Lecture 24 - What is a brain system and how do we identify one? Flashcards
What is a system?
A collection of structures in the brain that work together to perform a common function
What is a network?
a network loosely refers to the structure of circuits that connect areas of neurons together
System vs network?
A network is like the wiring/how things are pieced together whereas the system is more of a collective function/functional unit
Examples of systems
Examples: Visual system, auditory system, vestibular system, somatosensory system, motor system, reward system
Function of systems in general
Systems can work alone or in concert with other systems to bring about changes in behaviour – analysis of how this is achieved is called systems neuroscience
Sensory information can lead to
motor output
Core theme of systems neuroscience
We have many input sensory ‘systems’ but essentially ONE behavioural output motor ‘system’
Lots of information coming in but the way we respond/act is caused by only one output system because we only have one set of muscles that can work at any one particular time
Example of Sensory information and how it might be processed to produce a behavioural output
Sensory information comes in via the thalamus from the effectors out in the body such as limbs and it comes in and ends up via various pathways to the thalamus and then come from the thalamus and end up on the primary sensory cortex and from here it is just raw information so you need to process it so you send it backwards in your brain to the parietal lobe which is behind the primary sensory cortex which is in the post central gyrus and you send it back and then you can put things together from other inputs of sensory information such as visiaul information coming in via the thalamus and then to other visual parts or auditory information coming in via the thalamus to part of the auditory cortex on the temporal lobe and all of these come together to form the idea of a perception and memory which all occurs in the temporal and parietal cortices, from there when this sensory information comes in and forms/triggers a memory or triggers the wish for a particular response then this gets passed to the motor area so first to the frontal cortex (planning) which is preparing for an action and then to the primary motor cortex to relay that movement and then it will go down the different tracts to the muscles and drive them to bring about the behaviour that is required / respond to the sensory stimulus (very generic)
How do we identify the components of a brain system?
Neuroimaging
e.g. XRAY, MRI, CT, PET scan etc
What neuroimaging technique uses X-ray radiation?
CT
What neuroimaging technique uses Radio Waves?
MRI
What neuroimaging technique uses Radiolabelling?
PET
CT summary
CTs are reliant on X-rays which are impeded via very dense structures but are not impeded by non-dense structures so non-dense structures get lots of X-rays going through it
structural (density) imaging
MRI summary
High fidelity structural images where you can look diffusion of water molecules through tracts to pick up white matter tracts etc or functional where you can actually look at the brain while it is functioning and see what parts of the brain light up based on what you are thinking and doing
Types
- Structural (high fidelity)
- Diffusion
- Functional
PET summary
PET requires you having a radiolabelled molecule floating around inside that can be picked up based on metabolic activity in the brain or wherever you are messaging and can also look at molecular processes
Types
- Metabolic
- Molecular
CT full name
Computerised Tomography
CT summary of process behind it (orange)
- Tomography – imaging by creating ‘slices’ using any kind of penetrating wave
- Principles of X-ray radiation: x-rays absorbed to different degrees by tissues of different densities – dense tissues like bone absorb most X-rays so photographic film would be minimally exposed -> ‘white’; whereas low density fat/water passes X-rays easily and exposes the film to appear dark
- CT - rotation of a source of X-rays and a detector separated 180 deg from the source – very thin slices - computer integrates to form an image (lots of slices are bought together to form an image)
Big machine that generates x-rays usually by a rotating anode and blasts x-rays that hit the body and if they get through they are met on the other side by a detector
X-ray source that is rotating around head an emitting x-rays and the detector is always right on the other side
Way of telling it is a CT scan
