Modern methods in neuroscience I Flashcards
What is magnetic resonance imaging used for?
To produce high resolution images of under the skin
Why is MRI safer than X-rays and CT scans?
Doesn’t use ionising radiation
What does MRI scanning use to get an image of under the skin?
How?
Powerful magnets
Elicit a signal from the protons contained in the water molecules of the body
What type of particle emits electromagnetic waves?
What does this produce?
An ACCELERATING, CHARGED particle
Produces electromagnetic radiation which can be detected
What is acceleration?
The rate of CHANGE of velocity
What does velocity comprise of?
Magnitude(size) AND direction
How FAST something moves in a particular DIRECTION
If a particle is accelerating, what has changed?
The magnitude or direction of a CHARGED particle
What type of particle is CONSTANTLY accelerating?
What does this particle release?
A spinning CHARGED particle - A PROTON
Constantly releases electromagnetic radiation
What does any spinning particle have?
A magnetic dipole (North and South)
How are the protons arranged in a given body tissue?
What does this cause?
All the north poles are orientated in DIFFERENT directions
Cancel each other out
Causes NO NET magnetic effect
NO electromagnetic radiation can be detected (it is 0)
What happens when a powerful VERTICAL, STATIC external magnetic field is applied?
What does this produce?
Protons line up with the magnetic field (vertically) - no longer cancel each other out
Forms a weak electromagnetic field in the tissue
All the spins of the protons add together - produces electromagnetic radiation
What happens when a powerful HORIZONTAL, PULSING external magnetic field is applied? (after the vertical one)
What does this produce? Why?
Pushes the protons over, causing them to wobble on their axis
Produces a VARYING, DETECTABLE magnetic field
As change in direction - acceleration in the the change in velocity (magnitude/direction)
What is radio frequency?
When is this used?
Magnetic field that is rapidly pulsed on and off
Used as a horizontal magnetic field to push the protons over on their axis
What does precess mean?
When is this achieved?
When the protons wobble on their axis
Occurs when there is a vertical static magnetic field and a horizontal radio frequency
What component of the magnetic field of the protons is easier to detect? Why?
The horizontal component - gets bigger and smaller as the protons are knocked over
Changes the magnetic field
Easier to measure change
In a MRI machine, what causes the vertical magnetic field?
Super-cooled, super-conducting coil
In a MRI machine, what causes the horizontal magnetic field?
What is this magnetic field also called?
The head coil
RF - radio frequeny
Which magnetic field is static?
Vertical
What happens to the protons when the RF is turned off?
Protons move out of phase - Dephase
How does dephasing occur?
Quickly
What does dephasing lead to?
A loss of horiztonal magnetisation and a weakened signal in the horizontal field - The ‘dephasing signal’
What is T2?
The time constant of the weakening horizontal field
What is a T2 weighted image based upon?
The dephasing part of the cycle
What happens to the protons if the horizontal radio frequency pulse remains off?
What does this result in?
Protons realign themselves with the strong VERTICAL magnetic field
Resulting in the restoration of the vertical magnetisation
Increase in the vertical signal
Does the realignment happen faster or slower than the dephasing phase?
Much slower
How is the return to the original alignment of the protons measured?
Indirectly
What is T1?
The time constant of the recovery of the longitudinal magnetisation
What is a T1 weighted image based upon?
When all the protons are aligned in the vertical plane (no horizontal magnetic field)
When is the signal in T2 strongest?
When does this signal reduce?
When the protons are in phase
Reduces when the protons dephase
When is the signal in T1 weakest?
When does this signal increase?
When the protons are horizontal
Increases when the protons stand up and realign with the vertical field
What is useful in getting information about the tissue the protons are in?
Changes
Especially in the RATE OF CHANGE of the signal strength
What do the values of T1 (realignment) and T2(dephasing) of the protons depend upon?
The surrounding matter
How quick do protons dephase and realign in DENSE tissue? (eg. bone)
Why?
Quicker than if in a less dense tissue
Because the protons are closer together
What is used to construct an MRI image?
A computer system attached to the scanner, which assigns a particular brightness value to the various T1 and T2s measured
In T2, what colour do dense substances appear?
Bright
In T1, what colour do dense substances appear?
Black
What is the point in having T1 and T2 images?
Can easily see different structures
Which one is used depends upon what the researcher wants to visualise
What information does MRI give?
Structural information
What can be used to determine what parts of the brain is functioning in different tasks?
fMRI
functional magnetic resonance imaging
What is fMRI and what is it used for?
Functional MRI
Used to determine what parts of the brain are functional during different tasks
What did James Williams do?
What did this show?
Placed a subject on a balanced table that could tip downwards at the head or foot, if the weight at either end was increased
- Showed that if emotional/intellectual activity began in the subject - the head end went down
- As a result of blood rush
What happens to the blood in the brain?
Blood flow SLOWLY increases to metabolically active areas of the brain
What do some people suggest about the blood flow to metabolically active parts of the brain?
Suggest it is to supply area working hard with nutrients and oxygen
And to flush away the metabolic byproducts
What does oxygenated blood do in the brain?
What happens after this
Enters the capillary bed supplying an area of metabolically active brain
Tissue then extracts the O2 from the blood
Deoxygenated blood leaves the capillary bed
Is oxygenated or deoxygenated blood paramagentic (has magnetic properties??
What does this allow?
Deoxygenated blood
Magnetic field of the deoxygenated blood can be detected in an MRI scanner
Can see the difference between oxygenated and deoxygented blood on the MRI image
What is a ‘BOLD’ signal?
Blood Dependant Oxygenation Level Dependant signal
If there is more deoxygenated blood present, what happens to the signal?
It is brighter
If there is more deoxygenated blood coming out of a specific brain area, what is the assumption?
That more oxygenated blood has travelled to that area of the brain
So, that part of the brain is more active
What do computer programmes such as ‘Statistical Parametric Mapping’ and ‘FSL’ do?
How do they do this?
What does this ‘map out’
Calculate the probability that a small chunk of brain has a DIFFERENT BOLD signal to the one next to it (Probability of increased blood flow to that tiny area)
- Compare the voxel of one brain area to the voxel of the neighbouring area using a t-test
- Assign a colour to each probability
- Overlay the colours on the ANATOMICAL MRI image
Maps out activation of the brain
What is a VOXEL?
A 3D pixel (volumetric pixel)
Cube shaped pixel of the brain
Why is the nature of the BOLD signal controversial?
Why?
Some think it is may be a non-specific ‘cooling system’ of the brain
Because the increased blood flow to the brain area FAR EXCEEDS the requirements of the active neurons
Why is the usefulness of fMRI in question?
- Patterns of activation can be DIFFERENT in different patients performing the SAME task
How can you make comparisons between the fMRI of different people?
What are the disadvantages of this?
Apply non-linear registration to a STANDARD template
Disadvantages:
- Produces an average picture of the brain which distorts the original image and makes it appear fuzzy
- As each brain is different
What is fMRI good and bad for?
Good at telling you WHERE the brain is activated (good spatial resolution)
Bad at telling you WHEN the brain is activated (bad temporal resolution)
Why does fMRI have poor temporal resolution?
It takes several seconds for the arterioles to dilate and allow increased blood flow (detected in fMRI)
BOLD signal takes several seconds to build up
What is fMRI not idea for?
Example?
Experiments in which timing is important
Eg. investigating rapid cognitive tasks
If investigating tasks where timing is important, what must fMRI be coupled with?
How is this technique different to fMRI?
ELECTROENCEPHALOGRAPHY:
- High TEMPORAL resolution
- Low SPATIAL resolution
