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fMRI: functional (nuclear) magnetic resonance imaging
Neuroimaging: get the structure of the brainWant to know how it works: connection brain
parts and brain functionsAim: measure the local „thinking activity”
Usage (criticism):Lie detectorNeural- and psychological-modell checking
(think/know experiment)
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How we use it
Human attempts frequent Well-planned tasks or questions Measures: order of minutes
One measure: order of 5 secondsMeasure with and without tasks or question,
further investigation based on the difference
Overlap the intensity map and brain image
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The basics of fMRI
MRI: interaction between spins and magnetic field
QM based phenomenaClassical view is almost satisfactory
Find a „think-activity”-sensitive MRI measureable quantity, measure it, and then reflect to think activity
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Get the signal - BOLD
Blood-oxygen-level dependence Hemoglobin: Fe2+ can absorb O2
Hemoglobin + O2 : diamagnetic molecule
Hemoglobin – O2 : paramagnetic (S=2)
Measure the oxygen-flow differencies in vein
Determine the connection between BOLD singal and toughts
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Get the signal – O2 flow
The oxygen flow depends on the communicating intenstiy
Communication needs energyNeuron cells don’t have repositoryIncreased activity needs more energyBOLD signal decreasing and then
increasing
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Time, accuracy and resolution Time: depends on the BOLD signal,
about less than half a minuteDo experiments with the same patient, same
time
Accuracy: easily detect maximum of BOLD
Space: resolution: in order of mm×mm×mmProblem: the motion of the patient
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Increasing space resolution No new information with increasing resolution
Signal comes from multiple capillars Solution: BOLD signal minimum: more
localised Longer time or Bigger magnetic field
No (?) news: cerebral tasks are not well localised
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Temporal resolution
Increasing time resolution does count (more details) usable image Two tricks to improve:
Spin echo – gradient echoEPI: echo planar imaging
x y z t const × × × »
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Measurement: EOM
First: apply Short RF pulse:
( ) ( )0ddt = ´ + -M M B M M τ
( ) 01
zz z M MdMdt T-
= ´ +M B( ) { }/ ,2i iidM M T i x ydt = ´ - ÎM B
( ) ( ),, , , ,0 0 00 0 0 0zB M= Þ =B M( ) ( ) ( )( )sin ,cos ,1 0t B t t =B
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Measurement: relaxation and decoherence Solutions
after„excitation”:
We can measure
Important: New rotating cylindrical coordinates with
frequency:
iM( )/ 10 1 t TzM M e-= -
/ /0 2 20 0 Li B t t T i t t TiM M e M e - -= × = ×
1 2 2 1T T T T¹ >
0L B =
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Measurement: spin-echo
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Measurement: spin-echo
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Measurement: spin-echo
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Measurement: slice-excitation
( )
( )( ) ( )
( )
*
*
,
000 1
zz
ii B G zii B i G zt
dM z tdt t eM B t e
+ ×
- ×
=
= ´ ×
=
M B( )
( )*
,
0 1 zr
i G z tdM z tdtM B t e - ××
=
= × ×
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Measurement: slice-excitation and spin-echo Find for the desired excitation-
distribution:( )*1B t
( ) ( )* sin /1 1 zB t B x x x G d t= = ×
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Measurement: another way, the gradient echo
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Measurement: EPI
Echo Planar Imaging: increase time resolution even more!
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What we get: images
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An experiment: caffeine
caffeine as a contrast booster
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Limits
We cannot answer any why question – only answer the question where
The brain is 3D – the image is 2D, so more experiments needed
We can measure only the neurons firing, but not the real actyvity ( block or stimulate?)
High degree of cerebral plasticity: places may vary
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Mind reading The map of optic nerves to vissual cortex is
so „localised”, „continous” Training set: known videos and those fMRI
signal Unknown video: fMRI signal → video
images Ability to guess what patient think Numerical problem Further aim: movement of implants Success: camera-eye
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http://sites.google.com/site/gallantlabucb/publications