Download GRASE (Gradient- and spin-echo) MR imaging: A new fast clinical

Magnetic
David
A. Feinberg,
GRASE
A New
PhD,
MD
Koichi
#{149}
(GradientFast
Clinical
A novel technique
of magnetic
resonance (MR) imaging,
which
cornbines gradient-echo
and spin-echo
(GRASE)
technique,
accomplishes
T2-weighted
multisection
imaging
in
drastically
reduced
imaging
time,
currently
24 times
faster than spinecho imaging.
The GRASE
technique
maintains
contrast
mechanisms,
high
spatial
resolution,
and image
quality
of spin-echo
imaging
and is compatible with clinical
whole-body
MR systems without
modification
of gradient hardware.
Image
acquisition
time
is 18 seconds
for 11 rnultisection
body
images
(2,000/80
[repetition
time
msec/echo
time msec])
and 36 seconds for 22 brain images
(4,000/104).
With a combination
of multiple
Hahn
spin echoes
and short gradient-echo
trains, the GRASE
technique
overcomes
several
potential
problems
of
echo-planar
imaging,
including
large
chemical
shift, image distortions,
and
signal
loss from field inhomogeneity.
Advantages
of GRASE
over the
RARE
(rapid
acquisition
with
relaxation enhancement)
technique
indude
faster acquisition
times and
lower deposition
of radio-frequency
power
in the body. Breath
holding
during
18-second
GRASE
imaging
of
the upper
abdomen
eliminates
respiratory-motion
artifacts
in T2-
weighted
images.
A major improvement in T2-weighted
abdominal
imaging
is suggested.
Index
terms:
Abdomen,
MR studies,
70.1214
Brain, MR studies,
13.1214
#{149}
Magnetic
resonance
(MR), comparative
studies
. Magnetic
resonance
(MR), echo planar
. Magnetic
resonance
(MR), rapid imaging
. Magnetic
resonance
(MR), technology
Radiology
1991;
Oshio,
181:597-602
MD,
Resonance
PhD
and Spin-Echo)
MR Imaging:
Imaging
Technique’
W
oven the last decade,
it is remarkable
that multisection
two-dimensional
Fourier-transform
spin-echo
imaging
(1) with T2 weighting
has remained
a
standard
of reference
for routine
clinicat MR imaging
studies
of the body
and head. Since the earliest
studies
in
ben of image
sections
and in S/N, the
hybrid
RARE technique
currently
has
a fourto 16-fold
reduction
in imaging time from that of spin-echo
imaging with one excitation
and has tissue
contrast
similar
to that of the spinecho technique.
In hybrid
RARE imaging,
the rapid
application
of a large
number
of 180#{176}
radio-frequency
(RF)
1982,
pulses
HILE
there
resonance
magnetic
imaging
has
has
(MR)
improved
been
rapidly
a progressive
reduction
in image
acquisition
time.
Improvements
in MR-imager
hardware and pulse-sequence
design
have
permitted
faster imaging
times by neducing
the number
of signals
averaged (NSA) used to raise the signalto-noise
ratio (S/N) in images.
Conjugate
synthesis
of data (half Fourier or NSA = Y2) yields
nearly
another factor-of-two
reduction
in imaging time by taking
advantage
of a
natural
symmetry
in the k-space
data
to halve the number
of phase-encoded
signals
(2,3). For clinical
imaging, it is significant
that the conjugate
synthesis
contrast,
method
chemical
produces
shift,
and
tissue
spatial
resolution
identical
to that of spinecho imaging
with one excitation.
Whereas
the conjugate
synthesis
method
produces
an expected
35%41 % reduction
in image
S/N, this S/N
is acceptable
for
many
T2-weighted
examinations.
An alternative
approach
for fast
T2-weighted
imaging
is the rapid acquisition
with relaxation
enhancement (RARE)
technique
(4). The hybrid RARE technique,
also called fast
spin-echo
imaging,
reduces
imaging
time by performing
phase
encoding
both in trains
of multiple
spin echoes
and during
multiple
cycles (repetition
times [TRs]) of signal
excitation.
