Reduced data requirements for phase-contrast MR angiography

grantor: University of TorontoA class of magnetic resonance angiography (MRA) methods, called time-of-flight (TOF), seems to be preferred over a second class of methods called phase-contrast (PC), even if the latter provides a greater suppression of static tissues surrounding blood vessels. A major reason for the lack of popularity of PC methods is the significantly longer imaging time required in PC angiography. In this thesis, it is recognized that a large portion of the information acquired in PC MRA is either redundant, or simply discarded. This suggests that faster PC methods could be designed if one can avoid acquiring unnecessary information. PC methods having reduced data requirements are developed in chapters 2 and 3; these methods achieve such a reduction by encoding velocity information in both the phase and the magnitude of a voxel, instead of using only the phase, as in conventional PC MRA. The basic idea that allows the encoding of velocity information in both phase and magnitude is very simple: if two complex numbers are added together, the phase of one of them affects both the phase and the magnitude of the sum. As in conventional PC MRA, velocity information is encoded into the phase of the magnetization (which is represented by a complex number). However, unlike conventional PC MRA, the methods introduced in chapters 2 and 3 generate a sum of complex numbers, forcing the velocity-induced phase shifts to be encoded in both phase and magnitude in the resulting image. In chapter 2, this sum occurs as ghosts of different orders are superposed in the object domain. In chapter 3, the sum occurs as echoes having different flow sensitizations overlap in the same acquisition window. It is recognized in this thesis that there are degeneracies in the acquisition process, in phase-contrast MR angiography. The methods presented in chapters 2 and 3 prove that these degeneracies are not a fundamental limitation of the phase-contrast approach, since they can be partly eliminated.Ph.D