This thesis investigates the combination of Nuclear Magnetic Resonance (NMR) and Ultrasound (US), referred to as Acoustic NMR (ANMR), to modulate longitudinal relaxation rates (R₁) in aqueous solutions of superparamagnetic iron oxide nanoparticles (SPIONs). By enabling the modulation of relaxation rates, ANMR could serve as a promising technique for low-field MRI. The aim of this work is twofold. Firstly, a theoretical model is developed to describe the dynamics of SPION clusters under the influence of US waves to estimate the effect of rotational and translational motion on the fluctuation in the local magnetic field. Secondly, experimental ANMR measurements are conducted on three aqueous SPION suspensions with particle diameters of 50, 130, and 300 nm to investigate the effect of particle size on the longitudinal relaxation rate.

The model predicts that translational motion of SPION aggregates contributes more significantly to the longitudinal relaxation rate than rotational motion. The modeled spectral density confirms a distinct peak at the US driving frequency, suggesting that the SPION clusters exhibit resonant magnetic field fluctuations with the Larmor frequency. Experimental ANMR results show no significant change in the longitudinal relaxation rate for all three SPION solutions, indicating that the delivered acoustic pressure is potentially insufficient. By overcoming current experimental limitations, ANMR holds great promise as a novel contrast mechanism for low-field MRI, potentially enabling localized contrast enhancements.