Structural MRI vs' Functional MRI

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Structural MRI vs. Functional MRI
Structural MRI reveals brain anatomy.
Functional MRI (fMRI) reveals brain function.
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History of MRI (previously NMR)
Edward Purcell1946 Showed that motion of
protons can interact with radio waves Felix Block
1952 Nobel prize in physics. Crucial insights
in Magnetism. NMR Images Nuclear a product of
spinning atomic nuclei Magnetic strong magnetic
field required Resonance synergistic behavior of
strongly magnetized protons and radio waves
Bloch
Purcell
Homsar
NMR ? MRI Why the name change?
Or maybe because NMR sounds like enema.
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Step 1, Magnetization Protons align with
magnetic field
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Step 2, Resonance Spinning Protons Boost Radio
Signal

• Resonance happens when
• Wave cycle proton spin cycle.
• Protons are at just the right angle for wave to
  get boosted.
• The more protons behaving as above, the more
  resonance.
• Spin behavior varies in different types of
  biological tissues.
• This is what allows imaging.

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Structural MRI Image
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Stacking the slices together for a 3D Image
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MRI vs. fMRI
MRI
fMRI
low resolution (3 mm per slice)
One full head image
high resolution (1 mm per slice)
fMRI Blood Oxygenation Level Dependent (BOLD)
signal indirect measure of neural activity

Series of many images (e.g., one full head every
2 sec for 5 mins)
? neural activity ? ? blood oxygen ? ?
fMRI signal
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A pretty good slide about the BOLD signal
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Another slide about the BOLD effect
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The Hemodynamic Response Function (HRF)
signal change (point baseline)/baseline usu
ally 0.5-3 initial dip -more focal and
potentially a better measure -somewhat elusive so
far, not everyone can find it
time to rise signal begins to rise soon after
stimulus begins time to peak signal peaks 4-6
sec after stimulus begins post stimulus
undershoot signal suppressed after stimulation
ends
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Typical fMRI ExperimentThe Blocked or
Boxcar Design
By using as the rest condition, any
active areas that are also used to perceive other
visual things will be subtracted out (e.g.,
primary visual areas).
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Activation Statistics
Functional images
Time
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Typical fMRI activations for reading words.
Subject J.P.
Subject M.A.
Areas of the left hemisphere activated by reading
words in 30second blocks. No subtraction.
Reading words minus looking at symbols.
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The Blocked Design
Cond1
Cond1
Cond1
Cond1
Cond2
Cond2
Cond2
Cond2
Subject does one task for about 30 seconds, then
another task for 30 seconds, etc. E.g. read
a list of words for 30s, look at symbols for
30s. Produces a relatively strong BOLD signal.
The Event-Related Design
Cond2
Subject does not have to repeat task several
times in a row.
Allows for more natural
test conditions.
Events
can occur randomly (unexpected).

Weaker signal. Signal is
strengthened by averaging together signals from
same condition.
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Event-Related fMRITesting words within in
sentences.
Sentence appears one word at a time .5 seconds
each word. Temporal Lag in the BOLD Response
2seconds
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ERP doesnt have lag. Effects are Instantaneous.
She noticed the banana that the
monkey had.
Scalp Potential
100 200 300 400 500 600 700 800 900
Milliseconds
banana
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ERP Source Localization

• Tomographic Estimation (LORETA)
• A method used in Dr. As lab.

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Banana
500ms