While
there is a reduction
in the num-
produces
deposition
body
From the Department
of Radiology,
Harvard
Boston.
Received
April 18, 1991; revision
requested
1. Address
reprint
requests
to D.A.F.,
Department
ter, 560 First Ave. New York, NY 10016.
© RSNA,
1991
Medical
School,
Brigham
and Women’s
Hospital,
June 1; revision
received
June 25; accepted
July
of Radiology,
New York University
Medical
Cen-
considerably
higher
of RE energy
at the
standard
in the
human
absorption
rate
(SAR).
shorter
pulses,
ultimately
ments
Although
SAR increases
with
time intervals
between
RE
the speed
of RARE imaging
is
limited
by the time requireof section-selective
180#{176}
RF
pulses
during
which
signal
cannot
be
acquired.
In echo-planar
imaging
(5), signals
are produced
by rapid
switching
of
gradient
polarity
in place of the
slower
selective
180#{176}
RF pulses.
In this
way,
echo-planar
imaging
and
its
modern
variants,
modulus-blipped
echo-planar
single-pulse
technique
(MBEST)
(6) and Instascan
(Advanced
NMR,
Woburn,
ing use
produce
msec.
Mass)
of a single
an image
At high
strengths,
imaging
(7), both
involv-
180#{176}
RF pulse,
can
in less than 100
magnetic
methods
field
of echo-planar
produce
a large
amount
of
chemical
shift on the phase
axis of
images,
typically
10-15-pixel
misregistration
between
water
and fat. This
problem
can be circumvented
by using fat-suppression
methods.
These
single-shot
images,
however,
have
lower spatial
resolution
and S/N than
spin-echo
images.
This can be improved
by using multiple
excitation
Abbreviations:
boom-Gill,
GRASE
echo, MBEST
I
Imaging
single-pulse
nals averaged,
relaxation
SAR =
to-noise
time.
=
CPMG
= Carr-Purcell-Mei= gradient
echo and
modulus-blipped
technique,
RARE
=
NSA
rapid
=
spin
echo-planar
number of sigacquisition
with
enhancement,
RF = radio frequency,
standard
absorption
rate, S/N = signalratio,
TE = echo
time,
TR = repetition
597
cycles
(multiple
imaging
TRs)
times.
quirements
that
Other
lengthen
stringent
of echo-planar
reimaging
include
static magnetic
fields with
good homogeneity
(better
than 0.3
ppm for head imaging
with a 1.5-T
system)
to avoid
signal
loss and image
Phase
distortions
and themlcal
resulting
from
inhomoge-
neity-induced
phase
errors.
All echoplanar
imaging
variants
require
strong
magnetic
gradients
with fast
rise times,
which
to date has limited
their application
to a handful
of research
centers.
It is significant
that the
subsecond
imaging
time of echo-planar techniques
eliminates
artifacts
caused
by both respiratory
and cardiac motion.
In this article,
a novel
technique
of
MR imaging,
which
combines
gradient echo and spin echo (GRASE)
techniques,
enables
T2-weighted
abdominat imaging
in tess than 18 seconds.
This is sufficiently
fast to permit
breath
holding
during
image
acquisitions to eliminate
respiratory-motion
artifacts,
currently
not possible
in dinical T2-weighted
spin-echo
studies
of
the abdomen
performed
with acquisition times of 4-8 minutes.
MATERIALS
GRASE
AND
METHODS
Technique
The
technique
of combining
echoes with spin echoes
pulse-sequence
diagram
number
of RF refocused
is produced
by using
sequence.
gradient
is shown in the
(Fig 1). A train, or
spin echoes (NSE),
the CPMG
Centered
about
a number
of gradient-recalled
are produced
by switching
the readout
gradient.
The
each
echoes
(NGR)
the polarity
of
speed
advan-
In GRASE
is sampled,
train.
imaging,
shift.
it1
RF&slgnal
(Read
vu tt
Gy
(Phase)
Pulse-sequence
Carr-Purcell-Meiboom-Gill
multiple
signal
reproduced
near
the
the use of 180#{176}
RE
read
echo.
way, the modern
echo-planar
techniques,
MBEST
(6) and
involve
with
use of a single
greater
proportional
ent-echo
128
and the center
In a similar
gradient
(Gx)
GRASE
the
selective
With
the
An echo
train is formed
180#{176}
RE pulses
(rr, ‘rr,
multiple
180#{176}
pulses,
phase
shift are greatly
echo
train.
