David A. Feinberg, PhD, MD #{149}Koichi Oshio, MD, PhD

GRASE (Gradient- and Spin-Echo) MR Imaging:A New Fast Clinical Imaging Technique’

I From the Department of Radiology, Harvard Medical School, Brigham and Women’s Hospital,Boston. Received April 18, 1991; revision requested June 1; revision received June 25; accepted July1. Address reprint requests to D.A.F., Department of Radiology, New York University Medical Cen-ter, 560 First Ave. New York, NY 10016.

© RSNA, 1991

597

Magnetic Resonance Imaging

A novel technique of magnetic reso-nance (MR) imaging, which corn-bines gradient-echo and spin-echo(GRASE) technique, accomplishesT2-weighted multisection imaging indrastically reduced imaging time,currently 24 times faster than spin-echo imaging. The GRASE techniquemaintains contrast mechanisms, highspatial resolution, and image qualityof spin-echo imaging and is compati-ble with clinical whole-body MR sys-tems without modification of gradi-ent hardware. Image acquisition time

is 18 seconds for 11 rnultisection bodyimages (2,000/80 [repetition timemsec/echo time msec]) and 36 sec-onds for 22 brain images (4,000/104).With a combination of multiple Hahnspin echoes and short gradient-echotrains, the GRASE technique over-comes several potential problems ofecho-planar imaging, including largechemical shift, image distortions, andsignal loss from field inhomogeneity.Advantages of GRASE over theRARE (rapid acquisition with relax-ation enhancement) technique in-dude faster acquisition times andlower deposition of radio-frequencypower in the body. Breath holdingduring 18-second GRASE imaging ofthe upper abdomen eliminates respi-ratory-motion artifacts in T2-weighted images. A major improve-ment in T2-weighted abdominalimaging is suggested.

Index terms: Abdomen, MR studies, 70.1214

Brain, MR studies, 13.1214 #{149}Magnetic reso-

nance (MR), comparative studies . Magnetic

resonance (MR), echo planar . Magnetic reso-

nance (MR), rapid imaging . Magnetic reso-

nance (MR), technology

Radiology 1991; 181:597-602

W HILE magnetic resonance (MR)imaging has improved rapidly

oven the last decade, it is remarkablethat multisection two-dimensionalFourier-transform spin-echo imaging(1) with T2 weighting has remained astandard of reference for routine clini-cat MR imaging studies of the bodyand head. Since the earliest studies in1982, there has been a progressive

reduction in image acquisition time.Improvements in MR-imager hard-ware and pulse-sequence design havepermitted faster imaging times by ne-ducing the number of signals aver-aged (NSA) used to raise the signal-to-noise ratio (S/N) in images.Conjugate synthesis of data (half Fou-rier or NSA = Y2) yields nearly an-

other factor-of-two reduction in imag-ing time by taking advantage of anatural symmetry in the k-space datato halve the number of phase-en-coded signals (2,3). For clinical imag-ing, it is significant that the conjugatesynthesis method produces tissue

contrast, chemical shift, and spatial

resolution identical to that of spin-echo imaging with one excitation.Whereas the conjugate synthesismethod produces an expected 35%-41 % reduction in image S/N, this S/Nis acceptable for many T2-weighted

examinations.An alternative approach for fast

T2-weighted imaging is the rapid ac-quisition with relaxation enhance-ment (RARE) technique (4). The hy-brid RARE technique, also called fastspin-echo imaging, reduces imagingtime by performing phase encodingboth in trains of multiple spin echoesand during multiple cycles (repetitiontimes [TRs]) of signal excitation.While there is a reduction in the num-

ben of image sections and in S/N, the

hybrid RARE technique currently hasa four- to 16-fold reduction in imag-ing time from that of spin-echo imag-ing with one excitation and has tissuecontrast similar to that of the spin-echo technique. In hybrid RARE im-aging, the rapid application of a largenumber of 180#{176}radio-frequency (RF)pulses produces considerably higher

deposition of RE energy in the humanbody at the standard absorption rate

(SAR). Although SAR increases withshorter time intervals between REpulses, the speed of RARE imaging isultimately limited by the time require-ments of section-selective 180#{176}RFpulses during which signal cannot be

