Basics of Functional Imaging Flashcards
Which came first? Structural or Functional?
Structure gave pictures.
Functional came hundreds of years later.
Angelo Mosso 1891
Looked at individuals with exposed brain from war. Noticed a change in exposed brain tissue - pulsating and changed in colour depending on task.
Built set of scales. Knew something to do with blood floor. More blood flow would tip the scales to see if he could measure weight of blood flow change.
Pre-neuroimaging. Mosso’s human circulation balance.
Early imaging of structure
Weak xrays that could not penetrate the skull.
Dandy 1919 - Genrate contrast within head to see if they could go through the skull - Pneumoencephalography. Replace entire CSF with air then gas. Used till 1970’s!
Early imaging of function
First human recording EEG recording - Berger 1924.
1970’s imaging
Structure - First CT scan - 1971
Function using PET - basic.
Structural MRI
Functional MRI
Difference in quality. Blurry raw data for fMRI and distorted - makes many artifacts.
Structural really high image quality.
Use same hardware in different ways.
Through different programs called pulse sequences.
MEG
MagnetoEncephaloGraphy - measures tiny changed in membrane repolarising.
Liquid helium on top to cool down detectors.
Detect on outside and mathematically see where it is coming from.
Not really clinically useful - could use EEG.
Expensive to buy and set up but cheaper to use. (£250/hour)
PET
Positron Emission Tomography.
Used a lot for cancer. Give radioactive label to glucose, inject into patient and see where it goes to in the brain.
Many risks and limitations.
Very expensive - cylcotron - radio activity. (£1500-£10,000/hour)
Only real technique to measure at receptor levels but MRI becoming better so may be replaced.
fNIRS
Functional Near InfraRes Spectroscopy - optic fibres, shine light and measure amount of like being refracted or reflected. Children have thin skin on head so great for babies.
Infrared penetrated deeper than normal light.
From 80’s but not as advanced and now increasing use - making more portable.
Lower expense.
fMRI
Functional Magnetic Resonance Imaging - most prominent technique used.
Mid 90’s.
Good for spatial resolution. Small voxels. Every second measure. Relatively slow - good for cognition measuring.
Higher expense with running, purchase and running.
(£500/hour)
It’s a contrast technique. Not absolute measurements.
EEG
ElectoEncephaloGraphy - Every millisecond. Very spatially competent.
Cheapest.
MRI timings
Functional needs to be quick so scan very fast. Can scan fast, cover different amounts of brain in terms of voxels.
Can get an image in 3 minutes. Can get thinner slices or thicker slices if you know why or where you need to measure.
In research normally always scan the whole brain.
BOLD
Blood Oxygenation Level Dependent effect.
Oxygenated bright red.
Deoxygenated blood is darker red.
Also have different magnetic properties.
Blood vessels - OXY not magnetic (diamagnetic)
DEOXY is magnetic (paramagnetic).
Deoxyheamoglobin perturbs the local magnetic field.
Brightness in fMRI image is linked to the level of local magnetic perturbation.
More magnetic perturbation = darker the image.
What is fMRI measuring?
Local increase in brain activity. More oxygen more glucose, needs ATP. to make AP’s and uses local pool of oxygen. More deoxyheamoglobin than at rest, more magnetic perturbation than at rest and gives a darker image than at rest.
Followed by larger increase of regional oxygen delivery than needed.
Local area flooded by oxyhaemoglobin. Meaning image brighter at rest from overcompensation of oxygen - 14/15 seconds for whole cycle.
fMRI processing stream
Raw data
Pre-processing - Head movement, background noise (electronic), physiological noise (heartbeat, respiration).
Subject level analysis
Group level analysis
