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Huda > Chapter 15 Magnetic Resonance I > Flashcards

Chapter 15 Magnetic Resonance I Flashcards

1
Q

how is magnetism created?

A

moving electric charges

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2
Q

how do magnetic fields exist

A

as dipoles

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3
Q

what is magnetic field strength

A

tesla
number of magnetic lines per unit area

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4
Q

what nuclei have magnetization

A

nuclei with odd number of protons or odd number of neutrons

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5
Q

other names for magnetic nuclei

A

-dipoles
-spins
-magnetic moments

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6
Q

magnetic field strength of protons vs electrons

A

proton fields are 1000X weaker than electron fields

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7
Q

what do protons do in the absence of an external magnetic field?

A

orient randomly
no net magnetization

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8
Q

what happens to magnetic nuclei when they are placed in a magnetic field

A

precess at the larmor frequency

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9
Q

what is larmor frequency proportional to?

A

magnetic field strength

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10
Q

larmor frequency of protons

A

42 MHz/T

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11
Q

parrallel vs anti-parrallel proton orientation

A

parrallel = lower energy level- slghtly more protons oriented this way

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12
Q

what component contributes to MR signal?

A

z component only

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13
Q

net tissue magnetization

A

difference between parrallel and anti-parralel magnetization along z-axis

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14
Q

what is magnetic susceptibility

A

extent to which matter becomes magnetized when placed in an external magnetic field
-local magnetic fields change because of the effect of atomic electrons

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15
Q

diamagnetic materials

A

small negative susceptability
small decreases of the local magnetic field
tissues and plastic

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16
Q

what happens at tissue interfaces

A

changes in susceptibility result in changes in local magnetic fields
-likely yields to signal loss due to spin de-phasing

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17
Q

paramagnetic materials

A

-increase local magnetic field
-caused by magnetism of unpaired atomic electrons
-gadolinium
-deoxyhemoglobin

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18
Q

ferromagnetic materials

A

-dramatically increase local magnetic field
-large susceptibility

-steel
-some implanted medical devices

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19
Q

net magnetization at 1 T

A

3 in a million (low SNR)

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20
Q

what is resonance

A

RF field interacts with net nuclear magnetization
applied RF must be at Larmor frequency
causes Mz to rotate at a rate proportional to RF intensity

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21
Q

what happens after Rf is switched off

A

Mz has rotated through a flip angle
have longitudinal and perpendicular magnetization

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22
Q

what does doubling RF pulse duration do?

A

doubles flip angle

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23
Q

free induction decay

A

-transverse magnetization rotates at Larmor frequency
-detected as induced voltage in coil wrapped around tissues
-voltages detected in coil oscillate at larmor freqneyc = FID

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24
Q

how to increase FID signal frequency

A

increase magnetic field

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25
Q

does longitudinal magnetization lead to signal?

A

NO

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26
Q

T1

A

longitudinal magnetization grows from 0 to Mz exponentially
-after T1, 64% of Mz will have formed
-full Mz forms after 4T1

-spin-lattice interactions

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27
Q

T1 and T2 of bone

A

molecules are large and slow
long > 1000 ms T1
short < 0.01 ms T2

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28
Q

T1 and T2 of soft tissue

A

molecules are medium size
moderate 500 ms T1
short 50 ms T2

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29
Q

T1 and T2 of fluid

A

molcules are small and fast
long > 1000 ms T1
long > 1000 ms T2

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30
Q

how does T1 change with magnetic field strength

A

increases with increasing strength
quadrupling magnetic field doubles T1 time

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31
Q

what determines T1 and T2?

A

-molecules that move at the larmor frequency encourage nuclei to return to equlibrium (T1 shorter)

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32
Q

T2

A

exponential decay of transverse magnetization
-at time T2, FID has decayed to 37% of its original value
-after 4X T2, transverse magnetization is null

-spin-spin interactions dephase transverse M when they experience each other’s magnetic field

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33
Q

how to shorten T2

A

increase spin-spin interactions

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34
Q

does T2 depend on field strength

A

no

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35
Q

is T2

A

Mt cannot be present when Mz fully recovered

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36
Q

what increases T2*

A

any magnetic field inhomogeneity that increases spin dephasing in the transverse plane

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37
Q

where do inhomogeneities come from

A

susceptibility diffrrences at tissue boundaries
MR magnets always have some
vicinity of magnetic tissues

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38
Q

what is T2*

A

dephasing of Mxy from field inhomogeneities

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39
Q

paramagnetic contrast agent

A

gadolinium DTPA

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40
Q

what does gadolinium do

A

reduces T1 by increasing spin-lattice interctions

-makes hyperintensity on T1 weighted images
-positive contrast agent

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41
Q

what is Fe3O4

A

superparamagnetic
-when placed in external magnetic field, develop a strong internal magnetization

42
Q

paramagnetic contrast agents

A

disrupt local field inhomogeneity
shorten T2 and T2*
hypointensity on T2 weighted images
negative contrast agent

43
Q

where is super-paramagnetic iron oxide used?

A

image liver and reticuloendothelial system

ultrasmall SPIO are used to differentiate begign and inflammatory lesions

44
Q

describe spin echo

A

90-180-acquire
180 at TE/2 generates a spin echo at time TE, canceling out T2* dephasing effects

45
Q

why do the spins spins at different speeds?

A

slightly higher local field= slightly faster spin

46
Q

what does signal localization require?

A

magnetic field gradients
gradient changes Larmor frequenc along the gradient direction

47
Q

what permits MR signal to be extracted?

A

fourier analysis

48
Q

how is the voxel where signal is coming from selected?

A

use 3 gradients
slice select
frequency encode
phase encode

each echo is obtained with a unique set of combinations of 3 gradients

49
Q

what is needed to generate 128x128 MR image?

