harrison Flashcards
Where is the nucleus contained?
Cell body
What do dendrites do?
Allow other cells to synapse on + communicate w/ the cell
What do axons do?
Conduct electrical signals and are surrounded by myelin
Myelin def + use in MRI
Fatty sheaths that help conduct the signal, major factor in determining MR signal + contrast (makes axon look v diff from cell body)
2 types of brain tissue n definitions:
- Grey matter - made up of densely connected cell bodies
- White matter - long axons surrounded by myelin live in the white matter
What comes under grey matter?
Subcortical nuclei - densely packed areas of cell bodies, but deeper into the brain.
These folds full of packed neurons differentiate human brain from those of animals’.
Cerebrospinal fluid (CSF) def
- Fluid that bathes brain + lives in holes/pockets of middle of brain, goes all around spinal cord
- Is important in: shock absorption, bringing in nutrients, taking away waste products
Name the components involved in a MRI scan:
- Patient
- Patient table
- Scanner
- Magnet
- Gradient coils
- Radio frequency coil
What r u (in very simple terms) doing w/ a MRI scan?
Putting someone in a big magnet, creates magnetic field in the middle (B0 field - base magnetic field, always on)
How can u measure tissues based on a magnetic field?
U can do it bc some tissues have magnetic properties (e.g. hydrogen nucleus)
1. If you put someone in a strong magnetic field, those hydrogen ions tend to move in direction of that magnetic field (helpful if u want 2 read those signals)
2. Fatty parts (myelin, white matter) will have diff proportions of hydrogen ions than grey matter
Structural MRI def (mainly looking @ GM)
- Imaging gross or macro-scale brain anatomy (what u can see w/ naked eye)
- Basically measuring tissue types indirectly via magnetic properties
What is typical structural MRI resolution + can u change that?
Resolution is typically 1mm or less, can take finer details (go down to .2 or .3 mm) but takes longer for each scan
By changing magnetic sequence, can detect other features of tissue. Can change sequence to be susceptible to:
- Iron content (primarily) and myelin/white matter
- Bound or free water
- Could look @ vessel structure, contrast agent (injecting smth into someone’s brain 2 get better signal) might b useful here
Structural MRI analysis
Can segment different tissue types (WM, GM, CSF)
1. Look @ cortical thickness (measure thickness between outer and inner ribbon of cortex @ every point)
2. Look at local grey matter density
3. Could look @ sub-cortical shape n structure - whether or not its deformed inwards or outward according 2 some pathology or behaviour we’re looking at
Structural MRI analysis: sub-cortical shape n structure (examples)
- Bromis et al., 2018 - grey matter decreases w/ PTSD
- Akiki et al., 2017 - shape of hippocampus (memory formation n learning) changed w/ presence of PTSD
Structural MRI cons:
- Doesn’t measure tissue type directly
- Absolute values aren’t the same across scanners or sessions (bc measurements r relative)
- Measurement is done in mm - thousands of underlying cells per 3-dimensional pixel, vv broad
- Doesn’t always distinguish bone from air
- Contrast can b poor/variable in subcortical (deep) brain regions
- A single sequence does not show all pathologies
- Some noise present in the data, and maybe artefacts as well (e.g. if some1 doesn’t shut the door)
Complementary techniques 2 structural MRI:
- Computed tomography (CT)
- Histology
CT description
- Shows bone, membrane, vessels, + tumours
- MRI better for soft tissues
- Uses X-rays rather than magnetisation
- Lower spatial resolution than MRI (not as much fine detail)
Histology description
Take slice of post-mortem brain + directly measure cells
1. Shows brain microstructure, much finer spatial resolution (micrometres)
2. Typically only a localised brain region or slice
Diffusion MRI def (WM)
Based on the movement/diffusion of water
1. Measures direction of WM fibres in the brain
2. Can give indication of WM integrity
3. Can provide info on the connections between brain structures (e.g. limbic + frontal area of brain)
4. Need to acquire many ‘directions’ - 5-30 min scan
Principles of diffusion
- Diffusion of water is restricted more in some directions than others (water in cell body n terminals can move wherever, water in axon can only move along axon) –> gives most info abt axon directions in WM
- Gives info abt microstructure - if we know how quickly n well smth diffuses, can draw conclusions abt how many/how dense axon projections r (measurements r abt 1-3mm, so lots of axons w/ in that)
- Take lots of images + each one is sensitive to 1 direction
Internal capsule
Where lots of WM/axons r running in the same direction
Structural vs. diffusion MRI
- Diffusion scan has artefacts @ the top due 2 how quickly images r taken (@ front of our brain we have air pockets, so get distortion there)
- Abt 5 mins 4 one structural MRI image, 1-3 seconds for diffusion MRI (although whole process is abt 5 mins, yields 100-300+ images)
Corpus callosum def
Highway of WM tracts between brain’s hemispheres where all axons r pretty much in same direction, allows 4 communication
Gradient coils can create magnetic field changes in any direction. What r the implications of this?