In GRASE
proportional
through
of fre-
the echo
reduced
In echo-planar
in magnitude
imaging,
with
.
the
.) and
.
errors
and are peri-
with
the
imaging,
the chemical
to the time interval
small
phase
increments
are
ing each
180#{176}-180#{176}
interval
train.
shift is
of the
absence
short echo train with NCR echoes,
whereas
in echo-planar
imaging,
the chemical
shift
is proportional
to the longer
time of the
gradient-echo
train
composed
of 64-128
echoes.
The GRASE
pulse sequence
is more
complicated
than a simple
combination
spin echo with gradient
echo, since the
small
field-inhomogeneity
errors
recur identically
in each
NCR echoes between
successive
pulses,
as shown
in Figure
1.
of
phase
group
of
180#{176}
RE
If phase en-
coding
is continuously
incremented
the total echo train, as in echo-planar
aging or the RARE technique,
there
along
imwould
of chemical
shift and T2*
axis of k space. After Fourier-
but
ing artifacts
in the image.
This problem
is eliminated
with use of a
discontinuous
phase-encoding
order duning the echo train. As more fully described
elsewhere
(9), phase
encoding
sweeps
error,
of 64-
through
during
these
image
reconstruction,
would
three
each
large
result
large
group
phase
these
in severe
increments
increments
ghost-
of k space
of NCR echoes.
Holding
constant,
imposed
along
the
echo train. Subsequent
computer
ing or interleaving
of the signals
180#{176}
RE pulse,
composed
technique.
with
reversals.
and chemical
axis as a result
sensitivity
transform
signals.
#{149}
Radiology
sequence
modulations
Common to both GRASE and echo-planan imaging,
chemical
shift is greaten on
the phase axis of the image than on the
598
of the
(CPMG)
(7),
field-inhomogeneity
typically
diagram
imaging
Instascan
to half the time of the graditrain
n
throughout
(frequency)
quency
be modulation
on the phase
echo
spin
vu
of
180#{176}
pulses there would be a large accumulation
of phase errors and chemical
shift (dashed
line). The 24 echoes are phase-encoded
differently
by the gradient
pulses in the phase-encoded direction (Gy). In each excitation
cycle
(TR), the strength
of certain
Gy pulses
are
changed
(down-directed
arrows)
to offset the phase position
in k space, while the pairs of
negative
Gy pulses maintain
constant
large jumps in the k-space
trajectories.
(The k-space
location
of signals
[sl-s9] is shown
in Fig 2.) By using
refocusing
pulses
(up-directed
arrows),
the net accumulated
Gy phase shift is returned
to zero before each 180#{176}
RF pulse. Here, three
readout
gradient
reversals
(NCR = 3) are shown,
and only three
of the eight
RE refocused
spinecho periods
(N5) are shown.
each
respective
readout
due to field inhomogeneity
fi
vu
n
Gz
(slice select)
odically
‘k_I I\_[\
f\_/\
1.
s7s8s9
JVJ’\lJ(J&
out)
Figure
s4s5s6
JVJ’tAJ&
ir/2
relatively
gradient-recalled
2
s2s3
Si
pulses
to nutate
on reverse
the position
of
spins leads to the nulling
of field inhomogeneity
errors
at the Hahn
spin-echo
time
(8), eliminating
image
distortions
and loss
of S/N. Small field-inhomogeneity
errors
evolve within each group of N(;R echoes
during
the relatively
short time between
of their
T.....
(nhomogenetty
spin-echo
spin echo,
tage over standard
spin-echo
imaging
is
proportional
to the total number
of echoes
pen 90#{176}
excitation
and equals NGR X N5.
The effective echo time of the image is the
time at which the origin of k space (zero
order
phase
encode)
middle
of the echo
-i
error ottleld
(Fig 2) effectively
removes
duntotal
reorderin k space
the peniodicity
of field inhomogeneity
errors
on the phase
axis.
