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yeni
Yeni Symposium 41 (3): 152-153, 2003 M
152
Functional MRI (fMRI) is currently one of the most widely used techniques for basic and clini-cal neuroscience. It is an extension of MRI that measures changes in cerebral blood flow and oxygen level that reflect localized changes in bra-in activity bra-induced by various tasks bra-includbra-ing mo-tor, sensory or cognitive. Unlike other MRI ima-ges used to form high resolution pictures of bra-in structure, the series for functional images are specifically tuned to contrast chemical changes
that are dependent to neural activity. The fMRI measures changes in blood flow and oxygen level by measuring small differences in MR signal.
FMRI studies are divided into two separate classes:
1. The contrast techniques, those that require the intravenous administration of a paramag-netic agent;
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* Uz. Dr., Ege University School of Medicine Department of Psychiatry, ‹zmir
Figure 2: The image of a 36 years old healthy woman, during a highly complicated mental (mathematical-working memory) task. Middle frontal (mainly left) and occipital cortices are active during the task
(1.5 T Siemens Magneton, Ege University School of Med. Neuroradiology Dept)
increased in blood flow
Decrease ratio: deoxy-Hb/oxy-Hb
Increased metabolic rate
Figure 1: A simplistic mechanism of changes in the blood oxygen level dependent
Increased MR signal Local vasodilatation localized increase in
neural activity Sensory, motor, or cognitive task
2. Noncontrast techniques, those that make use of endogenous physiological factors to detect changes in cerebral activation.
As far back as 1936, it has been known that de-oxy-hemoglobin is a paramagnetic factor in the blood though oxy-hemoglobin is magnetic. T2-weighted pulse sequences are generally used to detect changes in the local concentration of para-magnetic deoxy-hemoglobin. The latter method has been referred to as BLOOD OXYGEN-LEVEL DEPENDENT (BOLD) imaging. Thus, a decrease in the T2 decay rate, which is measured as an
inc-reased MR signal, is largely due to localized incre-ase in the ratio of oxy-hemoglobin to deoxy-he-moglobin a few seconds after increase in the ne-uronal activity in the region.
The basic principle of BOLD is largely depen-ding on neuronal activity related to increase in blood flow (Fig.1).
Increase in neuronal metabolism, by direct or indirect mechanisms causes a local vasodilatati-on. One should keep in mind that any vascular di-sease may have direct effect on local vasodilatati-on.
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