The hydrodynamics of magnetic fluids, often termed ferrofluids, has been an active
area of research since the mid 1960s. However, it is only in the past twenty years
that these fluids have begun to be used in magnetic resonance imaging (MRI) where
they have found application as contrast agents, improving the contrast to noise ratio
of the received MR intensity.
The major goal of this work was to assess the feasibility of modulating the signal
intensity in MRI in regions near superparamagnetic iron oxide contrast agents by
applying a rotating magnetic field. Analytical study and numerical simulations of
the effects of such a rotating magnetic field are presented. Simulated variations in
fluid magnetization are achieved by changing the frequency of the rotating magnetic
field. A linearization of the equilibrium magnetization Langevin relation results in a
small signal magnetic susceptibility tensor that can be used to influence fluid flow.
The feasibility of the approach under the real-life constraints of MR imaging are
assessed. Also examined is the potential for coupling changes in magnetization due
to rotating fields with hyperthermia treatment of cancerous tissue.
In addition to theoretical and simulated analysis, considerable experimental work
was undertaken at the MRI systems of the HST Athinoula A. Martinos Center for
Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts.
This work evaluated the passive behavior of both commercial ferrofluids and MRI
contrast agents. The ability of MRI to serve as a highly accurate indicator of the
fluid┐s physical and magnetic properties is shown. The thesis represents the first
investigation into the use of the hydrodynamic properties of magnetic fluid in the
presence of a rotating field as a mechanism for contrast in MRI. The results show the
possibility of varying MR image intensity under careful selection of field conditions.