acquired.In echo-planar imaging (5), signals

are produced by rapid switching ofgradient polarity in place of theslower selective 180#{176}RF pulses. In thisway, echo-planar imaging and its

modern variants, modulus-blippedecho-planar single-pulse technique(MBEST) (6) and Instascan (AdvancedNMR, Woburn, Mass) (7), both involv-

ing use of a single 180#{176}RF pulse, canproduce an image in less than 100msec. At high magnetic field

strengths, methods of echo-planarimaging produce a large amount of

chemical shift on the phase axis ofimages, typically 10-15-pixel misregis-tration between water and fat. Thisproblem can be circumvented by us-ing fat-suppression methods. Thesesingle-shot images, however, havelower spatial resolution and S/N thanspin-echo images. This can be im-proved by using multiple excitation

Abbreviations: CPMG = Carr-Purcell-Mei-boom-Gill, GRASE = gradient echo and spinecho, MBEST = modulus-blipped echo-planarsingle-pulse technique, NSA = number of sig-nals averaged, RARE = rapid acquisition withrelaxation enhancement, RF = radio frequency,SAR = standard absorption rate, S/N = signal-

to-noise ratio, TE = echo time, TR = repetitiontime.

�-i� �Phase error ottleld (nhomogenetty T.....and themlcal shift.

it1 �2Si s2s3 s4s5s6 s7s8s9

RF&slgnal ir/2 � JVJ’tAJ&� JVJ’\lJ(J& �

(Read out) � f�\_�/�\ ‘��k_I��I��\_�[�\

Gy � tt� � � �fi �(Phase) vu vu vu

Gz n n(slice select)

Figure 1. Pulse-sequence diagram of the GRASE technique. An echo train is formed with the

Carr-Purcell-Meiboom-Gill (CPMG) sequence with selective 180#{176}RE pulses (�rr�, ‘rr�, . . .) andmultiple signal readout gradient (Gx) reversals. With the multiple 180#{176}pulses, phase errors

due to field inhomogeneity and chemical shift are greatly reduced in magnitude and are peri-odically reproduced throughout the echo train. In echo-planar imaging, with the absence of

180#{176}pulses there would be a large accumulation of phase errors and chemical shift (dashedline). The 24 echoes are phase-encoded differently by the gradient pulses in the phase-en-coded 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 ofnegative Gy pulses maintain constant large jumps in the k-space trajectories. (The k-space lo-cation 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, threereadout gradient reversals (NCR = 3) are shown, and only three of the eight RE refocused spin-

echo periods (N5�) are shown.

598 #{149}Radiology November 1991

cycles (multiple TRs) that lengthenimaging times. Other stringent re-

quirements of echo-planar imaging

include static magnetic fields withgood homogeneity (better than 0.3ppm for head imaging with a 1.5-Tsystem) to avoid signal loss and imagedistortions resulting from inhomoge-

neity-induced phase errors. All echo-planar imaging variants requirestrong magnetic gradients with fastrise times, which to date has limitedtheir application to a handful of re-search centers. It is significant that thesubsecond imaging time of echo-pla-nar techniques eliminates artifactscaused by both respiratory and car-diac motion.

In this article, a novel technique ofMR imaging, which combines gradi-ent echo and spin echo (GRASE) tech-niques, enables T2-weighted abdomi-nat imaging in tess than 18 seconds.This is sufficiently fast to permitbreath holding during image acquisi-tions to eliminate respiratory-motionartifacts, currently not possible in din-ical T2-weighted spin-echo studies ofthe abdomen performed with acquisi-tion times of 4-8 minutes.

MATERIALS AND METHODS

GRASE Technique

The technique of combining gradientechoes with spin echoes is shown in thepulse-sequence diagram (Fig 1). A train, ornumber of RF refocused spin echoes (NSE),is produced by using the CPMG spin-echosequence. Centered about each spin echo,a number of gradient-recalled echoes (NGR)are produced by switching the polarity ofthe readout gradient. The speed advan-tage over standard spin-echo imaging isproportional to the total number of echoespen 90#{176}excitation and equals NGR X N5�.The effective echo time of the image is thetime at which the origin of k space (zeroorder phase encode) is sampled, near themiddle of the echo train.

