L2 - Neuroimaging Flashcards
Why is MRI good compared to X-rays and CAT scans?
Revolutionised clinical imaging by safely producing high resolution images of all tissues
Safe because it does not use ionising radiation
- Uses magnets to elicit a signal from protons contained in the water molecules of the body
What is acceleration?
An accelerating charged particle emits electromagnetic waves
Acceleration - rate of change of velocity
What is velocity?
Velocity - magnitude and direction
If magnitude or direction of a charged particle changes it is accelerating
A particle spinning in its axis is?
Accelerating
If you apply static external magnetic field to random proton domains?
Protons line up
Weak electromagnetic field produced by the tissue
If you apply horizontal magnetic flux that switches on and off at radio frequencies to random proton domains?
Protons precess - wobble on their axons
Produces a varying detectable magnetic field
Wobble can be divided into horizontal and vertical components
MRI measure the changes in these two components as the protons respond
These changing signals are the basis of the image reconstruction
Typical MRI sequencing method
- Subject placed in a strong vertical magnetic field – protons line up
- Super cooled, super conducting coil produces the static magnetic field
- Horizontal radio frequency pulse is applied, tipping protons over so they rotate in synchrony in the horizontal plane
- Head coil produces the RF pulse - RF pulse is turned off and protons begin to move out of phase – dephase
- RF off horizontal field decays - Dephasing occurs quickly and leads to a loss of horizontal magnetism - T2
- If horizontal radio frequency pulse remains off, the protons slowly realign themselves with the vertical magnetic field - restoration of vertical magnetisation
- The time constant of recovery of longitudinal magnetisation - T1
What is T2?
The time constant of loss of horizontal magnetism
What is T1?
The time constant of recovery of longitudinal magnetisation
What does T2 time constant curve look like?
Curved downwards
What does T1 time constant curve look like?
Curved upwards
Changes in T1 and T2 values are useful in getting information about?
The tissue protons are in
What do he values of T1 and T2 depend on?
The surrounding matter and whether it causes dephasing or realignment to happen quickly or slowly
How are the various T1 and T2 values recorded?
Computer attached to scanner assigns a brightness value to the various T1 and T2 being measured
Used to construct an image
Can look at T1 or T2 weighted images depending upon what you’re interested in visualising
Different tissues have?
Different T1 and T2 values
Is CSF dark or light in T1?
Dark - weak signal
Is CSF dark or light in T2?
Light - strong signal
What if fMRI good for?
Clinical application
Anatomical studies
First fMRI method
The subject to be observed lay on a delicately balanced table
- When emotional or intellectual activity began in the subject the balance went down at the head-end - consequence of blood in his system
Why does blood flow increase to metabolically active areas?
To supply the area working hard with nutrients & oxygen and flush away metabolic by-products
What characteristic does deoxygenated blood have that oxygenated blood does not have?
Deoxygenated blood is paramagnetic
What are the implication of paramagnetic deoxygenated blood?
Slight variation on MRI scanning
BOLD signal
What is the BOLD signal?
Blood oxygenation level dependent signal
What does the BOLD signal show?
Shows almost same anatomical detail plus indicates which area of brain is active - by assumption that more flow to an area means it must be active
Computer program can differentiate these differences and display image accordingly
What is the Statistical Parametric Mapping programme?
Calculates probability that an area of brain has a different BOLD signal than the one next to it
Statistical Parametric Mapping programme method
Compare a VOXEL with the voxel next to it - t-test
Gives a probability of increased blood flow to that area compared to those around it
All probabilities assigned colours which are overlaid on anatomical MRI – maps out activation
Why do some people think the BOLD signal is wrong?
Think it
- Has nothing to do with nutrition since the increased blood flow exceeds the requirements of active neurones
- May be a non-specific cooling system
Usefulness of fMRI
Patterns of activation can be different in different subjects performing same task
Comparisons in different people - non-linear registration applied on to a standard template
- Causes distortion of original image - fuzzy
fMRI temporal and spatial resolution
Low temporal resolution
High spatial resolution
fMRI is good at?
Telling where the brain is activated
fMRI is bad at?
Telling you when its occurring
Takes several seconds for arterioles to dilate and allow increased blood flow which is detected in fMRI
- Therefore, BOLD signal takes several seconds to build up
- Bad for experiment in which timing of events is important
If you’re investigating rapid cognitive tasks then?
fMRI may need to be coupled with another technique - electroencephalography
Electroencephalography spatial and temporal resolution
High temporal resolution
Low spatial resolution
