Welcome! In this course learners will develop expertise in basic magnetic resonance imaging (MRI) physics and principles and gain knowledge of many different data acquisition strategies in MRI. In particular, learners will get to know what is magnetic resonance phenomenon, how magnetic resonance signals are generated, how an image can be formulated using MRI, how soft tissue contrast can change with imaging parameters. Also introduced will be MR imaging sequences of spin echo, gradient echo, fast spin echo, echo planar imaging, inversion recovery, etc.
Lecture 6-2 Gradient Echo Imaging
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Do curso por Korea Advanced Institute of Science and Technology
MRI Fundamentals
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6 MRI DATA ACQUISITION STRATEGIES
In this module, you will learn about mri data acquisition strategies. Also, you will be exposed to MR imaging sequences of spin echo, gradient echo, fast spin echo, echo planar imaging, and inversion recovery.
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Sunghong ParkAssociate Professor
Department of Bio and Brain Engineering
Here we will talk about the gradient echo imaging again.
So, we talked about the gradient echo imaging in the previous week,
but we will talk about gradient echo imaging here
again but in a slightly different viewpoint.
So, we just talked about two different ways
of filling k-spaces for the two dimensional multi-slice imaging.
So, in case we fill all slices for k-space
from one slice all together and then move to next slice,
in that case, TR should be shorter to maintain a clinically reasonable scan time.
So in that case, gradient echo imaging require special,
some module to accommodate transverse magnetization.
So, that is going to be discussed in
this video lecture let's try to review the gradient echo imaging concept again.
So, the gradient echo is produced with
a single RF pulse in conjunction with
prephase and the frequency encoding gradients as you know.
So, prephasing gradient with opposite polarity are
applied first for half the duration of readout time.
So, this is not exactly true,
but it's roughly true if we ignore some other event,
but it can be roughly true.
When gradient is reversed afterwards
and then the spins start to refocus and form an echo,
so we have it as shown here.
So, flip angle is typically below 90 degree.
So as shown here, the signal decays following T_2 star.
So this is free induction decay right after slice excitation.
Then if there is no gradient and we can see some free induction decay signal.
But if we apply for readout prephasing gradient and readout gradient then
the signal waveform is going to be changing like that
because this gradient accelerate sooner decay.
So, it decays much faster as shown here.
Then it refocuses and deburses the gradient decay induced by gradient.
So, now we can see higher signal intensity and then signal decay after
the middle of this readout gradient and then the signal decay faster again,
and which forms an echo called gradient echo.
This is procedure to lead one k-space line, as we mentioned.
Again, this is gradient echo pulse sequences.
We have RF pulses and slice selection refocusing gradient,
and phase encoding gradient,
and readout prephasing gradient,
and readout gradient, and data location, so analog-to-digital converter.
This time is defined as echo time and this time is defined
as a time to repeat if this excites the same slice.
In this case, we assumed this TR is quite long,
much longer than time T_2 relaxation of the tissue of interest..
In that case, we don't need to do anything if that time is relatively long.
But if it's time to repeat is comparable to
the T_2 decay of a signal then when we apply for next RF pulse,
after this RF excitation we acquire data
but the transverse magnetization may still remain and it may not be zero.
Then next RF pulse is applied,
then we assumed there will be
only longitudinal magnetization but the transverse magnetization may affect
signal for the next excitation if this time to repeat is quite
short compared to the T_2 decay of a signal.
So, because of that we have to deal with some special procedures.
We will talk about that later.
But let's try to review the gradient echo characteristics.
Again, we just talked about that in the previous week,
but let's try to review this concept again.
So, if we make time to repeat relatively short,
so comparable to the T_1 of tissue of interest and
then flip angle is relatively large like ernst angle,
and also echo time is pretty short,
as short as possible,
and then that gives T_1-weighted imaging because
T_2 star contrast is almost gone and it will give T_1-weighted imaging.
T_2 star weighted imaging case,
we can use long TR which is twice or three times longer than T_2,
T_1 or flip angle is quite short compared to the ernst angle and
then there will be almost no contrast for the T_1 because after excitation all of them,
longitudinal and magnetization is going to be recovered back to original.
But we can use relatively long TE which is comparable
to tissue T_2 star that is optimal echo-time maximize the T_2 star contrast.
