Origin of signal: Excitation, Relaxation

Question 1

What is the main field?

Answer 1

B0

Along the Z direction

Question 2

What is found inside the scanner?

Answer 2

1. Precession

2. Bulk/net magnetization

Question 3

What is Larmor frequency?

Answer 3

Every proton/every spin, every nuclear magnet moment rotates about B0

Question 4

What is Larmor frequency?

Answer 4

Product of general magnetic ratio and B0

Question 5

What is bulk magnetization?

Answer 5

Creation of magnetization vector M which is the sum of every nuclear magnetic moment

Question 6

What is parallel to the Z axis, same axis in the equilibrium state?

Answer 6

The M vector

Question 7

Where can you not measure any signal?

Answer 7

At equilibrium

Question 8

How can you rotate M vector?

Answer 8

Using Radiofrequency pulse and B1 field

Question 9

What is perpendicular to Z?

Answer 9

B1 field

Question 10

What are the same?

Answer 10

Z and B0

Question 11

Where does B1 field rotate?

Answer 11

About Z axis at Larmor frequency

Question 12

What is the flip angle?

Answer 12

Angle just after excitation between M vector and Z axis

Question 13

What does the amplitude depend on?

Answer 13

Scanner you are using e.g. 1.5T or 3T

Question 14

What is perpendicular to B0?

Answer 14

Magnetisation vector M

Question 15

What is used to measure signal?

Answer 15

Coil

Question 16

What is the coil able to measure?

Answer 16

Transverse component of M vector

Question 17

What can M vector be decomposed into?

Answer 17

Transverse component and Longitudinal component

Question 18

What is needed to be able to rotate M towards the transverse plane?

Answer 18

1. Radio Frequency (RF) field
2. Transmit field
3. B1 field

Question 19

What creates a variation in magnetic transverse component through the coil?

Answer 19

M rotates about B0 all the time

Question 20

How can you avoid having just only one signal?

Answer 20

Measure one M per voxel

Question 21

What is each M vector?

Answer 21

The sum of every nuclear magnetic moment, every proton within one voxel

Question 22

What is the total magnetic field?

Answer 22

No gradient (B0) + GzZ gradient

Question 23

What does B1 field used for excitation have?

Answer 23

Frequency

This frequency is required to excite the protons - rotate the M vector - create a signal

Question 24

What happens if B1 field is not at the Larmor frequency or frequency of M vector around B0?

Answer 24

It will not be able to rotate M

Question 25

With a gradient

Answer 25

Frequency of the protons will be different one side of the table than the other because the frequency of protons is proportional to B0?

Question 26

when will all the protons be excited?

Answer 26

Apply radio frequency pulse at omega 0

Question 27

What happens if you apply a gradient to the radio frequency pulse at omega 0?

Answer 27

Just a slice will be excited when the gradient is 0

Question 28

What is omega 0?

Answer 28

Gamma B0

Question 29

What do you do if you want to select a different slice?

Answer 29

1. Change the slope of the gradient (amplitude) | 2. Change the bandwidth of the pulse

Question 30

What do you do if you want to change the width of the slice?

Answer 30

1. Change frequency of the pulse 2. Change the slope of the gradient (amplitude) 3. Change the bandwidth of the pulse

Question 31

What happens once a detectable signal is generated?

Answer 31

It decays

Question 32

What wants to be aligned with the strong main field?

Answer 32

Magnetisation vector

Question 33

What occurs once detectable signal is generated?

Answer 33

T1 Recovery/Relaxation of longitudinal magnetisation

Question 34

RF pulse at 90 degrees

Answer 34

Transverse magnetisation is at its maximum | There is no longitudinal magnetisation

Question 35

Relaxation

Answer 35

1. Precession (static in rotating frame) 2. Return to equilibrium - Mz regrows - Mxy decays

Question 36

What is the definition of T1?

Answer 36

Time taken to reach 63% of the equilibrium value

Question 37

What does the time taken for T1 depend on?

Answer 37

Ability of the local environment to faciliate relaxation | Depends on the micro structure of the tissue

Question 38

What decays more rapidly than the longitudinal component

Answer 38

Transverse component

Question 39

What is T2 decay of transverse component?

Answer 39

1. A more rapid process | 2. Nuclei alter the field experienced by each other and lead to 'dephasing' and signal loss

Question 40

What is M?

Answer 40

Sum of protons in one voxel

Question 41

What is the definition of T2?

Answer 41

Time taken to decay to 37% of its original value

Question 42

What does the time taken for T2 depend in?

Answer 42

How quickly the spins get out of phase

Question 43

3 T2 - 90milliseconds

Answer 43

No signal left

Question 44

What is another phenomenon of Transverse component?

Answer 44

Additional T2' decay | Due to inhomogeneity of the main field B0

Question 45

T2*

Answer 45

T2 Decay + T2' Decay | 1/T2* = 1/T2 + 1/T2'

Question 46

T2 Decay

Answer 46

The signal loss cannot be recovered

Question 47

T2* Decay

Answer 47

The additional signal loss can be recovered

Question 48

How can we recover the signal lost because of B0 homogeneity?

Answer 48

RF excitation

Question 49

What is B1 pulse characteristics?

Answer 49

``` B1 oscillates in x-y plane with frequency 𝜔_𝑟𝑓, and only excites magnetization precessing at 𝜔_0=𝜔_𝑟𝑓. 𝑀 ⃗ rotates about B1 at rate 𝜔_1. Flip angle: 〖𝜃=𝜔〗_1∙𝑡 𝑀_𝑧=𝑐𝑜𝑠𝜃 𝑀_𝑥𝑦=𝑠𝑖𝑛𝜃 ```

Question 50

What are the types of RF pulse?

Answer 50

90º excitation 180º excitation 180º inversion

Question 51

What is present during excitation?

Answer 51

There is only B0 and B1 | B0 is around Z and B1 is rotating

Question 52

What T and Omega 1?

Answer 52

1. T= duration of the pulse | 2. Omega 1 = the frequency about which M vector rotates about B1

Question 53

What can you do to change theta?

Answer 53

1. Increase time of the pulse (duration) | 2. Change Omega 1 - Change B1

Question 54

What is 180 degree Inversion?

Answer 54

Pulse that inverts the spin magnetisation

Question 55

What does spin echo sequence include?

Answer 55

slice selective 90-degree pulse followed by one or more 180 degree refocusing pulses