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306 MEDIMAGE > Lecture 2: Basic physical principles, MR equipment and safety issues > Flashcards

Lecture 2: Basic physical principles, MR equipment and safety issues Flashcards

1
Q

at body temp when in a magnetic field what is the behaviour of the H atoms in our bodies

what does this produce

A

align with main magnetic field

gives net magnitization

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2
Q

if we add in radiation at special frequency when the person has been placed in the MRI machine what does it do to the net magnitization

A

tips the net mag to become perp to the main mag field

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3
Q

what is the energy we put into the system

A

radiofreq waves

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4
Q

what does the imaging gradient do

A

increase mag field slightly at one end and decrease the mag field at another end of the body

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5
Q

what is special resonant freq proportional to

A

the main mag field

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6
Q

what is the magnetic susceptibility

A

degree of magnetization of a material in response to an applied mag field

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7
Q

what are differences in mag susceptibility a source of in MRI

A

contrast

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8
Q

what are the 3 types of mag susceptibility

A

diamag
paramag
ferromag

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9
Q

what is diamagnetism’s mag susceptibility and what does it do in the applied field

A

negative mag susceptibility

opposed to applied field

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10
Q

what are examples of diamagnets

A

water, copper, tissue

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11
Q

what is paramagnetism’s mag susceptibility and what does it do in the applied field

A

small positive mag susceptibility

small attractive force

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12
Q

what is ferromagnetism’s mag susceptibility and what does it do in the applied field

A

high positive magnetic susceptibility

strong attractive forces

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13
Q

what are examples of paramagnets

A

aluminium

gadolinium

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14
Q

what are examples of ferromagnets

A

iron
nickle
cobalt

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15
Q

what is the precessionary motion aligned to

A

the main mag field

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16
Q

what freq do H atoms precess at

A

at the larmor freq

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17
Q

what are H atom spins

A

moving positive charge gives rise to magnetic moment

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18
Q

what happens when H protons with mag moments are outside and inside the mag field

A

outside = randomly distributed

inside = align with mag field, some align parallel and some align anti parallel (all precess in direction of mag field but point in opp directions)

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19
Q

which way do lower energy state H atoms point in the main mag field

A

in direction of main mag field

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20
Q

what produces the net energy mag

A

when the H atoms that point opposite directions cancel out and its the excess protons that produce the net mag

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21
Q

Radiofreq at special freq has to match what energy

A

energy difference between the 2 states

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22
Q

describe excitation/RF transmit

A

Put energy in resulting in the effect of net mag being tipped from longitudinal to trans plane

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23
Q

describe induction/RF recieve

A

As it’s a changing mag field, if we put the same RF coil next to the sample that changing magnetization will induce a current or voltage across the coil also oscillating at the lamor freq

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24
Q

what is T2* decay

A

When we tip net mag into transverse plane the individ spins that make up the net mag were all precessing at the same freq as we used the same freq to excite them

They can start to precess at diff freq so not all are in phase and making up net magnitization so effect of that is that when the Vectors added together they don’t make the same large net mag anymore and the net mag goes down and the sig in the coil goes down in the exponential way

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25
Q

what does T2* time characterize

A

time taken for signal to decrease when the protons dephase

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26
Q

what is T1 recovery what happens

A

longitudinal mag starts to recover even though the signal decays away

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27
Q

what plane does the T2 decay in

A

transverse plane

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28
Q

what happens to T1 recovery rate for different tissues

A

tissues with different T1 recover at a different rate

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29
Q

what doe the superconducting magnet provide

A

very strong and uniform magnetic field B0

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30
Q

what does the RF coils tuned to resonant freq provide

A

a way to transmit and detect the emitted RF waves

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31
Q

what type of magnetism does a permanent magnet produce

A

intrinsic

32
Q

what is a tesla

A

unit of mag field strength - mag field density

33
Q

what is a gauss

A

unit of magnetic induction - mag field density

34
Q

what are the 2 positives of permanent magnets

A

easily shielded

less claustrophobic

35
Q

what are resistive mag fields

A

reg wound copper coil carrying direct current gen mag field along axis

36
Q

what are the 2 types of main mag fields

A

permanent and resistive

37
Q

what is the drawback of resistive mag field

A

high operating costs due to large power to gen current

50% of power dissipated as heat

38
Q

what is the positives of using resistive mag field

A

easily turned on/off

39
Q

what is a superconducting magnet similar to and how is it different

A

similar to resistive magnets but uses liquid helium to cool coil windings

40
Q

what happens to the electrical resistance in superconducting magnets and why

A

decreases and becomes super conducting (zero electrical resistance) when temperature falls below critical value ~263 degrees celsius

