Mapping Brain Activity in Humans using Structural and Functional MRI Flashcards
Name the 3 types of MRI and what they show
Structural - gross brain anatomy
Diffusion - white matter tracts
Functional - brain activity
What are the 2 acquisition sequences of structural MRI and how do they colour different tissues?
T1-weighted - CSF black, blood white, fat white
T2-weighted - CSF white, blood black, fat black
What is the difference between T1 and T2-weighted structural MRI in terms of the interference of H+ ion magnetic fields with each other during realignment?
T1 - recover longitudinal component
T2 - lose transverse component - go to 0 in transverse plane
What are the steps of structural MRI image analysis?
Remove skull and CSF from image
Divide brain into white/grey matter
Divide brain by structures
Align image to others to compare
What can structural MRI be used to measure?
Between grey and white matter boundaries - cortical thickness
Sub-cortical structure shape changes
Cortical thickness changes
Grey matter changes
What are the two types of water diffusion and where do they occur?
Isotropic - equal movement in all directions - grey matter
Anisotropic - preferred movement direction - parallel to axon tract direction - white matter
What are the uses of diffusion MRI?
Tractography - traces anatomical connections - see changes in white matter tracts
Probabilistic tractography - estimate most likely fibre orientations for every voxel - form sets of paths - connection probabilities between areas
Connectivity-driven segmentation - segment seed region - follow connections to other regions based on highest target probability - segment target regions - pair with connected seed regions
What are the disadvantages of diffusion MRI?
Does not measure single fibres - only average fibres within group - crossing/kissing fibres not continuous tract but create loss of signal in voxel
Relies on estimating quantitative local measurements - not connections
How can the magnetic properties of deoxy-Hb be described and what is the effect of this?
Paramagnetic
Creates local distortions in magnetic field - causes spins of surrounding water molecules to interfere with each other - decreases signal
How can the magnetic properties of oxy-Hb be described and what is the effect of this?
Diamagnetic
No distortion of magnetic field - increases signal
What are the stages of the haemodynamic response function?
Initial dip due to oxygen metabolism - time for response by vasodilation
Increases oxy-Hb - positive BOLD response
Overshoot then steady state
Remove stimulation - post-stimulus undershoot (unclear why)
Return to baseline
What are the disadvantages of fMRI?
Indirect measure of neuronal activity
Low spatial resolution - as fast acquisition needed
Excitatory and inhibitory neuronal activity metabolically demanding - cannot distinguish between
What are the 3 main macroscopic changes across brain development?
Increased brain size
Decreased white matter brightness from early third trimester - increased myelination
Increased cortical folding
How does the Haemodynamic Response Function change over development?
Decreased latency - from pre-term to term to adult
Increased magnitude - in adult compared to pre-term and term - adult neurovascular coupling tighter
How does the proprioceptive functional activity change over development?
Wrist movement
Pre-term infant - only activity in wrist area of contralateral somatomotor cortex
Adult - bilateral somatomotor cortex activation - due to growth of interhemispheric connections
