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.