41
Q

larger currents in windings of a superconducting magnet leads to what effect on the magnetic field

A

stronger mag field

42
Q

what are the 2 positives of having a superconducting magnet

A

large homogenous magnetic field

little power input

43
Q

what are the 2 cons of having a superconducting magnet

A

costly due to cooling system - liquid He

always on

44
Q

what is the image quality of a 1.5T look like compared to a 3T

A

1.5T is a bit grainier than 3T

45
Q

what is SNR used to measure

A

evaluates image quality

46
Q

what is SNR proportional to

A

field strength

47
Q

what do RF coils generate and what are they tuned to

A

generates and detects the RF pulses

tuned to the selected resonant freq

48
Q

what are the 2 types of RF coils

A

body and surface coils

49
Q

where does the body coil sit and what does it do

A

The body coil sits just on inside of magnet to do transmission that makes it nice and uniform - long cylinder that puts equal amount of radiofreq into the bore

50
Q

where do we want the surface coil to be located

A

as close as possible to the object

51
Q

what is done to correct the small signal of surface coils

A

amplifiers are used

52
Q

do body RF coils transmit or receive

A

transmit

53
Q

do surface RF coils transmit or receive

A

receive

54
Q

what is resonance freq proportional to

A

main mag field

55
Q

what happens to the spin precessions at either end of the body when you apply the gradient

A

spins precess faster at the higher and slower at the lower end where there is lower mag field

56
Q

what are the 3 main safety concerns of the static field

A

cryogen
equipment malfunction
displacement effects

57
Q

when does quench occur

A

cryogen is boiled off when need to shutdown mag field or if there is a mag fault

58
Q

what is the safety measure to combat cryogen risks

A

quench pipe regularly checked

59
Q

what are the 2 things that are considered in equipment malfunction

A

electrical equipment or active implants

60
Q

what are the 3 safety measure to combat equipment malfunction

A

5Gauss line is marked
controlled areas for access
procedures in place for scanning conditional implants

61
Q

what are the 2 things that are considered in displacement effects of static fields

A

projectile effect and rotation effect

62
Q

what 2 things does projectile effect due to attractive force depend on

A

field strength and spatial mag gradient

63
Q

what does the rotation due to torque depend on

A

B^2

64
Q

what are the 2 safety measure to combat displacement effects of the static field

A

effective shielding

controlled access of people and objects to area

65
Q

what are the 2 types of shielding for the fringe field

A

passive and active

66
Q

what is passive shielding

A

added ferromag material that channels the field lines

67
Q

what is active shielding

A

secondary gradient coils are used which oppose the field created outside the magnet bore

68
Q

how do RF waves cause burns and heating

A

energy of RF wave is absorbed and converted to heat

69
Q

what are some tissue factors that influence the degree of heating effects from RF waves

A

some tissues lack heating control mechanisms

depends on patient size/weight/age/health

depends on ambient conditions

70
Q

what are the 2 safety measure to combat heating effects of RF waves

A

limit exposure of the patient with RF using SAR - specific absorption rate (W/kg)

keep room temp finely controlled

71
Q

what are the 3 safety measure to combat burn effects of RF waves

A

good patient positioning (prevent skin-skin contact)

effective screening (remove conductive materials)

make sure any required materials/leads are well insulated

72
Q

what are the 2 main safety concerns of the gradient field

A

nerve stimulation

acoustic noise

73
Q

what causes the gradient field’s nerve stimulation

A

changing mag field with time will induce current in a conductor

in the body currents can be induced along nerves which can stimulate things

74
Q

what are the 4 things that can be stimulated by the nerves by the gradient field and order them in order of seriousness

A

retina < peripheral nerve < peripheral muscle < cardiac

75
Q

what 2 things does stimulation thresholds of the gradient field rely on

A

shape of gradient pulse
patient

conductivity/implants/size

76
Q

what is a safety measure for gradient field nerve stimulation

A

limits on dB/dt - how fast the switching can go and limits set to never exceed peripheral nerve stimulation

306 MEDIMAGE (2 decks)

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