Download MR cha5 SpinEchoImaging

M R I Physics
Course
Jerry Allison Ph.D., Chris Wright B.S.,
Tom Lavin B.S., Nathan Yanasak Ph.D.
Department of Radiology
Medical College of Georgia
M R I Physics Course
Spin Echo Imaging
Hahn Spin Echo
Spin - Warp Imaging
Carr Purcell (CP) Pulse Sequence
Carr Purcell Meiboom Gill (CPMG) Pulse Sequence
Spin Echo Imaging
Multiplanar Imaging
Multislice Imaging
Oblique imaging
Spin Echo Variations
Spin Echo Imaging
The Spin Echo imaging technique
has the advantage that it is not as
sensitive to inhomogeneity of the
magnet and inhomogeneity caused
by magnetic susceptibility of
patient tissue.
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Hahn Spin Echo
The concept of Spin Echo production was
developed by Hahn. Spin echoes are sometimes
referred to as “Hahn spin echoes”.
The Hahn Spin Echo technique consists of a
90o flip (see Lecture 3), followed by an interpulse
delay time, τ, a 180o flip, followed by a second
interpulse delay (τ). A “spin echo” occurs at
echo time TE (2τ) following the initial 90o flip as
shown in the figure.
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Hahn Spin Echo (continued)
The Hahn Spin Echo was developed in 1950 for use
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in nMR long before the advent of MRI.
Hahn Spin Echo (continued)
The Hahn Spin Echo was used for the
measurement of T2 values. It is possible to
measure T2 since spin echo techniques
compensate for local inhomogeneities of the
static magnetic field. In order to measure T2 , the
sequence had to be repeated with several
different values of the interpulse delay (τ). It
was necessary to wait approximately 5 x T1
(~4sec) for relaxation between sequences.
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Hahn Spin Echo (continued)
Due to the length of some interpulse delays,
diffusion of nuclei (Brownian motion)
contributed to relaxation and corrupted the
measured T2 values. Diffusion errors depend on
gradient strength, diffusion coefficient (D) and
diffusion time. In a spin echo technique,
τ determines the diffusion time.
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Spin-Warp Imaging
Spin-warp imaging is a basic spin echo technique
which reduces corruption of T2 data due to magnet
inhomogeneity and tissue susceptibility. In 2DFT
techniques the variations in phase encoding associated
with each TR can be accomplished by varying the
magnitude of the gradient or the duration of the gradient.
Spin-warp imaging varies the magnitude of phase
encoding gradients (not the duration).
Relaxation caused by flow, diffusion or proton
exchange are not compensated by the spin echo
technique, as mentioned before.
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Here is a pulsesequence diagram.
This shows a
timeline for: 1) RF
pulses; 2) gradient
amplitudes for Gx,
Gy, Gz; 3) the
readout (i.e., A/D),
and 4) the signal of
the excited nuclei.
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Spin-Warp Imaging (continued)
******************
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Carr-Purcell (CP) Pulse Sequence
A modified spin echo technique is the Carr
Purcell spin echo technique. The Carr-Purcell
technique (CP pulse sequence) uses a 90o RF
pulse followed by a train of evenly spaced 180o
RF pulses. A series of echoes which have
alternating signs are produced. The first echo is
negative, the second echo is positive, the third
echo is negative, etc.
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Carr-Purcell (CP) Pulse Sequence
An envelope connecting the echo amplitudes
decays exponentially with a rate constant
accurately reflecting the T2 of the sample (as
opposed to T2* ). Since τ is relatively short in the
CP technique, diffusion errors in T2 measurement
are much smaller.
One can use the exponential decay envelope to
calculate T2 values.
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Carr-Purcell-Meiboom-Gill (CPMG)
Pulse Sequence
The Carr-Purcell-Meiboom-Gill (CPMG sequence)
technique is a modification of the Carr-Purcell
technique. The CPMG technique applies the
180o RF pulses along the Y axis of the rotating
frame (rather than the X axis as in the CP
technique). This modification makes the
accuracy of the 180o RF pulse much less critical.
Each echo signal is positive in the CPMG
technique. Variants of the CPMG technique
have been widely used in MRI.
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Carr-Purcell-Meiboom-Gill (CPMG)
Pulse Sequence
Another note of interest: the signal envelope
following the 90o RF pulse reflects T2*, while the
signal envelope connecting the magnitude of
succeeding echoes reflects T2.
