fMRI Flashcards
front of the brain
anterior/rostral
top of the brain
superior/dorsal
back of the brain
posterior/caudal
bottom of the brain
inferior/ventral
what is the middle of the brain called?
medial
how are the sides of the brain called?
lateral
how can you name brain regions?
anatomical terms
functional terms
how is the middle frontal gyrus named?
anatomical term
how is the dorsolateral prefrontal cortex named?
functional term
what can MRI be divided into?
structural MRI and functional MRI
what does structural MRI measure?
structural properties of the brain:
grey matter
white matter
fibre tracts
what does functional MRI measure?
functional properties of the brain
what does functional MRI provide?
indirect measure of brain
activity
what are the main parts of a MRI scanner? (5)
powerful magnet, RF coil, Computer and Monitor, RF generator and receiver, gradient coils
4 steps of MRI
no magnet: atoms are randomly positioned
magnet on: mostly aligned
radio pulse: moved orthogonally
protons relax: release radio signal
what happens in T1 relaxation time?
Proton spins will ‘relax towards their ’non-tilted’ alignment
what happens in T2 relaxation time?
Proton spins will lose synch and become out of phase
different tissues have …
different amounts of water – different magnetic properties – different relaxation time
What is TR (repetition time)?
The time it takes to scan one volume of the brain
basis for functional MRI
- oxygen from lungs
- oxygen bonds to hemoglobin
- oxygen released to tissue cells
what ae most fMRI studies based on?
the BOLD signal (Blood Oxygenation Level Dependent)
what does BOLD stand for?
Blood Oxygenation Level Dependent
what do metabolic changes in nerves require?
energy
what is metabolism based on?
oxygen consumption, i.e. increased neural activity causes an increased demand for oxygen
what does the drop of oxygen level lead to?
an increase of the local blood flow
what does the compensatory mechanism do?
relative concentration of oxygenated hemoglobin increases
how can oxygenated and de-oxygenated hemoglobin be differentiated?
oxygenated and de-oxygenated hemoglobin have different magnetic properties
5 steps of fMRI
1) change of neuronal activity
2) oxygen consumption (initial decrease in oxygen level)
3) increase of local blood flow (increase of oxygen level)
4) changes of the magnetic properties of hemoglobin (causing local inhomogeneities in the magnetic field)
5) change in signal intensity
BOLD response over time (3)
1) initial dip
2) main BOLD response
3) post stimulus undershoot
what time span is the TR usually?
1 or 2 seconds
how often does a volume happen?
every 1 or 2 seconds
what is a volume?
scan of the whole brain
what is the time series?
the sequence of all the scans
what are the problems with motion?
- if the participant moves, voxels change their position
- movement can cause artefacts if they are correlated with the design (e.g. when the participant
always moves in a specific condition)
what is the solution to the motion problem?
Rigid-body transformation
-> Different images are re-aligned to a reference image
what is co-registration?
alignment and overlay of functional image and structural image
what is Spatial normalization?
warp images from different subjects on a template brain -> averaged activation
what is smoothing?
Spatial smoothing replaces the signal of each voxel with a weighted average of that voxel’s neighbors
what are the advantages of smoothing?
- While spatial smoothing decreases the spatial resolution of the measurement it reduces noise.
- Furthermore, it makes averaging of functional activation across participants easier because it increases
potential spatial overlap.
what os statistical modeling of fMRI data?
time series of activation in a specific voxel
what can the BOLD response be modeled with?
a specific response function, the so called HRF (Hemodynamic Response Function)
what does HRF stand for?
Hemodynamic Response Function
what does bold overlap refer to?
In some experimental designs, the BOLD response to specific stimuli might overlap.
what does contrast refer to?
one can compare specific conditions and test whether there is a significant difference between the
conditions in a specific brain area
what is the block design?
the same stimulus type is presented repetitively in blocks
what are the advantages of block design?
- gives a strong signal-to-noise ration
- stimuli can be presented in rapid succession
what are the disadvantages of block design?
- psychological not always possible
- if trials are longer, the advantage of blockdesigns
vanish
what are slow event-related designs?
stimuli are presented with a long inter-trial interval to allow the HRF to
return to baseline
what are the advantages of slow event-related designs?
- Experimental conditions can be randomly mixed
- HRF for each condition and even each stimulus can
be determined
what are the disadvantages of slow event-related designs?
- Inter-trial intervals are very long
- Participants might get bored
what are rapid event-related designs?
stimuli are presented with short inter-trial intervals that do not allow the HRF to return to baseline
what are advantages of rapid event-related designs?
- Experimental conditions can be randomly mixed
- HRF for each condition can be determined
- Stimuli can be presented fast
what are the disadvantages of rapid event-related designs?
- The HRF strongly overlaps therefore one can not
easily determine the HRF for each condition and
event
