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P2 PHYSICS > Why is MRI so popular? > Flashcards

Why is MRI so popular? Flashcards

1
Q

Why MRI?

A
  1. Based on magnetic resonance of water molecules
  2. Non-ionising
  3. Full 3D imaging technique
  4. Has excellent contrast in soft tissue
  5. Very versatile
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2
Q

What are the MRI Field strength?

A 
1T = 1 Tesla
0.00005T = Average Earth's field
1.5T = Typical clinical MR scanner
3T = becoming standard MR in UK
7T = many in US, Europe, Asia, Australia
10.5T = Highest human MR scanner
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3
Q

What does MRI look at?

A

Molecular level =10-9 m
Atomic level =10-10 m
Nucleus of hydrogen atom=10-13 m

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4
Q

Where does spin rotate (or precess)?

A

About applied magnetic field B0 i.e. along z axis

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5
Q

What is the frequency of this precession proportional to?

A

Applied field

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6
Q

What does each water molecule within body have?

A

Small magnet associated with it

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7
Q

What is Larmor equation?

A

v= γB0

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8
Q

What does MRI rely on?

A

Magnetic properties of hydrogen atom to produce images

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9
Q

What is H nucleus composed of?

A
  1. Single proton
  2. Spinning charged particle
  3. Produces a magnetic field = magnetic moment
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10
Q

How are the protons orientated?

A

Randomly orientated with no applied field

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11
Q

What are the MRI components?

A
  1. Primary magnet
  2. Gradient magnet
  3. Radiofrequency (RF) coils
  4. Computer system
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12
Q

What does the primary magnetic field refer to?

A

The strength of the static permanent field

e.g. 1.5 or 3T

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13
Q

How does hydrogen atoms align?

A

parallel or antiparallel to primary field (B0)

known as longitudinal magnetisation

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14
Q

How does a greater proportion of the hydrogen protons align?

A

Direction parallel to the primary magnetic field or low energy state than align anti-parallel to the primary magnetic field (high energy state)

net result = net magnetic vector (M)

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15
Q

What is precession?

A

The protons spin on their axis

protons spin around long axis of primary magnetic field

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16
Q

What is the precession rate termed?

A

Larmor frequency

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17
Q

What is in phase?

A

When protons precess together?

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18
Q

What is out of phase?

A

When protons precess separately

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19
Q

What does frequency change with?

A

In proportion to the magnetic field strength

At 1.5T = 63.9 MHz

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20
Q

What do gradient coil generate?

A

Secondary magnetic field over the primary field
located within the bore of the primary magnet
They are arranged in opposition to each other to produce positive and negative poles

21
Q

Why do gradient coil give MRI the capacity to do?

A

Image directionally along the z, x and y axis

22
Q

What do gradient magnet alter?

A

the strength of primary magnetic field changing the precession frequency between slices
this can be used for slice selection and localised in the x, y and z axis called spatial encoding of MR images

23
Q

Where does the z gradient run along?

A

Long axis to produce axial images

24
Q

Where does the y gradient run along?

A

Along the vertical axis to produce coronal images

25
Q

Where does the x gradient run along?

A

Along the horizontal axis to produce sagittal images

26
Q

What is RF coils used for ?

A

Transmitting radio frequency or RF pulse and receiving signals in MR

27
Q

What are RF coils designed for?

A 
Specific body regions e.g.
Head
Body
Knee
Shoulder
Wrist
Ankle
produce best diagnostic images
28
Q

What is RF coil used to transmit/

A

Second magnetic field/RF pulse which results in a disturbance of the proton alignment

29
Q

How is longitudinal magnetisation decreased?

A

Some low energy parallel protons flip to a high energy state

30
Q

When does net magnetization vector turn toward transverse plane?

A

When protons become synchronised and precess in phase

31
Q

What is the transverse plane?

A

Right angles to the primary magnetic field = transverse magnetisation

32
Q

What is radio frequency or RF coil for?

A

Receive signal to create images as protons resume their normal state in primary magnetic field prior to the transmission of the RF pulse = this is called relaxation

33
Q

What is relaxation in the longitudinal axis?

A

T1 relaxation

34
Q

What is relaxation in the transverse axis?

A

T2 relaxation

35
Q

What happens after the RF pulse?

A

Several protons flip back to their low energy state parallel to primary magnetic field z axis giving energy to surroundings the lattice
This results in changes to the longitudinal relaxation known as T1/ or spin lattice relaxation

36
Q

What increases with time?

A

Magnetisation

T1 curve

37
Q

What is spin-spin relaxatin?

A

After RF pulse, protons that were in phase begin to dephase out of larmor frequency in the tranverse (x-y) plane
Reduction in tranverse magnetisation

38
Q

Why does spins dephase much quicker than T2?

A

Inhomogeneities in the magnetic field (B0)

39
Q

What is T2*?

A

T2 relaxation + field inhomogeneities

40
Q

What is net magnetic vector?

A

Sum of longitudinal and transverse magnetisation

41
Q

Where does the net magnetic vector spiral around?

A

Z axis with net precession

42
Q

What results in free induction decay (FID)?

A

Changing magnetic moment of net magnetic vector
Induces an electrical signal
Signal received by the RF coil in transverse plane

43
Q

What is contrast ?

A

Difference in appearance of different tissues in an image

44
Q

What is X-ray contrast based on?

A

Transmission

45
Q

What is one of the most important characteristic of MRI?

A

Range of contrasts available

Can visualise aspect of tissue structure, composition and function

46
Q

What is magnetic susceptibility?

A

Degree of magnetization of a material in response to applied magnetic field

47
Q

What is magnetisation transfer?

A

physical process by which macromolecules and their closely associated water molecules (the “bound” pool) cross-relax with protons in the free water pool.

48
Q

What are the advantages of MRI?

A
  • MRI does not use ionizing radiation;
  • MRI has a much greater range of available soft tissue contrast;
  • MRI scanning can be performed in any imaging plane;
  • MRI uses safer contrast agents;
  • MRI can produce quantitative images that reflect underlying biophysical properties;

P2 PHYSICS (16 decks)

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