Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Principles of CN Proper > Lecture 4: Methods - MRI - Basics Principles of MR Signal Prod > Flashcards

Lecture 4: Methods - MRI - Basics Principles of MR Signal Prod Flashcards

1
Q

Diagram of MRI image

A

This is T1 weighted contrast - structural image

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Lecture focuses on - (2)

A
  • how structural MRI image generated
  • How functional MRI image (showing different regions of brain activity ) is developed
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

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

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

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

A

X,Y and Z - dirrect orientations

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

The radio-frequency (RF) coils in MRI scanner is also known as

A

head coils

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

The purpose of head coils (RF coils) in MRI scanner is to..

A

stimulate the sample (brain) and also to get info from the sample (brain) - excite and receive

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

MRI typically measures what atom?

A. Carbon
B. Hydrogen
C. Iron
D. Oxygen

A

B. Hydrogen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

The earth has a

A

magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Why does the Earth have a magnetic field? - (2)

A
  • The core of the Earth is iron so it is charged - free electrons going around
  • The Earth also rotates - rotation of charged particles produces a magnetic field–> fact of physics (electromagnetism)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

M in MRI is for

A

magnetic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Magnetic field of earth created by the - (2)

A

earths rotation and iron in the inner core.

It is about 50 000 x smaller than the MRI magnet

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

The static magnet field (principle) in MRI is generated by an

A

large electrical coil - getting current around a coil

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What does this diagram show in terms of static magnetic field? - (2)

A
  • In A (simple) has two coils (rings) and current going around both and producing static magnetic field going through centre - right-hand rule
  • In B (similar to MRI scanner) producing static magnetic field and multiple coils
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Strength of the magnetic field in MRI is determined by the

A

amount of current following through the coil.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

3 or 7 tesla magnet in MRI are

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Most MRI scanners in hospitals is

A

1.5 T

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Homogeneity of static magnetic field in MRI means

A

it should be same throughout and not fluctuating which gices better MRI signal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Ensure homeogenity of magnetic field in MRI which is determined by - (2)

A

the number of turns in the coil and by the ratio of length to diameter of coil (diameter/length)

B is better for MRI scanner than A as only 2 coils

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

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

A

people will get claustrophobic in scanner

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

The coils in MRI scanner need to be cooled to

A

prevent overheating by pumping helium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Ampheres Law/Biot-Savart describes how current creates a magnetic field

Equation:

A

= no. turns/radius * μ * I (current)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What does this diagram show?

A

Brain is made up of 75% of water

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is water made up of?

A

H20 - Two hydrogen atoms and one oxygen atom

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

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

A

nuclear

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Essentially what we are measuring in MRI scanner is

A

nucleus of the hydrogen atom

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

People used to call MRI

A

Nuclear MRI- dropped due to connotations like nuclear weapons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

All matter is composed of

A

atoms

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

The nucleus of each atom in matter contains

A

contains positively charged protons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

The nucleus of hydrogen atom contains…

special feature it does it…

A

One positive charge - protons and its spinning –> creating magnetic field

30
Q

What does B show in diagram? - (2)

A
  • IMagine each hydrogen ion is like a magnet in our bodies
  • We got loads of magnets in our body (since we have a lot of water in our bodies) and pointing at different directions so cancel each other out and don’t attract or repulse
31
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.

