fMRI/fNIRS/structural MRI Flashcards
what are the brain lobes ?
frontal lobe; parietal lobe; occipital lobe; temporal lobe; limbic lobe
what are the main gyri and sulci?
superior precentral sulcus; central sulcus; superior frontal sulcus; inferior precentral sulcus; intermediate frontal sulcus, inferior frontal sulcus.
precentral gyrus; superior frontal gyrus; middle frontal gyrus; inferior frontal gyrus
what does structural MRI measure?
measures structural properties of the brain
- grey matter
- white matter
- fibre tracts
How does an MRI scanner work?
it uses protons that are stimulated by a strong magnetic field to align with that field. After that a radio frequency pulse is introduced that forces the protons to either 90 or 180 degree realignment with the static magnetic fields (against his nature). As the radio frequency is turned off the proton realign with the static magnetic field releasing electromagnetic energy ⇒ differentiation of the tissues based on how slow the energy is released after the pulse is turned off.
What is measured with MRI?
different tissues have different amounts of water and therefore different magnetic properties and different relaxation time.
What is measured with MRI?
different tissues have different amounts of water and therefore different magnetic properties and different relaxation time.
How is the spatial localisation determined with MRI?
- through gradients ( = additional magnetic fields)
- 3 mutually orthogonal gradients are used to localise signal in three spatial dimensions
- y axis: phase encoding
- x axis: frequency encoding
- pixels = voxels.
What does functional MRI measure?
Most fMRI studies are based on BOLD signal (= blood oxygenation level dependent).
What are the metabolic bases of fMRI?
- metabolic changes require energy
- metabolism is based on oxygen consumption
- neural activity increase the demand for oxygen
- drop of oxygen levels leads to an increase of the local blood flow
- compensatory mechanism: relative concentration of oxygenated haemoglobin increases.
Oxygenated and de oxygenated haemoglobin have different magnetic properties which causes change in signal intensity.
what is TR?
TR = time required to process one image of the brain (normally 1/2 sec) ⇒ brain scan every 1 or 2 second. time series: sequence of all the scans.
What is motion correction useful for after fMRI and how is it performed?
- if the participant moves, voxels would change their position
- movement can cause artefacts
It is performed through rigid body transformation: different images are re-aligned to a reference image.
What is spatial normalisation?
it consists on wrapping images from different subjects on a template brain and the activation of brain regions is averaged.
what is the smoothing process?
it replaces the signal of each voxel with a weighted average of that voxel’s neighbours.
- spatial smoothing decreases the spatial resolution of the measurement but reduces noise.
- it makes averaging across participants easier because it increases spatial overlap.
What are the main statistical modelling used to interpret fMRI data?
- stimulus compared to a control
- BOLD response can be modeled with a specific response function (HRF: hemodynamic response function)
- BOLD overlap: one can determine the contribution of each experimental condition to the fMRI time series.
-
contrasts: compare whether there is a significant difference between two conditions in a specific brain area.
- Problem: Because this is done for each voxel (mass univariate analysis), one has to correct for multiple comparisons. Univariate analysis assumes that data in neighboring voxels have no relationship to each other.
what are the most common experimental designs using fMRI?
block design
the same stimulus type is presented repetitively in blocks
- gives a strong signal-to-noise ration
- stimuli can be presented in rapid succession
- not useful with longer trials
- not always psychologically possible
event-related designs
slow event related designs
stimuli are presented with a long inter-trial interval to allow the HRF to return to baseline.
- experimental conditions can be randomly mixed
- HRF for each condition and stimulus can be determined
- inter-trial intervals are long
- participants might get bored
rapid event related designs
stimuli are presented with short inter-trial intervals and do not allow for HRF to return to baseline.
- experimental conditions can be randomly mixed
- HFR for each condition can be determined
- stimuli can be presented fast
- HFR strongly overlaps for different conditions
