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Thesis Format

Integrated Article

Degree

Master of Science

Program

Medical Biophysics

Supervisor

Baron, Corey A.

Abstract

Oscillating gradient spin-echo (OGSE) is an implementation of diffusion MRI that enables shorter effective diffusion times than the conventional pulse gradient spin-echo (PGSE) by periodically modulating the diffusion gradient. Measurements of the diffusion kurtosis, which reflects the degree of restricted diffusion, have previously been prohibited with OGSE due to technical limitations of clinical gradient systems. This thesis presents a novel oscillating gradient waveform that enables the measurement of kurtosis using OGSE without requiring advanced gradient hardware. Decreases of kurtosis are observed in OGSE acquisitions of healthy human subjects relative to PGSE, demonstrating the dependence of the kurtosis on oscillation frequency. This frequency dependence is exploited to generate novel contrast based on the difference between PGSE and OGSE kurtosis measurements acquired with an optimized protocol. This work demonstrates the first in vivo measurements of kurtosis in the human brain using OGSE without the aid of advanced gradient hardware.

Summary for Lay Audience

Magnetic resonance imaging (MRI) is a diagnostic imaging modality that can image soft tissues for the characterization of anatomy and diagnosis of disease. Diffusion MRI is a particular type of MRI that can image water molecules in the body that are randomly moving due to thermal energy. When molecules diffuse they often bounce off boundaries like cell membranes and other structures they encounter as they move, therefore by following the diffusion of water molecules it is possible to gain insight into cellular structures that make up the surrounding environment. Oscillating gradient spin-echo (OGSE) is a specialized diffusion MRI method that can control the time the tracked molecules are allowed to diffuse for. By looking at diffusion using different diffusion times more insight about cellular structures can be acquired; a short diffusion time gives less time for the molecule to explore surrounding structures while a long diffusion time means the molecules will interact with many structures. This thesis presents a method to apply OGSE in the human brain to measure a quantity called the diffusion kurtosis, which provides information about how restricted diffusion is. Previously, measuring the kurtosis with OGSE was not possible due to technical limitations however in this work a new implementation of OGSE is proposed that allows the kurtosis to be measured on clinical MRI scanners. By using OGSE a new type of contrast can also be generated by calculating the difference in kurtosis values between the images taken with two different diffusion times. This type of contrast has been observed in animals before but never in humans and is a candidate for being sensitive to stroke and brain tissue degeneration. The new method presented here will allow us to begin exploring new diagnostic applications of the diffusion kurtosis with OGSE in future human studies.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 4.0 License.

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