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P2 PHYSICS > Origin of signal: Excitation, Relaxation > Flashcards

Origin of signal: Excitation, Relaxation Flashcards

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1
Q

What is the main field?

A

B0

Along the Z direction

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2
Q

What is found inside the scanner?

A
  1. Precession

2. Bulk/net magnetization

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3
Q

What is Larmor frequency?

A

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

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4
Q

What is Larmor frequency?

A

Product of general magnetic ratio and B0

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5
Q

What is bulk magnetization?

A

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

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6
Q

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

A

The M vector

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7
Q

Where can you not measure any signal?

A

At equilibrium

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8
Q

How can you rotate M vector?

A

Using Radiofrequency pulse and B1 field

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9
Q

What is perpendicular to Z?

A

B1 field

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10
Q

What are the same?

A

Z and B0

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11
Q

Where does B1 field rotate?

A

About Z axis at Larmor frequency

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12
Q

What is the flip angle?

A

Angle just after excitation between M vector and Z axis

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13
Q

What does the amplitude depend on?

A

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

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14
Q

What is perpendicular to B0?

A

Magnetisation vector M

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15
Q

What is used to measure signal?

A

Coil

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16
Q

What is the coil able to measure?

A

Transverse component of M vector

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17
Q

What can M vector be decomposed into?

A

Transverse component and Longitudinal component

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18
Q

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

A
  1. Radio Frequency (RF) field
  2. Transmit field
  3. B1 field
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19
Q

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

A

M rotates about B0 all the time

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20
Q

How can you avoid having just only one signal?

A

Measure one M per voxel

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21
Q

What is each M vector?

A

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

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22
Q

What is the total magnetic field?

A

No gradient (B0) + GzZ gradient

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23
Q

What does B1 field used for excitation have?

A

Frequency

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

24
Q

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

A

It will not be able to rotate M

25
Q

With a gradient

A

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

26
Q

when will all the protons be excited?

A

Apply radio frequency pulse at omega 0

27
Q

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

A

Just a slice will be excited when the gradient is 0

28
Q

What is omega 0?

A

Gamma B0

29
Q

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

A
  1. Change the slope of the gradient (amplitude)

2. Change the bandwidth of the pulse

30
Q

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

A
  1. Change frequency of the pulse
  2. Change the slope of the gradient (amplitude)
  3. Change the bandwidth of the pulse
31
Q

What happens once a detectable signal is generated?

A

It decays

32
Q

What wants to be aligned with the strong main field?

A

Magnetisation vector

33
Q

What occurs once detectable signal is generated?

A

T1 Recovery/Relaxation of longitudinal magnetisation

34
Q

RF pulse at 90 degrees

A

Transverse magnetisation is at its maximum

There is no longitudinal magnetisation

35
Q

Relaxation

A
  1. Precession (static in rotating frame)
  2. Return to equilibrium
    - Mz regrows
    - Mxy decays
36
Q

What is the definition of T1?

A

Time taken to reach 63% of the equilibrium value

37
Q

What does the time taken for T1 depend on?

A

Ability of the local environment to faciliate relaxation

Depends on the micro structure of the tissue

38
Q

What decays more rapidly than the longitudinal component

A

Transverse component

39
Q

What is T2 decay of transverse component?

A
  1. A more rapid process

2. Nuclei alter the field experienced by each other and lead to ‘dephasing’ and signal loss

40
Q

What is M?

A

Sum of protons in one voxel

41
Q

What is the definition of T2?

A

Time taken to decay to 37% of its original value

42
Q

What does the time taken for T2 depend in?

A

How quickly the spins get out of phase

43
Q

3 T2 - 90milliseconds

A

No signal left

44
Q

What is another phenomenon of Transverse component?

A

Additional T2’ decay

Due to inhomogeneity of the main field B0

45
Q

T2*

A

T2 Decay + T2’ Decay

1/T2* = 1/T2 + 1/T2’

46
Q

T2 Decay

A

The signal loss cannot be recovered

47
Q

T2* Decay

A

The additional signal loss can be recovered

48
Q

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

A

RF excitation

49
Q

What is B1 pulse characteristics?

A 
B1 oscillates in x-y plane with frequency 𝜔_𝑟𝑓, and only excites magnetization precessing at 𝜔_0=𝜔_𝑟𝑓.
 𝑀⃗  rotates about B1 at rate 𝜔_1.
Flip angle: 〖𝜃=𝜔〗_1∙𝑡
𝑀_𝑧=𝑐𝑜𝑠𝜃
𝑀_𝑥𝑦=𝑠𝑖𝑛𝜃
50
Q

What are the types of RF pulse?

A

90º excitation
180º excitation
180º inversion

51
Q

What is present during excitation?

A

There is only B0 and B1

B0 is around Z and B1 is rotating

52
Q

What T and Omega 1?

A
  1. T= duration of the pulse

2. Omega 1 = the frequency about which M vector rotates about B1

53
Q

What can you do to change theta?

A
  1. Increase time of the pulse (duration)

2. Change Omega 1 - Change B1

54
Q

What is 180 degree Inversion?

A

Pulse that inverts the spin magnetisation

55
Q

What does spin echo sequence include?

A

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

P2 PHYSICS (16 decks)

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