Origin of Image contrast

Question 1

Where is histological slice obtained from?

Answer 1

Post-mortem

Question 2

Where is there good contrast between?

Answer 2

Grey matter periphery and white mater centrally

Spaces that would’ve been filled by CSF

Question 3

What does MRI allow you to look at?

Answer 3

Structure and function

Question 4

What does longitudinal plane involve?

Answer 4

Z axis

Question 5

What is Z axis typically used to denote?

Answer 5

Orientation of B0

Question 6

What is the Transverse plane?

Answer 6

x-y plane

Orthogonal to the B0 direction

Question 7

What plane can you get a measurable MR signal?

Answer 7

Transverse plane

Question 8

How is the transverse magnetisation generated?

Answer 8

Manipulating the equilibrium condition

Question 9

What is the Bloch Equation?

Answer 9

A phenomenological equation that was created and developed to describe behaviour that was observed for MRI/MR signal

Question 10

Magnetisation in a magnetic field

Answer 10

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

Question 11

Longitudinal component

Answer 11

Difference to M0 (equilibrium magnetisation) and Mz component

Question 12

What is T1?

Answer 12

Time constant that is describing the temporal behaviour of the magnetisation that is aligned along Z

Question 13

What is T2 dependent on?

Answer 13

How large the transverse component is

Question 14

What is the decay of transverse magnetisation described by?

Answer 14

T2

Question 15

What is the approach to equilibrium in a static field described by?

Answer 15

T1

Question 16

What is excitation?

Answer 16

The generation of Transverse component

Question 17

What is the Bloch Equation broken down into?

Answer 17

1. Precession

2. Relaxation

Question 18

What does the rotating frame do?

Answer 18

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]

Question 19

What are the features of Longitudinal Magnetisation Time Course?

Answer 19

1. Exponential saturation curve
2. The growth depends on amount present (protons)
3. The time taken is characterised by a time constant T1

Question 20

After T1

Answer 20

we will have 99% of the magnetisation recovered T=5 T1

Question 21

What does different tissues have?

Answer 21

Different T1 values

Question 22

What is the amount of recovery that occurs dependent on?

Answer 22

Tissue type we are looking at

Question 23

What happens when different times that allow for different times of recovery?

Answer 23

There will be different signal intensity in the image

Generate an image that has different appearance depending on T1 time of the tissue

Question 24

What is a sequence parameter than can be used for different tissues?

Answer 24

Repitition time

Question 25

What maximises the differences between different tissue types?

Answer 25

TR

Question 26

What is TR?

Answer 26

Time between 2 RF excitation pulses

Question 27

Long TR

Answer 27

A lot of recovery happening

Question 28

Short TR

Answer 28

Little recovery happening

Question 29

Where does most recovery happen?

Answer 29

White Matter It has the shortest TR Appears bright

Question 30

Where does least recovery happen?

Answer 30

CSF It has a very long TR appears dark Little magnetisation

Question 31

Where is there excellent contrast between?

Answer 31

Grey and White matter

Question 32

What will TI generate?

Answer 32

Transverse magnetisation component

Question 33

What happens in Inversion recovery?

Answer 33

Get an image where there is no signal from one tissue

Question 34

Choosing TI that goes to the crossing point for white matter

Answer 34

Know the cortical ribbon

Question 35

What is the important timing parameter for Inversion recovery?

Answer 35

Inversion time Dictates what the amplitude of longitudinal magnetisation is and therefore what size transverse magnetisation that can be created

Question 36

When will you get no signal?

Answer 36

Choosing a TI that corresponds to 0 crossing of a particular tissue such as white matter No transverse magnetisation is created

Question 37

What does FLAIR do?

Answer 37

Suppress the signal from CSF to make other pathologies visible

Question 38

What are the features of Transverse Magnetisation Time Course?

Answer 38

1. Exponential decay curve 2. The decay depends on the amount present 3. The time taken is characterised by a time constant T2

Question 39

What is the relative length of T1 and T2?

Answer 39

T1 is longer | Transverse magnetisation must decay faster than the longitudinal magnetisation

Question 40

How can we exploit the differences in T2 times?

Answer 40

Sequence parameter used to capitalise on this difference: TE (echo time)

Question 41

What is Echo time?

Answer 41

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

Question 42

Why is T2 a bit shorter?

Answer 42

Various dephasing effects that lead to a rapid decay of the signal

Question 43

What happens during a 90 degree pulse?

Answer 43

- No longitudinal magnetisation - Maximum transverse magnetisation - Longitudinal magnetisation is going to recover when transverse magnetisation is going to decay

Question 44

What happens after a particular echo time?

Answer 44

We can sample the signal | ADC

Question 45

How can you manipulate the signal further?

Answer 45

weight some TR before we apply another excitation pulse

Question 46

What do you choose for T2 weighted imaging?

Answer 46

Fixed TE

Question 47

What will fixed TE determine?

Answer 47

The amplitude of the signal

Question 48

In T2 weighted Imaging what does CSF have?

Answer 48

Long T2 | Bright

Question 49

In T2 weighted Imaging what does White matter have?

Answer 49

Short T2 | Dark

Question 50

What has excellent contrast in T2 weighted Imaging?

Answer 50

Tissue and fluid e.g. oedema

Question 51

What happens when there is no magnetic field inhomogeneity?

Answer 51

There will be decays of the signal according to T2

Question 52

What does T2' do?

Answer 52

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

Question 53

In our blood what is the relative proportion between deoxygenated and oxygenated haemoglobin depend on ?

Answer 53

Neuronal activity

Question 54

Longer T2*

Answer 54

Higher signal intensity

Question 55

What leads to BOLD contrast?

Answer 55

Brain activation increases oxygen demand and overall results in a higher concentration of oxygenated haemoglobin in the activated area

Question 56

What can different tissues have?

Answer 56

Different number of protons per unit volume that results in different net magnetisation