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Principles of CN > Lecture 4: Methods - MRI > Flashcards

Lecture 4: Methods - MRI Flashcards

1
Q

Diagram of MRI image

A
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2
Q

Lecture focuses on - (2)

A
  • how MRI image generated
  • How functional MRI image (different regions of brain) is active
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3
Q

There is key components in MRI scanner which is - (3)

A

Static magnetic field
Gradient coils
Radio-frequency (RF) coils

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4
Q

There are 3 different gradien coils in MRI scanner , important for creating an image, which is - (3)

A

X,Y and Z

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5
Q

MRI measures the atom

A

hydrogen

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6
Q

The earth has a

A

magnetic field

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7
Q

M in MRI is for

A

magnetic

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8
Q

Magnetic field of earth created by the

A

earths rotation and iron in the inner core.

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9
Q

The static magnet field in MRI is generated by an

A

large electrical coil.

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10
Q

Strength of the magnetic field is determined by the

A

amount of current following through the coil.

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11
Q

3 or 7 tesla magnet in MRI are

A

different strength of magnet and higher magnet gives better signal to noise

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12
Q

Most MRI scanners in hospitals is

A

1.5 T

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13
Q

Magnetic field of Earth is …. smaller than MRI magnet

A

50 000

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14
Q

Ensure homeogenity of magnetic field in MRI which is determined by

A

the number of turns in the coil and by the ratio of length to diameter.

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15
Q

We need to ensure homeogenity of magnetic field in MRI as it gives

A

better MRI signal

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16
Q

For a homegenous coil we want a long and narrow coil but not feasible as

A

people will get claustrophobic in scanner

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17
Q

The MRI is developed from the technique NMR (nucleus magnetic resonance) with N representing

A

nuclear

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18
Q

All matter is composed of

A

atoms

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19
Q

Example of matter composed of atoms

A

Brain is made of up 75% of water atoms

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20
Q

The nucleus of each atom in matter contains

A

contains positively charged protons.

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21
Q

Hydrogen (mostly in the form of water) is the most and its nucleus contains… - (2)

A

abundant atom in the body.

Its nucleus contains a single proton.

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22
Q

Because the proton in a hydrogen atom has a single electrical charge that spins, it creates a corresponing

A

magnetic field

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23
Q

MRI commonly measures magnetic signals from

A

hydrogen

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24
Q

The R in MRI is

A

resonance

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25
Q

When (hydrogen) protons are placed in the static magnetic field of an MRI, they

A

resonate around the main axis of the external field

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26
Q

The precision field of H in a 7T magnet is

A

298 MTS

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27
Q

The frequency at which the hydrogen protons precess around main axis of external field is known as

A

Larmor frequency

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28
Q

The Larmor frequency depends on the .. - (2)

A

atom and the field strength

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29
Q

The Larmor frequency for Hydrogen is

A

42.58 MHz/T.

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30
Q

In a 3T magnet a hydrogen atom precesses about

A

127 million times per second

31
Q

When hydrogen atoms are placed in a static magnetic field, they align to the

32
Q

If RF at precession freq (see Larmor) apply perpendicular to z direction it can

A

knock hydrogen atoms to x and y direction

33
Q

A receieve coil measure the signal we get in

A

x and y plane

34
Q

When hydrogen atoms precess at 90 degrees to main magnetic field and transerve (or x,y plane) then

A

coil produces electrical current

35
Q

The T2 decay is that - (2)

A

hydrogen atoms transerving across x and y plane together (coherent magnetic field) then spread out

Causing reduction in fMRI signal you get in x and y direction

36
Q

Equal magnetic signals that point at all directions will

A

cancel out

37
Q

T2 is important for especially

38
Q

After T2 takes place, static magnetic field takes dominant and pulls hydrogen atoms in direction of static field which is

39
Q

Schematic of T2 decay - (3)

A
  • Blue - hydrogen atoms in different directions
    • We still get average direction - some signal
  • After time, some spread out spreading into different direction - then signal is zero
40
Q

T2 delay happens in

A

milliseconds - quickly

41
Q

T1 takes … to happen

42
Q

Schematic of T1 recovery shows that

A

Magnetic field takes dominant and pulls hydrogen atoms to z direction

43
Q

The gradient coils chages the

A

strength of static magnetic field in x,y and z direction

44
Q

If the magnetic field was increased along the z axis (sometimes referred as B0) the RG to excite

A

top of the brain would be different to the bottkm

45
Q

If the magnetic field was increased along the z axis (sometimes referred as B0) the RG to excite top of the brain would be different to the bottom - why?

