Chapter 9 Flashcards
(105 cards)
1
Q
magnetism is a class of __ mediated by __
A
physical phenomena; B fields
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
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
4
Q
by Faraday’s law, a __ induces a magnetic field, and vice versa
A
changing electric current
5
Q
images from slide 3
A
6
Q
the magnetic susceptibility of a material is determined by __
A
the magnetic moment of the atoms which constitute the material
7
Q
the motion of an electrically charged particle induces a __ which is proportional to the __
A
magnetic moment (miu); spin angular momentum (S)
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
9
Q
electrons orbiting the nucleus are __
A
organized in shells according to the laws of quantum physics
10
Q
images from slide 4
A
11
Q
image from slide 5
A
12
Q
in each shell, electrons __ so that their net magnetic moment __
A
pair up; has the lowest possible value
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
14
Q
diamagnetic = __
A
all electrons are paired up
15
Q
paramagnetic = __
A
a few electrons are unpaired
16
Q
ferromagnetic = __
A
many electrons are unpaired because the outermost shell is only half filled
17
Q
image from slide 6
A
18
Q
diamagnetic materials have __ electrons
A
paired
19
Q
if no external B field is present, diamagnetic substances __
A
have no net magnetic moment
20
Q
5 examples of diamagnetic materials
A
bismuth (Bi), carbon (C), lead (Pb), copper (Cu), and water (H2O)
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
22
Q
(diamagnetism) the presence of a small magnetic moment implied the presence of __
A
a small non-zero magnetic susceptibility
23
Q
(diamagnetism) the fact that the material is repelled by the external field implies that __
A
the susceptibility is negative
24
Q
image from slide 7
A
25
paramagnetic materials have __ electrons
unpaired
26
even if no external B field is present, paramagnetic substances __
have a small net magnetic moment
27
4 examples of paramagnetic materials
aluminum (Al), lithium (Li), magnesium (Mg), sodium (Na)
28
if an external B field is present, paramagnetic substances exhibit __. this causes the substances to be __, implying that the susceptibility is __
a small magnetic moment which is aligned with the applied B field; slightly attracted by the applied magnetic fields; positive
29
many materials consist of both __ atoms and __ atoms - such materials are __ overall because it is a stronger effect
diamagnetic; paramagnetic; paramagnetic (because paramagnetism is a stronger effect)
30
images from slide 8
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31
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
unpaired; magnetic spins; in the same direction spontaneously; intrinsic magnetic field
32
2 examples of ferromagnetic materials
iron (Fe) and gadolinum (Gd)
33
ferromagnetic materials have __ susceptibilities
very large
34
when a ferromagnetic material comes into contact with an external magnetic field, the result is __
a strong attraction of the material to the source of the field
35
in the absence of an external magnetic field, ferromagnetic materials __
retain their permanent magnet property
36
images from slide 9
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37
permanent MR magnets are typically made of __
an alloy of Al, Ni, and Co (AlNiCo)
38
2 advantages of permanent MR magnets
no need for a power supply or cryogenic cooling; B field is confined to the bore (no fringe field)
39
4 disadvantages of permanent MR magnets
extremely heavy; very sensitive to ambient temperature; field strengths are low; difficult to deploy in the field due to weight and permanent magnetic properties
40
the main reason that permanent MR magnets are still used is __
cost
41
image from slide 10
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42
Faraday's law of induction: the __ in any closed circuit is equal to the negative of the __
induced electromotive force (voltage); time rate of change of the magnetic flux enclosed by the circuit
43
equation from slide 11
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44
a solenoid is a __ which allows a __ magnetic field to be generated in the space __
long wire wound in the form a helix; fairly uniform; surrounded by the turns of the wire
45
images from slide 11
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46
the efficiency of __ is determined by Ohm's law (__)
a electric current passage through a solenoid's coil; V=IR
47
Ohm's law: V=IR. define these variables. how does it relate to meltdown of permanent magnets?
