Origin of MR signal 1

Question 1

What is main field, B0?

Answer 1

E.g. 1.5T, 3T or 7T
It is a vector 
A vector has:
1. Magnitude e.g. 3T
2. Direction: Defined as Z

Question 2

What is always on?

Answer 2

Main field B0

Question 3

Where does the axis of B0 point?

Answer 3

Along the axis of z direction

Build a frame of reference: Z along B0 and X and Y

Question 4

Where is Z?

Answer 4

Table Along the MR scanner and points out of the scanner

Question 5

what does each proton act as ?

Answer 5

Little magnet

Question 6

What is the name of vector of proton?

Answer 6

Nuclear magnetic moment

Question 7

Outside scanner

Answer 7

Not experiencing magnetic field

Question 8

Insider the scanner: M is the vector

Answer 8

Can be decomposed into 3 components: x,y,z

Question 9

What is B0 applying?

Answer 9

Force on little protons

Name of force = torque

Question 10

What does the Bloch equation dictate?

Answer 10

Motion of the protons

Question 11

What was the assumption of Bloch equation?

Answer 11

Millions of individual nuclei in a sample could be represented by a single vector M

Question 12

What does external field (B) produce?

Answer 12

A torque (T) or twisting force on M resulting in its precession  at angular frequency
ω = γ B

Question 13

What is frequency of precession about B0 axis proportional to?

Answer 13

Main magnetic field B0

Question 14

What is Larmor equation?

Answer 14

ω = γ B
constant is called gyromagnetic ratio
-depends on the nucleus
-equivalent to 42.5MHz/T for hydrogen nuclei

Question 15

What happens when someone is put in the scanner?

Answer 15

1. Precession
   - the nuclei precess about the field
   - precessional frequency:ω = γ B
2. Bulk magnetisation

Question 16

What happens when you enter the scanner?

Answer 16

All of the protons align with the B0 field

Question 17

What is quantum physics?

Answer 17

One state is in higher energy state than the other

Question 18

Where will there be more protons?

Answer 18

Lower energy state

Question 19

What is the summary for when you enter the scanner?

Answer 19

1. A magnetisation vector is created pointing in the same direction B0 axis

Question 20

Amplitude of the magnetisation vector

Answer 20

is proportional to the magnetic field

Question 21

What is Faraday’s law?

Answer 21

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.

Question 22

Signal Generation

Answer 22

1. Strong magnetisation due to abundance of 1H
2. M is aligned along B0 and static
3. No signal yet

Question 23

How do you measure a signal?

Answer 23

Variation in the magnetic field

Question 24

What is the second magnetic field called?

Answer 24

1. Transmit field
2. RF field
3. B1 field

Question 25

What is RF field: B1?

Answer 25

This field is turned on to disturb the system from equilibrium

2. This process of disturbance is called ‘‘Excitation’’
3. The amount of ‘‘excitation’’ is the flip angle
4. This field is not static but oscillates and has a frequency

Question 26

Signal generation - Laboratory Frame of Reference

Answer 26

1. Equilibrium alignment of Magnetisation vector
2. Apply additional magnetic field (B1): Magnetisation precesses
3. When the B1 field is switched off the transverse magnetisation remains

Question 27

Turn magnetic field for a short period of time

Answer 27

This magnetic field is perpendicular to the z axes

Rotates about the B0 axes

Question 28

What does the Bloch equation combine?

Answer 28

B0 axes and B1 axes

Precession about z axes

Question 29

Where does B1 field also rotate around?

Answer 29

B0

Question 30

Where does the M vector rotate?

Answer 30

Around magnetic field axes that is experiencing

Question 31

Frequency of precession of B1

Answer 31

frequency of B1 has to be exactly the same as the frequency in which M is rotating about B0 [Larmor frequency]

Question 32

Rotating frame of reference

Answer 32

Rotating at the same frequency as the B1 field

Question 33

What do you change to increase the signal?

Answer 33

1. Duration

2. Amplitude

Question 34

What is benefit of having a higher flip angle?

Answer 34

T1 + T2 contrast

Question 35

How do you measure a signal?

Answer 35

1. Flipped to transverse plane - disturbed the equilibrium state
2. M vector is rotating about B0
3. Induce a change in the magnetic field in the coil
4. Create a current in the coil