Journal
MAGNETIC RESONANCE IMAGING
Volume 28, Issue 8, Pages 1234-1243Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.mri.2010.01.005
Keywords
Astrocytes; BOLD; Brain; Cell swelling; Contrast mechanisms; fMRI; Proton-density; SEEP; Spinal cord
Funding
- International Spinal Research Trust (UK)
- Canada Research Chairs
- National Sciences and Engineering Research Council of Canada
- Craig H. Nielsen Foundation
- Canadian Institutes of Health Research Canada
- Queen's University Faculty of Health Sciences Harry Botterell Foundation
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Despite the popularity and widespread application of functional magnetic resonance imaging (fMRI) in recent years, the physiological bases of signal change are not yet fully understood. Blood oxygen level-dependant (BOLD) contrast - attributed to local changes in blood flow and oxygenation, and therefore magnetic susceptibility - has become the most prevalent means of functional neuroimaging. However, at short echo times, spin-echo sequences show considerable deviations from the BOLD model, implying a second, non-BOLD component of signal change. This has been dubbed signal enhancement by extravascular water protons (SEEP) and is proposed to result from protondensity changes associated with cellular swelling. Given that such changes are independent of magnetic susceptibility, SEEP may offer new and improved opportunities for carrying out fMRI in regions with close proximity to air tissue and/or bone tissue interfaces (e.g., the prefrontal cortex and spinal cord), as well as regions close to large blood vessels, which may not be ideally suited for BOLD imaging. However, because of the interdisciplinary nature of the literature, there has yet to be a thorough synthesis, tying together the various and sometimes disparate aspects of SEEP theory. As such, we aim to provide a concise yet comprehensive overview of SEEP, including recent and compelling evidence for its validity, its current applications and its future relevance to the rapidly expanding field of functional neuroimaging. Before presenting the evidence for a non-BOLD component of endogenous functional contrast, and to enable a more critical review for the nonexpert reader, we begin by reviewing the fundamental principles underlying BOLD theory. (C) 2010 Elsevier Inc. All rights reserved.
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