7: gradient echo sequences
residual transverse magnetization
transverse mag doesn't have time to decay, it's magnitude accumulates over successive TR periods
purpose of balanced gradient echo
use large flip angle w short TR while reducing saturation & increasing T2*w
coherent gradient echo is used to create...
very fast T2* w images
larger flip angles create more...
T1 contrast on an image
TR & flip angle control
T1/PD weighting
incoherent gradient echo used to create...
T1W & PDW images
in coherent gradient echo...
T2* effects dominate especially if TE is long
T2 weighted gradient echo images are called...
T2* weighted
contrast for steady state free precession
T2w
steady state is affected by..(2)
TR & flip angle (more or less equal each other)
flip angle determines...
amount of energy given to pt via excitation
TR determines...
amount of energy lost during excitation
quantity of energy applied during excitation is determined by...
amplitude & duration of RF pulse and results in flip angle of certain magnitude
in incoherent gradient echo TE is...
as short a possible
uses a rewinder to preserve the residual transverse mag
coherent gradient echo
small flip angle...(2)
decreases T1 contrast & SNR
spoiling & reqinding happen after...
echo is collected to prepare the next TR time period for proper contrast
In a steady state free precessional sequence the RF pulse...
excites spins in a slice & rephases residual transverse mag present from previous slice
angle through which Voves away from Bo during resonance
flip angle
the degree the flip angle is pushed into transverse plane
flip angle
in balanced gradient echo, what is used on all 3 gradients to maintain coherency
flow compensated balanced gradient system
rephasing is done by...
gradient
what causes change in mag field strength that changes precessional frequency & phase of spins
gradient
RF excitation pulse followed by a relaxation per & application of gradient scheme to produce rephasing of spins
gradient echo
the type of gradient echo pulse sequence we are creating determines...
how we manipulate residual transverse magnetization
large lip angle...(2)
increases T1 contrast & SNR
in coherent gradient echo, T2* effects dominate bc...
it uses tissues with long T2 decay time
gradient echo uses a flip angle that is...
less than 90°
in gradient echo, SNR is...
less than spin echo
TR must be ...
long enough to prevent saturation
mag moments of the spins are dephased with a...
negative gradient pulse
gradient polarity is reversed to...
positive
in balanced gradient echo TR must be
reduced to a minimum so there's less time for spins to exit slice
flip angles less than 90°...
reduces TR so it reduces scan time
after dephasing e negative pulse, mag moments of the spins...
rephase & produce a gradient echo
built up transverse mag is...
rephased by each RF excitation pulse
spoiling destroys...
residual trans mag so protons stay incoherent (put of phase)
a reversal of the phase-encoding gradient & preserves coherency
rewinder gradient
in steady state free precession what must be used
rewinder to reposition echo so it occurs sooner than excitation pulse
spoiling is opposite of....
rewinding
incoherent gradient echonuses RF...
spoiling to apply RF excitation pulses at different phases every TR
stable condition that doesn't change over time
steady state
sequence that obtains images w long TE to measure T2 decay along with short TR
steady state free precession
rewinding preserves...
steady state so protons stay coherent (in phase)
incoherent gradient echo elimates...
the echo to images w longitudinal T2 times can't dominate image contrast
the longer the TE...
the more T2* contrast in image
3 uses for incoherent gradient echo
•3D volume acquisitions •2D breath hild T1W sequences •dynamic contrast enhanced images
5 uses for coherent gradient echo
•breath-hold T2*w images •joint imaging •cine of heart •MR angiography •T2*w volume imaging
2 uses for steady state free precession
•cardia studies to view heart tissues •cine studies
4 uses for balanced gradient echo
•central nervous system to reduce flow from CSF •cardiac imaging •abd to reduce flow artifacts in biliary/cardiac systems •volume imaging
balanced gradient echo gives us (2)
•reduced flow artifacts •increased SNR