Chapter 16 Magnetic Resonance II Flashcards
describe spin echo
90-180-echo
180 is at TE/2
-eliminate T2 * effects so signal depends on T2
describe gradient echo
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)
describe inversion recovery
180 - 90 pulse at TI - 180- echo
apply 90 pulse when the Mz of a given tissue is at 0 to suppress that tissue
fast spin echo
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
echo train length
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
short TR times
T1 weighting
long T1 tissues appear dark
short T1 tissues appear bright (recover magnetization on 2nd, 3rd, subsequent acquisitions)
long TE
T2 weighting
tissues with long T2 appear bright
tissues with short T2 appear dark
TR and TE for T1 contrast
TE = 20 ms TR = 500 ms
TR and TE for T2 contrast
TE = 100 ms TR = 2,000 ms
TR and TE for proton density contrast
TE = 20 ms TR = 2,000 ms
how long does SE sequence with 128 x 128 take to acquire?
128 X TR
How can we speed up data acquisition for multiple slices
in time interval between TE and TR, MR hardware is used to acquire additional images at different slice locations
longer dimension
usually frequency encode direction to minimise imaging time
why use a low flip angle?
offers both Mxy and Mz so you can shorten the TR time (keep some z signal to flip)
why does GRE have lower SAR than SE
no 180 pulse
flip angles, TR and TE for GRE sequences
T1 weighting = larger flip angles,
TE weighting = longer TE times
T2* weighting = shorter TE times
what appears dark on GRE images
areas of T2* dephasing (blood clots)
example GRE sequences
FLASH (fast low-angle shot)
FISP (fast imaging with steady-state precession)
GRASS (gradient recalled acquisition in the steadu state)
where are T2* weighted GRE sequences used?
hemorghage, calcification, and iron deposition in tissues
STIR sequence
short time inversion recovery
suppresses signal from fat
-TI of 250 ms at 1.5 T to eliminate fat signal
FLAIR sequence
fluid-attenuated inversion recovery
suppreses signal from fluids
-TI of 1700 ms at 1,5 T to eliminate CSF signal
diffusion
random motion of water molecules
diffusion weighted imaging
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
what determines the effect of the diffusion gradient?
gradient strength
gradient duration
time between gradients
b parameter
diffusion gradients are more effective at dephasing as magnitude of b increases
what is a bright area on DWI image plus high ADC value?
T2 shine through
relfects tissues with very long T2 values
high diffusion on ADC map
brighter values = more diffusion
MR angiography contrast media
Gd
what does Gd contrast do
shortens T1 values of blood to 250 ms
blood with Gd increases signal on T1 weighted images
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
blood pool agents
contrasts that remain in circulation up to an hour
-permit longer imaging times and thereby higher resolution
max intensity projection
used in MRA
for whom is MRA useful?
patients who cannot tolerate iodinated contrast agents
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
limitation of TOF
signal loss because of turbulent or slow flow
common techniques to perform TOF MRA
GRASS
FISP
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
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
benefit of phase contrast angiography
quantitative measurements of blood flow can be obtained
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
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
imaging time for 3D MRI
phase encode z times phase encode y times TR
disadvantages of 3D MRI
longer acquisition time- more motion artifact
gaps can occur between the slices (or overlap depending on how it is done)
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
how fast can images be acquired with EPI?
<100 ms
good for cardiac imaging
multi-shot EPI
phase steps are divided into several shots or TR periods
what is a big problem in EPI
susceptibility effects degrade EPI
what does functional MRI rely on?
blood oxygenation, blood volume, or blood flow changes associated with neuronal activity
what effect does oxygenated hemoglobin have on T2*
reduces T2* effects because O2 “shields” the hemoglobin iron atoms and reduces dephasing of adjacent protons
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
what sequences are commonly used in BOLD?
EPI with T2* weighting
intensity changes with BOLD
5 %
increase at higher magnetic fields
data are often corrupted by noise and require statsd to extract underlying signal
fMRI vs PET
fMRI has better temporal and spatial resolution than PET
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
where is magnetization transfer used?
highlight abnormalities in brain
ex. multiple sclerosis
describe MRS
- makes use of difference in resonance frequency of protons and other nuclei found in metabolites
- measured in ppm
what kind of field does MRS prefer
-usually stronger and more uniform than conventional MRI
STEAM and PRESS
MRS
Stimulated Echo Acquisition Mode
Point RESolved Spectroscopy
voxel sizes in MRS
1 cc for 1H and 8 cc for 31 P
31 P spectroscopy is used where?
studies of cellular metabolism
shieft and use for phosphocholine
3.2 ppm
cell proliferation
shift and use for creatine
3 and 3.9 ppm
energy rich phosphates
shift and use for NAA
2 ppm
glineural structures
shift and use for lactate
1.3 ppm
anarobic glycolysis
limiting spatial resolution in MRI
0.3 lp/mm
10X worse than planar imaging
Fe2O3
increases lseion contrast in T2 weighted images
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
how does RF coil affect image noise
highest noise in large body coils and less noise in surface coils
most important determinant of MR image quality
SNR
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
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
truncation artifact
Gibbs
dark and bright bands adjacent to a sharp edge- don’t have high enough frequency to model it
wrap-around artifact
aliasing
FOV is smaller than structure
-caused by under-sampling
-remove by increasing FOV
ghost images
in phase encode direction
from patient motion
flowing blood and CSF can also give artifacts in phase encode direction
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
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
what limit does FDA recommend to prevent peripheral nerve simulation
3 T/s
RF burns
- account for 70% of MRI adverse effects
- RF heating can occur in conducting loops, tattoos, bone screws
for what patients should Gd contrast be avoided?
patients with reduced kidney function
evidence of MR harming fetus?
limited
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
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
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
noise levels in MRI
hearing protection is mandatory
noise ranges from 65 to 120 dB
fats are bright on?
T1 weighted images
fluids are bright on?
T2 weighted images
phase encode direction in head
lateral
phase encode direction for abdomen
AP
what does increasing flip angle in GRE do?
increases T1 weighting
magnetization transfer from CSF
likely 0%
