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Huda > Chapter 15 Magnetic Resonance I > Flashcards

Chapter 15 Magnetic Resonance I Flashcards

(102 cards)

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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
does longitudinal magnetization lead to signal?
NO
26
T1
longitudinal magnetization grows from 0 to Mz exponentially -after T1, 64% of Mz will have formed -full Mz forms after 4T1 -spin-lattice interactions
27
T1 and T2 of bone
molecules are large and slow long > 1000 ms T1 short < 0.01 ms T2
28
T1 and T2 of soft tissue
molecules are medium size moderate 500 ms T1 short 50 ms T2
29
T1 and T2 of fluid
molcules are small and fast long > 1000 ms T1 long > 1000 ms T2
30
how does T1 change with magnetic field strength
increases with increasing strength quadrupling magnetic field doubles T1 time
31
what determines T1 and T2?
-molecules that move at the larmor frequency encourage nuclei to return to equlibrium (T1 shorter)
32
T2
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
33
how to shorten T2
increase spin-spin interactions
34
does T2 depend on field strength
no
35
is T2
Mt cannot be present when Mz fully recovered
36
what increases T2*
any magnetic field inhomogeneity that increases spin dephasing in the transverse plane
37
where do inhomogeneities come from
susceptibility diffrrences at tissue boundaries MR magnets always have some vicinity of magnetic tissues
38
what is T2*
dephasing of Mxy from field inhomogeneities
39
paramagnetic contrast agent
gadolinium DTPA
40
what does gadolinium do
reduces T1 by increasing spin-lattice interctions -makes hyperintensity on T1 weighted images -positive contrast agent
41
what is Fe3O4
superparamagnetic -when placed in external magnetic field, develop a strong internal magnetization
42
paramagnetic contrast agents
disrupt local field inhomogeneity shorten T2 and T2* hypointensity on T2 weighted images negative contrast agent
43
where is super-paramagnetic iron oxide used?
image liver and reticuloendothelial system ultrasmall SPIO are used to differentiate begign and inflammatory lesions
44
describe spin echo
90-180-acquire 180 at TE/2 generates a spin echo at time TE, canceling out T2* dephasing effects
45
why do the spins spins at different speeds?
slightly higher local field= slightly faster spin
46
what does signal localization require?
magnetic field gradients gradient changes Larmor frequenc along the gradient direction
47
what permits MR signal to be extracted?
fourier analysis
48
how is the voxel where signal is coming from selected?
use 3 gradients slice select frequency encode phase encode each echo is obtained with a unique set of combinations of 3 gradients
49
what is needed to generate 128x128 MR image?
acquire 128 echoes and sample each echo 128 times
50
bandwidth
narrow range of frequencies included in RF pulse determines slice thickness -increasing RF bandwidth results in thicker slices
51
slice-select gradient strength
mT/m increasing this strength gives thinner slices for the same BW
52
frequency vs phase encode
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
what does sampling the echo more do to imaging time?
when echo sampling rate doubles, signals are halved but imaging time is the same
54
what gradients are on during 90 and 180 degree pulses for spin echo?
slice select
55
what determines the matrix size in the phase encode direction?
number of phase encode gradients
56
what does each number in k-space refer to?
spatial frequency
57
center vs periphery of k-space
center = low spatial frequency periphery = high spatial frequency
58
what do low vs high spatial frequencies provide
low = image contrast high = small features and edges
59
how are MR images obtained from k-space?
2D FT of k-space
60
strength of earth's magnetic field
50 uT
61
what do MR machines use to generate uniform magnetic field
superconducting MR magnets -kept cold using liquid helium
62
is magnetic field always on?
yes for superconducting magnets
63
magnet quench
wire temperature rises system loses its superconducting properties stored magnetic energy is converted to heat
64
homogeneity of magnetic fields for MRI
a few parts per million
65
what can elevators and other large metal structures due to MRI machine
disrupt uniformity of main magnetic field and degrade image quality
66
shimming
used to correct main field to improve uniformity
67
how do metallic objects show up in spin echo>
bright area (pulse pile) due to magnetic field distortions
68
missile effect
ferromagnetic objects can be pulled into the magnet
69
why 3 orthogonal gradient coils?
allows gradients to be oriented in any arbitrary direction
70
what kind of coils produce z gradients and x/y gradients
hemholtz coils for z saddle coils for x/y
71
gradient strenths on 1.5 T scanner
30 mT/m
72
slew rate
time to achieve required magnetic field amplitude
73
what do nonuniform gradients cause
image distortions
74
how fast are gradients switched on and off
< 500 us
75
eddy currents
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
transmitter vs receiver coils
transmitter: send RF pulses receiver: detect radio waves from patients
77
transmit bandwidth
range of emitted frequencies
78
can some coils do both transmit and receive?
yes they can switch
79
what is receiver BW proportional to?
-gradient strength -noise goes up with BW and therefore gradient strength
80
volume coils
designed to transmit and receive uniform RF signal throughout a volume
81
surface coil
more sensitive close to coil signal drops off as you go away from coil
82
linear vs quadrature coils
linear- receive signal from only x or y of rotating magnetization quadrature: uses signal in both x and y axes- increases SNR
83
phased array coils
-used for parrallel imaging -combination of many surface coils around body part
84
dieletric artifacts
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
how do we prevent RF signals from getting into coils and adding background noise?
RF shielding also prevents RF pulses from interfering with outside electronic equipment
86
what is the RF shiedling
faraday cage -copper -all utility services like electrical must be routed through special filters
87
what does RF leakage into the MR suite cause?
zipper artifacts along phase encode direction
88
fringe field
peripheral magnetic field that extends from the magnet
89
static magnetic shiedling
thick iron plates or layers of special steel sheet metal in the MR magnet room walls
90
distance of the 0.5 mT line (5 G) from isocenter of 1.5 T magnet
along central bore axis: 12 m unshielded, 4 m shielded perpendicular to bore: 9.5 m unshielded, 2.5 m shielded
91
active shielding
-uses coils to make magnetic fields to cancel out ambient field -take away need for static magnetic shielding
92
what magnetic fields can deactivate pacemakers
0.5 mT
93
where is access restricted
inside 5 G (0.5 mT) line
94
S distortions
magnetic fields may result in S distortions in fluoro images
95
how does time and RF strength affect flip angle?
flip angle is proportional to time * RF strength
96
FID frequency is proportional tp?
applied magnetic field
97
T2* vs T2 vs T1 times
T2 * are 10 times shorter than T2 and 100 X shorter than T1 times
98
where are inhomogeneities most likely to affect signal intensity?
gradient recalled echoes SE removes T2* effect
99
why does fat saturation failure ok?
main magnetic field has non-uniformities
100
T2* comes from?
inhomogeneities in main magnetic field this is why we can remove T2* with SE- because the inhomogeneitu is constant in the field
101
Does T2 depend on field strength?
no
102
what do contrast agents do to relaxation times?
reduce them

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