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Imaging Review CCPM > MRI > Flashcards

MRI Flashcards

1
Q

MRI resolution

A

1-2 mm FWHM (0.3 lp/mm)
depends on acquisition parameters

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2
Q

MRI matrix size

A

256x256, 2 bytes/pixel, 1/8 MB

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3
Q
A
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4
Q

does T2 depend on FS?

A

no

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5
Q

what do contrast agents generally do

A

reduce relaxation times

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6
Q

as BW increases, noise..

A

increases

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7
Q

BW relation to gradient strength

A

BW is proportional to G

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8
Q

electron magnetic field vs proton

A

electron is 1000X stronger

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9
Q

paramagnetic

A

increase magnetic field
deoxyhemoglobin, gadolinium

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10
Q

diamagnetic

A

decrease magnetic field
-tissues and plastic

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11
Q

what is magnetic field strength

A

number of magnetic lines per unit area

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12
Q

gyromagnetic ratio of H

A

42.6 MHz/T

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13
Q

soft tissue T1 and T2

A

T1 = 500 ms
T2 = 50 ms

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14
Q

fluid T1 and T2

A

both > 1000 ms

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15
Q

bone T1 and T2

A

T1 > 1000 ms
T2 < 0.01 ms

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16
Q

what molecules have short T1?

A

those that move at larmor frequency

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17
Q

T2 time, FID is where?

A

FID decayed to 37%

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18
Q

net magnetization at 1 T

A

3 in a million

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19
Q

what is magnetic susceptibility?

A

local M changes due to electron interactions

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20
Q

paramagnetic contrast agents work how?

A

by disrupting local field homogeneity, therefore T2* decreases

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21
Q

what does quadrupling field strength do to T1?

A

doubles it

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22
Q

super-paramagnetic

A

develops magnetization when placed in a magnetic field

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23
Q

T2 time, where is Mz?

A

64% of Mz has formed
full Mz forms after 4T1

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24
Q

after what time is Mxy null?

A

after 4T2

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25
Q

earth’s magnetic field strength

A

50 uT

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26
Q

typical gradient strength

A

30 mT/m
higher gradient strength = thinner slices

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27
Q

how many echoes do you need to obtain 128x128 MRI image?

A

128 echoes and sample the echo 128 times

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28
Q

ferromagnetic

A

dramatically increases local susceptibility

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29
Q

homogeneity of MR magnet

A

should be within a few ppm

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30
Q

what is used to select the voxel?

A

slice select, frequency encode, phase encode

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31
Q

shimming

A

correct main field to improve homogeneity

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32
Q

type of coil for x, y, z gradients

A

Helmholtz= z
saddle = x and y

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33
Q

quench

A

loses superconducting properties, magnetic energy converted to heat

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34
Q

dielectric artifacts

A

-occurs at high fields because RF waves are short compared to body size
-standing waves can have constructive and destructive interference

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35
Q

linear coil vs quadrature coil

A

linear receives signal only from x or y
quadrature receives from both, increases SNR by root2

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36
Q

distance of 5 G line from isocenter of 1.5 T magnet

A

-12 m unshielded and 4 m shielded along bore
-9.5 m unshielded and 4.5 m shielded perpendicular to bore

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37
Q

RF shielding used

A

faraday cage

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38
Q

where is MRI access restricted?

A

within 5 G line

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39
Q

T2 times vs T1 times

A

T2* 10x shorter than T2 and 100X shorter than T1

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40
Q

BW vs slice thickness

A

larger BW = thicker slices

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41
Q

signal of surface coil

A

signal drops off as you go away from coil

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42
Q

slew rate

A

time required to achieve desired magnetic field amplitude

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43
Q

flip angle formula

A

gamma * B1 * time

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44
Q

what artifact does RF leakage into MR suite cause?

A

zipper artifact

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45
Q

active vs passive shielding

A

active: use coils to make magnetic fields to cancel out main field
passive: sheets of metal are permanently installed

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46
Q

why can fat saturation failure occur?

A

due to non-inhomogeneities

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47
Q

equation for slice thickness

A

transmit BW/ (gamma*gradient strength)

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48
Q

In STIR, what IR is used to suppress fat?

A

250 ms

49
Q

brighter values on ADC map is more or less diffusion?

A

more diffusion

think it is opposite…

50
Q

FLAIR

A

fluid attenuated IR
suppresses signal from fluids
TI = 1700 ms

51
Q

3D MRI imaging time

A

phase encode x * phase encode y * TR

52
Q

TOF

A

ex, GRASS, FISP
-stationary tissue becomes saturated (no Mz to flip)
-fresh blood always has Mz to flip

53
Q

MR angiography

A

subtract GD contrast from non contrast- get image of veins

-useful for patients who can’t have iodinated contrast agent

54
Q

fMRI

A

relies on blood O2 or flow changes with activity

55
Q

BOLD

A

oxyhemoglobin has longer T2 than deoxyhemoglobin
-brain activity increases local venous blood oxygenation, which increases T2*
-change from diamagnetic oxyhemoglobin to paramagnetic deoxyhemoglobin that happens with brain activity yields decreased signal intensity on MRI

56
Q

magnetization transfer

A

-bound protons(short T2) are saturated- exchange signal with free water
-free water becomes partially saturated due to coupling between the pools
-free water image free water before and after- drop in signal is related to bound proton content

57
Q

what is the bright area on trace DW images in combination with high ADC value?