Ways of telling it is a CT scan - very light and bright because it is the most dense and the X-rays have not gotten through the skull, least dense thing is air so the black around the head shows air, slightly more dense than air is fat so is slightly less dark than air, CSF is less dark than fat, white matter looks grey (slightly more dense) therefore starting to block the X-rays making it a lighter colour, grey matter even lighter because more dense, acute bleeding appears almost as dense as blood because globin in the blood is quite dense
Relative densities of parts of the head on CT
air (‘darkest’=least dense) < fat < CSF < white matter < grey matter < blood from haemorrhage (dense globin) < bone (‘brightest’=most dense)
Advantages of CT
Good for showing ‘acute’ bleeding or fracture of the skull
Relatively quick
Cheap
Less ‘scary’ for people (not enclosed or as noisy as MRI)
Disadvantages of CT
Structure only, not function
Not good for detail in the brain because of minimal contrast between areas (not very good demarcation of Frey and white matter)
dose of radiation
PET scanning summary of principles (ORANGE)
- Principles of Positron Emission Tomography (PET): radiation emitted from a radioisotope injected intravenously is registered by external detectors. ‘Positrons’ from isotope travel short distance (2-3 mm), combine with electrons and ‘annihilation’ results in energy release picked up by detectors = Just need to know than an isotope is injected and emits something which can be picked up by a detector
positrons are ions that come from the radioisotope
- Used to highlight areas of increased metabolism of glucose
(e. g. by cancers) – commonly use F18 radiolabelled fluoro-2- deoxyglucose (FDG) injected intravenously and enters organs where it highlights area of high glucose transport/phosphorylation
Radioisotopes in PET scan will go to
areas of high utilisation therefore can do functional measures with PET scanning
PET metabolism -
functional
PET molecular -
gene therapy
can also use PET to measure gene therapy/measure a molecule
PET advantages
Functional
Can identify and characterise tumours as benign or malignant
PET disadvantages
Poor resolution of brain tissue (can combine with CT) (poor resolution means that you cannot really resolve anything)
But also area localised is only approximate (within 5mm)
Requires radiation dose in the form of radio isotopes
MRI stands for
Magnetic Resonance Imaging
PET stands for
Positron Emission Tomography
MRI general principles of the machinery
Person placed within a high-powered magnet (scanner)
- 60,000 x greater than earth magnetic field
- Aligns hydrogen atoms in the body
The magnet is always on
Scanner consists of a very strong magnet, aligns up the hydrogen atoms in the body part that are exposed to the magnetic field therefore no metal inside or outside your person
MRI and magnetic fields with their relationship to the scanner (4 general principles)
1) Magnetic field aligns hydrogen atoms
- (BOTH the parallel component = axis orientation and perpendicular component=rotation)
2) Radiofrequency pulse is applied
- Moves the hydrogen atoms out of alignment
3) Measure time it takes for the hydrogen atoms to recover their alignment and spin after the radiofrequency pulse is turned off.
- Hydrogen ions in different tissues take different amounts of time to recover.
4) The recovery times are mapped to form an anatomical image Units of the image = voxel (volume pixel or 3D pixel)
General principles of how MRI works - 1) Magnetic field aligns hydrogen atoms
- (BOTH the parallel component = axis orientation and perpendicular component=rotation)
Hydrogen atoms are everywhere
General principles of how MRI works - 2) Radiofrequency pulse is applied
- Moves the hydrogen atoms out of alignment
Perturb the magnetic field with a radio frequency pulse and moves the hydrogen atoms out of alignment when the pulse is hit, out of magnetic alignment and do not get the rotating in synchrony that they were before
Magnetic field off
hydrogen atoms are not magnetised - spinning around in various directions and facing various directions, not aligned at all
Outside the scanner
Magnetic field on
now magnetised, spinning in sync - magnetic field on leads to being magnetised, axis aligns and lines up
In the scanner
Radio pulse on
Out of magnetic alignment and not rotating in synchrony
in the scanner
Outside the scanner
magnetic field off
Inside the scanner
magnetic field on
radio pulse on
General principles of how MRI works - 3) Measure time it takes for the hydrogen atoms to recover their alignment and spin after the radiofrequency pulse is turned off.
