Selective positive contrast of subvoxel field-disturbers using off-resonance excitation

Introduction: Positive contrast by means of selective excitation is a helpful way of visualizing objects of clinical interest, like iron-labeled cells, paramagnetic agents or labeled therapeutics. In regular gradient echo imaging, these entities cause negative contrast due to intra voxel dephasing effects which cannot be distinguished from signal voids caused by other effects, and is subjected to partial volume effects. Positive contrast can be generated by selectively exciting the offresonance protons surrounding the paramagnetic material while suppressing the on-resonance background. This mechanism has been shown to be effective for e.g. iron-labeled cells [1] and gadolinium containing structures [2,3]. However, in these cases, contrast was based on macroscopic effects from relatively large clusters of paramagnetic particles occupying several voxels. In this work we will show that the same mechanism is also effective on a microscopic scale, i.e. we will show that it is feasible to selectively obtain signal from protons that reside in the vicinity of Holmium-166 loaded microspheres, used for internal radiation therapy [4].