optically scattering, elastic media (such as human and animal tissues) subject to a sinusoidally-varying magnetic gradient force. This dissertation describes the design and implementation of an MMOCT system composed of a novel combination of a line-field configuration with a supercontinuum light source and a faster MMOCT imaging scheme. The combination of the line illumination with a high-speed 2D camera and the low-noise, high-power supercontinuum light source produces the best combination of axial resolution, optical SNR, and imaging speed of any line-field-OCT (LFOCT) system to date. This system is then converted to a LF-MMOCT system with additional hardware and software components. The performance of the LF-OCT system combined with the faster magnet modulation scheme results in a LF-MMOCT system with the highest volumetric imaging speed of any MMOCT system to date. High volumetric imaging speed is essential for the problem of endogenous magnetite detection, as is high magnetic sensitivity. The LF-MMOCT system is optimized to produce the best possible magnetic SNR at kilohertz framerates. We then demonstrate the ability to detect a single magnetic point particles, measure the vibration amplitude produced by the application of an external magnetic gradient force on each point particle, and finally compare that vibration amplitude to a theoretical value. The ability to image a single magnetic point particle with a high-resolution, high-sensitivity, and high-speed LF-MMOCT system provides a key proof of concept that this system may be used for endogenous magnetite detection.