A

T2 shine through
-reflects tissues with very long T2 values

58
Q

b values in DWI

A

the higher the b value, the stronger the diffusion effect - controls degree of diffusion weighting

59
Q

trace DW image and ADC map

A

diffusion-weighted map
-mathematically remove T2 effects
-can obtain ADC map from here- ADC map intensities are opposite to those of DW

-lesions with low diffusion have low ADC- they appear bright on DW and dark on ADC

-onbtain 2 images: one with diffusion gradients and one without

60
Q

MRS voxel sizes for 1H and 31P

A

1 cm3 for 1H
8 cm3 for 31P

61
Q

advantage of surface coils

A

less noise
-closer to anatomy (more sensitive)

62
Q

temporal and spatial resolution of PET vs fMRI

A

fMRI has better temporal and spatial resolution

63
Q

echo train length

A

of echoes acquired per each TR in fast spin echo

64
Q

chemical shift artifact type II

A

fat and water protons are in and out of phase- get light or dark bands

65
Q

what is aliasing caused by

A

-caused by undersampling
-remove by increasing FOV

66
Q

images required to get complete view of flow

A

need 3 images

67
Q

what do susceptibility effects do to EPI

A

Degrade it

68
Q

what does Fe2O3 do?

A

increases lesion contrast in T2 weighted image

69
Q

Gibbs artifact

A

bands adjacent to edges
-there isn;t high enough frequency to model the edge or interface

70
Q

most important determinant of MR quality

A

SNR

71
Q

time dependence on receiver BW

A

as receiver BW increases, time decreases

72
Q

noise dependence on receiver BW

A

noise increases when receiver BW increases

73
Q

what does taking 4 acquisitions do to SNR and imaging time

A

-4 acquisitions = 4 x signal but 2x noise, therefore get 2x SNR
-4X imaging time

74
Q

concerns about 10T

A

-can get hazardous biological effects

75
Q

what does scan time depend on?

A

scan time = TR *Np * signal averages

76
Q

effect of FOV on pixel size

A

-smaller FOV = smaller pixel size if matrix is unchanged

77
Q

SNR dependence on Bo for body noise dominant and electronic noise dominant

A

SNR ~ Bo for body noise
~Bo^7/4 for electronic noise

78
Q

SNR equation

A

~ proton density
~voxel volume
~ Bo (body noise dominant)
~square root (NSA * TR * Np)
NSA TRNp is total time
if it is 3D MRI then total time is NSA * Nz * Ny *TR

79
Q

what sequence does BOLD use

A

EPI with T2* weighting

80
Q

hearing noise level in MRI

A

65-120 dB

81
Q

fats on T1 images

A

bright

82
Q

max ramp speed to prevent nerve stimulation

A

3T/s

83
Q

what is the point of using a gradient echo?

A

-T2* but instead of getting FID you get a centered signal

84
Q

what causes ghost image

A

periodic patient motion in phase encode direction

Using faster sequences and respiratory-ordered phase encoding can help eliminate artifacts, as can navigator echo gating to track the motion of the diaphragm to time image acquisition

85
Q

TE and TR for T1 contrast

A

TE = 20 ms
TR= 500 ms

86
Q

TE and TR for T2 contrast

A

TE=100 ms
TR= 2000 ms

87
Q

TE and TR for density contrast

A

TE= 20 ms
TR= 2000 ms

88
Q

how to increase T1 weighting in GRE

A

increase flip angle

89
Q

whole body SAR heating limits

A

-normal- 0.5 degrees C
1st level controlled- 1 degree C
2nd level controlled- > 1 degree C

90
Q

phase encode direction for abdomen

A

AP

91
Q

phase encode direction for head

A

lateral

92
Q

issues with 3D MRI

A

longer acquisition time = more motion artifact
-gaps or overlaps between slices

93
Q

rules for average SAR in head and body

A

3 W/kg head
4 W/kg body

94
Q

fluids on T2 weighted images

A

bright

95
Q

chemical shift artifact type I

A

mis-registration of fat leads to bonds at interfaces

96
Q

magic angle

A

tendons align at 55 degrees to the main field and yield longer T2 times that can appear bright
-still has short T2 time that only appears for short TE though

97
Q

4 zones of MRI

A

free access, interface, restricted, MR room

98
Q

bright and dark areas near a foreign object is due to what artifact?

A

magnetic susceptibility

99
Q

how to minimize chemical shift artifacts

A

-increase BW
-reduce matrix size (ie reduce number of pixels)
-suppress fat-frequency signal

100
Q

flow artifact

A

Due to the nature of GRE sequences, blood flow will produce a bright signal. Using saturation bands can minimize this artifact, saturating the slice upstream so the blood will not produce a signal. Gradient moment nulling can also be applied to try to correct flow artifacts.

