Characterization of biological tissue with magnetization transfer
Magnetic resonance techniques that exploit the transfer of magnetization between coupled spins in different relaxation environments may be used to investigate the intrinsic relaxation parameters of the individual spin environments as well as to describe the exchange characteristics of the system. This work was undertaken to investigate analytical models that have been proposed to describe magnetization transfer, leading to the application of these models to the characterization of biological tissue, particularly through the use of experimental techniques that may be applied in vivo. The observable effects of magnetization transfer are reflected in the steady state value of longitudinal magnetization following an appropriate preparatory pulse sequence. A technique employing pulsed off-resonance application of radiofrequency energy was used to obtain magnetic resonance images with magnetization transfer contrast. This technique was applied in animal models and human subjects in order to determine the effects on the observed signal of variation of the experimental parameters that define the imaging experiment. Equations for the steady state magnetization of spin systems in the presence of partial or complete saturation through continuous constant irradiation were derived, incorporating the assumption of the exchange of transverse as well as longitudinal magnetization. The inclusion of the exchange of transverse magnetization in the model implies that the exchange of magnetization may be reflected in the observed transverse relaxation rate, while this assumption has a negligible effect on the predicted value of the steady state longitudinal magnetization. Numerical simulations were carried out to compare the effects of pulsed and continuous irradiation. The results of these simulations suggested that pulsed magnetization transfer preparation is equivalent to continuous preparation as long as the average power deposited by the sequences is equal. The qualitative behavior of multi-site systems was investigated, predicting the existence of oscillations in the transient response of the longitudinal magnetization in systems with greater than two exchange sites. Two and three-site analytical models were fitted to experimental data obtained with pulsed preparation, allowing characterization of tissue with regard to the relaxation environment of individual spin pools through the use of spectra describing the variation of observed magnetization transfer effect with saturation frequency offset.
McGowan, Joseph Charles, "Characterization of biological tissue with magnetization transfer" (1993). Dissertations available from ProQuest. AAI9413877.