Could apply magnetic field change 2 the left –> water disappears where axons that r going 2 the left r clumped (shows really good WM integrity in that area; diffusion MRI signal reduced there)
Once you’ve got diffusion MRI scans, u gotta analyse WM microstructure:
- Tensors give us surrogate measures of microstructure
(the pointier they r, the greater the diffusion in that direction; the fatter they r, less directed the diffusion is) - Can indicate axonal integrity - if there’s lots + lots of strong axons, tensors become slimmer + pointier. Summarised by: fractional anisotropy - pointedness, mean diffusivity - how much does it diffuse in every direction.
Analysis of WM tracts (diffusion MRI)
- Follow direction of voxels so we can create + recreate tracts in some1’s brain
- Can start a seed in one part of brain + see where that part of the brain connects to w/ WM tracts (tractography, probabilistic tractography)
Tractography + probabilistic tractography
Not only assessing WM integrity, but also locating tracks in brain w/ this measure
Liao et al., 2014
Decrease in WM integrity for adolescents w/ Generalised Anxiety Disorder (GAD)
Tromp et al., 2012
- Looked @ fronto-limbic tract (limbic system important 4 emotion, frontal cortex important 4 logic)
- Less WM in this specific tract 4 individuals w/ GAD
What r the implications of looking @ signal associated w/ tissues n not measuring tissues directly in diffusion MRI?
Different underlying changes could cause same changes that we find. Examples:
1. Could get increased fractional anisotropy (FA) + mean diffusivity (MD) due to either swelling or higher density of axons + myelin
2. Could get decreased FA + MD due to either myelin loss or cell death
Diffusion MRI cons
- Doesn’t measure axon size/density directly
- Doesn’t measure single fibres (only average groups)
- More difficult to deal with crossing/kissing fibres
- More difficult to do in pulsatile/moving regions (e.g. brainstem)
- More restricted by scanner hardware - scanners still improving
- Sensitive to fast imaging artefacts (feature appearing in image that isn’t present in original object)
Crossing fibres def
Areas of the brain where one tract crosses another
Kissing fibres def
Fibres that come together then come apart again
Complementary techniques 4 diffusion MRI (other measures in neuroscience that can tell us abt axonal integrity n direction):
Tracer studies - inject dyes into cell bodies or termini
1. Can track individual fibres across the brain, track direction of connections
2. Can have very fine spatial resolution
Histology
1. Can directly measure axonal + myelin dimensions
Math 4 diffusion n structural MRI:
- Maths for structural MRI analysis: t-statistic
- Maths for diffusion MRI analysis: Pearson r (looking at correlation between integrity of white matter and, in this example, how much some1 enjoyed ice cream)
Functional MRI (fMRI) description (looking @ brain activity instead of GM or WM)
- MR signal is dependent on blood flow
- Need to take image vv quickly - want to know how that MRI signal changes across time, cause ur getting participant 2 do smth while they’re in the scanner
- Can either look @ brain activity during task or resting
- Lower spatial resolution (1-3mm)
- Takes lots of fast images like diffusion MRI (one per 1-3 seconds) for ~5 minutes (100-300+ images)
Describe the physiology relevant 2 the fMRI
- Neurons need oxygen + glucose 2 work
- With fMRI, take advantage of changes in fuel that neuron needs - if neuron works harder, needs more fuel
- Haemodynamic response - oxygenated haemoglobin has a diff magnetic property than deoxygenated haemoglobin –> lil bit of a diff signal from when ur blood has more oxygen vs. less (what fMRI is measuring)
Describe haemodynamic response in more depth
- When neuron is in its baseline state, blood vessels bring in oxygenated haemoglobin + neurons take up some of the oxygen → out the other side there’ll be more deoxygenated haemoglobin (bc neurons used the oxygen)
- When neurons r more active, body helps by opening up blood vessels n bringing in wayy more blood → get change in MR signal bc more oxygenated blood comes in
- Haemodynamic response is SLOW
fMRI analysis
Single subject regression
1. Can help determine what voxel (chunk of brain) is important in, for example, milkshake tasting
2. Do this by looking @ haemodynamic response of a certain voxel 2 milkshake - is it being more active when its been given milkshake?
fMRI continued analysis/results
Group regression
fMRI cons:
- Doesn’t measure electrical activity
- Doesn’t measure metabolic activity (not measuring how much glucose they’re taking up)
- Change from baseline is what’s important, not the baseline itself or absolute numbers
- Sensitive to fast imaging artefacts (like diffusion)
Complementary techniques 4 fMRI (also looking @ the function/activity in some1’s brain)
- Positron emission tomography (PET) scanning
- Electroencephalography (EEG)
EEG description
- Cap on ur head w/ electrodes all over it
- Measures electrical brain activity directly
- Much faster temporal resolution (ms), reduced spatial resolution (cm)
PET scanning
- Inject radioactive tracer that can b taken up by all the active neurons - measure where that glucose tracer goes (measures brain metabolism directly)
- Only want 2 expose ppl to these in rlly necessary scenarios
- Much slower temporal resolution (5-10s for a single image)
- Reduced spatial resolution (4-5 mm)
