Investigation of NMR properties of gel volume change with magnetic resonance imaging

Ferromagnetic or supetparamagnetic particles as MRI contrast agent present many advantages for bringing about soft tissue contrast as compared to single-ion complexes. Based on the dynamic frequency scale 1/τ and the magnetic frequency scale δω (perturber strength) of the system the relaxation behavior is categorized into five diffusion regimes. Two empirical models are proposed, one for the spin echo and the other the gradient echo sequence, to account for the relaxation dependence of such variables as sphere radius R, δω , and diffusion coefficient D in these regimes. The models are verified with the results of our spectroscopic measurements as well as simulations and experiments in the literature. Through proper scaling of the sphere radius and the relaxation rate normalized models are obtained, which maybe used to quantitatively estimate l/T2 for various combinations of the variables. The models are then extended to account for effects of sphere size change upon relaxation rate of surrounding spins. The predicted l/T2 are close to simulation points for small R but not for large R. Experimental approaches were used to investigate NMR physical properties of the interior of gel network for polyacrylamide gels. Longitudinal relaxation time T1, T2 , and D of proton spins trapped within polymer network decrease as gels reduce in size during the volume phase transition. The values fall shatply around acetone concentration of 30% to 40%, at 50% the gels appear to complete the phase transition. For gels in the contracted state T1 is one order and D and T2 less than two orders of magnitude smaller than when in the swollen state. The internal component therefore contribute very little as compared to the iii external component to the total relaxation, making this situation resemble to that of the impermeable polystyrene beads where our extended empirical models seem to apply well.U of I OnlyThesi