FMRI is a technique that measures brain activity by detecting changes in blood flow and oxygenation levels in the brain in response to neural activity. It has both advantages and disadvantages compared to other brain imaging methods like PET. FMRI uses strong magnetic fields to detect the magnetic signal from hydrogen atoms in water molecules in the brain. Changes in the magnetic properties of oxygenated versus deoxygenated hemoglobin allow FMRI to detect brain activity based on the blood oxygenation level dependent (BOLD) response to neural activation.

1. FMRI

2. Further Readings
• FMRIBC, University of oxford Ed: FMRI: Int.
• Functional MRI: An Introduction to Methods, Peter Jezzard et all. 1-3 CHL.
• FMRI: Basic and Clinical Applications, Stephen Ulmer et all.
• MRI From A to Z: A Definate Guide for Medical Professionals, Gary Liney
• Youtube has nice illustrative videos
• http://www.fmrib.ox.ac.uk/education/fmri/introduction-to-fmri/

3. Introduction
• A technique for measuring brain activity.
• How? Detects the changes in blood
oxygenation and flow that occur in response
to neural activity

4. Advantages & Disadvantages
• Pros:
1. Can see activation in addition to high res brain
structures
2. Scanners can be fitted to present stimuli
3. Higher spatial and temporal resolution than PET
• Cons:
1. Cannot trace neurotransmission like PET
2. Blood flow is, again, only an indirect correlate of
brain activity

5. MRI
• MRI houses a very powerful electro-magnet.
• Field strength of 3 teslas (T)
• The magnetic field influence atomic nuclei
• The stronger the field the greater the degree of alignment.
• When pointing in the same direction, the tiny magnetic signals
from individual nuclei add up coherently resulting in a signal that is
large enough to measure.
• In FMRI it is the magnetic signal from hydrogen nuclei in water
(H2O) that is detected
• The key to MRI is that the signal from hydrogen nuclei varies in
strength depending on the surroundings. This provides a means of
discriminating between grey matter, white matter and cerebral
spinal fluid in structural images of the brain.

7. Measuring FMRI
• Haemoglobin has oxygen, and increase neuronal
activity increases demand for oxygen rendering in local
increase of blood flow near the area of activity.
• Haemoglobin is diamagnetic when oxygenated but
paramagnetic when deoxygenated.
• Difference in magnetic properties leads to differences
in MR signal of blood depending on the degree of
oxygenation.
• Since blood oxygenation varies according to the levels
of neural activity these differences can be used to
detect brain activity [blood oxygenation level
dependent (BOLD) imaging]

8. One point to note is the direction of oxygenation change with increased
activity. You might expect blood oxygenation to decrease with activation,
but the reality is a little more complex.
There is a momentary decrease in blood oxygenation immediately after
neural activity increases, known as the “initial dip” in the haemodynamic
response. This is followed by a period where the blood flow increases, not
just to a level where oxygen demand is met, but overcompensating for the
increased demand. This means the blood oxygenation actually increases
following neural activation. The blood flow peaks after around 6 seconds
and then falls back to baseline, often accompanied by a “post-stimulus
undershoot”.