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530: Data Acquisition > Chapter 9 > Flashcards

Chapter 9 Flashcards

1
Q

magnetism is a class of __ mediated by __

A

physical phenomena; B fields

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2
Q

magnetic susceptibility (__) is a physical property of materials which indicates whether they are __

A

chi; attracted into or repelled out of a magnetic field

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3
Q

by the Law of Biot & Savart, __ give rise to a magnetic field which acts upon __

A

electric currents; other current and magnetic moments in materials

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4
Q

by Faraday’s law, a __ induces a magnetic field, and vice versa

A

changing electric current

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5
Q

images from slide 3

A
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6
Q

the magnetic susceptibility of a material is determined by __

A

the magnetic moment of the atoms which constitute the material

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7
Q

the motion of an electrically charged particle induces a __ which is proportional to the __

A

magnetic moment (miu); spin angular momentum (S)

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8
Q

in the classical view, an atom’s magnetic moment is determined by the __ and by the __

A

orbital angular momentum (L); spins (S) of its electrons

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9
Q

electrons orbiting the nucleus are __

A

organized in shells according to the laws of quantum physics

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10
Q

images from slide 4

A
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11
Q

image from slide 5

A
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12
Q

in each shell, electrons __ so that their net magnetic moment __

A

pair up; has the lowest possible value

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13
Q

depending on the extent to which its electrons are __, and atom can be __(3)

A

paired up (i.e. the extent to which atomic shells are filled); diamagnetic, paramagnetic, or ferromagnetic

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14
Q

diamagnetic = __

A

all electrons are paired up

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15
Q

paramagnetic = __

A

a few electrons are unpaired

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16
Q

ferromagnetic = __

A

many electrons are unpaired because the outermost shell is only half filled

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17
Q

image from slide 6

A
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18
Q

diamagnetic materials have __ electrons

A

paired

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19
Q

if no external B field is present, diamagnetic substances __

A

have no net magnetic moment

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20
Q

5 examples of diamagnetic materials

A

bismuth (Bi), carbon (C), lead (Pb), copper (Cu), and water (H2O)

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21
Q

if an external B field is present, diamagnetic substances __. this causes diamagnetic substances to be __

A

exhibit a small magnetic moment which opposes the applied B field; slightly repelled by applied magnetic fields

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22
Q

(diamagnetism) the presence of a small magnetic moment implied the presence of __

A

a small non-zero magnetic susceptibility

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23
Q

(diamagnetism) the fact that the material is repelled by the external field implies that __

A

the susceptibility is negative

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24
Q

image from slide 7

A
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25
Q

paramagnetic materials have __ electrons

A

unpaired

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26
Q

even if no external B field is present, paramagnetic substances __

A

have a small net magnetic moment

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27
Q

4 examples of paramagnetic materials

A

aluminum (Al), lithium (Li), magnesium (Mg), sodium (Na)

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28
Q

if an external B field is present, paramagnetic substances exhibit __. this causes the substances to be __, implying that the susceptibility is __

A

a small magnetic moment which is aligned with the applied B field; slightly attracted by the applied magnetic fields; positive

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29
Q

many materials consist of both __ atoms and __ atoms - such materials are __ overall because it is a stronger effect

A

diamagnetic; paramagnetic; paramagnetic (because paramagnetism is a stronger effect)

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30
Q

images from slide 8

A
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31
Q

ferromagnetic materials also have __ electrons, however the __ of their atoms align themselves __, such that they generate a __ even in the absence of an external magnetic field

A

unpaired; magnetic spins; in the same direction spontaneously; intrinsic magnetic field

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32
Q

2 examples of ferromagnetic materials

A

iron (Fe) and gadolinum (Gd)

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33
Q

ferromagnetic materials have __ susceptibilities

A

very large

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34
Q

when a ferromagnetic material comes into contact with an external magnetic field, the result is __

