AMRI has continued its investigation into the mechanisms that generate functional and anatomical contrast in MRI scans. In susceptibility-weighted MRI a technique that provides unique contrast and high spatial resolution at high field, the major contributors to contrast between healthy brain tissues have been revealed and quantified, and AMRI has contributed substantially to this progress. Remaining uncertainties are 1) what is the distribution of susceptibility compounds on the cellular level;and 2) what is the molecular mechanism that creates exchange-induced NMR frequency shifts. Research is underway to investigate these issues. Specifically, experiments on chemical compounds are being performed to investigate the relationship between exchangeinduced frequency shifts and the classical magnetization transfer effect. Preliminary data suggest both effects are predominantly caused by hydroxyl rather than amide groups. In functional MRI, substantial signal variations originate from spontaneous brain activity, and AMRI has further investigated the the origin, role and characteristics of this activity. Specifically, a study with high density MEG found widespread spatial correlations in activity in specific frequency bands;the fluctuation frequency showed strong similarities to the frequencies dominating the spontaneous fMRI activity. In a collaboration with David Leopold from the Unit of cognitive Neurophysiology (NIMH), we found that these fluctuations are also present and widespread in intra-cortical recordings from awake monkeys. We are currently investigating whether the observed spontaneous activity represents cortico-cortical connectivity, or alternatively may be generated from a sub-cortical drive.
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