Origin of MR signal 1 Flashcards
What is main field, B0?
E.g. 1.5T, 3T or 7T It is a vector A vector has: 1. Magnitude e.g. 3T 2. Direction: Defined as Z
What is always on?
Main field B0
Where does the axis of B0 point?
Along the axis of z direction
Build a frame of reference: Z along B0 and X and Y
Where is Z?
Table Along the MR scanner and points out of the scanner
what does each proton act as ?
Little magnet
What is the name of vector of proton?
Nuclear magnetic moment
Outside scanner
Not experiencing magnetic field
Insider the scanner: M is the vector
Can be decomposed into 3 components: x,y,z
What is B0 applying?
Force on little protons
Name of force = torque
What does the Bloch equation dictate?
Motion of the protons
What was the assumption of Bloch equation?
Millions of individual nuclei in a sample could be represented by a single vector M
What does external field (B) produce?
A torque (T) or twisting force on M resulting in its precession at angular frequency ω = γ B
What is frequency of precession about B0 axis proportional to?
Main magnetic field B0
What is Larmor equation?
ω = γ B
constant is called gyromagnetic ratio
-depends on the nucleus
-equivalent to 42.5MHz/T for hydrogen nuclei
What happens when someone is put in the scanner?
- Precession
- the nuclei precess about the field
- precessional frequency:ω = γ B - Bulk magnetisation
What happens when you enter the scanner?
All of the protons align with the B0 field
What is quantum physics?
One state is in higher energy state than the other
Where will there be more protons?
Lower energy state
What is the summary for when you enter the scanner?
- A magnetisation vector is created pointing in the same direction B0 axis
Amplitude of the magnetisation vector
is proportional to the magnetic field
What is Faraday’s law?
a law stating that when the magnetic flux linking a circuit changes, an electromotive force is induced in the circuit proportional to the rate of change of the flux linkage.
Signal Generation
- Strong magnetisation due to abundance of 1H
- M is aligned along B0 and static
- No signal yet
How do you measure a signal?
Variation in the magnetic field
What is the second magnetic field called?
- Transmit field
- RF field
- B1 field
What is RF field: B1?
This field is turned on to disturb the system from equilibrium
- This process of disturbance is called ‘‘Excitation’’
- The amount of ‘‘excitation’’ is the flip angle
- This field is not static but oscillates and has a frequency
Signal generation - Laboratory Frame of Reference
- Equilibrium alignment of Magnetisation vector
- Apply additional magnetic field (B1): Magnetisation precesses
- When the B1 field is switched off the transverse magnetisation remains
Turn magnetic field for a short period of time
This magnetic field is perpendicular to the z axes
Rotates about the B0 axes
What does the Bloch equation combine?
B0 axes and B1 axes
Precession about z axes
Where does B1 field also rotate around?
B0
Where does the M vector rotate?
Around magnetic field axes that is experiencing
Frequency of precession of B1
frequency of B1 has to be exactly the same as the frequency in which M is rotating about B0 [Larmor frequency]
Rotating frame of reference
Rotating at the same frequency as the B1 field
What do you change to increase the signal?
- Duration
2. Amplitude
What is benefit of having a higher flip angle?
T1 + T2 contrast
How do you measure a signal?
- Flipped to transverse plane - disturbed the equilibrium state
- M vector is rotating about B0
- Induce a change in the magnetic field in the coil
- Create a current in the coil