A

a strong attraction of the material to the source of the field

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35
Q

in the absence of an external magnetic field, ferromagnetic materials __

A

retain their permanent magnet property

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36
Q

images from slide 9

A
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37
Q

permanent MR magnets are typically made of __

A

an alloy of Al, Ni, and Co (AlNiCo)

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38
Q

2 advantages of permanent MR magnets

A

no need for a power supply or cryogenic cooling; B field is confined to the bore (no fringe field)

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39
Q

4 disadvantages of permanent MR magnets

A

extremely heavy; very sensitive to ambient temperature; field strengths are low; difficult to deploy in the field due to weight and permanent magnetic properties

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40
Q

the main reason that permanent MR magnets are still used is __

A

cost

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41
Q

image from slide 10

A
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42
Q

Faraday’s law of induction: the __ in any closed circuit is equal to the negative of the __

A

induced electromotive force (voltage); time rate of change of the magnetic flux enclosed by the circuit

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43
Q

equation from slide 11

A
44
Q

a solenoid is a __ which allows a __ magnetic field to be generated in the space __

A

long wire wound in the form a helix; fairly uniform; surrounded by the turns of the wire

45
Q

images from slide 11

A
46
Q

the efficiency of __ is determined by Ohm’s law (__)

A

a electric current passage through a solenoid’s coil; V=IR

47
Q

Ohm’s law: V=IR. define these variables. how does it relate to meltdown of permanent magnets?

A 
V = applied electric voltage
I = applied electric current
R = resistance of the solenoid wire
48
Q

to generate large magnetic field through an electromagnet, a __ is required and this can increase __, ultimately leading to meltdown

A

huge current; the temperature of the coil

49
Q

though __ than permanent magnets, room-temperature electromagnets have __ and their B fields are __

A

lighter; large electric power requirements; weak

50
Q

image from slide 12

A
51
Q

to circumvent the limitations of permanent magnets and electromagnets, MR engineers have designed __

A

superconducting electromagnets

52
Q

super conducting electromagnets have coils made of __ which exhibits __ when cooled below __

A

titanium-niobium alloy; superconductivity; critical temperature Tc

53
Q

superconductivity is the ability of materials to __

A

conduct electric currents with extremely low resistance

54
Q

the critical temperature is the absolute temperature below which a metal __

A

superconductive

55
Q

2 advantages of superconducting magnets

A

can generate very strong magnetic fields; have low electric power requirements

56
Q

3 disadvantages of superconducting magnets

A

can be very expensive to manufacture; require periodic refills of liquid He (rare on Earth, difficult to extract and process, challenging to cool down and transport, thus very expensive); because He stays in liquid form at such a low temp, requires additional thermal shielding using liquid N (also expensive)

57
Q

superconducting magnets: thermal shielding must be implemented using __ which contain __ (__)

A

cryostats; cryogens (He and N)

58
Q

3 reasons liquid He is expensive

A

rare on Earth, difficult to extract and process, challenging to cool down and transport

59
Q

image from slide 14

A
60
Q

the stray magnetic field outside the bore of an MR magnet is called a __ and __

A

fringe field; its magnitude can be significant

61
Q

due to safety concerns regarding the presence of fringe fields, __ must be implemented

A

magnetic shielding

62
Q

two types of magnetic shielding

A

passive and active

63
Q

passive shielding involves __

A

surrounding the magnet and lining the magnet room with steel plates to attenuate the fringe field

64
Q

3 disadvantages to passive shielding

A

very heavy (steel plates), expensive, and inconvenient

65
Q

active shielding involves installing additional __ and __

A

superconducting electromagnet coils just outside the main magnet coils; running a current through them whose direction is opposite the direction of the current through the main coil

66
Q

superconducting MR magnets produce static fields with inhomogeneities due to __

A

tiny imperfections in the geometry of the magnet

67
Q

because MR imaging is very sensitive to inhomogeneities, __ must be used to obtain high-quality images

A

shim coils

68
Q

passive shimming involves placing __ around the circumference of the inner wall of the __. once these have been placed, a __ is scanned and the __ is measured. the positions of the __ are then __.

