Origin of Image contrast Flashcards
Where is histological slice obtained from?
Post-mortem
Where is there good contrast between?
Grey matter periphery and white mater centrally
Spaces that would’ve been filled by CSF
What does MRI allow you to look at?
Structure and function
What does longitudinal plane involve?
Z axis
What is Z axis typically used to denote?
Orientation of B0
What is the Transverse plane?
x-y plane
Orthogonal to the B0 direction
What plane can you get a measurable MR signal?
Transverse plane
How is the transverse magnetisation generated?
Manipulating the equilibrium condition
What is the Bloch Equation?
A phenomenological equation that was created and developed to describe behaviour that was observed for MRI/MR signal
Magnetisation in a magnetic field
There will be some change of magnetisation with time [pattern of behaviour over time]
Related to magnetisation vector and effective field that is applied (e.g. B0 and B1)
cross product
forces that are applied orthogonal to these directions
Longitudinal component
Difference to M0 (equilibrium magnetisation) and Mz component
What is T1?
Time constant that is describing the temporal behaviour of the magnetisation that is aligned along Z
What is T2 dependent on?
How large the transverse component is
What is the decay of transverse magnetisation described by?
T2
What is the approach to equilibrium in a static field described by?
T1
What is excitation?
The generation of Transverse component
What is the Bloch Equation broken down into?
- Precession
2. Relaxation
What does the rotating frame do?
Takes away all of the rotational component and allows us to focus solely on the relaxation parts
[How the magnetisation is recovering or decaying over time]
What are the features of Longitudinal Magnetisation Time Course?
- Exponential saturation curve
- The growth depends on amount present (protons)
- The time taken is characterised by a time constant T1
After T1
we will have 99% of the magnetisation recovered T=5 T1
What does different tissues have?
Different T1 values
What is the amount of recovery that occurs dependent on?
Tissue type we are looking at
What happens when different times that allow for different times of recovery?
There will be different signal intensity in the image
Generate an image that has different appearance depending on T1 time of the tissue
What is a sequence parameter than can be used for different tissues?
Repitition time
What maximises the differences between different tissue types?
TR
What is TR?
Time between 2 RF excitation pulses
Long TR
A lot of recovery happening
Short TR
Little recovery happening
Where does most recovery happen?
White Matter
It has the shortest TR
Appears bright
Where does least recovery happen?
CSF
It has a very long TR
appears dark
Little magnetisation
Where is there excellent contrast between?
Grey and White matter
What will TI generate?
Transverse magnetisation component
What happens in Inversion recovery?
Get an image where there is no signal from one tissue
Choosing TI that goes to the crossing point for white matter
Know the cortical ribbon
What is the important timing parameter for Inversion recovery?
Inversion time
Dictates what the amplitude of longitudinal magnetisation is and therefore what size transverse magnetisation that can be created
When will you get no signal?
Choosing a TI that corresponds to 0 crossing of a particular tissue such as white matter
No transverse magnetisation is created
What does FLAIR do?
Suppress the signal from CSF to make other pathologies visible
What are the features of Transverse Magnetisation Time Course?
- Exponential decay curve
- The decay depends on the amount present
- The time taken is characterised by a time constant T2
What is the relative length of T1 and T2?
T1 is longer
Transverse magnetisation must decay faster than the longitudinal magnetisation
How can we exploit the differences in T2 times?
Sequence parameter used to capitalise on this difference: TE (echo time)
What is Echo time?
Time after we have generated the transverse magnetisation – we can choose one that gives us large difference between the different tissue type
- Further along we go in time, the lower all of our signals are becoming
Why is T2 a bit shorter?
Various dephasing effects that lead to a rapid decay of the signal
What happens during a 90 degree pulse?
- No longitudinal magnetisation
- Maximum transverse magnetisation
- Longitudinal magnetisation is going to recover when transverse magnetisation is going to decay
What happens after a particular echo time?
We can sample the signal
ADC
How can you manipulate the signal further?
weight some TR before we apply another excitation pulse
What do you choose for T2 weighted imaging?
Fixed TE
What will fixed TE determine?
The amplitude of the signal
In T2 weighted Imaging what does CSF have?
Long T2
Bright
In T2 weighted Imaging what does White matter have?
Short T2
Dark
What has excellent contrast in T2 weighted Imaging?
Tissue and fluid e.g. oedema
What happens when there is no magnetic field inhomogeneity?
There will be decays of the signal according to T2
What does T2’ do?
accelerates decay such that 1/T2* is equal to 2 contributions: spin-spin relaxation [natural loss of coherence due to spins interacting with each other] and additional component that makes the decay happen more quickly
In our blood what is the relative proportion between deoxygenated and oxygenated haemoglobin depend on ?
Neuronal activity
Longer T2*
Higher signal intensity
What leads to BOLD contrast?
Brain activation increases oxygen demand and overall results in a higher concentration of oxygenated haemoglobin in the activated area
What can different tissues have?
Different number of protons per unit volume that results in different net magnetisation
