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Huda > Chapter 16 Magnetic Resonance II > Flashcards

Chapter 16 Magnetic Resonance II Flashcards

1
Q

describe spin echo

A

90-180-echo
180 is at TE/2
-eliminate T2 * effects so signal depends on T2

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2
Q

describe gradient echo

A

echoes are formed by reversing an initial dephasing gradient
echo depends on T2*
initial flip angle is < 90 degrees
(basically instead of getting an FID you get an echo- signal is centred- really only pro of using this technique)

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3
Q

describe inversion recovery

A

180 - 90 pulse at TI - 180- echo

apply 90 pulse when the Mz of a given tissue is at 0 to suppress that tissue

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4
Q

fast spin echo

A

acquire multiple echoes in a TR interval
-different phase-encode gradient is applied to each echo

-use it to flip only certain spins that have recovered at TI

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5
Q

echo train length

A

number of echoes acquire per each TR in fast spin echo
-reduces imaging time by factor of echo train length

-intensities of later echoes get progressively weaker

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6
Q

short TR times

A

T1 weighting

long T1 tissues appear dark
short T1 tissues appear bright (recover magnetization on 2nd, 3rd, subsequent acquisitions)

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7
Q

long TE

A

T2 weighting

tissues with long T2 appear bright
tissues with short T2 appear dark

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8
Q

TR and TE for T1 contrast

A 
TE = 20 ms
TR = 500 ms
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9
Q

TR and TE for T2 contrast

A 
TE = 100 ms
TR = 2,000 ms
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10
Q

TR and TE for proton density contrast

A 
TE = 20 ms
TR = 2,000 ms
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11
Q

how long does SE sequence with 128 x 128 take to acquire?

A

128 X TR

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12
Q

How can we speed up data acquisition for multiple slices

A

in time interval between TE and TR, MR hardware is used to acquire additional images at different slice locations

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13
Q

longer dimension

A

usually frequency encode direction to minimise imaging time

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14
Q

why use a low flip angle?

A

offers both Mxy and Mz so you can shorten the TR time (keep some z signal to flip)

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15
Q

why does GRE have lower SAR than SE

A

no 180 pulse

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16
Q

flip angles, TR and TE for GRE sequences

A

T1 weighting = larger flip angles,
TE weighting = longer TE times
T2* weighting = shorter TE times

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17
Q

what appears dark on GRE images

A

areas of T2* dephasing (blood clots)

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18
Q

example GRE sequences

A

FLASH (fast low-angle shot)
FISP (fast imaging with steady-state precession)
GRASS (gradient recalled acquisition in the steadu state)

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19
Q

where are T2* weighted GRE sequences used?

A

hemorghage, calcification, and iron deposition in tissues

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20
Q

STIR sequence

A

short time inversion recovery
suppresses signal from fat
-TI of 250 ms at 1.5 T to eliminate fat signal

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21
Q

FLAIR sequence

A

fluid-attenuated inversion recovery
suppreses signal from fluids
-TI of 1700 ms at 1,5 T to eliminate CSF signal

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22
Q

diffusion

A

random motion of water molecules

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23
Q

diffusion weighted imaging

A

uses standard SE with 2 additional gradients to provide info about diffusion
-gradients are applied on either side of 180 pulse (in opposite phases)

two images are obtained- one with diffusion gradients (b ~ 1,000) and one without

  • with no diffusion,, the gradients cancel out
  • when there is diffusion, diffusion image echoes become weaker and appear darker on the diffusion gradient image
  • the two images are used to computer apparent diffusion coefficient at each pixel
  • bright areas plus low ADC values indicate little diffusion
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24
Q

what determines the effect of the diffusion gradient?

A

gradient strength
gradient duration
time between gradients

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25
Q

b parameter

A

diffusion gradients are more effective at dephasing as magnitude of b increases

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26
Q

what is a bright area on DWI image plus high ADC value?

A

T2 shine through

relfects tissues with very long T2 values

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27
Q

high diffusion on ADC map

A

brighter values = more diffusion

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28
Q

MR angiography contrast media

A

Gd

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29
Q

what does Gd contrast do

A

shortens T1 values of blood to 250 ms

blood with Gd increases signal on T1 weighted images

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30
Q

MRA procedure

A
  • inject contrast media in vein
  • images obtained pre-contrast and during first-pass through arteries
  • subtraction of the 2 acquisitions shows image of only the blood vessels
  • tissues with no Gd generate same signal in both images
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31
Q

blood pool agents

A

contrasts that remain in circulation up to an hour

-permit longer imaging times and thereby higher resolution

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32
Q

max intensity projection

A

used in MRA

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33
Q

for whom is MRA useful?