In this way, GRASE
phase
encoding
orden is significantly
different
from that of
RARE
and echo-planar
imaging,
which
continuously
ing steps
train
increase
as a function
(Fig 2). Similar
technique,
formed
shifts
Multiple
their
phase-encodof time
in the
to the
multiple
with
hybrid
excitations
small
to fill in additional
sections
are
incremental
TR cycle of GRASE
per-
phase
areas
are
echo
RARE
of k space.
imaged
during
each
echo-train
excitation.
The acquisition
time (T) of the GRASE
technique
can be directly expressed
as T
[TR x (NL X NSA)/(NGR
X N SE)]
+ TR,
=
where
NL
number
of acquired
phaseencoded
image
lines. The additional
TR is
required
to establish
steady-state
zation.
This initial excitation
a template
of echoes
without
coding,
which
can
tude normalization
timing
corrections
echoes.
ments
In the
in which
be
used
magneti-
also produces
phase
enfor
T2*
magni-
and gradient-echo
in the phase-encoded
presented
NL
imaging
192, NcR
expen3, and
November
1991
ky
s4
s7
I
III
III
s2
III
uu
S5&
Ii’
II
I
III
I I I
III
II
s3
s6
kx
L
I___
s9
L
b.
a.
Figure
2.
The k-space
and Instascan.
Numbers
tal arrows)
reverse
jectony
of the
except
for a
their
first
slightly
are continuously
space
trajectory
on the
by interleaving
N5E
8, the
direction
right
on the phase
signals
total
from
data
in both
acquisition
GRASE
and
Methods
1.5-T
tems,
were
MR system
Milwaukee)
performed
(Signa;
using
with
a
GE Medical
Sysmaximal
gradient
of 1 C/cm (10 mT/rn). The readout periods were 2 msec on 4 msec with 0.6
msec gradient
ramp times for a respective
18- or 24-msec interval between
each pair
of 180#{176}
RE pulses. Free-induction-decay
spoiler pulses were used on the read-gradient axis and are not shown in Figure 1.
The effective echo time (TE) is the time at
which
the origin of the k space is sampled:
80 on 104 msec.
With NGR
3 and NSE
8, there is a totat of 24 signals per 90#{176}
excitation
to give
exactly 192 phase-encoded
signals in eight
excitations.
The 192 x 256 k-space data set
acquired
in this way is symmetrically
zerofilled to 256 x 256 before two-dimensional
strength
Fourier-transform
image
reconstruction.
A
standard
multisection
excitation
scheme
(1) was performed
by using selective RE
excitations
with frequency
offsets.
Head and body images of the same
healthy volunteer
were obtained.
Also,
two patients
with a known radiologic
and
clinical
diagnosis
of multiple
sclerosis
un-
the GRASE technique at the time of their routine T2weighted
spin-echo
studies.
derwent
imaging
with
GRASE,
tissue
obtained
spin-echo,
and
RARE
intentionally
equal
(NSA
Volume
set
181
Number
#{149}
synthesis
and NSA
2
are
similar
among
the
three
techniques,
the
ment is affected
accuracy
of measureand limited
by van-
ability
structure.
in tissue
of the abdomen
(Fig 4) were
in 18 seconds,
during
a sinhold. Therefore,
respiratory
artifact
is totally
absent.
kidney
=
shift
are
on the
sharply
phase
defined.
axis
The
Chemical
(horizontal)
Multiple
excitation
cycles
(a),
MBEST
(horizonthe
tra-
of signals
trajectories
fill in k
axis.
1.6 pixels
(vertical)
and on the
is 0.8 pixel.
Typical
tong-TR
5, 6) obtained
with
frequency
axis
head
studies
(Figs
a pulse
sequence
of 4,000/104
and NSA = 2, require
a
72-second
imaging
time; those obtamed
with NSA = 1 require
a 36second
imaging
time. Comparison
of
the image
quality
and S/N can be
made
at similar
brain levels in these
NSA
=
1 and NSA = 2 studies.
