TANG, XUEYAN2021-09-242021-09-242021-07https://hdl.handle.net/11299/224556University of Minnesota Ph.D. dissertation. 2021. Major: Biophysical Sciences and Medical Physics. Advisor: Michael Garwood. 1 computer file (PDF); 85 pages.In magnetic resonance imaging, the signal intensity is directly affected by the external field strength and the spin system properties. It is challenging to acquire signal from electron spins at ultra-low field because of their short relaxation times: the weak signal would decay away before it can be captured. The longitudinal detec- tion (LOD) of electron paramagnetic resonance (EPR) offers an alternative solution. Instead of measuring the transverse magnetization, LOD measures the longitudinal magnetization along the polarizing field direction. By placing the receive coil or- thogonal to the transmit coil and tuning it to a different frequency range, an LOD EPR system can have superior resilience against transmit cross-talk and enables si- multaneous transmit and receive. As a result, the EPR signal from spins with short relaxation times can be recorded with high signal to noise ratio. This thesis presents an ultra-low field LOD EPR measurement system. A novel method of inducing signal from longitudinal magnetization is described: fictitious field modulation. When viewed in a rotating frame of reference, a transverse-plane radiofrequency (RF) field manifests as a longitudinal field component called the fic- titious field. By modulating the RF field and thus the fictitious field, detectable longitudinal magnetization patterns are measurable. Furthermore, via combining fictitious-field modulation and longitudinal detection, this work demonstrates EPR spectroscopy and one-dimensional imaging. This custom-built LOD system oper- ates at an ultra-low frequency (24 MHz) for detecting electron spins with short (∼nanoseconds) relaxation times. Simultaneous transmit and receive with low trans- mitter leakage level (∼80 dB isolation) is also achieved. Finally, a different version of this system is introduced for measuring short relaxation times at ultra-low magnetic field, the principle of which is derived from the fictitious field theory.enElectron paramagnetic resonanceFictitious fieldIron-oxide nanoparticleLongitudinal detectionSimultaneous transmit and receiveSpectroscopyThesis or Dissertation