Lecture 6: Fundamentals of fMRI - Part 1 Flashcards
What does fMRI stand for?
- functional Magnetic Resonance Imagining
MRI is tuned to detect changes in
blood flow that are associated with neural activity
“functional” imaging allows us to begin to understand what
different parts of the brain do when we perceive, think, remember, understand, act etc…
fMRI complements other neuroimagning methods?
Yes
fMRI is non-invasive and applicable to healthy humans?
Yes
To resupply oxygen and nturients, blood flow increases to an active region flooding it with fresh
oxygenated blood
Whats blood-oxygenation dependent (BOLD) contrast?
Differences in signal on T2* weighted images as a function of the amount of deoxygenated Hb
Fresh oxygenated blood generates a stronger ….. than stale dexoygenated blood
T2
What does the BOLD contrast depend on? - (2)
total amount of deoxygenated hemoglobin present in brain region which in turn depends on balance between oxygen consumption and oxygen supply
the former is dependent
on neural activity and the latter is dependent on blood flow.
If the amount of deoxygenated Hb increases locally, the …
BOLD signal decreases
If the amount of deoxygenated haemoglobin decreases locally, the…
BOLD signal increases
What does BOLD stand for?
Blood Oxygen Level Dependent
BOLD ssignal is a key ….
DV in almost all fMRI research
The BOLD signal is correlated with
neural activity
Charles Coryell and Linus Pauling disocvered that the
haemoglobin molecule has magnetic properities that differ whether it is bounded to oxygen or not
Oxygenated Hb is diamagnetic meaning
it has 0 magnetic moment
Dexoygenated hemoglobin dHb is paramagnetic meaning
it has unpaired electrons and a significnat magnetic moment
Deoxygenated and fresh oxygenated blood have different magnetic properities such that - (2)
- Fresh oxygenated blood has less signal decay due to T2 effects and remains brighter in T2* images as compared to deoxygenated blood and takes longer for signal to die away
- So brighter voxels in part of brain that is supplied with fresh blood and that fresh blood is BOLD signal
What does this diagram of Thulborn’s study show? - (2)
- MR pulse sequences sensitive to T2* should show more MR signal where blood is highly oxygenated and less MR signal where blood is highly deoxygenated.
- At low field strength (i.e., less than 0.5 T), there was little difference between transverse relaxation values (T2) for oxygenated blood and deoxygenated blood but in higher fields (i.e., 1.5 T or greater) , their values differed significnatly.
- So stronger magnetic fields necessary for MR imagining of T2* Weighted contrast in blood.
What does increased neuronal activity lead to? - (4)
- increases in blood flow and supply of oxygen that exceed oxygen demand
- As excessive oxygenated blood flows through active regions, it flushes the deoxygenated hemoglobin from the capillaries supporting the active neural tissue and from
the downstream venules. - This process is consistent with the experience of
neurosurgeons, who have long observed regions of the brain “pinking up” (due to the red color of oxygenated hemoglobin) in response to stimulation. - Lead to increased BOLD signal in area
Increased BOLD signal following neuronal activity occurs not because ….. but because… - (2)
oxygenated Hb increases MR signal
but because oxy Hb displaced the deoxygenated HB that has been suppressing MR signal intensity
Summary of BOLD signal generation - (2)
- (A) Under normal conditions, oxygenated hemoglobin is
converted to deoxygenated hemoglobin at a constant rate within the capillary bed. - (B) When neurons become active, however, the vascular system supplies more oxygenated hemoglobin than
is needed by the neurons through an overcompensatory increase in blood flow. The result is a decrease in the amount of deoxygenated hemoglobin and a corresponding decrease in the signal loss due to T2* effects, leading to
a brighter MR image.
Increased BOLD signal following neuronal activity occurs due to oxy Hb displaces deoxygenated Hb that suppress MR signal intensity
As a result, increased neuronal activity increases
the signal of T2* images and results in positive BOLD signal
What does this diagram show? - (5)
- Rat’s somtatosensory cortex is being simtulated indirectly by stimulating a neuron that activates the cortex
- As you stimulate the cortex, you can record an increase in blood velocity (speed at which blood flow increases) going from 30 to 60 arbitary units , doubling, a short period after neural activiy begins
- It declines over time but it remains steadly increased
- What happens at same time is aertial diameter is increasing so blood vessels supplying oxygen to that part of active brain are dilating to let more blood to go to that area - going from 35 to 50 arbitary units
- Rate of blood flow/velocity and aertial diameter increases when area is active to let more oxy Hb in but (Not in diagram) but blood pressure stays the same when area is active