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Spin Echo Imaging
First, let’s go through the spin echo imaging
sequence, see a demonstration of this, then finish up
by discussing some timing issues of image
acquisition:
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1. Using a Z
gradient for “slice
selection”, the
macroscopic
magnetization is
nutated into the
transverse plane
using a 90o flip.
Nutation is about
the X axis. The Z
gradient is reversed
briefly to rephase
the spins within the
selected slice. 18
2. The Y “phase
encode gradient” is
applied with the
first phase encode
value.
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3. After the interpulse
delay time, τ , a slice
selective 180o RF pulse
is applied to flip the
transverse plane about
the X axis (Spin-Warp,
CP) or the Y axis
(CPMG). The effect of
the 180o RF pulse is to
retard the spins that
were ahead in phase and
to advance the spins that
had retarded phase.
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3. (continued):
At time τ after
the 180o RF pulse (2τ
after the 90o RF pulse),
the slow spins having
advanced phase and the
fast spins having
retarded phase briefly
reestablish phase
coherence and a spin
echo occurs. After
phase coherence occurs
the fast spins once again
advance in phase and
the slow spins fall
behind in phase.
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3. (continued):
A
second 180o RF
pulse at time 3τ
after the 90o RF
pulse can cause
a second echo to
occur at time 4τ
after the 90o RF
pulse. The
second echo is
smaller than the
first echo,
primarily due to
T2 relaxation.
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4. During each
echo period, the
frequency
encoding
gradient is
applied, during
the collection of
the signal
induced in the
RF coil. Notice
that the Analog
to Digital
converter (A/D)
is enabled while
the frequency
encode gradient
is active. 23
Spin Echo Imaging (continued)
5. Pulse sequences are repeated.
Consider a 256 x 256 acquisition matrix. 256
pulse sequences are executed with a different value of
the phase encoding gradient to fill k-space (raw data).
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Spin Echo Imaging (continued)
It’s movie time again… before
proceeding, let’s see the spin-echo
sequence in action to visualize how it
works.
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Spin Phase Plot Discussion
We can overlay all excited spins onto one “orbit”, to
show phase differences easily between all of them.
The overlay is plotted on the right, from the
perspective of the lab frame.
Excited
spins
precessing
in slice
Overlay
image,
lab frame
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Spin Phase Plot Discussion
Phase differences between the spins are easier to see
if we plot spin position while rotating the slice.
Compare a plot on the left of the excited slice in
rotation, to the simple overlay plot on the right.
Excited
spins in
slice,
showing
rotation of
the slice.
Overlay
image,
rotating
frame
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Spin Phase Plot Discussion
Because the total MRI signal is a sum of
the signals from all spins, we see a
maximum echo amplitude when all of the
phases are nearly the same.
Same
phase,
large
echo
Disparate
phases,
no echo
Similar phases,
noticeable echo
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Spin Echo Movies
Basic Carr-Purcell sequence (90ox, 180ox)
Spin in red leads in
phase, and
always
progresses
clockwise.
Spin in blue lags in
phase, and
always
progresses
counterclockwise. 29
Spin Echo Movies (continued)
Basic Carr-Purcell-Meiboom-Gill sequence
(90ox, 180oy)
Spin in red leads in
phase, and
always
progresses
clockwise.
Spin in blue lags in
phase, and
always
progresses
counterclockwise. 30
Spin Echo Movies (continued)
Basic Carr-Purcell-Meiboom-Gill sequence
(90ox, 180oy), including T2 decay.
Spin in red leads in
phase, and Spin
in blue lags in
phase, but notice
the “jitter” in
phase (T2).
Spin-echo
sequence cannot
correct for this,
and echo is
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smaller.
Spin Echo Imaging (continued)
The time interval between each execution of the pulse
sequence is termed the Repetition Time (TR).
In the previous examples, each movie showed <1 TR
worth of the sequence.
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Spin Echo Imaging (continued)
6. The value of the repetition time (TR) and the echo
time (TE) can be varied to control contrast in spin echo
imaging. For example:
TR = 2000 msec, TE = 20 msec Proton Density Weighting
TR = 2000 msec TE = 80 msec T2 Weighting
TR = 600 msec TE = 20 msec T1 Weighting
Note that the echo time is typically short compared to
the repetition time. We will return to this point in our
discussion of multislice imaging.
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Spin Echo Imaging (continued)
7. The image acquisition time can be calculated
as follows:
TS = NY x TR x NEX
where
NY = number of phase encodings (512, 256, 192, 128, n)
TR = repetition time
NEX = number of excitations (1, 2, n).