```
V = applied electric voltage
I = applied electric current
R = resistance of the solenoid wire
```
48
to generate large magnetic field through an electromagnet, a __ is required and this can increase __, ultimately leading to meltdown
huge current; the temperature of the coil
49
though __ than permanent magnets, room-temperature electromagnets have __ and their B fields are __
lighter; large electric power requirements; weak
50
image from slide 12
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51
to circumvent the limitations of permanent magnets and electromagnets, MR engineers have designed __
superconducting electromagnets
52
super conducting electromagnets have coils made of __ which exhibits __ when cooled below __
titanium-niobium alloy; superconductivity; critical temperature Tc
53
superconductivity is the ability of materials to __
conduct electric currents with extremely low resistance
54
the critical temperature is the absolute temperature below which a metal __
superconductive
55
2 advantages of superconducting magnets
can generate very strong magnetic fields; have low electric power requirements
56
3 disadvantages of superconducting magnets
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
superconducting magnets: thermal shielding must be implemented using __ which contain __ (__)
cryostats; cryogens (He and N)
58
3 reasons liquid He is expensive
rare on Earth, difficult to extract and process, challenging to cool down and transport
59
image from slide 14
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60
the stray magnetic field outside the bore of an MR magnet is called a __ and __
fringe field; its magnitude can be significant
61
due to safety concerns regarding the presence of fringe fields, __ must be implemented
magnetic shielding
62
two types of magnetic shielding
passive and active
63
passive shielding involves __
surrounding the magnet and lining the magnet room with steel plates to attenuate the fringe field
64
3 disadvantages to passive shielding
very heavy (steel plates), expensive, and inconvenient
65
active shielding involves installing additional __ and __
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
superconducting MR magnets produce static fields with inhomogeneities due to __
tiny imperfections in the geometry of the magnet
67
because MR imaging is very sensitive to inhomogeneities, __ must be used to obtain high-quality images
shim coils
68
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 __.
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
active shimming involves using __ to shim the system for __
an electromagnetic coil; each patient and each sequence used
70
typical MRI scanners use __ shimming to attenuate the largest inhomogeneities, and __ shimming to further reduce remaining inhomogeneities and optimize image quality
passive; active
71
the gradient set is a __ structure which contains __
cylindrical; 3 individual electromagnets, one for each gradient direction (X, Y, Z)
72
gradient coils are used for __ (3)
spatial encoding, dephasing, and rephasing spins
73
by changing the direction of the current which passes through a gradient coil, one can __
alter the polarity of the corresponding gradient
74
image from slide 18
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75
image from slide 19
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76
gradient strength (__) defines gradient __
amplitude; steepness
77
gradient rise time is the time needed for the gradient to __
reach maximum magnitude
78
reducing the rise time results in a __ and subsequent __
power overshoot; undershoot
79
slew rate is the time it take the gradient to __
some given magnitude
80
duty cycle is the __
percentage of time the gradient is allowed to work
81
image from slide 20
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82
balanced gradient systems involve each gradient pulse __
being balanced by an equal but opposite gradient pulse
83
gradient lobe amplitude is limited by __ (2) of the sequence
bandwidth and acquisition window
84
gradient duty cycle is determined by __
the receiver bandwidth
85
the application of symmetric/balanced gradients is not very __, so asymmetric gradient systems have been developed which decrease the __, thus shortening __ and increasing __
time-efficient; amount of time that the gradient is on; scan time; turbo factor
86
images from slide 21
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87
in conventional sampling, data are read out after the gradient __
has achieved maximum magnitude
88
a disadvantage of conventional sampling is __
wasting time while gradients are turned on
89
to alleviate wasting time while gradients are turned on in conventional sampling, one can implement __, where data are acquired __, although data must be __
ramp sampling; while the gradient is still changing; weighted using complicated mathematical techniques
90
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 __
sinusoidal sampling; varies nonlinearly; scan times; artifacts
91
images from slide 22
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92
RF coils allow scanner operators to generate RF pulses via __, which instructs the coil to __
pulse control unit; transfer energy to the body, tip the magnetization at an angle with respect to the static magnetic field, etc.
93
in neuroimaging, the __ coil is the most important, though other coils may also be used for imaging the __ (__ coil) or __ (__ coils)
head; spine (body); peripheral nerves (extremity coils)
94
images from slide 23
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95
receiver coils are the coils which __
exhibit an induced EMF due to the MR signal
96
there are two types of receiver coils: __
volume coils (go all the way around the body/ body part) and surface coils (sit atop the body part)
97
a transceiver is a __
receiver coil combined with an RF coil (very common)
98
the smaller the RF coil, the __ the SNR
better
99
using more coils __ the SNR
increases
100
disadvantage of small coil
small area of coverage
101
advantage of small coil
better SNR
102
to alleviate the drawbacks of small coils, MRI engineers have designed phased array coils, where __
multiple small coils are paired with multiple receivers
103
with phased array coils, each coil __
receives signal from its own FOV
104
phased array coils: MRI software is used to automatically __ in order to produce an MR volume which __
combine the signals from all the coils; covers all the anatomy of interest
105
images from slide 24
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