MRI - Olivia Harrison Flashcards
dendrites
openings for other cells to communicate
cell body
powerhouse, motor of the cell, energy production
axon
neurones main way of talking to other cells in other parts of the brain
myelin
fatty substance that covers the axons
axon terminals
branch onto other dendrites
grey matter
grey part of the brain, in a ribbon around the edge of the brain where the cell body sit
white matter
axons make up white matter, long projection, myelin makes that projection fast
what is the main difference in human and animal brain
animals is much smoother their not as deep folds, don’t have as many folds
what is cerebrospinal fluid important for
cushioning the brain, delivering nutrients and taking away waste products
Where and what are the ventricles
the holes located in the middle of the brain
T1-weighted structural MRI
very common, can see white matter grey matter and CSF
provides information about the relaxation times of tissues. It is widely used in clinical practice to assess the structural characteristics of various organs and tissues.
T2-weighted structural MRI
colours are flipped, highlights CSF
provides information about the relaxation times of tissues. It is commonly used in clinical practice to evaluate the structural characteristics of various organs and tissues.
Proton Density structural MRI
provides information about the density or concentration of protons in tissues
what are the cons of getting a really good quality image
more time consuming, longer patient has to lie completely still
FLAIR
specific type of magnetic resonance imaging (MRI) sequence that is particularly useful in assessing certain brain pathologies. It is a T2-weighted sequence that suppresses the signal from fluids, such as cerebrospinal fluid (CSF), while maintaining good contrast for other tissues.
WM nulled
specific type of magnetic resonance imaging (MRI) sequence that is designed to suppress the signal from white matter tissue in the brain. It is often used in neuroimaging research to selectively visualize and study other brain structures or pathologies without the interference of white matter signals.
Double Inversion Recovery (DIR)
ability to provide improved contrast and better visualization of gray matter structures. It allows for enhanced differentiation between different brain tissues, including gray matter, white matter, and CSF. This can be particularly helpful in detecting subtle gray matter lesions, assessing brain atrophy, and studying neurodegenerative disorders.
higher resolution
what MRI are good for highlighting certain tissues, visualising lessons/pathologies or nuclei and measuring issue types indirectly via magnetic properties
FLAIR
WM nulled
Double Inversion Recovery (DIR)
Structural MRI
T1-weighted, T2-weighted and Proton Density
Structural MRI (2)
FLAIR, WM Mulled and Double Inversion Recovery (DIR)
Structural MRI (3)
SWI/QSI, MT and Veno/Angio-grams
Structural MRI Analysis
Quantify tissue volumes and structure shape and size
tissue types - GM,WM, CSF
cortical surfaces and thickness
sub-cortical structure and shape
local GM changes
Example of a research question based on structural MRI
how is the volume of grey matter related to age?
hypothesis: older individuals will have less grey matter as a proportion of head size
null hypothesis: older individuals will not demonstrate any differences in grey matter
Structural MRI limitations
- does not measure tissue type (GM,WM,CSF) directly
- the absolute values are not the same across scanners or sessions
- measurement is in the order of millimetres - thousands of underlying cells per 3-dimensional pixel
- does not always distinguish bone from air
- contrast can be poor/variable in subcortical (deep) brain regions
- a single sequence does not show all pathologies
- lots of artefacts and noise
Structural MRI complimentary techniques
- CT (with/without contrast) (shows bones/membranes/vessels/tumours)
- Histology (shows microstructure)
Histology
postmortem brain-staining for something specific
What does diffusion MRI measure
measures the direction of WM fibres in the brain
based on movement (diffusion) of water
restricted in some directions than others which gives more information about axon directions in white matter
in which direction is there more movement in axons
more movement lengthways rather than widthways, water can diffuse along but is hard to diffuse sideways
Differences between structural and diffusion MRI
structural takes 5 minutes to acquire one image
diffusion takes lots of fast images (one every 1-3 secs) for a total of 5 minutes (100-300+ images)
diffusion has a lower resolution (1-3mm) to make scanning faster compared to structural with (<1mm)
Corpus Callosum
the highway between the two hemispheres, a dense bundle of white matter
3 ways to look at the brain
dorsal - looking from the top
sagittal - side on
coronal - front on
Corpus Callosum tensors
pointier and in the same direction of the diffusion then there is more diffusion
not as pointy and have less diffusion in those areas, which might mean fewer axons or don’t have strong boundaries
what kind of tensors would have a higher mean diffusivity
the wider ones
White Matter tract tracing
Fit tensors and trace tracks, follow the tracks and see where we end up
2 types of white matter tract tracing
tractography traces connections via local directions
probabilistic tractography where the different axons might be projecting to
Yang et al. 2017 white matter integrity
greater mean diffusivity in patients with trigeminal neuralgia (type of headache when cranial nerves are hypotensive) compared to healthy controls
Example research question based on Diffusion MRI
how is the mean diffusivity of the corpus callous related to age?
hypothesis: older individuals will have greater mean diffusivity than younger individuals, reflecting age-related decline in structural integrity
null hypothesis: there will be no relationship between age and mean diffusivity of the corpus callous
Diffusion MRI limitations
- does not measure axon size/density directly
- does not measure single fibres (only average groups)
- more difficult to deal with crossing/kissing fibres
- more difficult to do in pulsatile regions (brainstem)
- more restricted by scanner hardware
- sensitive to fast imaging artefacts
Diffusion MRI complimentary techniques
Tracer studies (invisible fibres) - inject dye into specific neurone and see where that projection goes on a postmortem brain
histology (myelin/axon dimensions/gila) - really look at structure
The Haemodynamic Response
When neurons in the brain become active during cognitive or sensory processing, they require an increased supply of oxygen and nutrients to meet their metabolic demands. The hemodynamic response is the complex chain of events that ensures this increased blood flow and oxygen delivery to the activated brain regions.
FMRI experiment word generation
noun is presented and verb is generated from the word, ball - catch etc
FMRI experiment word shadowing
just has to repeat the word that is presented, swim - just repeats swim verb presented verb repeated
FMRI experiment null event
baseline might change depending on different factors which can determine the baseline - we compare the word events to these
- contains a visual component
- no word component
Results of functional MRI
compare to see where in the brain is actually trying to produce a word rather than just repeating, showing what is in the word generation and not in word showing, found Broca’s area, speech production and fluency
Broca’s area
contributes to speech production and fluency
Functional MRI limitations
- does not measure electrical activity
- does not measure metabolic activity or neurons
- does measure changes in blood oxygen levels resulting from the electrical and metabolic activity of active neutrons
- BOLD-FMRI is qualitative (change from baseline important not baseline itself, or absolute numbers)
- sensitive to fast imaging artefacts (like diffusion)
complimentary techniques for functional MRI
Positron Emission Tomography (PET) - inject radioactive glucose, go around the brain and lock onto cells that are more active and radiate back, only can do it once and shouldn’t be to often
- measures brain metabolism directly
- can use radioactive tracers
- slower temporal resolutions vs FMRI
- reduced spacial resolution vs FMRI
Electroencephalography (EEG)
- measures electrical brain activity directly
- much faster temporal resolution vs FMRI (milliseconds)
- reduced spatial resolution vs FMRI
very good at measuring deep structures in the brain
can’t localise where it comes from
