Relaxation Theory & Measurement - T1 & T2 Flashcards
What is the NMR recap?
- Nuclei have ‘spin’
- Quantum mechanical quantity
- Relates to the angular momentum of the nucleus
- Restricted to nuclei with odd no. of protons or neutrons or both
- Mostly use the hydrogen nucleus (single proton) for MRI
What is highly abundant in human tissue?
Hydrogen 1H
What is a proton?
A spinning sphere with charge and mass
What does spin give the proton?
- Magnetic moment
2. Angular momentum
When will spins align?
When placed in an external magnetic field (B0)
Where does the spin not ‘snap to’?
To the B0 axis, due to its angular momentum, but precesses around it
Where does precession occurs at?
Larmor frequency
w= yB0
Y= gyromagnetic ratio = 42.57 MHz/T (protons)
What is established in the presence of a magnetic field for hydrogen?
Two energy levels
E= -mHyB-
m=1/2 or -1/2, spin up or spin down
What orientation is spins most likely to be found in and what does it represent?
Parallel than anti-parallel (~1 in 100,000 protons, B0=1.5T, 37)
This represents ‘net magnetisation’ of the system M0
What does M0 precess at?
Frequency w=yB0
64 MHz at 1.5T, 128 MHz at 3.0T (radiofrequency)
What is the orientation of Mz?
Stays the same
Changing the field in XY (transverse) plane
How do you detect M0?
Place Receive coils on X and Y axis
Change Flux
Induced EMF
NMR Signal
What is the RF Excitation pulse?
Expose spins to alternating magnetic field (B1)
B1 frequency must match the Larmor frequency, and is applied in the transverse plane
Spins absorb energy, and nutate away from longitudinal axis, creating time-varying transverse component
What can be determined by using two coils?
The direction of the rotation of the magnetisation, and the phase of the spins
What do parallel state have?
Slightly lower energy level than the anti-parallel state
What is M0 used to generate?
the MR signal
What is net magnetisation?
When more of the spins are aligned with the magnetic field than anti-parallel
What is frame of reference?
- B0 is the main magnetic field [direction in which the main magnetic field is pointing]
- The spins are spinning around main field
- X/Y plane is perpendicular to the main magnetic field
- In the presence of main magnetic field and the spins is precessing around it, there is a change in signal in X/Y plane – place coil and measure as it will induce the change in magnetic field going through a coil induces a voltage – measure it as a voltage
What is the basis of magnetic resonance imaging?
RF frequency pulse are placed into the system that is of the same frequency as the Larmor frequency
What happens when you hit the resonance frequency and put RF energy in?
it transfers energy into the system and tips the magnetisation down into transverse plane and continues to sweep around the Larmor frequency and get more signal in the coils that are placed to detect the signal from transverse axes
What is the benefit of using more coils?
- Detects more antenna [get more signal]
2. Get the phase of the signal
What is rotating frame of reference?
- Z is the main magnetic field
- X/Y are the transverse plane
- The angle in which we flip is the flip angle
What is T1 relaxation also known as and what does it describe?
Spin-lattice relaxation
It describes the process of M0 gradually returning to equilibrium [Regrowth of the magnetisation back in the Z direction]
What is the recovery of Mz given by?
Mz = M0 . (1-e-t/T1)