Karin
Shmueli1, Steve Dodd2, T-Q Li3, Jeff H. Duyn1
1Advanced MRI Section, Laboratory of
Functional and Molecular Imaging, National Institute of Neurological
Disorders and Stroke, National Institutes of Health, Bethesda, MD, United
States; 2Functional and Molecular Metabolism Section, Laboratory
of Functional and Molecular Imaging, National Institute of Neurological
Disorders and Stroke, National Institutes of Health, Bethesda, MD, United
States; 3Department of Medical Physics, Karolinska Huddinge,
Stockholm, Sweden
Water-macromolecular
exchange has been proposed to explain brain gray and white matter frequency
(phase) contrast. We extended previous observations of exchange-induced
frequency shifts (fexch) in protein solutions by performing
chemical shift imaging experiments using reference chemicals (TSP and
dioxane) to observe positive fexch in fixed human and fresh pig
brain tissue. Substantial negative GM-WM δfexch was observed
which was similar for all tissues and references but opposite to in-vivo
GM-WM frequency contrast, implying that tissue magnetic susceptibility may
have a greater contribution. Exchange should therefore be included in
frequency contrast models but is insufficient to explain in-vivo GM-WM phase
contrast.