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Zusammenfassung:
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 a contrast agent. The intravascular contrast agent MION (monocrystalline iron oxide
nanoparticles) has been recently applied in a number of studies (Dubowitz et al., 2001; Leite et al., 2002;
Mandeville et al., 1997; Mandeville et al., 1998; Mandeville and Marota, 1999; Tsao et al., 2003a; Tsao et al.,
2003b; Vanduffel et al., 2001). In comparison to BOLD, imaging with MION results in higher contrast to noise
ratios (CNR) (Leite et al., 2002;Vanduffel et al., 2001) which may provide improved fMRI sensitivity, making
MION especially appealing at low to moderate magnetic field strengths. Moreover, it has been suggested that
MION may result in improved spatial specificity, since it appears to arise primarily from small parenchymal
vessels (Mandeville et al., 1998; Mandeville and Marota, 1999) as opposed to BOLD which, at low magnetic
fields, is known to be influenced by larger vessels that run along the cortical surface (Gati, Menon, et al., 1997).
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, Marota, et al., 1998; Vanduffel, Fize, et al., 2001).
In a first set of experiments we measured the distribution of BOLD/MION as a function of gray matter depth in
macaque primary visual cortex (V1). Stimulation was done by alternating a rotating polar checkerboard with a
field of uniform light intensity. Voxel size was 0.2 x 0.2 x 1mm 3 . Correlation coefficients were computed voxel
by voxel and then averaged for voxels lying at the same cortical depth. Figure 1 plots the average correlation
coefficients for BOLD (blue) versus MION (red) as a function of depth in the primary visual cortex (V1) of one
macaque. Note that the correlation coefficients for MION appear to peak deeper in gray matter as compared to
BOLD.
To compare the spatial extent of functional activation seen with BOLD versus MION along the cortical surface,
we used a similar stimulation paradigm as before, except that now the rotating polar checkerboard pattern
contained a 6 deg diameter occluder (artificial scotoma) centered at 6 deg from the fovea. This assured that the
corresponding cortical area in V1 received no direct visual stimulation. Figure 2 shows the correlation maps
obtained with BOLD and MION under these stimulation conditions in a slice through the left V1 of one macaque
(voxel resolution: 1 x 1 x 2 mm 3 ). Note that the spread of functional activity into the cortical area corresponding
to the artificial scotoma (i.e. where there is no visual stimulation) appears to be markedly greater for MION than
for BOLD.
