94. MRI: RELAXATION Flashcards
1
Q
- Define: Relaxation.
A
- it is a term that refers to Energy Loss
- it is when we remove the Radio Frequency Pulse
- the Net Magnetisation Vector will try to return to the
Longitudinal Direction
2
Q
- When do we remove the Radio Frequency Pulse?
A
- when the Radio Frequency Excitation Pulse has been
applied - and then Resonance and Flip angle are achieved
3
Q
- What happens once we remove the Radio Frequency Pulse?
A
- the signal in the Receiver Coil begins to decrease
- the amplitude of the Voltage in the coil gradually
decreases
4
Q
- Why does the Net Magnetisation Vector in the Transverse Plane begin to decrease?
A
- Relaxation Processes
- Field Inhomogenities
- Susceptibility Effects
5
Q
- Define: T2* Decay.
A
- this is the cumulative dephasing of the spin-spin
interactions - it is also the dephasing of the inhomogenities
- the field becomes more homogeneous
6
Q
- What happens to the magnetisation in each tissue?
A
- it relaxes at different rates
- this is one of the factors that contribute to image
contrast
7
Q
- What are the effects that take place when the Radio Frequency is withdrawn?
A
- THE NUCLEI EMIT ENERGY
- this energy is absorbed from the Radio Frequency
Pulse
- this is known as Spin Lattice Energy Transfer - THE NUCLEI SHIFT THEIR MAGNETIC MOMENTS
- from the high energy state
- to the low energy state
- THE NET MAGNETISATION VECTOR
- recovers to its original position
- it aligns with the direction of the magnetic field (B₀)
- this is the 90° Longitudinal Plane
NB:
- this whole relaxation process is known as the
T1 Recovery
8
Q
- Define: T2 Decay.
A
- THE NUCLEI
- lose their precessional coherence
- they dephase - THE NET MAGNETISATION VECTOR
- decays in the Transverse Plane
- this is a Relaxation Process
9
Q
- How do Nuclei lose their consistency?
A
- THEY HAVE INTERACTIONS
- with the naturally-occurring magnetic fields
- of the adjacent nuclei
- spin-spin energy is transferred from one nuclei to the
other - THIS CAUSES INHOMOGENITIES
- of the external magnetic field
10
Q
- What does Relaxation result in?
A
- T1 Recovery
- T2 Decay
11
Q
- Briefly state what T1 Recovery is.
A
- this is the Recovery of the magnetisation
- this happens in the Longitudinal Field
NB:
- there is no Radio Frequency signal in the Longitudinal
Plane
- the paths are removed
12
Q
- Briefly state what T2 Decay is.
A
- this is the Decay of Magnetisation in the Transverse
Plane
NB:
- the Net magnetisation Vector is aligned to the External
Magnetic Field
- the Net Magnetisation Vector is at 90°
- energy has been lost due to T1 Recovery
13
Q
- Can we avoid Inhomogenities of the External Magnetic Field?
A
- no
- they are inevitable
- they directly affect the T2 Decay
NB:
- we can become more homogenous by using the Skin-
shining effect
14
Q
- What do Inhomogenities in the External Magnetic Field alter?
A
- they alter the magnitude of the Magnetic Field (B₀)
THIS IS BECAUSE:
- some small areas in the field will have a magnetic field
strength that is slightly more or slightly less
- than the main field strength
15
Q
- How do we get areas in the Magnetic Field that are weaker than others?
A
- they have a lower frequency compared to the applied
Magnetic Field - they will be weaker
16
Q
- What is the Precessional Frequency of a Spin proportional to?
A
- it is proportional to the Magnetic Field (B₀)
17
Q
- What happens when a Spin passes through inhomogenities?
A
- it will experience a magnetic Field strength that differs
from the direction of the External Magnetic Field (B₀)
THIS CAUSES:
- the Precessional Frequencies to change
18
Q
- What happens when the Precessional Frequencies of the Spin change?
A
- the Spins change their phase
- the Net Magnetisation Vector is now dephased
19
Q
- What is the results of the Net Magnetisation Vector dephasing?
A
- the Transverse Magnetisation Decays
- this decay occurs exponentially
- this means that it continues more and more rapidly
- this is known as T2* Decay
20
Q
- What do Magnetic Field Inhomogenities result in?
A
- they cause the Net Magnetisation Vector to dephase
- this happens before the intrinsic Magnetic Fields
influence the Dephasing
THIS MEANS THAT:
- T2* Decay happens before T2 Decay
21
Q
- Can you summarise T2 Decay in three simple points?
A
- This is the decay in the Transverse Plane
- It is caused by inhomogenities
- It removes the Radio Frequency signals
- It is the decay of the Net Magnetisation Vector in the
Transverse Plane
22
Q
- What do we use in order to produce images where the T2 Contrast can be visualised?
A
- we make use of Pulse Sequences
THESE PULSE SEQUENCES:
- allow us to control the way in which the system
- applies the Radio Frequency Pulses and gradients
