(A) study of dynamic NMR imaging and its applications-bulk and microscopic flow imaging

Abstract

In this thesis, the effects of flow on NMR images are studied. The effects of flow appear as a change of phase as well as signal intensity. Since the flows of blood and cerebrospinal fluid (CSF) of human body are pulsatile due to heart pumping, their velocities are not constant during NMR imaging. This type of velocity fluctuation induces irregular flow-dependent phase shifts which have been the main causes of flow artifacts in NMR imaging. In order to reduce the flow artifacts, two kinds of flow artifact reduction schemes are proposed and studied in this thesis. One is the flowinduced phase compensating technique. This technique utilizes back-to-back symmetric gradient waveforms which can remove flow-induced phase shift. The flow compensating pulse sequence for double spin echo is also proposed for the simultaneous acquisition of spin density and heavily $T_2$-weighted images. The technique seems to be promising for the reduction of eyeball and respiratory motion artifacts. The other approach is the cardiac cycle ordered phase encoding method. This technique utilizes the cardiac cycle as a precursor for the phase encoding gradients in such a way as does the respiratory ordered phase encoding (ROPE) technique which has been used for respiratory motion artifact reduction. This approach seems to be advantageous in the sense that the method is easy to implement in any pulse sequences without additional hardware requirements. Both of the proposed techniques were verified by the experimental results obtained with human vounteers using the KAIS 2.0 tesla whole-body NMR imaging system. The effects of diffusion which can be regarded as a kind of microscopic flows are also studied. In particular, the signal attenuation due to the diffusion in the fast steady state free precession (SSFP) sequence is theoretically analyzed and it is shown that the signal attenuation is dependent on spin-spin relaxation time $T_2$ and flip angle. Finally, a new fast diffusion coefficient m...