A

acquire 128 echoes and sample each echo 128 times

50
Q

bandwidth

A

narrow range of frequencies included in RF pulse
determines slice thickness
-increasing RF bandwidth results in thicker slices

51
Q

slice-select gradient strength

A

mT/m
increasing this strength gives thinner slices for the same BW

52
Q

frequency vs phase encode

A

when echoes are formed, a frequency encode gradient is applied to encode spatial info along the x axis. Perpendicular to slice-select direction

a phase encode is used to encode spatial info along the y-axis. Perpendicular to slice-select and frequency-encode direction

53
Q

what does sampling the echo more do to imaging time?

A

when echo sampling rate doubles, signals are halved but imaging time is the same

54
Q

what gradients are on during 90 and 180 degree pulses for spin echo?

A

slice select

55
Q

what determines the matrix size in the phase encode direction?

A

number of phase encode gradients

56
Q

what does each number in k-space refer to?

A

spatial frequency

57
Q

center vs periphery of k-space

A

center = low spatial frequency
periphery = high spatial frequency

58
Q

what do low vs high spatial frequencies provide

A

low = image contrast
high = small features and edges

59
Q

how are MR images obtained from k-space?

A

2D FT of k-space

60
Q

strength of earth’s magnetic field

A

50 uT

61
Q

what do MR machines use to generate uniform magnetic field

A

superconducting MR magnets
-kept cold using liquid helium

62
Q

is magnetic field always on?

A

yes for superconducting magnets

63
Q

magnet quench

A

wire temperature rises
system loses its superconducting properties
stored magnetic energy is converted to heat

64
Q

homogeneity of magnetic fields for MRI

A

a few parts per million

65
Q

what can elevators and other large metal structures due to MRI machine

A

disrupt uniformity of main magnetic field and degrade image quality

66
Q

shimming

A

used to correct main field to improve uniformity

67
Q

how do metallic objects show up in spin echo>

A

bright area (pulse pile) due to magnetic field distortions

68
Q

missile effect

A

ferromagnetic objects can be pulled into the magnet

69
Q

why 3 orthogonal gradient coils?

A

allows gradients to be oriented in any arbitrary direction

70
Q

what kind of coils produce z gradients and x/y gradients

A

hemholtz coils for z
saddle coils for x/y

71
Q

gradient strenths on 1.5 T scanner

A

30 mT/m

72
Q

slew rate

A

time to achieve required magnetic field amplitude

73
Q

what do nonuniform gradients cause

A

image distortions

74
Q

how fast are gradients switched on and off

A

< 500 us

75
Q

eddy currents

A

generated in coils or metal structures from the switching of the gradients
-eddy currents create image artifacts
-actively shielding gradient coils helps to reduce effects of eddy currents

76
Q

transmitter vs receiver coils

A

transmitter: send RF pulses
receiver: detect radio waves from patients

77
Q

transmit bandwidth

A

range of emitted frequencies

78
Q

can some coils do both transmit and receive?

A

yes they can switch

79
Q

what is receiver BW proportional to?

A

-gradient strength
-noise goes up with BW and therefore gradient strength

80
Q

volume coils

A

designed to transmit and receive uniform RF signal throughout a volume

81
Q

surface coil

A

more sensitive close to coil
signal drops off as you go away from coil

82
Q

linear vs quadrature coils

A

linear- receive signal from only x or y of rotating magnetization
quadrature: uses signal in both x and y axes- increases SNR

83
Q

phased array coils

A

-used for parrallel imaging
-combination of many surface coils around body part

84
Q

dieletric artifacts

A

occurs at high fields because RF waves are short compared to size of body\
-standing wave currents can arise and cause destructive and constructive interference

85
Q

how do we prevent RF signals from getting into coils and adding background noise?

A

RF shielding
also prevents RF pulses from interfering with outside electronic equipment

86
Q

what is the RF shiedling

A

faraday cage
-copper

-all utility services like electrical must be routed through special filters

87
Q

what does RF leakage into the MR suite cause?

A

zipper artifacts along phase encode direction

88
Q

fringe field

A

peripheral magnetic field that extends from the magnet

89
Q

static magnetic shiedling

A

thick iron plates or layers of special steel sheet metal in the MR magnet room walls

90
Q

distance of the 0.5 mT line (5 G) from isocenter of 1.5 T magnet

A

along central bore axis: 12 m unshielded, 4 m shielded
perpendicular to bore: 9.5 m unshielded, 2.5 m shielded

91
Q

active shielding

A

-uses coils to make magnetic fields to cancel out ambient field
-take away need for static magnetic shielding

92
Q

what magnetic fields can deactivate pacemakers

A

0.5 mT

93
Q

where is access restricted

A

inside 5 G (0.5 mT) line

94
Q

S distortions

A

magnetic fields may result in S distortions in fluoro images

95
Q

how does time and RF strength affect flip angle?

A

flip angle is proportional to time * RF strength

96
Q

FID frequency is proportional tp?

A

applied magnetic field

97
Q

T2* vs T2 vs T1 times

A

T2 * are 10 times shorter than T2 and 100 X shorter than T1 times

98
Q

where are inhomogeneities most likely to affect signal intensity?

A

gradient recalled echoes

SE removes T2* effect

99
Q

why does fat saturation failure ok?

A

main magnetic field has non-uniformities

100
Q

T2* comes from?

A

inhomogeneities in main magnetic field

this is why we can remove T2* with SE- because the inhomogeneitu is constant in the field

101
Q

Does T2 depend on field strength?

A

no

102
Q

what do contrast agents do to relaxation times?

A

reduce them

Huda (20 decks)

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