fMRI and long term memory Flashcards
what 3 key discoveries does MRI depend on ?
- computers (control and data analysis)
- superconductivity (high field strength magnet)
- nuclear magnetic resonance/quantum mechanics (absorption and release or electromagnetic energy)
how does MRI work?
electricity running through it creates a suer strong magnetic field, needs to made of material that is capable of sustaining this conduction. The magnetic field aligns the proton ‘spin’ amongst the water in the human body
2 safety concerns of the strong magnetic field
- projectile effect (biggest)
- movement of objects
what is the super conductor quench?
helium evaporation- cooled wit helium so that is it conductive but the if breaks it will stop conducting and helium will rapidly evaporate pushing air out of the room
6 safety concerns of MRI
- strong magnetic field
- superconductor quench
- changing magnetic fields (current)
- radio frequency coil (absorption rate)
- claustrophobia (the head coil)
- noise
energy at specific radio frequencies is absorbed and reemitted by nuclei with non zero spin
nuclear magnetic resonance
the amount of signal depends on the property of the tissue, what 3?
- protein density
- T2 decay (dephasing) - changes how the energy is released
- -> normally the H atoms all spinning and then they get oh out phase and then rephase together, the tissue type changes how fast this happens - T1 recovery (return to resting state)
what else does the signal depend on?
the timing of the RF pulse (TR) and data acquisition (TE)
for a given TR and TE, differences in proton density, T1 and T2 produce ______. 3 types
contrast
- proton density weighted
- T1 weighted
- T2 weighted
2 properties of oxygenated hemoglobin
- diamagnetic
2. zero magnetic moment
4 properties of deoxygenated hemoglobin
get a blurring image- interferes with the MRI signal
- paramagnetic
- significant magnetic moment
- greater magnetic susceptibility
- causes faster T2 decay
nearby blood vessels get about ___% wider. the blood vessels in the one tiny area of the brain where the neurons are firing
- this allows for?
25
- allows for more oxygenated hemoglobin to that part of the brain (flushes out deoxygenated)
reflect action potentials
multi unit activity and single unit activity
reflect post synaptic potentials
local field potentials
the 5 typical steps in fMRI preprocessing
- slice timing correction
- realignment
- coregistration
- normalisation
- smoothing
collecting an image from the whole brain but we dont collect it all at one time, we collect slices the time from which we collect at the top of the brain and the bottom of the brain is not the same - so we correct for this
slice timing correction
(all slices are collected at slightly different times)
the brain isnt staying in the same spot in the image so we have to correct for this.
realignment
- record how much we had to the brain to get it lined up again
6 parameter rigid body transformation (3 translations and 3 rotations) is the typical method for what?
realignment
after realignment you need to _____ to match ___
reslice , voxels
align functional images to structural image
- uses an affine transformation
- coregistration
wrap each subjects brain into a standard space using a linear combination of smooth basic functions
normalisation (each persons brain is a slightly different shape)
in normalisation, you segment into ______, ________ and _____, using tissue probablility maps and bayesian estimation
grey matter, white matter, CSF
deal with the remaining inconsistencies by ???
smoothing the images (blurring) (4, 7 or 10 mm FWHM)
- giving up pin point accuracy in able to maintain generalization
what is the goal for fMRI results?
learn which brain regions are more active in one experimental condition than in another