In GRASE imaging, the use of 180#{176}REpulses to nutate on reverse the position ofspins leads to the nulling of field inhomo-geneity errors at the Hahn spin-echo time(8), eliminating image distortions and lossof S/N. Small field-inhomogeneity errorsevolve within each group of N(;R echoesduring the relatively short time betweeneach gradient-recalled echo and the centerof their respective spin echo. In a similarway, the modern echo-planar imagingtechniques, MBEST (6) and Instascan (7),involve use of a single 180#{176}RE pulse, butwith greater field-inhomogeneity error,proportional to half the time of the gradi-ent-echo train typically composed of 64-

128 signals.

Common to both GRASE and echo-pla-nan imaging, chemical shift is greaten onthe phase axis of the image than on the

read (frequency) axis as a result of fre-quency sensitivity through the echo train.In GRASE imaging, the chemical shift isproportional to the time interval of theshort echo train with NCR echoes, whereasin echo-planar imaging, the chemical shiftis proportional to the longer time of thegradient-echo train composed of 64-128echoes.

The GRASE pulse sequence is morecomplicated than a simple combination ofspin echo with gradient echo, since therelatively small field-inhomogeneity phaseerrors recur identically in each group ofNCR echoes between successive 180#{176}REpulses, as shown in Figure 1 . If phase en-coding is continuously incremented alongthe total echo train, as in echo-planar im-aging or the RARE technique, there wouldbe modulation of chemical shift and T2*on the phase axis of k space. After Fourier-transform image reconstruction, thesemodulations would result in severe ghost-ing artifacts in the image.

This problem is eliminated with use of adiscontinuous phase-encoding order dun-ing the echo train. As more fully describedelsewhere (9), phase encoding sweepsthrough three large increments of k spaceduring each group of NCR echoes. Holding

these large phase increments constant,

small phase increments are imposed dun-

ing each 180#{176}-180#{176}interval along the total

echo train. Subsequent computer reorder-ing or interleaving of the signals in k space(Fig 2) effectively removes the peniodicityof field inhomogeneity errors on the phaseaxis.

In this way, GRASE phase encoding or-den is significantly different from that ofRARE and echo-planar imaging, whichcontinuously increase their phase-encod-

ing steps as a function of time in the echo

train (Fig 2). Similar to the hybrid RAREtechnique, multiple excitations are per-

formed with small incremental phaseshifts to fill in additional areas of k space.

Multiple sections are imaged during each

TR cycle of GRASE 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 phase-encoded image lines. The additional TR isrequired to establish steady-state magneti-

zation. This initial excitation also producesa template of echoes without phase en-coding, which can be used for T2* magni-

tude normalization and gradient-echotiming corrections in the phase-encodedechoes. In the presented imaging expen-

ments in which NL 192, NcR 3, and

s4s7

s2S5�&

s3s6

s9

a.

�IIIIIII

III

III

uu�

Ii’�

IIII I IIII

II

I___

�L

��L

kx

b. c.

Figure 2. The k-space trajectory of (a) the GRASE technique, (b) RARE technique, and (c) echo-planar imaging techniques, including MBEST

and Instascan. Numbers on the left side in a-c correspond to the time order of the signal in the echo train. The phase-encoded signals (horizon-tal arrows) reverse their direction from right to left during the negative-polarity readout gradients (Fig 1). In the GRASE technique (a), the tra-

jectony of the first group of three signals scans over nearly the entirety of k space, with identical trajectories for subsequent groups of signalsexcept for a slightly displaced starting position between each group. In RARE imaging (b) and echo-planar imaging (c), the k-space trajectories

are continuously displaced on the phase axis sequentially in time (thick arrows), unlike GRASE imaging. Multiple excitation cycles fill in kspace by interleaving signals in both GRASE and hybrid RARE imaging. ky = phase axis, kx = frequency axis.

ky

Volume 181 #{149}Number 2 Radiology #{149}599

N5E 8, the total data acquisition time is

(9 x TR x NSA).

Methods

Experiments were performed with a

1.5-T MR system (Signa; GE Medical Sys-tems, Milwaukee) using maximal gradientstrength of 1 C/cm (10 mT/rn). The read-out periods were 2 msec on 4 msec with 0.6msec gradient ramp times for a respective18- or 24-msec interval between each pairof 180#{176}RE pulses. Free-induction-decayspoiler pulses were used on the read-gra-dient axis and are not shown in Figure 1.The effective echo time (TE) is the time atwhich the origin of the k space is sampled:80 on 104 msec.