A

z gradient changes magnetic field strength from top to bottom

Strength at top of brain is 3.1 T and bottom of brain is 2.9 T and strength in middle is 3

What we do in order to excite sample, we apply RF which has to be matched with precession frequency of hydrogen atoms

That precision frequnency determined by magnets

Top is fastest as compared to bottom

If we change RF we can excite different slides of brain

46
Q

Once excited slice of brain along z axis we can also excite it on the

A

x and y axis

47
Q

MR pulse sequences is sequence of

A

creating an MR image

48
Q

MR Pulse sequences - (4)

A
  • RF = Radio frequency pulse which is matched with particular z gradient
  • Apply Z gradient coil that selects a specific slice
  • Apply different levels of x and y gradient which allows spatial info along x,y, plane of that specific slice (Gx, Gy)
  • We then do the measurement of measuring magneitc signal as it goes through the pip
  • TE i(short)s time from RF pulse to measurement
  • TR (long) is time between succession radio frequency pulses
  • Can vary TE and TR to pick out different types of tissue
49
Q

Different MRI pulse sequences leading to different

A

types of images

50
Q

Different types of MRI images you can obtain - (3)

A
  • Proton density imaging
  • T1 contrast
  • T2 contrast
51
Q

For proton density imagining we would want in terms of TE and TR - (2)

A

long TR
short TE

52
Q

The T1 contrast is the most image that is commonly

53
Q

The T1 contrast used for ..

A

anatomical images of the brain

54
Q

We can’t do proton density in T1 contrast as - (2)

A
  • density of water same so signal would be the same
  • Here is interested in T1 recovery
55
Q

T2 contrast has long and intermediate

A

long TR and intermedite TE

56
Q

TE gives biggest difference between

57
Q

T2 used most clinically as picks up … and also sued for

A

a lot of fluied and used in fMRI

58
Q

Oncograph measures expansion of a

A

somatosensory cortex when the sciatic nerve from the leg was stimulated

59
Q

Experiment by Roy and Sherrington in which revealed

A

link between blood flow and brain function

60
Q

Experiment by Roy and Sherrington show

A

change in blood flow goes along the activity in region of somatosensory cortex

61
Q

The core sources of energy we have for brain is - (2)

A

glucose and oxygen

62
Q

Anaerobic respiration is when

A

glucose is only used for energy reserve

63
Q

glucose not that efficient in

A

producing energy

64
Q

Aerobic (oxygen + glucose) resp produces more ATP (34) than

A

anaerobic respiration (glucose)

65
Q

Oxygenated haemoglobin has no

A

magnetic momement - dimagnetic

66
Q

Haemglobin is a protein that can allows to

A

carry oxygen around body

67
Q

Diagram of MRI signal when oxygenated vs deoxygenated

68
Q

deoxygenated haemoglobin is paramagnetic and disrupt the

A

MRI signal

69
Q

The magnetic properties of deoxyHb causes spin dephasing (loss of syncrhonisation) of

A

hydrogen atoms in the transverse direction.

70
Q

The magnetic properties of deoxyHb causes spin dephasing of hydrogen atoms in the transverse direction.
This results in the

A

T2 decay being significantly shorter in the presence of deoxyHb than oxyHB (T2* decay)

71
Q

If there is less oxygen in Hb then less

A

MRI signal

72
Q

BOLD stands for

A

blood oxygen level dependent

73
Q

When region is active in brain in MRI there is a higher concentration of

A

oxygenated Hb than dexoygenated Hb

Principles of CN (3 decks)

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