NMR imaging toward in vivo ultra high resolution imaging

Abstract

The limitations and problems in nuclear magnetic resonance (NMR) imaging for NMR microsocopy is analyzed. The analysis of the ultimate resolution which is dependent on the diffusion effect is presented. A generalized formulation of the diffusion related NMR signal is derived from a random walk model, and is extended to the two dimensional imaging case. In this thesis, based on the knowledge from the NMR microscopy study, the application to high resolution NMR imaging of the large object or a human in vivo is attempted together with the development of a new spatial localization technique. Since, in in vivo high resolution imaging, it is required to localized a volume of interest in order to remove the aliasing effect, a new localization technique which can be used in high resolution imaging is propoed. In contrast to other localization techniques which use a series of RF pulses to define a volume of interest, only one RF pulse is utilized in the proposed method for selection of a region to be imaged. Instead of RF pulses for region selection, sub-encoding gradient puleses are used for the localization together with convolution process on each of phase encoding gradient by a set of additional gradients (e.g.y-direction). Then the 2-D localization is completed by restricting the bandwidth in the readout direction (e.g.x-direction). The latter is simply achieved by using a low pass filter in the receiver system. A combination of the multislice imaging technique and the localization technique also proposed to obtain many slices of images during the same imaging time. Multislice imaging can be achieved by using a slice encoding technique which uses a series of Hadamard encoded RF pulses. Finally to improve SNR in obtaining high resolution images of the region which is located deep inside the body, a new type of surface coil which consists of triple surface RF coils are constructed and used in imaging a spinal cord. By applying this technique on a human body, localize...