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