This thesis deals with phase information in magnetic resonance imaging, with a focus on local changes in the static magnetic field produced by structures of different magnetic susceptibility, and their manifestation in the phase of the complex magnetic resonance (MR) signal. Such structures are e.g. venous vessels, where changes in venous blood oxygenation are reflected in the local field distribution and are thus detectable by MR methods.
Following two introductory chapters on the theory of NMR and MR-imaging (MRI), the properties of blood are discussed. Using a two compartment model describing the vein as a straight cylinder of infinite length embedded in a medium of different magnetic susceptibility, the effects of parameters such as vessel diameter, blood oxygenation, and orientation of the vein with respect to the magnetic field are simulated. These parameters influence the local magnetic field near venous vessels and hence the signal in gradient echo sequences.
Modulations of blood oxygenation induced by breathing of oxygen or carbogen (95-98% O2 + 2-5% CO2) lead to a modulation of the signal in susceptibility weighted images (SWI) and the reaction of the human brain (as detected by SWI) to such modulation was investigated in healthy subjects and in patients with cerebral tumors. Improvements (phase unwrapping) in post processing of SWI allowed to assess the variation of extravascular field inhomogeneities caused by variations in blood oxygenation. This variation is the basis of the blood oxygenation signal dependency (BOLD) effect. Based on phase unwrapping the technique of susceptibility weighted phase imaging was developed and applied to neuroimaging.
High resolution phase maps are also the basis of of the investigation of spin dephasing in macroscopic field inhomogeneities. A phase map based method to optimize scan parameters in echo planar imaging was developed.
The concept of spins precessing at different frequencies can also be applied to the chemical shift between fat and water. In dynamic MR mammography the MR-signal of fat was used to improve visibility of small tumors adjacent to or embedded in fatty tissue.