In this case, this ocassion mode is going to be T_2 star weighted image.
In proton density weighted imaging is long TR or
a small flip angle compared to the ernst angle
that minimizes the T_1 contrast and short TE.
So, as short as possible,
TE as short as possible that minimizes T_2 star contrast.
In that case, this is going to be a proton density weighted imaging.
Large flip angle/short TR increases T_1 effect and long T increases T_2 star effect.
Again, this long TE is about tissue T_2 star and
this larger flip angle or short TR is about T_1 of tissue of interest.
Then advantages fast acquisition is possible,
but disadvantage is susceptible to magnetic field inhomogeneity.
This can be a disadvantage because it may cause problems,
but this can be also advantage too because that can be a source of
contrast like a functional MRI which we will not discuss in detail.
So, this gradient echo imaging should be combined with special module called spoiling.
When transverse magnetization remain and may effect for the following excitation,
if time to repeat is not long enough compared to
the T_2 of tissue of interest or a T_2 star of tissue of interest.
So if TR is short,
transverse magnetization from one RF excitation
may remain when the next RF pulse is applied.
So, this can cause problem or this can be utilized either way.
So, there are two main classes of GRE sequences and depending on how
these residual magnetization is managed then one
is nulling of residual and transverse magnetization.
So, removing that before next excitation that is called spoiling.
Then this transverse magnetization can be utilized for
the next excitation to enhance the signal to noise ratio.
This occasion mode is called steady state.
We'll not talk about steady state in detail in this course,
but we will talk about this spoiling case,
spoiled gradient echo imaging here.
So, this is spoiled gradient echo imaging.
So we have RF, all the other part remain the same as
the previous pulse sequence diagram for
the gradient echo with the only difference is here,
spoiling gradient that is applied for the selection.
So again, please remember that after the allocation there may be still T_2
decay time and that decay can be accelerated if there exist gradient.
So, we can apply for a spatial gradient along slip selection direction which could
be randomized to equally distribute some residual energy.
So, that's a little bit beyond the concept here.
But anyway, the spoiling gradient can be used along
the slip selection direction to accelerate signal decay of the transverse component.
How can we remove the transverse magnetization for the gradient echo imaging?
So, one simple way is just to use long TR that will make
a transverse magnetization de-phase overtime
and that is one easy way but it takes longer time.
But RF spoiling is one way to avoid transverse magnetization affecting the images.
So, RF spoiling case is not removing the transverse magnetization,
but avoiding the contributional transverse magnetization in the imaging.
So, that is conceivable RF spoiling.
So, this is about phase offset,
so I added to each RF pulse and then each transmission RF pulse and
also reception phase both of them can be manipulated to phases are changed,
and then that suppresses the effects
from residual transverse magnetization on the imaging side.
That is one way of avoiding gradient transverse magnetization for the next excitation.
Gradient spoiling is actually killing
or trying to suppressing the transverse magnetization
itself to about the application of gradients to
de-phase or accelerate the transverse magnetization decay.
With that is this gradient spoiling mechanism is just I showed in the previous slide.
So, RF pulses or gradients can be used to de-phase
the transverse magnetization and this mechanism is called spoilers.
An application of this spoiled gradient echo imaging is
a T_1-weighted Anatomical Imaging or MR Angiography like
Time-of-Flight MR Angiography or a Contrast Enhanced
MR Angiography or Phase Contrast MRA.
So, we can use this spoiled gradient echo for some special MR Angiography techniques.
In the T_2 star weighted imaging or susceptibility weighted imaging,
for this applications gradient echo imaging can be used.
We will not talk about these applications in detail in this course.
But we'll try to remember that this gradient echo imaging,
spoiled gradient echo imaging can be used for this kind of special applications.
These are the spoiled gradient echo images acquired in human brain at 3Test law.
This is T_1-weighted imaging which gives
good contrast between grey matter and white matter.
T_2 star weighted imaging is very useful if there is
some blood hemorrhage or some blood clots then they code
some susceptibility differences compared
to the surrounding tissues so that gives us a pretty high contrast.
So, it's very useful,
also T_2 star weighted imaging,
and also proton density weighted imaging.
So, these can be used for
some clinical images that can be acquired from spoiled gradient echo imaging.
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