With NGR 3 and NSE 8, there is a to-tat of 24 signals per 90#{176}excitation to giveexactly 192 phase-encoded signals in eightexcitations. The 192 x 256 k-space data setacquired in this way is symmetrically zero-filled to 256 x 256 before two-dimensionalFourier-transform image reconstruction. Astandard multisection excitation scheme(1) was performed by using selective REexcitations with frequency offsets.

Head and body images of the samehealthy volunteer were obtained. Also,two patients with a known radiologic andclinical diagnosis of multiple sclerosis un-derwent imaging with the GRASE tech-nique at the time of their routine T2-weighted spin-echo studies.

RESULTS

To compare tissue contrast, brainimages were obtained with theGRASE, spin-echo, and RARE tech-

niques, each with similar pulse se-quences: 2,500/100-104 (TR msec/TEmsec). For this comparison, the S/Nwas intentionally set to be nearly

equal by using conjugate synthesis(NSA = V2) for spin-echo and NSA =

2 for the GRASE and RARE tech-mques (Fig 3). In all three images, thephase axis is horizontal. The contrastamong gray matter, white matter, andcerebrospmat fluid is similar in theseimages. The signal intensity of skin inthe RARE image (Fig 3c) is noticeablymore intense than that in either theGRASE or spin-echo image due to ahigher signal of lipid in the RARE im-age. Greater flow artifact is presentacross the central region of the spin-echo image (Fig 3b).

Quantitative measurements of tis-sue signal intensity (pixel intensity)obtained from the comparative headstudy involving the spin-echo, RARE,and GRASE techniques are providedin the Table. Comparison of tissuecontrast is somewhat affected by the

different TE values. To obtain similar

S/N in images, acquisitions of GRASE

and RARE images involved use ofNSA = 2 and a signal readout period

of 4 msec, whereas acquisition ofspin-echo images involved use ofNSA = ‘/2 and a signal readout periodof 8 msec. Although estimates of im-age S/N, calculated from the ratio oftissue signal intensity to air signal in-

tensity, are similar among the three

techniques, the accuracy of measure-ment is affected and limited by van-ability in tissue structure.

Images of the abdomen (Fig 4) wereobtained in 18 seconds, during a sin-gle breath hold. Therefore, respiratorymotion artifact is totally absent. The

renal arteries branching from the ab-dominal aorta are well demonstratedwithout respiratory motion artifact.The contours of the liver, spleen, andkidney are sharply defined. Chemical

shift on the phase axis (horizontal) is

1.6 pixels and on the frequency axis(vertical) is 0.8 pixel.

Typical tong-TR head studies (Figs

5, 6) obtained with a pulse sequence

of 4,000/104 and NSA = 2, require a72-second imaging time; those ob-tamed with NSA = 1 require a 36-second imaging time. Comparison ofthe image quality and S/N can bemade at similar brain levels in theseNSA = 1 and NSA = 2 studies. Theimage quality is sufficient to demon-strate many small radial arteries in the

neocortex. By using the GRASE tech-

nique with NSA = 1, a multiple scle-rosis plaque is readily demonstratedin the right frontal white matter (Fig6b) of a patient with multiple sctero-sis. Direct comparison by a neuroradi-ologist of the number of multiple scle-rosis plaques in T2-weighted spin-echo images with that in GRASEimages at the same levels demon-strated an equal number of plaques in

the two patients studied.

Sagittal GRASE images of the lum-bosacral spine (Fig 7) demonstrated

the myetographic effect of cerebrospi-nat fluid with a TR of 3 seconds. De-generative disk disease was noted inthe lumbar spine.

DISCUSSION

The development of computed tomo-graphic (CT) technology for abdominalimaging, not unlike that of MR imaging,required a large leap in image acquisitionspeed to move from the early head imag-ing times of 3-5 minutes to the presentimaging times of 1-3 seconds. CT bodyimaging is now of great clinical utility,while the problems of respiratory motionhave not been overcome in routine clinicalMR imaging. To date, respiratory motion

a. b. C.