32
Q

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

A

magnetic field

33
Q

MRI commonly measures magnetic signals from

A

hydrogen

34
Q

The R in MRI is

A

resonance

35
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

36
Q

What does this diagram show? - (2)

A
  • When placing hydrogen atom in static magnetic field they allign along axis of magnetic field (when lying down)
  • They precess or resonate around the main axis of magnetic field like a spinning top.
  • They precess or resonate at a specific frequency –> important in producing spatial fMRI image
37
Q

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

A

Larmor frequency

38
Q

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

A

atom and the field strength

39
Q

The Larmor frequency is dependent on

A

strength of magnet

40
Q

The Larmor frequency for Hydrogen is

A

42.58 MHz/T. (megahertz per Telsa)

41
Q

In a 3T magnet a hydrogen atom precesses about

A

127 million times per second

42
Q

The precession frequency of H atom in a 7T magnet is - (2)

A

298 MHz

42.58 per T so multiply by 7

43
Q

There are 3 axis to think about in MRI scanner - (3)

A
  1. Z axis : Along the bore of magnet and direction of magnetic field (green arrow)
  2. Y axis and X axis - orthonogonal to magnetic field
44
Q

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

A

main axis - Z axis

45
Q

After hydrogen atoms are alligned to main axis, we are going to

A

knock them down from current state (alligned to z magnetic field into y and x position) – > excitation

46
Q

What does this diagram of excitation show? - (5)

A
  • In A, hydrogen ions alligned to z direction (M) and precessing around but their common magnetic direction is Z direction
  • We have to excite sample (brain) using radio-frequency (RF) pulse
  • If we apply RF pulse perpendicular to Z direction, knock hydrogen ions from Z direction to Y and X direction - shown in B and precess around X and Y plane
  • We have a reciver coil in A which has no current going through the coil (i.e., do/dt = 0)
  • When hydrogen ions go through a certain point in Y and X plane, the magnetic signal they are generating - magnetic field goes through the trreciver coil which generates an electrical signal (works both ways can put current through coil and produces magnetic field OR put magnetic field through coil and produces electrical current) - which is what MRI scanner detects
47
Q

What is radio-frequency (RF) pulse? - (2)

A
  • It is electromagnetic radiation
  • Same frequency we use for radios
48
Q

The frequency of RF pulse is matched to the frequency of the Lamour frequency so RF pulse in 3T and 7T MRI is what.. ? - (2)

A

In 7T MRI, the RF has to be about 298 (7 * 42.58)
In 3T , RF has to be 127 (3 * 42.58)

49
Q

A radiofrequency (RF) pulse at precesson frequency (see Larmor) at excitation will…

then transverse…

a change in magnetic signal in x/y plane…

just as an electrical current flowing through… - (4)

A

knock hydrogen atoms to x and y direction

They then transverse to 90s to main magnetic field (x,y plane)

A change in magnetic signal in x/y plane can then be detected in MRI scanner

Just as an electrical current flowing through a coil causes a magnetic field (see Static magnetic field), a magnetic field passing through a coil causes an electric current to flow

50
Q

A receieve coil measure the signal we get in

A

x and y plane

51
Q

The net magnetization changes in in two directions following excitation - (2)

A

Transverse (x, y) - T2 decya
Longitudinal (z) - T1 recovery

52
Q

The T2 decay is that - (2)

A

hydrogen atoms transerving across x and y plane together (coherent magnetic field) then spread out when knocked down to x and y plane (to all hydrogen ions pointing in different directions - cancel out)

Causing reduction in fMRI signal you get in x and y direction due to spreading out of hydrogen atoms

53
Q

What is T2*? - (2)

A

is the T2 decay in the presence of local magnetic field inhomogeneities (not same) due to changes in blood flow.

FMRI is based on this signal.

54
Q

T2 decay is the time for the decay of transverse signal due to

A

accumulated phase differences caused by spin-spin interactions

55
Q

What is T1 recovery?

A

– the time at which the spins gradually recover to precess in the longitudinal direction B0.

56
Q

Equal magnetic signals that point at all directions will

A. Create magnetic fields in opposite directions

B. Create a magnetic field in one direction

C. Cancel

A

cancel out

C.

