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

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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

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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