600 #{149}Radiology November 1991

Figure 3. Comparison of tissue contrast in (a) GRASE, (b) spin-echo, and (c) RARE MR images of the brain. The three multisection images

were acquired with a TR of 2.5 seconds and a section thickness of 5 mm. (a) The GRASE image was obtained with NSA = 2 and an image time

of 45 seconds for 13 sections. (b) The spin-echo image was obtained with conjugate synthesis (NSA = ‘/2) and an image time of 4.6 minutes for

25 sections of both proton-density and T2-weighted images. (c) The hybrid RARE image was obtained with NSA = 2 and an image time of 64seconds for 13 sections. RARE and spin-echo images were acquired with a TE of 100 msec, while the GRASE TE differed slightly (104 msec) due

to sequence timing differences.

artifacts have significantly limited T2-weighted spin-echo imaging, which other-wise has great sensitivity to abdominal

pathologic conditions. To eliminate respi-natony artifact, GRASE imaging times of 18seconds permit comfortable breath hold-ing for a large portion of the patient popu-lation.

Due to the discontinuous phase-encod-

ing method of GRASE imaging, Hahn spinechoes coven the entire central third por-

tion of k space where the highest signal

energy occurs (8). For this reason, the tis-sue contrast of GRASE, in theory andpractice, is indistinguishable from that of

spin-echo imaging, determined predomi-

nantly by these central gradient echoes

that in fact are spin echoes. Given theirsimilar contrast mechanism, GRASE and

spin-echo imaging demonstrate multiple

sclerosis plaques equally well, unlike low-flip-angle gradient-echo imaging, which issuboptimal for detection of some patho-

logic conditions.

As demonstrated in the head studies(Fig 3, Table), tissue contrast in GRASE

images is essentially the same as that inspin-echo images obtained with similar

TRs and TEs. The abdominal tissues (Fig4), including liver, spleen, kidney, muscle,and fat have the expected spin-echo T2-

weighted contrast. While breath holdingeffectively eliminates much of the motion-

related artifact, some residual artifact re-mains because of pulsations in the liver

and adjacent structures. Pulse sequencemodifications to reduce cardiac pulsationeffects and other related artifacts are cur-nently being investigated (10).

The primary determinant of S/N in

GRASE, RARE, and spin-echo imaging issignal bandwidth (S/N is proportional to

the square root of signal read time). While

the GRASE technique more efficiently uses

the T2 relaxation period to refocus or,, pump out” more signals pen acquisitiontime, this factor does not directly cause

S/N loss unless shorter read periods with

larger signal bandwidths are used. Conju-gate synthesis spin-echo imaging, not hay-ing the time constraint of many signal read

periods in each excitation, permitted twiceas long a read period as the GRASE andRARE techniques in these experiments,

compensating for its intrinsic 35%-41 %S/N loss. The demonstrated image quality

in head and body studies obtained withthe GRASE technique with NSA = I issuitable for routine clinical imaging.

It is significant that GRASE imaging in-volves use of about a third the number of

180#{176}RF pulses per multisection acquisitionthat are used in a similar RARE acquisi-tion, which has a higher SAR. The speed

advantage of the GRASE over the RARE

technique can be given in a simple expres-

sion for the average time of each phase-

encoded signal. The average time of a

phase-encoded echo can be directly calcu-

lated by using exemplary time values; theselective 180#{176}RF nefocusings and sun-

rounding free-induction-decay spoilergradients (T51) equal 5 msec; the net timeof the initial phase-encoding pulse andfinal rephasing pulse (TPE) equals 4 msec; a4-msec echo readout period plus gradient

rise times (TRw) equals 5.2 msec. Average

time per echo = [TRI + TPE + (NCR X T5�))]/N�,5.

For RARE imaging with NCR = L theaverage time per signal is (5 + 4 + 5.2)/I,

or 14.2 msec. For GRASE imaging withNGR _3, the average time per echo is [5 +

4 + (3 x 5.2)]/3, on 8.2 msec. For GRASE

imaging with N(;5 = 7, the average timeper echo is 6.5 msec. Although the effi-ciency of GRASE imaging is currently in-

C. d.

a. b. C.

a. b.

Volume 181 #{149}Number 2 Radiology #{149}601

Figure 4. Abdominal MR images.