Siemens terminology for NEX is Number of
Acquisitions (No. Acq.).
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Spin Echo Imaging (continued)
7. (continued):
MRI images are sometimes acquired with a
NEX greater than 1. For example, the number of
excitations (NEX) might be set to 4 for a particular
study. The result is that each line of k-space is
sampled 4 times in order to improve signal-to-noise
in the image. Image acquisition time is increased
by 4. Signal in this case is improved by the square
root of 4, (i.e., a factor of 2).
In essence, each image is acquired 4 times and
averaged together as 1 image.
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Multiplanar Imaging
Spin echo imaging techniques (as well as other
MRI techniques) can be used to acquire axial,
sagittal, coronal, or oblique images.
The spin echo technique described above used
the Z gradient for slice selection, the Y gradient
for phase encoding and the X gradient for
frequency encoding. This described the
acquisition of an axial image.
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Multiplanar Imaging (continued)
Axial, sagittal, and coronal images can be
acquired as follows:
Notice that for each plane, the choice of axis for phase and
frequency encoding can vary.
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Multiplanar Imaging (continued)
The MRI system usually chooses to apply the phase
encoding axis along the thinner body dimension. For
example, when acquiring an axial image of the thorax
the phase encoding gradient is applied along the Y axis
(anterior to posterior) since the AP dimension of the
thorax is smaller than the left to right dimension. This
selection helps to prevent wrap around in the phase
encoding direction and may enable use of a rectangular
field of view for faster scanning. The MRI system
operator can choose to swap the direction of phase
encoding and frequency encoding if necessary.
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Multiplanar Imaging (continued)
Phaseencode
direction is
A-P (longer
axis of
head),
creating
aliasing.
Aliasing example
Phaseencode
direction is
L-R
(shorter
axis of
head),
eliminating
aliasing.
Images from MRI Tutor website (Copyright © 1994-1996, All
Rights Reserved)
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http://www.mritutor.org/mritutor/alias.htm
Multiplanar Imaging (continued)
The MRI system operator may also choose to swap the
direction of phase encoding and frequency encoding to
minimize flow artifacts in particular organs.
Flow
artifact
Phase encode in the
A-P direction.
Phase encode in the
L-R direction. 40
Oblique Imaging
Imaging of oblique planes can be accomplished
by applying more than one gradient during the
slice selective 90o and 180o RF pulses.
If a Y gradient is applied during slice selection,
an axial slice is defined. If Z and X gradients of
equal magnitude are applied during slice
selection, an “axial oblique” slice is defined. The
“axial oblique” slice would be at an angle of 45o
to both the axial and sagittal planes.
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Multislice Imaging
As shown earlier, echo time TE is typically short
compared to the repetition time TR. The long
TR is necessary to allow the excited slice to relax
sufficiently between the first phase encoding
sequence and subsequent phase encoding
sequences. This time can be used efficiently by
performing the first phase encoding on other
slices while waiting to perform the second phase
encoding sequence on the first slice.
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Multislice Imaging (continued)
For a T1 weighted pulse sequence having a TR
of 600 msec and TE of 20 msec, it is possible to
perform the first phase encoding on
approximately 12 slices before performing the
second phase encoding on the first slice. For a
256 x 256 matrix, this means that data can be
acquired from 12 different anatomic slices in the
same time ( 2:41 minutes) required for a single
slice.
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Spin Echo Variations
1. MEMP (Multi Echo Multi Planar) techniques
on a GE Signa allow the production of up to 4
evenly spaced (e.g. TE = 20, 40, 60, 80) echo
images during image acquisition.
2. VEMP (Variable Echo Multi Planar)
techniques on a GE Signa allow the production of 2
echo images during image acquisition. The echo
times are variable and are not required to be evenly
spaced (e.g. TE = 30,80)
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Spin Echo Variations (continued)
3. Siemens supports single echo, dual echo and
triple echo spin echo sequences. Echo times can be
set by the operator (and need not be multiples).
There is also a multiple echo technique that allows
for production of up to 16 echoes for measurement
of T2 values.
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Spin Echo Variations (continued)
4. k-space variations
It is possible to reduce scan time by filling only part
of the k-space or raw data matrix. The “missing” data is
then synthesized using the symmetric properties of the
matrix. If half of k-space is filled (NEX = 0.5) the scan
takes less time to acquire but has a lower signal-to-noise
ratio.
We will discuss how this is possible in a later lecture.
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