The
image
quality
is sufficient
to demonstrate
many
neocortex.
small
radial
By using
the
arteries
GRASE
in the
tech-
nique
with NSA = 1, a multiple
sclerosis plaque
is readily
demonstrated
in the right frontal
white
matter
(Fig
6b) of a patient
with multiple
scterosis. Direct
comparison
by a neuroradiologist
of the number
of multiple
sclerosis plaques
in T2-weighted
spinecho images
with that in GRASE
images
at the same levels demonstrated
the
two
an equal
patients
number
studied.
of plaques
in
Sagittal
GRASE
images
of the lumbosacral
spine (Fig 7) demonstrated
the myetographic
effect of cerebrospinat fluid with a TR of 3 seconds.
Degenerative
disk disease
was noted
in
the lumbar
spine.
DISCUSSION
renal arteries
branching
from the abdominal
aorta are well demonstrated
without
respiratory
motion
artifact.
The contours
of the liver, spleen,
and
to be nearly
by using conjugate
= V2) for spin-echo
and RARE images
involved
use of
NSA = 2 and a signal
readout
period
of 4 msec, whereas
acquisition
of
spin-echo
images
involved
use of
NSA
= ‘/2 and
a signal readout
period
of 8 msec. Although
estimates
of image S/N, calculated
from the ratio of
tissue signal intensity
to air signal
in-
motion
tech-
niques,
each with similar
pulse sequences:
2,500/100-104
(TR msec/TE
msec).
For this comparison,
the S/N
was
contrast
is somewhat
affected
by the
different
TE values.
To obtain
similar
S/N in images,
acquisitions
of GRASE
Images
obtained
gle breath
contrast,
brain
with the
arrows),
unlike GRASE imaging.
ky = phase axis, kx = frequency
the GRASE
and RARE
techmques
(Fig 3). In all three images,
the
phase
axis is horizontal.
The contrast
among
gray matter,
white
matter,
and
cerebrospmat
fluid is similar
in these
images.
The signal
intensity
of skin in
the RARE image
(Fig 3c) is noticeably
more intense
than that in either
the
GRASE
or spin-echo
image
due to a
higher
signal of lipid in the RARE image. Greater
flow artifact
is present
across
the central
region
of the spinecho image
(Fig 3b).
Quantitative
measurements
of tissue signal intensity
(pixel intensity)
obtained
from the comparative
head
study
involving
the spin-echo,
RARE,
and GRASE
techniques
are provided
in the Table. Comparison
of tissue
tensity,
RESULTS
To compare
images
were
hybrid
in time (thick
RARE imaging.
2 for
is
(9 x TR x NSA).
Experiments
including
signals
the negative-polarity
readout
gradients
(Fig 1). In the GRASE
technique
over nearly
the entirety
of k space, with identical trajectories
for subsequent
groups
between
each group.
In RARE
imaging
(b) and echo-planar
imaging
(c), the k-space
axis sequentially
time
and (c) echo-planar
imaging techniques,
signal in the echo train. The phase-encoded
to left during
group
of three
signals
scans
displaced
starting
position
displaced
c.
of (a) the GRASE
technique,
(b) RARE technique,
left side in a-c correspond
to the time order of the
is
The development
of computed
tomo(CT) technology
for abdominal
not unlike
that of MR imaging,
required
a large leap in image acquisition
speed to move from the early head imaging times of 3-5 minutes
to the present
imaging
times
of 1-3 seconds.
CT body
imaging
is now
of great
clinical utility,
while the problems
of respiratory
motion
have not been overcome
in routine clinical
MR imaging.
To date, respiratory
motion
graphic
imaging,
Radiology
599
#{149}
a.
b.
Figure 3. Comparison
of tissue contrast
were acquired
with a TR of 2.5 seconds
C.
and (c) RARE MR images
of the brain.
and a section thickness
of 5 mm. (a) The GRASE
image
was obtained
of 45 seconds
for 13 sections.
(b) The spin-echo
image
was obtained
with conjugate
synthesis
(NSA
= ‘/2) and
25 sections
of both proton-density
and T2-weighted
images.
