MOLECULE SPECIFIC CONTRAST USING ULTRA-LOW-FIELD MAGNETIC RESONANCE IMAGING

Optically detected atomic magnetometers use the coherent precession of polarized atomic spins to detect and measure magnetic fields. Low-field MRI using atomic magnetometers with improvements in the spatial resolution and sensitivity can be made more competitive with conventional MRI systems. In the present thesis we report the improvement of spatial resolution of low-field MRI, contrast imaging generated by ligand-conjugated magnetic particles and improvements in selective polarization technique.

Magnetic resonance imaging (MRI) in an ultra-low magnetic field usually has poor spatial resolution compared to its high-field counterpart. The concomitant field effect and low signal level are among the major causes that limit the spatial resolution. Here, we report a novel imaging method, a zoom-in scheme, to achieve a reasonably high spatial resolution of 0.6 mm × 0.6 mm without suffering the concomitant field effect. This method involves multiple steps of spatial encoding with gradually increased spatial resolution but reduced field-of-view. We also demonstrate the use of a unique gradient solenoid to improve the efficiency of optical detection with an atomic magnetometer. The enhanced filling factor improved the signal level and consequently facilitated an improved spatial resolution. Ultra-low-field magnetic resonance imaging usually cannot provide chemical information, because of the loss of chemical shift information. By using ligand-conjugated magnetic particles, we show contrast imaging corresponding to the particles binding their specific molecular target. A 10% signal decrease was observed when the streptavidin-biotin bonds were formed between the magnetic particles and the surface. Our method provides a unique approach for probing molecules on surfaces, especially under opaque conditions where optical-based imaging techniques are not applicable. The last part of our study is selective polarization, which is unique advantage of low-field MRI. By selectively pre-polarizing the sample in a specific channel provides significant information for studying flow and mixing behaviour in chemical reactions. For the first time we conducted the selective polarization experiment on two different liquids. By selectively polarizing water, we report a 5% difference in the signal intensity at different flow rates. The improvements mentioned in this thesis can contribute for further developments of low-field MRI.