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neuroimaging 1 > MRI Physics Extra > Flashcards

MRI Physics Extra Flashcards

1
Q

Why do more protons align with the magnetic field?

A

The direction in line with the magnetic field is slightly more energetically favourable than aligning away from the magnetic field

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2
Q

What creates a magnetic vector?

A

The sum of all the protons in an area (some aligned with the magnetic field and some aligned in the opposite direction) will be in line with the magnetic field creating a magnetic vector in the direction of the magnetic field

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3
Q

What happens when a person is placed into the magnetic field (B0)?

A

All the protons in the body are effected by the magnetic field

Some protons align with the magnetic field

Some protons align is the opposite direction to the magnetic field

There are more protons in the upper energy state (aligned with magnetic field) and therefore the total sum creates a net magnetisation in the same direction as B0

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4
Q

What happens to the protons when we adjust the magnetic field?

A

The protons stay in their alignment but begin to rotate or precess around B0

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5
Q

What is Larmor frequency?

A

The rate of precession of protons about B0

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6
Q

How do we push the net magnetisation away from B0?

A

We introduce a new oscillating or rotating magnetic field at the Larmor frequency through the use of an electromagnetic field

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7
Q

Why do we need to apply a radio frequency pulse?

A

For a 1.5T magnet, the Larmor frequency will be 63 MHz which is in the radio-frequency range

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8
Q

Why does the RF pulse need to be at the Larmor frequency?

A

Actually all electromagnetic waves will impart a degree of energy to the system

However, things applied at the natural frequency of the system (Larmor frequency) will most efficiently transfer the energy of the wave to the system

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9
Q

What is the function of the RF pulse?

A

To apply an alternating magnetic field which excites the spins at the Larmor frequency which causes the net magnetic vector inside the body precess at the Lamor frequency

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10
Q

How do we alter the amplitude of the detected signal?

A

If you change the angle of the precession around B0, the size of the detected signal changes

This is called the flip angle - the angle of precession in regard to B0

The bigger the flip angle, the bigger the signal
If the precession is in line with B0 you get a very small signal

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11
Q

How do we change the frequency of the detected signal?

A

Increasing or decreasing the magnetic field (B0)

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12
Q

What principle does MRI take advantage of?

A

Law of magnetic induction ==
An alternating magnetic field (our precessing net magnetic vector) induces a voltage and therefore, current in a nearby coil

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13
Q

Does the MR signal represent a continuous sin wave?

A

The MRI machine records a decaying sinusoidal current coming solely from the XY plane as we lose energy

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14
Q

What is the name for the decaying sinusoidal wave?

A

T2 decay

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15
Q

What does T2 decay represent?

A

The decay of each individual voxel summed together

–> Fat voxels may create a larger signal whilst tissue may create a smaller signal but the T2 decay signal represents the sum of all these voxels together

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16
Q

How do we build a contrasted image based on T2 decay?

A

We can choose a time to compare and calculate the current coming from each individual voxel to build an image

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17
Q

What is the reason for “echo”?

A

Free induction decay isn’t long enough to be able to create a good image so an echo is needed to build the signal back up and then decay again

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18
Q

To create a larger flip angle how much energy does it take?

A

It takes more energy to create a larger flip angle

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19
Q

To create an image what is the main goal of measuring the signal?

A

Need to localise the signal in 3 dimensions i.e., the signal coming from each individual voxel

Z axis
X axis
Y axis

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20
Q

What are gradient coils?

A

Three sets of gradient coils in each axis

Used to excite the protons and vary the Larmor frequency throughout each axis and therefore, a current at Larmor frequency can be applied to detect a specific location in the body

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21
Q

How do we localise the signal in the z axis?

A

Slice select gradient

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22
Q

How is slice selection achieved?

A

Two coils of wire are at either end of the body

Current is placed through these wires to create and electromagnet

A reverse current (B1) is applied to the coil at the head producing a magnetic field in the opposite direction to B0 , B1 is subtracting from the main field B0

A direct current is applied to the coil at the feet which aligns with the main field and therefore adds to B0

The result is a magnetic field which goes in a diagonal line across the body which uniquely varies from head to tall

This also means the Larmor frequency will vary head to toe

23
Q

To excite a slice in the body what is needed?

A

Simply turn on the coils at the head and feet and apply an RF pulse at the desired Larmor frequency of the corresponding slice

Therefore, all signal recorded will only be from the specific slice you have picked

24
Q

What is the isocenter?

A

The point where the two magnetic fields cross and equals exactly that of the main magnetic field

25
Q

What process occurs in the X axis?

A

Frequency encoding

26
Q

How does frequency encoding work?

A

To vary the magnetic field throughout the voxels of tissue and CSF again we use two coils in the x axis direction to create a linearly variant magnetic field throughout the body (same principle as for slice selection)

However, we have already excited the slice, the protons are already spinning, generating signal in the receiver coil so don’t need to apply another RF pulse

Therefore, we turn the coils on after slice selection and then record the signal

This generates a more complex MR signal - we have encoded spatial information along the x axis through differences in frequency

27
Q

How do we retrieve the individual T2 signals from the CSF and fat voxels from the raw MR signal?