(a, b) Coronal studies of the upper abdomenobtained during single breath holds. The

total image time was 18 seconds for 11 multi-section images, pulse sequence was 2,000/80,section thickness was 10 mm, and field ofview was 38 cm. (c, d) Axial images obtainedwith parameters identical to those of thecoronal images except for a TE of 90 msec.

Figure 5. (a-c) GRASE MR images obtained in the head of a healthy volunteer with NSA = 2. Imaging time was 72 seconds, pulse sequence

was 4,000/104, field of view was 24 cm, and section thickness was 5 mm.

termediate between echo-planar andRARE imaging, application of the GRASE

technique with an imaging system withhigh performance gradients (fast rise

times and strong maximal gradient) will

further increase the efficiency of GRASE

imaging, making it closer to that of echo-

planar imaging. The RARE technique can-not benefit nearly as much from high per-formance gradients because of the absence

of gradient refocusing, its large time over-head in 180#{176}RF pulses, and SAR limita-

tions.

At first glance, in comparing GRASEwith MBEST and Instascan, the additional

time cost of the multiple 180#{176}RE refocus-

ings certainty appears to directly reduce

the efficiency of GRASE imaging. Theseadditional nefocusings, however, permit

signal acquisition to occur much earlier

than in these echo-planar techniques,which necessarily have a much longer

90#{176}-180#{176}time interval when no signals can

be acquired. This advantage of GRASEimaging may largely offset the efficiencydifferences between these techniques forT2-weighted imaging.

It is important to realize that GRASEimaging is a variant of spin-echo imagingthat involves use of the CPMG sequence

a. b.

Figure 6. GRASE MR images obtained in the head with NSA = 1, image time of 36 seconds,and pulse sequence of 4,000/104. (a) Image obtained in healthy volunteer and (b) image ob-tamed in a patient with multiple sclerosis. The image quality and S/N can be compared with

those of brain images obtained with NSA = 2 in Figure 5, which were otherwise acquired with

identical image parameters.

602 #{149}Radiology November 1991

to produce spin-echo contrast. GRASE im-aging lies on a continuum between RAREimaging, which involves use of multiplespin echoes, and single-shot echo-planar

imaging, which involves use of multiplegradient echoes. In GRASE imaging, asmall amount of chemical shift is ac-cepted with the decrease in imaging time

and SAR afforded by the use of gradient

echoes.Several obvious modifications of GRASE

imaging are possible: division of the echotrain into two identically phase-encoded

data sets for simultaneous proton-densityand T2-weighted (double-echo) imaging,use of different TEs and TRs for desiredimage contrast; extension to multislabthree-dimensional volume imaging; and

use of 512 x 512 high-resolution imaging

in clinically acceptable imaging times.Methods of performing diffusion and flow

imaging with the CPMG sequence have

been described (II). Another possible ap-plication of GRASE imaging in systemswith low magnetic field strength could be

to perform imaging with NSA = 4 or 8 toincrease image S/N in clinically acceptableimaging times.

In summary, multisection GRASE imag-ing is similar to long-TR spin-echo imag-

ing in terms of image fidelity, contrast,

and spatial resolution with a 24-fold re-duction in image acquisition time. The

time advantage of GRASE over spin-echo

imaging is accomplished by more effi-ciently using the T2 decay period. A pen-

alty of fewer image sections in the currentimplementation of the GRASE techniquecan be optimized with tradeoffs in T2weighting and imaging time. Combination

of the GRASE technique with high-penfor-mance gradient systems should producefurther large reductions in imaging timewhile maintaining image quality and spa-tial resolution. The ability to implement

the GRASE technique without the need to

change gradient hardware, however, is asignificant advantage of this new high-speed imaging technique. U

Acknowledgment: We are grateful to FerencJolesz, MD, for his assistance in comparing neu-

rologic imaging studies and for his comments in

preparation of this manuscript. We are verygrateful to Lawrence E. Crooks, PhD, for his edi-tonal comments.

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Figure 7. Sagittal GRASE images of the

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time was 56 seconds. Note the myelographic

effect of cerebrospinal fluid. Degenerativedisk disease is demonstrated at multiple disk

levels, with more severe central disk bulging

at the L2-3 level and loss of signal intensityin the L4-5 disk.

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