(A) study of susceptibility effect analysis in NMR and its application to imaging

Abstract

The susceptibility effects in NMR (nuclear magnetic resonance) produce some artifacts in conventional spin echo imaging as well as gradient echo imaging which is recently popular. In this thesis, the nature of susceptibility effects is exploited for both spin echo imaging and gradient echo imaging, and subsequently a correction method of the artifacts such as geometrical displacement and signal loss is presented. In gradient echo imaging techniques recently under investigation in various forms of SSFP (steady state free precession), FLASH (fast low angle shot), GRASS (gradient recalled acquisition steady states mode) and FISP (fast imaging steady state free precession), the signal loss phenomenon is specially severe because the phase dispersion of spins due to field inhomogeneity is not refocused with the gradient refocusing inherent in the method. Therefore, these techniques based on gradient echo imaging have been difficult to use in clinical situation. As a reduction of the susceptibility artifact in gradient echo imaging, a technique using a tailored RF pulse is proposed. It is applied to the case of high local magnetic field inhomogeneity due to the susecptibility. On the basis of slice selection, the signal loss and void phenomena due to susceptibility in a voxel are studied and a correction method is also discussed. The description of the tailored RF pulse for this purpose and its application are given and experimental results obtained using a human volunteer are presented. Furthermore the bloods containing deoxyhemoglobin such as veins are known as paramagnetic substances. These paramagnetic substances surrounding normal tissues which are known as diamagnetic substances produce the magnetic field inhomogeneity around their vicinities. Consequently, the signal from these vicinities where field inhomogeneities are created is reduced or void out due to the phase incoherency of spins. This signal loss phenomenon apparently appears in gradient echo ima...