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Huda > Chapter 16 Magnetic Resonance II > Flashcards

Chapter 16 Magnetic Resonance II Flashcards

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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
b parameter
diffusion gradients are more effective at dephasing as magnitude of b increases
26
what is a bright area on DWI image plus high ADC value?
T2 shine through | relfects tissues with very long T2 values
27
high diffusion on ADC map
brighter values = more diffusion
28
MR angiography contrast media
Gd
29
what does Gd contrast do
shortens T1 values of blood to 250 ms | blood with Gd increases signal on T1 weighted images
30
MRA procedure
- 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
31
blood pool agents
contrasts that remain in circulation up to an hour | -permit longer imaging times and thereby higher resolution
32
max intensity projection
used in MRA
33
for whom is MRA useful?
patients who cannot tolerate iodinated contrast agents
34
TOF
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
35
limitation of TOF
signal loss because of turbulent or slow flow
36
common techniques to perform TOF MRA
GRASS | FISP
37
phase contrast MRA
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
how many image acquisitions are required to provide a complete image of flow
- 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
benefit of phase contrast angiography
quantitative measurements of blood flow can be obtained
40
colors in phase contrast angiography
black = max flow in one direction white = max flow in other direction gray= stationary tissue -areas with little signal are mottled noise
41
explain how 3D MRI is used to image a volume
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
imaging time for 3D MRI
phase encode z times phase encode y times TR
43
disadvantages of 3D MRI
longer acquisition time- more motion artifact | gaps can occur between the slices (or overlap depending on how it is done)
44
explain how echo planar imaging works
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
how fast can images be acquired with EPI?
<100 ms good for cardiac imaging
46
multi-shot EPI
phase steps are divided into several shots or TR periods
47
what is a big problem in EPI
susceptibility effects degrade EPI
48
what does functional MRI rely on?
blood oxygenation, blood volume, or blood flow changes associated with neuronal activity
49
what effect does oxygenated hemoglobin have on T2*
reduces T2* effects because O2 "shields" the hemoglobin iron atoms and reduces dephasing of adjacent protons
50
describe BOLD
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
what sequences are commonly used in BOLD?
EPI with T2* weighting
52
intensity changes with BOLD
5 % increase at higher magnetic fields data are often corrupted by noise and require statsd to extract underlying signal
53
fMRI vs PET
fMRI has better temporal and spatial resolution than PET
54
how does magnetization transfer contrast work
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
where is magnetization transfer used?
highlight abnormalities in brain | ex. multiple sclerosis
56
describe MRS
- makes use of difference in resonance frequency of protons and other nuclei found in metabolites - measured in ppm
57
what kind of field does MRS prefer
-usually stronger and more uniform than conventional MRI
58
STEAM and PRESS
MRS Stimulated Echo Acquisition Mode Point RESolved Spectroscopy
59
voxel sizes in MRS
1 cc for 1H and 8 cc for 31 P
60
31 P spectroscopy is used where?
studies of cellular metabolism
61
shieft and use for phosphocholine
3.2 ppm | cell proliferation
62
shift and use for creatine
3 and 3.9 ppm | energy rich phosphates
63
shift and use for NAA
2 ppm | glineural structures
64
shift and use for lactate
1.3 ppm | anarobic glycolysis
65
limiting spatial resolution in MRI
0.3 lp/mm | 10X worse than planar imaging
66
Fe2O3
increases lseion contrast in T2 weighted images
67
what is MR signal proportional to?
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
how does RF coil affect image noise
highest noise in large body coils and less noise in surface coils
69
most important determinant of MR image quality
SNR
70
chemical shift artifacts type I
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
chemical shift artifacts type II
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
truncation artifact
Gibbs | dark and bright bands adjacent to a sharp edge- don't have high enough frequency to model it
73
wrap-around artifact
aliasing FOV is smaller than structure -caused by under-sampling -remove by increasing FOV
74
ghost images
in phase encode direction from patient motion flowing blood and CSF can also give artifacts in phase encode direction
75
magic angle
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
at what Tesla can magnetic field be hazardous bio effects
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
what limit does FDA recommend to prevent peripheral nerve simulation
3 T/s
78
RF burns
- account for 70% of MRI adverse effects | - RF heating can occur in conducting loops, tattoos, bone screws
79
for what patients should Gd contrast be avoided?
patients with reduced kidney function
80
evidence of MR harming fetus?
limited
81
classification of MR zones
zone I - freelyt accessible to public zone II - interface between 1 and 3 zone III - restricted area zone IV- MR magnet room
82
how are effects of RF field quantified
SAR W/kg average SAR in head and body must not exceed 3 W/kg and 4 W/kg
83
limit for whole body heating
0.5 degrees C for normal mode 1 degree C for first level controlled mode > 1 degree C for second level- requires approval
84
noise levels in MRI
hearing protection is mandatory | noise ranges from 65 to 120 dB
85
fats are bright on?
T1 weighted images
86
fluids are bright on?
T2 weighted images
87
phase encode direction in head
lateral
88
phase encode direction for abdomen
AP
89
what does increasing flip angle in GRE do?
increases T1 weighting
90
magnetization transfer from CSF
likely 0%

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