(c) The hybrid
RARE
image
was obtained
with
seconds
for 13 sections.
RARE
and spin-echo
images
were
acquired
with a TE of 100 msec,
while
the GRASE
to sequence
artifacts
weighted
wise
timing
in (a) GRASE,
(b) spin-echo,
The three
multisection
images
with
NSA = 2 and an image
time
an image
time of 4.6 minutes
for
NSA = 2 and an image
time of 64
TE differed
slightly
(104 msec)
due
differences.
have significantly
limited
T2spin-echo
imaging,
which
other-
has
great
sensitivity
to abdominal
pathologic
conditions.
To eliminate
respinatony artifact,
GRASE
imaging
times of 18
seconds
permit
comfortable
breath
holding for a large portion
of the patient
population.
Due
to the
ing method
discontinuous
phase-encod-
of GRASE
imaging,
echoes
coven
the entire
tion of k space
where
energy
sue
occurs
contrast
practice,
that
(8). For this reason,
of GRASE,
by
similar
spin-echo
echoes.
plaques
equally
gradient-echo
suboptimal
for
logic
conditions.
detection
3, Table),
that
of
predomiechoes
Given
their
GRASE
and
multiple
well, unlike
lowimaging,
which
is
the
of some
the T2 relaxation
patho-
,,
As demonstrated
(Fig
from
contrast
mechanism,
imaging
demonstrate
sclerosis
flip-angle
and
imaging,
determined
these
central
gradient
in fact are spin
spin
the tis-
in theory
is indistinguishable
spin-echo
nantly
Hahn
central
third
porthe highest
signal
in the
tissue
images
is essentially
spin-echo
images
head
contrast
studies
obtained
S/N
as that
with
in
similar
TRs and TEs. The abdominal
4), including
liver, spleen,
tissues
(Fig
kidney,
muscle,
and
spin-echo
fat
have
weighted
expected
contrast.
effectively
While
eliminates
related
mains
the
artifact,
because
much
some
of pulsations
being
The
GRASE,
signal
primary
RARE,
bandwidth
the square
600
investigated
Radiology
#{149}
liver
is proportional
read
in
time).
is
to
While
more
period
more
factor
loss
efficiently
uses
to refocus
signals
does
unless
or
pen acquisition
not
shorter
each
periods
in
as long
a read
S/N
directly
read
excitation,
period
techniques
loss.
periods
lated
with
permitted
in
these
suitable
image
for
routine
use
of about
clinical
that
the
180#{176}
RF pulses
per multisection
that are used in a similar
RARE
tion,
which
advantage
has a higher
of the GRASE
SAR.
over
with
I is
imaging
in-
number
of
acquisition
acquisiThe
the
can be given
in a simple
average
time
of each
speed
RARE
The
echo
by using
average
can be
exemplary
180#{176}
RF
time
directly
time
nefocusings
expresphaseof a
calcu-
values;
and
the
sun-
rounding
free-induction-decay
spoiler
(T51) equal 5 msec; the net time
of the initial phase-encoding
pulse and
final
rephasing
pulse
(TPE) equals
4 msec;
gradients
echo
readout
period
plus
a
gradient
times
(TRw) equals
5.2 msec.
Average
time per echo = [TRI + TPE + (NCR X T5))]/
N,5.
For RARE imaging
with NCR = L the
average
time per signal
is (5 + 4 + 5.2)/I,
or 14.2 msec. For GRASE
imaging
with
NGR
3, the average
time
per echo
is [5 +
rise
imaging.
GRASE
a third
%
quality
=
the
selective
4-msec
35%-41
demonstrated
It is significant
and
experiments,
for its intrinsic
The
twice
as the GRASE
technique
sion
for
encoded
signal.
phase-encoded
cause
in head and body studies
obtained
the GRASE
technique
with NSA
volves
imaging
out”
this
compensating
re-
(10).
of S/N
technique
larger signal bandwidths
are used. Conjugate synthesis
spin-echo
imaging,
not haying the time constraint
of many
signal
read
RARE
motion-
artifact
in the
and spin-echo
(S/N
holding
Pulse sequence
cardiac
pulsation
artifacts
are cur-
determinant
root of signal
T2-
of the
residual
and adjacent
structures.
modifications
to reduce
effects and other related
nently
breath
pump
time,
in GRASE
the same
GRASE
_
4 + (3 x 5.2)]/3,
imaging
per echo
ciency
on 8.2
msec.