A

patients who cannot tolerate iodinated contrast agents

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34
Q

TOF

A

time of flight
aka flow-related enhancement
-relies on blood being tagged in one region being detected in another region

  • stationary tissues become saturated when short TR times are used because Mz does not have time to recover
  • thus fresh blood entering a slice results in a higher signal than the saturated stationary tissue
  • because blood is continuously refreshed it never experiences enough excitation pulses to become saturated
  • requires selection of slices perpendicular to blood vessel
  • venous and arterial flow can be selected by using saturation pulses either above or below the slice of interest
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35
Q

limitation of TOF

A

signal loss because of turbulent or slow flow

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36
Q

common techniques to perform TOF MRA

A

GRASS

FISP

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37
Q

phase contrast MRA

A

encodes blood velocities by applying bipolar gradient between a standard excitation pulse and the readout

  • phase acccrued during gradient is 0 for stationary spins but nonzero for spins that move (flowing blood)
  • subsequent image is obtained where sequence of the bipolar gradient is reversed
  • difference of the two images is calculated
  • static tissues subtract out
  • moving blood acquires a different phase, enabling flow velocity to be calculated
38
Q

how many image acquisitions are required to provide a complete image of flow

A
  • phase contrast acquires flow in one direction at a time so need 3 to get complete image of flow
  • info is obtained in the direction of flow of the gradients
39
Q

benefit of phase contrast angiography

A

quantitative measurements of blood flow can be obtained

40
Q

colors in phase contrast angiography

A

black = max flow in one direction
white = max flow in other direction
gray= stationary tissue
-areas with little signal are mottled noise

41
Q

explain how 3D MRI is used to image a volume

A

non-selective RF pulse makes transverse magnetization for a volume
two sets of orthogonal phase-encoding gradients are applied along z and y directions
freequency encode is along x direction

42
Q

imaging time for 3D MRI

A

phase encode z times phase encode y times TR

43
Q

disadvantages of 3D MRI

A

longer acquisition time- more motion artifact

gaps can occur between the slices (or overlap depending on how it is done)

44
Q

explain how echo planar imaging works

A

90 pulse rotates Mz into transverse plane
rapidly switched gradients in frequency-encode direction
each echo is preceeded by a different phase encode gradient
image data are acquired line by line within a single acquisition
later echoes have more T2* weighting
basically drawing a line going back and forth through k-space at different y positions

in single shot, all phase-encoding steps are obtained in one TR interval

45
Q

how fast can images be acquired with EPI?

A

<100 ms

good for cardiac imaging

46
Q

multi-shot EPI

A

phase steps are divided into several shots or TR periods

47
Q

what is a big problem in EPI

A

susceptibility effects degrade EPI

48
Q

what does functional MRI rely on?

A

blood oxygenation, blood volume, or blood flow changes associated with neuronal activity

49
Q

what effect does oxygenated hemoglobin have on T2*

A

reduces T2* effects because O2 “shields” the hemoglobin iron atoms and reduces dephasing of adjacent protons

50
Q

describe BOLD

A

blood oxygenation level dependent imaging
brain activity increases local venous blood oxygenation, which increases the intensity of detected T2* weighted signal intensity

images are obtained during rest state and active state
functional info is superimposed on MRI images as color overlays

Oxyhemoglobin causes longer T2 than deoxyhemoglobin

Deoxyhemoglobin is 2 % in arterial blood and 40% in venous blood

51
Q

what sequences are commonly used in BOLD?

A

EPI with T2* weighting

52
Q

intensity changes with BOLD

A

5 %
increase at higher magnetic fields

data are often corrupted by noise and require statsd to extract underlying signal

53
Q

fMRI vs PET

A

fMRI has better temporal and spatial resolution than PET

54
Q

how does magnetization transfer contrast work

A

saturate a pool of protons in bound macromolecules
bound macromolecules have short T2 and aren’t observed
bound water exchanges magnetization with free water
-Depending on the degree of coupling between the pools, the free water pool becomes partially saturated. If the free water pool is subsequently imaged using routine RF pulses and gradients, its signal will be reduced secondary to an MT effect.
magnetization transfer ratio is obtained from signal in normal imaging and signal with the pulse applied

55
Q

where is magnetization transfer used?