Hydrogen ions in different tissues take different amounts of time to recover. - which is why you get a difference in each individual tissue as to what it looks like
Radiofrequency is turned off and then go back to an alignment due to the magnet but some things will take longer than others to get back to this state
radio pulse turned on (out of magnetic alignment and not rotating in synchrony) - after radio pulse turned off - magnetised again (aligned and spinning in sync)
Recovery times of structures in MRI (principle 3)
Fastest recovery time of orientation (rather than density) is the brightest ….first Fat (e.g. in bone marrow) then white matter (therefore looks whiter than grey matter) and then grey matter then CSF …Slowest recovery time which is CSF therefore shows up the darkest
How do MRI and CT differ in visualisation
MRI is quite different to CT which is density, MRI is about hydrogen ion recovery of their alignment
Fastest recovery time tissue in MRI scan
Fat (e.g. in bone marrow)
think - fat springy - abnormal spin stops/reverys quickly - big signal
Slowest recovery time tissue in MRI scan
CSF
Think - water less springy - gradually slows then corrects - small signal
On CT, more dense is
brighter
On CT, less dense is
darker
Darkest things on CT scan
least dense therefore air
Brightest thing on CT scan
most dense therefore bone
In MRI, fastest recovery time shows up
brightest
In MRI, slowest recovery time shows up
darkest
General principles of how MRI works - 4) The recovery times are mapped to form an anatomical image
Units of the image = voxel (volume pixel or 3D pixel)
Sometimes grey and white matter can appear REVERSED under T1 - look at the fat (bright) and CSF (dark)
Fastest on MRI =
brightest
Slowest on MRI =
darkest
CT scanning vs MRI scanning
CT scanning - quick (could take 15 minutes) and less cary
MRI scanning - Takes much longer (could take an hour); confusing/noisy/restrained and therefore more scary
CT vs MRI quality
CT quality - inferior for brain detail
MRI quality - superior for brain detail
CT vs MRI bone and blood
CT - Preferable for rapid assessment of bleeding; good for identifying skull fracture
MRI - inferior for rapid assessment of bleeding; skull not well identified so little help for fractures (but it is a great scan if you are looking at brain substance) (no bone showing in MRI but we have bone marrow (fat) which can indicate where the bone is)
T1 and T2 MRIs are about
the relative balance about how we get the grey and white matter to response
MRI methods (3)
Structure - volumes and surface
Diffusion - structural connectivity (for looking at white matter tracts and the connections between parts of the brain)
Functional - Functional connectivity (look at how things are connected together functionally)
Structural MRI methods include
T1 weighted
T2 weighted
Structural MRI - T1 weighted
T1 = recovery time for the magnetic vector of the hydrogen atoms to return to resting alignment
after the radiofrequency pulse
It is the time taken for the acid to realign after the radio frequency pulse is taken (time taken for out of magnetic alignment (after radio pulse is turned off) to be demagnetised and orientation aligned
Useful to highlight fat in the brain - CSF darker (long T1) and Fat brighter (short T1)
Structural MRI application of T1
T1-weighted images used for structural (anatomical) analysis
Volume of brain and/or specific brain regions (‘voxel based morphology’ or VBM)
- VBM is a unit of measurement inside these scans is called a voxel which is like a 3D pixel, counting these voxels gives you VBM
Look and compare different peoples volume or brain and specific regions
Here measuring changes in the grey matter volume in limbic system as related to mindfulness
Structural MRI -T2 weighted
T2 = recovery time for the axial spin (spin around the acid) of the hydrogen atoms to return to resting state after the radiofrequency pulse - can produce almost opposite image effects to T1
When you hit the radiofrequency some of the hydrogen ions are going to rotate in synchrony whereas others are not because you have perturbed it with the radiofrequency pulse and the time it takes for the spin to return is the T2
Not rotating in synchrony - after radio pulse turned off - spinning in sync
CSF brighter because long T2 - takes longer time so signal is larger
Fat darker because show T2 - takes shorter so signal smaller because quicker recover
Application of T2 weighted MRI
used to investigate water in the brain - because fat is darker and water is brighter
T1 vs T2 - Fat
T1 fat is bright (white matter is usually), T2 fat is dark (white matter too)
T1 vs T2 - Water (CSF)
T1 the CSF is dark, T2 the CSF is bright (eyes and CSF bright
T1 vs T2 - New blood (flow)
T1 is bright (in vessels - this can be tuned), T2 is dark (blood vessels are dark)
T2 good for …
Ventricles/lesions if they are far from the CSF
Eyes and CSF bright in what
T2
Blood in blood vessels shows up on T1 or T2
T1
Odema in T2
In the T2 image the tumour contains ‘oedema’ fluid (=inflammation) which demarcates tumour from tissue BUT does not distinguish tumour from CSF easily - (need to do ‘T2-FLAIR’)
T2 Flair =
Fluid attenuation inversion recovery
Normal T2 does not distinguish CSF from oedema of inflammation (e.g. from a tumour) easily. Need to do a ‘T2- FLAIR’ sequence where CSF is DARK because it is free flowing but non free-flowing fluid like oedema is bright!
