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.
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Turner, R. (1997). Signal Sources in Bold Contrast FMRI. In: Villringer, A., Dirnagl, U. (eds) Optical Imaging of Brain Function and Metabolism 2. Advances in Experimental Medicine and Biology, vol 413. Springer, Boston, MA. https://doi.org/10.1007/978-1-4899-0056-2_2
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