A

Fourier transform - allows us to separate out the individual frequencies making up the complex raw signal

28
Q

How can we generate an image using the T2 signals from CSF and fat found using Fourier transform?

A

We can select a time point to compare the signals (TE), calculate the corresponding amplitude of each signal at that time point and correctly place it within the appropriate voxel because we know the frequencies will go from low to high in a left to right direction

Therefore, which ever T2 decay curve has the lower of the frequencies most be the voxel on the left

29
Q

How do we locate the signal along the y axis?

A

Phase encoding

30
Q

How does phase encoding work?

A

Two gradient coils are placed at either end of the y axis, if we can measure how the signal changes from the x axis to the y axis we can locate the signal

In order to locate in the y axis, you need to be able to change the phase of the MR signal of each voxel

By doing this, you can use formulas to work out the amplitude of each signal to locate the voxel

When both signals are in phase, the sum signal will have a larger amplitude whereas out of phase, one wave will be have a peak and the other phase will have a trough and therefore the sum will be smaller

If you can measure the signal both in phase and out of phase you can create two formulas = in phase: A + B = 8Amps, out of phase: A - B = 4 Amps

You can then rearrange these equations to find out A and B

31
Q

In a simple 2x2 matrix how many phase encodings would need to be applied to work out each of the 4 voxels?

A

2 phase encodings = 1 for each column

This increases with an increase in matrix

32
Q

How do we change the phase of the signals?

A

By turning the gradient coil on and off in the y direction

When you turn the gradient on, a different magnetic field is created for the bone and tissue signal and therefore different Larmor frequencys from the CSF and fat signal

When the gradients are turned off, they return to the same Larmor frequency but are not the same phase

We have induced a phase shift by momentarily disrupting their Larmor frequencies

33
Q

What does the amount of phase shift depend on?

A

Depends on the strength of the gradient field as well as the length of time the gradient is turned on

34
Q

What is T2 decay?

A

Signal of initially in-phase protons dephasing after excitation into the XY plane

35
Q

What does T2 weighting represent?

A

Bone, soft tissue and CSF all have different T2 decay times

CSF has a very slow decay time as it is majority water

Soft tissue has a slightly slower decay time than CSF but still contains water

Bone has a very quick decay time

Therefore, a long TE will create a heavily weighted T2 weighted image where CSF looks bright (high signal), bone looks very dark (lowest signal) and soft tissue looks grey

36
Q

How do we change how heavily T2-weighted our image contrast will be?

A

The time we choose to compare our recorded signal, the time-of-echo, TE, dictates how heavily weighted the image contrast will be

The longer the TE, the heavier T2-weighting

37
Q

For a T1 weighted image, how do we minimise the effects of T2 contrast?

A

Pick a very short TE time i.e., straight after the RF pulse

38
Q

What is spin-spin relaxation?

A

Another name for T2 decay because it reflects how spinning protons speed up or slow down relative to surrounding spins in the XY plane

39
Q

What does T1 weighting represent?

A

Represents the time is takes for the hydrogen protons to align with the magnetic field

40
Q

What happens during T1 recovery?

A

Apply an RF pulse to knock the magnetic field into the xy plane

The magnetic field precesses about the z axis creating signal in the coils

The signal is then lost and then you have to wait until all the protons realign along the z axis and the net magnetisation has built back up

41
Q

What takes longer T2 decay or T1 recovery?

A

T1 recovery takes much longer than T2 decay

42
Q

What does contrast represent?

A

The differences in our measured values

43
Q

Explain the process of T1 recovery and T2 decay

A
  1. Net magnetisation is in the z axis
  2. Apply an RF pulse which knocks the magnetisation into the xy plane where it precesses
  3. This generates signal in the receiver coil showing T2 decay
  4. Eventually the signal is completely lost as it dephases
  5. Then the net magnetisation returns to the z axis after a time period which represents T1 recovery
44
Q

What happens if you apply the RF pulse earlier before waiting for T1 recovery?

A

The amplitude of T2 decay is smaller

45
Q

What is TR?

A

Time to repetition - time until the next RF pulse

46
Q

What dictates T1 weighting?

A

Time to repetition

A short TR maximises T1 image contrast
A long TR minimises T1 image contrast

47
Q

What is spin-lattice interaction?

A

Another name for T1 recovery

Refers to how quickly precessing protons can shed their energy into their surrounding environment and realign with B0

48
Q

How does the flip angle affect T1 recovery?

A

The greater the flip angle, the more energy must be shed into the surrounding tissues to realign back with B0

Therefore, flip angle significantly contributes to tissue heating

49
Q

What do we do to create either a T1 weighted or T2 weighted image?

A

Change the TE and TR times

50
Q

What TE and TR times do you need to create a T2 weighted image?

A

Long TR to reduce the contrasts of T1 recovery
Long TE to maximise signal differences from varying T2 decay rates

51
Q

What TE and TR times do you need to create a T1 weighted image?

A

Short TR to catch the tissues at different stages of T1 recovery
Short TE where each tissue has wide variation

52
Q

What is a proton density weighted image?

A

Long TR
Short TE

53
Q

Why is it called proton density?

A

Because there’s a better correlation on this sequence with the amount of protons the tissue contains and how much signal we see

neuroimaging 1 (16 decks)

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