Micron resolution 600 MHz NMR microscope

Abstract

This dissertation describes development and application of a 600 MHz NMR microscope which is capable of obtaining pixel resolution of 1 ㎛. The principles of NMR and MRI, and the sensitivity and resolution of MRI experiments are described. Since the pixel size of one micrometer is $10^3$ times smaller than that normally used in human body imaging, special investigation is needed concerning SNR and resolution. We studied factors determining SNR of a voxel in microscopic MRI, and designed an NMR microscope applicable to the 1 ㎛ resolution MRI experiments. Micro-coils tuned at 600 MHz were used in the rf part of the microscope and the rf tank circuit was designed to be as small and compact as possible to get high Q-factors. Gradient coils generating over 1000 G/cm were designed and fabricated. The gradient strength is 5 times larger than that of the highest gradient field commercially available. Special endeavor was given to get a rigid structure of the gradient coil since large current for generating high gradient field may destroy the rigidity of the gradient coil unit and result in gradient coil movement which degrades the resolution of MRI images. The pulse sequence and the image reconstruction method were thoroughly studied and the optimized ones were found. Diffusion induces severe signal attenuation in micron resolution MRI experiments if a normal pulse sequence used in the human body imaging is adopted. It is because a small pixel needs large gradient fields for imaging and in that environment molecular spin diffusion causes spins to loose their phase coherence. To overcome the problem, we devised a diffusion effect reduced spin echo sequence, calculated the signal attenuation from diffusion and $T_2$ relaxation time, and selected and used the optimized medium and bandwidth. To reconstruct images from the asymmetrically sampled data, we studied the partial Fourier reconstruction method and optimized it in our experiments. The developed microscope, pulse s...