A

about 15 small ferromagnetic shim plates in sophisticated non-ferrous metal trays; cryostat; phantom; amount of field inhomogeneities; shim plates; adjusted to minimize inhomogeneities

69
Q

active shimming involves using __ to shim the system for __

A

an electromagnetic coil; each patient and each sequence used

70
Q

typical MRI scanners use __ shimming to attenuate the largest inhomogeneities, and __ shimming to further reduce remaining inhomogeneities and optimize image quality

A

passive; active

71
Q

the gradient set is a __ structure which contains __

A

cylindrical; 3 individual electromagnets, one for each gradient direction (X, Y, Z)

72
Q

gradient coils are used for __ (3)

A

spatial encoding, dephasing, and rephasing spins

73
Q

by changing the direction of the current which passes through a gradient coil, one can __

A

alter the polarity of the corresponding gradient

74
Q

image from slide 18

A
75
Q

image from slide 19

A
76
Q

gradient strength (__) defines gradient __

A

amplitude; steepness

77
Q

gradient rise time is the time needed for the gradient to __

A

reach maximum magnitude

78
Q

reducing the rise time results in a __ and subsequent __

A

power overshoot; undershoot

79
Q

slew rate is the time it take the gradient to __

A

some given magnitude

80
Q

duty cycle is the __

A

percentage of time the gradient is allowed to work

81
Q

image from slide 20

A
82
Q

balanced gradient systems involve each gradient pulse __

A

being balanced by an equal but opposite gradient pulse

83
Q

gradient lobe amplitude is limited by __ (2) of the sequence

A

bandwidth and acquisition window

84
Q

gradient duty cycle is determined by __

A

the receiver bandwidth

85
Q

the application of symmetric/balanced gradients is not very __, so asymmetric gradient systems have been developed which decrease the __, thus shortening __ and increasing __

A

time-efficient; amount of time that the gradient is on; scan time; turbo factor

86
Q

images from slide 21

A
87
Q

in conventional sampling, data are read out after the gradient __

A

has achieved maximum magnitude

88
Q

a disadvantage of conventional sampling is __

A

wasting time while gradients are turned on

89
Q

to alleviate wasting time while gradients are turned on in conventional sampling, one can implement __, where data are acquired __, although data must be __

A

ramp sampling; while the gradient is still changing; weighted using complicated mathematical techniques

90
Q

to alleviate wasting time while gradients are turned on in conventional sampling, one can also implement __ where the gradient amplitude __, resulting in shorter __ though potentially more __

A

sinusoidal sampling; varies nonlinearly; scan times; artifacts

91
Q

images from slide 22

A
92
Q

RF coils allow scanner operators to generate RF pulses via __, which instructs the coil to __

A

pulse control unit; transfer energy to the body, tip the magnetization at an angle with respect to the static magnetic field, etc.

93
Q

in neuroimaging, the __ coil is the most important, though other coils may also be used for imaging the __ (__ coil) or __ (__ coils)

A

head; spine (body); peripheral nerves (extremity coils)

94
Q

images from slide 23

A
95
Q

receiver coils are the coils which __

A

exhibit an induced EMF due to the MR signal

96
Q

there are two types of receiver coils: __

A

volume coils (go all the way around the body/ body part) and surface coils (sit atop the body part)

97
Q

a transceiver is a __

A

receiver coil combined with an RF coil (very common)

98
Q

the smaller the RF coil, the __ the SNR

A

better

99
Q

using more coils __ the SNR

A

increases

100
Q

disadvantage of small coil

A

small area of coverage

101
Q

advantage of small coil

A

better SNR

102
Q

to alleviate the drawbacks of small coils, MRI engineers have designed phased array coils, where __

A

multiple small coils are paired with multiple receivers

103
Q

with phased array coils, each coil __

A

receives signal from its own FOV

104
Q

phased array coils: MRI software is used to automatically __ in order to produce an MR volume which __

A

combine the signals from all the coils; covers all the anatomy of interest

105
Q

images from slide 24

A

530: Data Acquisition (13 decks)

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