14.3 BOLD and Magnetic Susceptibility

Blood, brain tissue, and HbO are diamagnetic while Hb and HbO are paramagnetic. A decrease in Hb concentration reduces the difference in intensity between blood and brain tissue. Consequently, the magnetic field becomes more uniform (causing the MRI signal to be affected).

14.3.1 Spatial vs. Temporal Resolution

Spatial resolution refers to how well an fMRI can dsicriminate between two locations. This is measured in voxels - full brain studies use specific regions while specific regions use smaller voxel sizes.

Temporal resolution is the smallest time period of neural activity based on fMRI. One element deciding this is the time resolution. The resolution needed depends on brain processing for various events.

14.3.2 T2* and fMRI

T2* is the exponential decrease in M_xy (i.e., signal strength) following the initial excitation pulse as a function of the time constant. In any real NMR experiment, the tranverse magnetization decays much faster than predicted - this rate is then termed T2*.

The T2* is an observed T2 - the first T2 can be considered as the natural T2 of the tissue being imaged.

T2* results from inhomogeneities in the main magnetic field - this may be a result of intrinsic effects in the magnet or from distortions made by surrounding tissues or from materials in the field.