A

highlight abnormalities in brain

ex. multiple sclerosis

56
Q

describe MRS

A
  • makes use of difference in resonance frequency of protons and other nuclei found in metabolites
  • measured in ppm
57
Q

what kind of field does MRS prefer

A

-usually stronger and more uniform than conventional MRI

58
Q

STEAM and PRESS

A

MRS
Stimulated Echo Acquisition Mode
Point RESolved Spectroscopy

59
Q

voxel sizes in MRS

A

1 cc for 1H and 8 cc for 31 P

60
Q

31 P spectroscopy is used where?

A

studies of cellular metabolism

61
Q

shieft and use for phosphocholine

A

3.2 ppm

cell proliferation

62
Q

shift and use for creatine

A

3 and 3.9 ppm

energy rich phosphates

63
Q

shift and use for NAA

A

2 ppm

glineural structures

64
Q

shift and use for lactate

A

1.3 ppm

anarobic glycolysis

65
Q

limiting spatial resolution in MRI

A

0.3 lp/mm

10X worse than planar imaging

66
Q

Fe2O3

A

increases lseion contrast in T2 weighted images

67
Q

what is MR signal proportional to?

A

pixel volume
slice thickness
FOV (doubling FOV quadruples pixel area and thus spins)
magnetic field

noise is increased when received BW is increased

number of acquisitions- i.e. 4 acquisitions will quadruple signal but only double the noise. Thus it will double SNR and quadruple imaging time

68
Q

how does RF coil affect image noise

A

highest noise in large body coils and less noise in surface coils

69
Q

most important determinant of MR image quality

A

SNR

70
Q

chemical shift artifacts type I

A

difference in resonance frequency of water and fat- misregistration of fat and water

  • light and dark bands at interfaces
  • appears along frequency encode direction
71
Q

chemical shift artifacts type II

A

GRE sequences
fat and water protons are alterntely in and out of phase
-changing GRE echo time can yield bands that are dark or light depending on if the water/fat are in or out of phase

72
Q

truncation artifact

A

Gibbs

dark and bright bands adjacent to a sharp edge- don’t have high enough frequency to model it

73
Q

wrap-around artifact

A

aliasing
FOV is smaller than structure
-caused by under-sampling
-remove by increasing FOV

74
Q

ghost images

A

in phase encode direction
from patient motion

flowing blood and CSF can also give artifacts in phase encode direction

75
Q

magic angle

A

tendons align at 55 degrees to the main field and yield longer T2 times that can appear bright
-longer T2 times are still very short though and only appear on short TE sequences

76
Q

at what Tesla can magnetic field be hazardous bio effects

A

10 T

  • time varying magnetic fields created by gradients can induce currents in patients
  • induced currents can cause cutaneous sensations, involuntary muscle contractions, and cardiac arrythmias
77
Q

what limit does FDA recommend to prevent peripheral nerve simulation

A

3 T/s

78
Q

RF burns

A
  • account for 70% of MRI adverse effects

- RF heating can occur in conducting loops, tattoos, bone screws

79
Q

for what patients should Gd contrast be avoided?

A

patients with reduced kidney function

80
Q

evidence of MR harming fetus?

A

limited

81
Q

classification of MR zones

A

zone I - freelyt accessible to public
zone II - interface between 1 and 3
zone III - restricted area
zone IV- MR magnet room

82
Q

how are effects of RF field quantified

A

SAR
W/kg

average SAR in head and body must not exceed 3 W/kg and 4 W/kg

83
Q

limit for whole body heating

A

0.5 degrees C for normal mode
1 degree C for first level controlled mode
> 1 degree C for second level- requires approval

84
Q

noise levels in MRI

A

hearing protection is mandatory

noise ranges from 65 to 120 dB

85
Q

fats are bright on?

A

T1 weighted images

86
Q

fluids are bright on?

A

T2 weighted images

87
Q

phase encode direction in head

A

lateral

88
Q

phase encode direction for abdomen

A

AP

89
Q

what does increasing flip angle in GRE do?

A

increases T1 weighting

90
Q

magnetization transfer from CSF

A

likely 0%

Huda (20 decks)

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