57
Q

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

A

z axis - T1 recovery

58
Q

Schematic of T2 decay - (4)

A
  • Blue - hydrogen atoms in different directions
  • When first knock down in left, still going in same direction - average magnetic signal is quite high as atoms coherent with each other
  • After some time, some spread out spreading into different direction - then magnetic signal is zero as cancel out
  • After the net magnetization has been tipped into the transverse plane, it rapidly decays through the loss of coherence among the spinning hydrogen atoms.
59
Q

T2 delay happens in most types of tissues

A

within few hundred milliseconds - quickly

60
Q

T1 takes … to happen

A

1-2 seconds (slower than T2)

61
Q

Schematic of T1 recovery shows that - (2)

A

Static Magnetic field takes dominant and pulls hydrogen atoms to z direction and start resonating around that z direction

After the T2 decay, the static magnetic field, causes protons to precess in the z direction and thus net magnetization recovers in this direction.

62
Q

T2 decay results from

A. frequency of RF pulse
B. changes in syncrhonisation of hydrogen ions as they roate in x,y plane
C. Gradual recovery of magnetic signal in z or B0 direction
D. A reduction in overall frequency at which hydrogen ions rotate in x,y plane

A

B

63
Q

fMRI relies on the fact that

A. oxygenated haemoglobin disrupts homogenity of magnetic field

B. deoxygenated haemoglobin disrupts homogenity of magnetic field

C. Oxygenated haemoglobin reduces direction of magnetic field

D. Dexoygenated haemoglobin reduces direction of magnetic field

A

B

64
Q

TR refers to

A. Time interval between successive RF pulses, usually measured in seconds

B. Time itnerval between data collection and RF pulse

C. time for the decay of the transverse signal due to accumulated phase differences

D. Time taken for spins to return to longiutindal b0 direction

A

A

65
Q

The Larmor frequency for Hydrogen is:

A. 12 MHz/T
B. 22 MHz/T
C. 32 MHz/T
D. 42 MHz/T

A

D

66
Q

Advantages of MRI - (3)

A
  • Non-invasive as participants exposed to strong magnetic field and radio waves that are thought to be harmless
  • MR images of brain tissues are extremely high resolution compared to other techniques
  • By varying gradient and RF pulse parameters , MRI scanners can produce images that are sensitive to many different aspect of brain pictures e.g., certain imagining parameters yields images which gray and white matter invisible but vasculature stands out
67
Q

What does magnetic, resonance and imagning mean in MRI - (3)

A
  1. Magnetic. Similarly, when a person is inside an MRI
    scanner, protons in hydrogen atoms of the brain become aligned with the very strong main magnetic field of the
    scanner. Perturbations with respect to this alignment provide a source signal that can be measured, analyzed, and
    used to construct an image.
  2. Resonance. Resonance refers to the ability of a system
    to absorb energy delivered at a particular frequency. During
    a process called excitation, the MRI scanner emits energy in the form of radio waves at precisely the resonant
    frequency of protons. After a few milliseconds, the radio-wave energy is turned off, whereupon the protons begin to release the energy they absorbed. This released energy— the MR signal—is measured by electromagnetic detectors around the head or other part of the body.
  3. Imagining: In order to create an image from the MR signal, electromagnetic coils in the scanner can cause the local magnetic field to differ in strength along specific directions.
    By varying these magnetic-field gradients in a systematic way along the x-, y-, and z-axes of the volume to be imaged, the MR signal is caused to vary correspondingly and systematically along these axes.
68
Q

The spatial resolution of MR images depends on the - (3)

A

strength of the magnetic
field,

the strength of the gradient coils,

and the types of images being collected.

69
Q

1.5 Teslas can provide a

A

structural resolution of 1mm or less

70
Q

Functional magnetic resonance imaging (fMRI; Figure 2.12A) is based on the
fact that

A

oxyhemoglobin (hemoglobin carrying a bound oxygen molecule) and deoxyhemoglobin (the oxygen-depleted form of hemoglobin) have different magnetic resonance signals.

71
Q

Functional MRI offers better … than PET

A

temporal and spatial

Principles of CN Proper (19 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions