IRON fMRI measurements of CBV and implications for BOLD signal

MGH/MIT/HMS Athinoula A Martinos Center for Biomedical Imaging, Charlestown, MA 02129, USA.
NeuroImage (Impact Factor: 6.36). 01/2012; 62(2):1000-8. DOI: 10.1016/j.neuroimage.2012.01.070
Source: PubMed


Changes in cerebral blood volume (CBV) and blood magnetization each induce changes in the transverse relaxation rate of MRI signal that are associated with changes in cerebral activity. BOLD signal, the preeminent method for non-invasive localization of task-induced brain function in human subjects, reflects a combination of changes in CBV and blood magnetization. Intravenous injection of paramagnetic contrast media, usually iron oxide particles surrounded by larger macromolecules, can overwhelm the BOLD response and sensitize signal to blood plasma volume, a method we have deemed "IRON" fMRI. The practical advantage of this technique is the ability to optimize blood magnetization at any echo time, enabling high detection power and the use of short echo times; for these reasons, IRON fMRI has become a valuable imaging tool in animal models. The temporal response of blood plasma volume is quite different from blood flow and BOLD signal; thus, CBV has been identified as a prominent source of transient features of the BOLD response. This article reviews the methodological advantages of the IRON method and how CBV measurements have informed our understanding of the BOLD response.

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    • "First, we only imaged ~250 μm below the surface of the cortex, meaning that the entire depth of cortex we imaged over would be lumped together into a single 'upper layer' in an fMRI experiment, in which the contribution from intracortical vessels in the upper levels will likely overwhelm any blood volume changes due to pial vessels. In addition, the vascular density on the surface of the brain is ~3× higher than inside the brain, and the pial vasculature is almost entirely composed of large vessels (Tsai et al., 2009), which could cause greatly reduced sensitivity to pial vessel dilation when using supermagnetic particle fMRI techniques, which are sensitive to vessel size (Mandeville, 2012; S.-G. Kim et al., 2013a; J.H. Kim et al., 2013b). "
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    • "Although literature comparisons focus on preclinical results using the commonly employed IRON fMRI technique , the expectation is that the model calculations also will be applicable to future human studies based upon robust techniques like BOLD signal at very high field strengths, or the IRON method in standard clinical MRI scanners (Qiu et al., 2012). This model approach has been presented previously in preliminary form (Mandeville et al., 2012; Normandin et al., 2012a). "
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