For
GRASE
N(;5 = 7, the average
time
is 6.5 msec. Although
the effi-
with
of GRASE
imaging
is currently
in-
November
1991
b.
a.
Figure
4.
Abdominal
(a, b) Coronal
obtained
MR
studies
during
images.
of the upper
single
breath
abdomen
holds.
The
total image time was 18 seconds
for 11 multisection images,
pulse sequence
was 2,000/80,
section thickness
was 10 mm, and field of
view
with
coronal
a.
b.
Figure
5.
(a-c)
GRASE
MR images
obtained
was 4,000/104,
field of view
was 24 cm, and
termediate
technique
between
imaging,
with
echo-planar
Volume
except
obtained
of the
for a TE of 90 msec.
GRASE
an imaging
system
with
gradients
(fast rise
maximal
gradient)
will
the efficiency
of GRASE
making
it closer
to that
gradients
181
because
Number
#{149}
of the
2
of echo-
technique
from high
C.
in the head
of a healthy
volunteer
section
thickness
was 5 mm.
of gradient
of the
planar
imaging.
The RARE
not benefit
nearly
as much
formance
and
application
high
performance
times
and
strong
further
increase
imaging,
images
(c, d) Axial images
identical
to those
d.
C.
RARE
was 38 cm.
parameters
canper-
absence
head
tions.
refocusing,
its large
in 180#{176}
RF pulses,
At first glance,
and
SAR
in comparing
with
time
MBEST
and
Instascan,
cost of the multiple
ings
certainty
the efficiency
with
appears
of GRASE
NSA
time
over-
limita-
GRASE
the additional
180#{176}
RE refocus-
to directly
= 2. Imaging
reduce
imaging.
These
additional
nefocusings,
however,
signal
acquisition
to occur
much
permit
earlier
than
time
was
in these
72 seconds,
echo-planar
pulse
sequence
techniques,
which
necessarily
have
a much
90#{176}-180#{176}
time
interval
when
no
longer
signals
can
be acquired.
This advantage
of GRASE
imaging
may largely
offset the efficiency
differences
between
these techniques
for
T2-weighted
imaging.
It is important
to realize
that GRASE
imaging
is a variant
of spin-echo
imaging
that involves
use of the CPMG
sequence
Radiology
#{149}
601
Figure
7.
Sagittal
GRASE
images
of the
spine
obtained
with
a TR of 3 seconds,
NSA
2, and section
thickness
of 5 mm. Image
time was 56 seconds.
Note
the myelographic
=
effect
of cerebrospinal
disk disease
levels,
with
Figure
and
at the L2-3 level
in the L4-5 disk.
b.
a.
6.
GRASE
pulse
MR
sequence
images
obtained
of 4,000/104.
in the
(a) Image
tamed
in a patient
with
multiple
those
of brain
images
obtained
identical
image
parameters.
sclerosis.
with
NSA
head
with
obtained
The
NSA
=
in healthy
image
1, image
volunteer
quality
and S/N
5, which
were
= 2 in Figure
can be compared
otherwise
acquired
with
with
2.
imaging,
spin
which
echoes,
involves
and
use
single-shot
echoes.
Several
imaging
train
into
decrease
by the
of multiple
imaging
ciently
weighting
two
identically
of GRASE
of the echo
in clinically
acceptable
imaging
of performing
with
the CPMG
magnetic
field
and
imaging
could
be
to perform
imaging
with NSA = 4 or 8 to
increase
image
S/N in clinically
acceptable
imaging
times.
In summary,
multisection
GRASE
imaging
ing
602
is similar
in terms
to long-TR
spin-echo
imagof image
fidelity,
contrast,
Radiology
#{149}
imaging
Jolesz,
rologic
The
technique
with
imaging
studies
preparation
of this
grateful
to Lawrence
tonal comments.
