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Signal Sources in Bold Contrast FMRI

Chapter
Part of the Advances in Experimental Medicine and Biology book series (AEMB, volume 413)

Abstract

When a non-diffusible paramagnetic contrast agent is present in the blood, the mechanism of MRI signal loss is as follows. The contrast agent has a magnetic susceptibility higher than that of blood or brain tissue, and thus creates magnetic field inhomogeneities within and surrounding the vessels in which it passes. Gradient-echo imaging is highly sensitive to such quasi-random non-uniformities of the magnetic field, by allowing the dephasing effect of the magnetic field inhomogeneity to accumulate over a period of 40–80 ms before data acquisition. Within a voxel containing blood vessels the nuclear spins experience spatially varying magnetic fields, and consequently precess at different rates depending on position. The result is a loss of phase coherence within the voxel, and thus a decrease in signal relative to when the contrast agent is absent.

Keywords

Magnetic Field Inhomogeneity Oxygenation Change Human Visual Cortex Cranial Window Ocular Dominance Column 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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Copyright information

© Springer Science+Business Media New York 1997

Authors and Affiliations

  1. 1.Wellcome Department of Cognitive NeurologyInstitute of NeurologyLondonUK

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