Sensitivity of feedback-enhanced MRI contrast to macroscopic and microscopic field variations.

Nonlinear feedback interactions have been shown to amplify contrast due to small differences in resonance frequency arising from microscopic susceptibility variations. Determining whether the selectivity of feedback-based contrast enhancement for small resonance frequency variations remains valid even in the presence of macroscopic field inhomogeneity is important for transitioning this new methodology into in vivo applications in imaging systems with lower field strengths and poorer homogeneity. This work shows that contrast enhancement under the radiation damping (RD) feedback field is sensitive to microscopic intravoxel frequency variations. Feedback-enhanced contrast provides superior signal differentiation from voxels with distinct microscopic frequency distributions compared with T(2)*-weighted imaging, while remaining robust to macroscopic field gradients, which frequently give rise to artifacts by other frequency-sensitive methods. Applying multiple RF pulses during evolution under RD and actively adjusting the phase and amplitude of the feedback field are shown to further improve signal differentiation. Experimental results reveal that feedback-enhanced contrast can generate positive contrast, reflecting microscopic field variations induced by strong local dipole fields, such as those created by blood vessels and superparamagnetic iron oxide nanoparticles. Extensions to in vivo imaging at lower field strengths are discussed in the context of amplifying the RD field via active electronic feedback.