Biol
1984;
29:857-867.
J, Nauerth
a fast
A, Friedburg
imaging
MR. Magn
Reson
Mansfield
P, Maudsley
H.
method
RARE
for clinical
Med 1986; 3:823-833.
AA.
J Magn
Planar
spin
1977; 27:
high-penfor-
nan techniques.
7.
and spa-
however,
new
to
is a
8.
9.
high-
Magn
Reson
10.
to Ferenc
neu-
Crooks
Nuclear
L, Arakawa
magnetic
M, HoenningerJ,
et al.
resonance
whole-body
imager
operating
at 3.5 KGauss.
of
fast MR imaging
technique.
Magn
Reson
Med 1991; 20:344349.
Feinberg
DA, Oshio
K.
Gradient
echo
time shifting
in fast imaging
(abstr).
In:
Book of abstracts:
Society of Magnetic
Resonance in Medicine
1991. Berkeley, Calif:
in
We are very
PhD, for his edi-
References
Med 1989;
10:227-240.
Rzedzian
RR, Pykett
IL.
Instant
images
the body by magnetic
resonance.
Magn
Reson
Med 1987; 5:563-571.
Hahn
EL.
Spin echoes.
Phys Rev 1950;
50:580-594.
Oshio
K, Feinberg
DA.
GRASE
(Gradient
and Spin Echo): a novel
U
in comparing
NMR.
Reson
RJ, Howseman
A, Coxon
R, et al.
imaging
at 0.5 T using echo-pla-
Society
1 1.
1.
Med
DA, HaleJD,
Watts JC, Kauffman
A.
Halving
MR imaging
time by
demonstration
at 3.5 kG. Ra1986; 161:527-531.
Ordidge
Snapshot
6.
and for his comments
manuscript.
E. Crooks,
Phys
Feinberg
L, Mark
conjugation:
diology
Hennig
imaging:
Combination
ability
to implement
without
the need
assistance
Uijen CMJ, Holzcheres
NMR imaging
by
Fourier
zeugmatogra-
by
quality
hardware,
his
intensity
imaging
101-107.
We are grateful
for
5.
should
produce
in imaging
time
image
gradient
MD,
A pen-
in the current
time.
technique
Acknowledgment:
and
flow
have
4.
effi-
period.
significant
advantage
of this
speed
imaging
technique.
possible apin systems
strength
decay
sections
maintaining
resolution.
GRASE
re-
time.
The
over
spin-echo
of the GRASE technique
with tradeoffs
in T2
and
change
times.
diffusion
sequence
been described
(II). Another
plication
of GRASE imaging
low
imaging;
tial
the
T2
gradient
systems
large reductions
while
phase-encoded
volume
high-resolution
mance
further
a 24-fold
by more
image
of the GRASE
obvious
modifications
are possible:
division
three-dimensional
use of 512 x 512
with
the
of fewer
implementation
can be optimized
in imaging
time
use of gradient
with
acquisition
of GRASE
is accomplished
using
alty
data sets for simultaneous
proton-density
and T2-weighted
(double-echo)
imaging,
use of different
TEs and TRs for desired
image
contrast;
extension
to multislab
Methods
imaging
resolution
echo-planar
imaging,
which
involves
use of multiple
gradient
echoes.
In GRASE
imaging,
a
small amount
of chemical
shift is accepted
with
the
and
SAR afforded
spatial
duction
in image
time
advantage
at multiple
disk
disk bulging
central
loss of signal
denBoefJH,
van
CD.
Multiple-slice
three-dimensional
phy.
and
and
Degenerative
time of 36 seconds,
and (b) image ob-
3.
to produce
spin-echo
contrast.
GRASE
imaging lies on a continuum
between
RARE
fluid.
is demonstrated
more
severe
of Magnetic
cine, 1991; 1239.
Feinberg
DA, Jakab
in humans
studied
tnbution,
line scan,
ing.
Magn
Reson
Resonance
in Medi-
PD.
Tissue
perfusion
by Fourier
velocity
disand echo-planar
imagMed
1990;
16:280-293.
Radiology
1982; 143:169-174.
November
1991