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Abstract

Most macaque functional magnetic resonance imaging (fMRI) studies are based on monitoring the intrinsic blood oxygen level dependent signal (BOLD) or, alternatively, on measuring changes in cerebral blood volume (CBV) after injection of the intravascular contrast agent MION (monocrystalline iron oxide nanoparticles). In comparison to BOLD, imaging using MION results in higher contrast to noise ratios (CNR) (Leite et al., 2002; Vanduffel et al., 2001). However, the spatial specificity of MION is not well understood.
In order to directly compare the spatial resolution of BOLD versus MION, we conducted a series of experiments in the anaesthetized macaque monkey preparation (macaca mulatta) at a magnetic field strength of 4.7 T. We acquired data using 8-segment multishot EPI with flip angle = 40 deg, TE = 20 ms, TR = 805 ms, for both BOLD and MION experiments. We typically injected 8mg/kg MION which is known to give near optimal CNR (Mandeville et al., 1998; Vanduffel et al., 2001).
We measured the distribution of BOLD/MION as a function of gray matter depth in the primary visual cortex (V1) during stimulation with a full-field rotating polar checkerboard pattern alternating with uniform illumination (blank). Functional activation for MION peaked deeper in gray matter compared to BOLD, and appeared to sometimes even extend into white matter.
To compare the extent of spatial activation of BOLD versus MION along the cortical surface, we used a rotating polar checkerboard pattern containing a 3.7○ diameter blank area we refer to as an ¨artificial scotoma¨. Functional activity spread inside the cortical area corresponding to the artificial scotoma (where there is no visual stimulation) appeared to be markedly greater for MION than for BOLD. On-going experiments aim to directly compare the fMRI findings to single unit responses inside the artificial scotoma.