95. MRI: T1 RECOVERY & T2 DECAY Flashcards
- What is the cause of T1 Recovery?
- the exchange of energy
- this energy is exchanged from the nuclei to their
surrounding environment
THIS ENERGY CAN ALSO BE EXCHANGED:
- from the nuclei to their surrounding lattice
- this is called the Spin Lattice Energy Transfer
- What happens as the nuclei energy starts to dissipate?
- the energy of the magnetic moments of the nuclei
- starts to relax
- it returns to the original strength of the External
Magnetic Field (B⁰)
- What happens when the magnetic of the nuclei returns to the original strength of the External Magnetic Field (B⁰)?
- their Net Magnetisation Vector returns to the
longitudinal plane - this means that there will no longer be a signal emitted
from the nuclei - this happens at different rates in different tissues
- it is an exponential process
- What can be said about the T1 time of a tissue?
- it is specific to the tissue
- it is a contrast parameter
- Define: T1 Time?
- this is the time it takes
- for 63% of the longitudinal magnetisation to recover
- What is the time period in which 63% of the longitudinal magnetisation recovers?
IT IS THE TIME BETWEEN:
- one excitation pulse to the next excitation pulse
- this is known as the Repetition Time (TR)
- What does the Repetition Time determine?
- it determines how much T1 Recovery occurs in the
tissue
- Define: T1 Relaxation.
- this is a process
- it occurs when the nuclei exchange the energy that was
given to them - they exchange it with their surrounding environment
THIS ENERGY WAS GIVEN TO THEM:
- by the Radio Frequency pulse
- What does the efficacy of T1 Relaxation determine?
EFFICACY= the ability to reach the desired result
= in this case it is the exchange of energy from
the nuclei to the environment
- it determines the T1 Time of the tissue
- this is the tissue in which the nuclei are situated
- What can be said about the T1 time of fat?
- it is very short
- fat can absorb energy quickly
- What does a short T1 Relaxation Time mean in the case of Fat?
- the nuclei give off their energy to the surrounding fat
tissue over a short period of time - the Net Magnetisation Vector of fat rapidly returns to
the Longitudinal Plane - it returns to the direction and strength of the External
Magnetic Field (B₀)
- What can be said about the T1 Relaxation time of water?
- it is long
- water is very inefficient at receiving energy from nuclei
THIS MEANS THAT:
- the nuclei give off their energy to the surrounding
water over a long period of time
- the Net Magnetisation Vector of water slowly returns
to the Longitudinal Plane
- it returns to the direction and strength of the External
Magnetic Field (B₀)
- What is the T1 Relaxation time of water?
- 2500 ms
- What is the T1 Relaxation time of fat?
- 200 ms
- What is the T1 Relaxation time of Cerebro-Spinal Fluid?
- 2000 ms
- What is the T1 Relaxation time of White Matter?
- 500 ms
- What does the Repetition Time (TR) control?
- it controls how much of the Net Magnetisation Vector
has recovered to the Longitudinal plane - before the application of the next Radio Frequency
Pulse
THIS IS CHECKED IN BOTH:
- water
- fat
- What can be said about having a short Repetition Time (TR)?
- it does not permit the full longitudinal plane recovery
in most tissues
- What happens when the Repetition Time is short and there is no full longitudinal plane recovery?
- there will be a contrast in the images
- this contrast is between fat and water
- it is due to their different T1 Recovery Times
- What can be said about having a long Repetition Time (TR)?
- it allows for the full recovery to the Longitudinal plane
in most tissues - there is no difference in the magnitude of the
longitudinal components - there is no contrast difference in the images
- What causes T2 Decay?
- the exchange of energy from one nucleus to another
nucleus
THIS IS CALLED:
- the spin-spin transfer energy
THE T2 DECAY TIME:
- is specific to each tissue
- it is a contrast parameter
- What causes spin-spin transfer energy?
- the magnetic fields of the nuclei will interact with one
another
- What does spin-spin transfer energy result in?
- it results in the loss of phase coherence
- this means that it results in de-phasing
THIS RESULTS IN:
- the decay of the Net Magnetisation Vector in the
Transverse Plane
- this process is exponential
- it occurs at different rates
- What is the T2 Decay Time?
- it is the time is takes for 63% of the Transverse
Magnetisation to be lost due to de-phasing
THIS MEANS THAT:
- the transverse magnetisation is reduced to 63% of its
original value
- it leaves behind only 37%
- Over which period of time is 63% of the Transverse Magnetisation lost due to de-phasing?
THIS HAPPENS BETWEEN:
- the excitation pulse
- and the MR signal
- this is known as the Echo Time (TE)
- What does the Echo Time (TE) determine?
- it determines how much T2 decay occurs in the tissue
- What causes T2 Relaxation?
- the spins of adjacent nuclei will interact with each
other - they exchange energy
- What dictates the efficiency of T2 Relaxation?
EFFICACY= how much energy is exchanged between the
adjacent nuclei
- how closely packed the molecules are to each other
- What can be said about the T2 Relaxation time of fat?
- it is very short
- the molecules in fat are very closely packed together
- the spin-spin effect is efficient
- What can be said about the T2 Relaxation Time of Water?
- it is very long
- the water molecules are spaced very far apart
- the spin-spin effect is not efficient
- What is the T2 Decay time in the following:
31.1. Water
31.2. Fat
31.3. Cerebro-Spinal Fluid
31.4. White Matter
(31.1) 2500 ms
(31.2.) 100 ms
(31.3) 300 ms
(31.4) 100 ms
- What does the Echo Time control in Terms of T2 decay?
- it controls how much the transverse magnetisation has
been allowed to decay - this happens when the signal is read
THIS HAPPENS IN BOTH:
- fat
- water
- What happens when we have a short Echo Time (TE)?
- full dephasing cannot happen
- it cannot happen in water or in fat
THIS MEANS THAT:
- there will be very little contrast between the fat and the
water in the image
- this is due to the very small differences in the T2 Decay
times
- What happens when we have a long Echo Time (TE)?
- full dephasing takes place
- this happens in both the water and the fat
THIS MEANS THAT:
- there will be a contrast difference between the fat and
the water in the image
- this is because there is a large difference in the T2
Decay times
- Answer the following questions with regards to
T1 Weighted Images:
35.1. What is the Repetition Time (TR)?
35.2. What is the Echo Time (TE)?
(35.1) The Repetition Time is short
- EG: 500 ms
(35.2) The Echo Time is short
- EG: 14 ms
- Answer the following questions with regards to
T2 Weighted Images:
36.1. What is the Repetition Time (TR)?
36.2. What is the Echo Time (TE)?
(36.1) The Repetition Time is long
- EG: 4000 ms
(36.2) The Echo Time is long
- EG: 90 ms