101
Q

what does spike in k space cause

A

streaks in image
size and location determine angle and width of streaks

102
Q

gradient field distortions

A

produce image distortions

103
Q

eddy currents cause

A

geometric distortions

104
Q

metal artifacts

A

using spin echo improves issue (removes effect of static inhomogeneities), but still have distortion

105
Q

chemical shift difference btween water and fat

A

3.5 ppm
fat resonates at slightly lower frequency and is thus placed at lower voxel (chemical shift artifact)’

The size of the chemical shift artifact can be readily computed in advance based on two parameters selected prior to imaging: receiver bandwidth and size of the frequency-encode matrix. For example, if the total receiver bandwidth selected is 32 kHz and with 256 pixels in the frequency-encode direction, the bandwidth per pixel is 32,000/256 = 125 Hz. Since the fat-water frequency difference at 1.5 T is about 215 Hz, the size of the chemical-shift artifact will be (215 Hz) ÷ (125 Hz/pixel) = 1.7 pixels.

Reducing the bandwidth per pixel will accentuate this artifact. Narrow bandwidth techniques should therefore generally be avoided in locations where chemical-shift artifacts may obscure important interfaces (for instance, between the optic nerve and orbital fat).

106
Q

number of hydrogen protons per cm3 tissue

A

10^22

107
Q

spin density of lung, bone, fat

A

3%,5%,98%
this is why you cant see lung or bone on proton density weighted MRI

108
Q

what materials have long T1?

A

liquids and solids

109
Q

what are the tissue differences in proton density

A

10%

110
Q

how do we improve resolution in MRI

A

stronger gradients
high SNR
large data acquisition matrix

111
Q

examples of ferromagnetic materials

A

iron
nickel
cobalt

112
Q

equation for gyromagnetic ratio

A

eg/2m

charge/2m

113
Q

what causes SAR to increase?

A

-field strenght
-RF power and duty cycle
-transmitter coil type
-body size

114
Q

T2 of CSF vs bone

A

T2 of CSF is longer (more fluid like)

115
Q

what appears brightest on T1 weighted images?

A

fat

116
Q

what type of MRI is less sensitive to motion

A

radial

117
Q

what does SAR increase with

A

duty cycle
flip angle^2
Bo^2

118
Q

permanent vs resistive vs superconducting magnet

A

0.35 T - temperature sensitive
0.5 T - uses a lot of power
up to 20 T

Permanent MRI magnets use permanently magnetized iron like a large bar magnet that has been twisted into a C-shape where the two poles are close together and parallel.Their magnetic field homogeneity is also sensitive to ambient temperature so room temperature must be controlled carefully. The initial purchase price and operating costs are low compared to superconductive magnets

Resistive (air core) MRI magnets operate at room temperature using standard conductors such as copper in the shape of a solenoid or Helmholtz pair coil. A solenoid is a cylindrical-shaped coil of wire. The uniform magnetic field is found inside the coil, especially in the center. These magnets are relatively inexpensive to make but require a large constant flow of current while magnetized and imaging. The coil has an electrical resistance that requires cooling of the magnet. The operating costs are high because of the large power requirements of the magnetic coils and the associated cooling system.

Both permanent and resistive MRI scanners are limited to low-field applications, primarily open MRI and extremity scanners. These magnets are useful for claustrophobic patients

Superconductive MRI magnets use a solenoid-shaped coil made of alloys such as niobium/titanium or niobium/tin surrounded by copper. These alloys have the property of zero resistance to electrical current when cooled down to about 10 kelvin. The coil is kept below this temperature with liquid helium. The power supply is connected on either side of a short heated segment of the coil and the current to the coil is gradually increased over several hours until the desired magnetic field is reached. The heated segment is allowed to cool to superconducting temperature and the power supply removed and taken away. The current continues in the closed-loop of the coil for years without significant decline. A resulting property is that the magnetic field is always present.

The surrounding copper acts as an insulator at low temperatures compared to the zero resistance of the alloy. The copper also protects the alloy coil from being destroyed in case of a quench of the magnet. A quench can occur if the helium levels drop too low or if a large ferromagnetic object is brought into the fringe field of the magnet. A quench results in loss of superconductivity with a large amount of heat produced by the current and rapid boiling-off of the cryogen. The gas produced is vented out of the room but can occasionally enter the scanner room with life-threatening consequences

The cost of cryogen replacement is reduced on modern magnets, which incorporate a refrigeration system called a “cold head” to condense the cryogen gas. Startup costs for the scanner can run up to about $1.5 million for a 1.5 T MRI. Site preparation can frequently run into several $100,000s including room radiofrequency (RF) shielding, possible magnetic shielding, floor reinforcement, vibration mitigation and a very reliable uninterruptible power supply (UPS).

Superconducting magnets at 1.5 T and above allow functional brain imaging, MR spectroscopy and superior SNR and/or improved time and spatial resolution. Magnets above 1.5 T have additional challenges from RF heating of the subject, and increased artifacts from susceptibility and RF penetration among others.

119
Q

optimal flip agle for SSFP

A

cos theta= (T1-T2)/(T1+T2)

Imaging Review CCPM (11 decks)

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