Mapping Macaque Visual Cortex Organization with BOLD and MION fMRI

Our research aims to use high field (4.7T) functional magnetic resonance imaging (fMRI) to map changes in cortical organization as a function of time after de-afferenting part of the primary visual cortex by inducing homonymous retinal lesions (Smirnakis et al., Neuroscience 2002). In order to obtain detailed maps of cortical organization by fMRI it is essential to use a strategy that maximizes spatiotemporal resolution. The contrast agent monocrystalline iron oxide nanoparticle (MION) has recently been used in the rat (Mandeville et al., Magn Res Med 1998,99) as well as in the awake behaving macaque (W. Vanduffel et al., Neuron 2001) to increase the sensitivity of fMRI imaging as compared to imaging based on the blood oxygenation level dependent (BOLD) signal. It is unclear, however, to what degree the advantage persists at higher field strengths, as well as whether the spatiotemporal profile of the MION (blood volume) induced signal provides adequate resolution to map cortical organization. Here we looked at the benefit of MION versus BOLD at 4.7 Tesla in the anesthesized macaque preparation (Logothetis at al., Nat Neurosci 1999). Visual stimuli of various sizes were presented in block design against background illumination, as well as retinotopic mapping was performed, with and without MION. Preliminary results suggest that MION invariably increased the sensitivity of the technique at 4.7T, boosting the modulation of the signal by a factor of 3-7 above that seen with the BOLD. The effect of MION on the spatial resolution is under investigation