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Flat Maps
14
ured signal depend on the local environment of the protons, the measurements give informa-
tion about the type of tissue [Raichle, 2000 #23]. Magnetic gradient fields (often called B1)
can be used to code the location in all three spatial directions.
MRI is therefore a noninvasive in vivo medical imaging technique that uses various mag-
netic fields and radio frequency pulses. It can produce 2-dimensional images from a thin slice
through a body. It can also produce three-dimensional data sets. In contrast to computed to-
mography (CT), no ionizing x-rays are used. Hence, MRI can safely be used in brain research,
even with healthy subjects. MRI is very flexible since it allows controlling many parameters.
The soft tissue contrast of MRI is far superior to that in CT. For example, with the right set-
tings (suitable pulse sequence and suitable weighting among T1, T2, and proton density im-
ages), there is a good contrast between white and gray matter [Toga and Mazziotta, 1996 #24].
A more detailed introduction to MRI is beyond the scope of this report. The interested
reader is referred to the many texts that are available, for example [Dössel, 2000 #25]
[Hornak, 2000 #26] [Keller, 1988 #27] [Nishimura, 1996 #28] [Wright, 1997 #29].
3.4 Functional Magnetic Resonance Imaging
To measure functional data, functional magnetic resonance imaging (fMRI) can be used. The
most common fMRI technique is BOLD (blood oxygen level-dependent) fMRI [Seminowicz,
2001 #30]. One advantage of BOLD fMRI is that no contrast material has to be injected into
the subject [Ogawa, et al., 1992 #31]. Other techniques are not discussed in this report. How
does BOLD fMRI work?
The oxygenation of blood is increased in regions that are more active compared with
regions that are less active (see Figure 5). An increase of blood oxygenation results in an in-
creasing oxyhemoglobin-deoxyhemoglobin ratio. This effect is very important for functional
brain imaging because deoxyhemoglobin reacts like a magnet when placed in a magnetic field.
It is paramagnetic (a material that is slightly attracted to a magnet is called paramagnetic; low
positive susceptibility). The iron within the hemoglobin molecule causes this.
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