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fMRI User Guide
2 Revisiting the Bold Effect
fMRI uses the BOLD effect to detect neuronal activation. Let’s review the basic facts:
• Oxygen is delivered to the cells attached to hemoglobin
• Oxygenated hemoglobin is diamagnetic – very small magnetic moment
• Deoxygenated hemoglobin is paramagnetic – has a significant magnetic moment
On activation, stimulated tissue undergoes an increase in blood flow. This in turn:
• Increases the delivery of oxygenated hemoglobin
• Decreases the amount of deoxygenated hemoglobin within the tissue, thus reducing the concentration of
paramagnetic species
• Reduces the amount of susceptibility induced dephasing and therefore increases the T2* of the stimulated
tissue vs. the unstimulated tissue
• To detect these activations, brain activation studies are performed using T2* weighted sequences
• On a T2* image, the stimulated tissue will then have a brighter signal vs. the unstimulated tissue
Figure 1. Signal response from stimulated and unstimulated tissue. Increased blood flow and oxygenation
increases T2, and thereby the signal intensity in stimulated regions.
Signal
TE Time
Unstimulated tissue
Stimulated tissue
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