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SCIENTIFIC EXHIBIT

357

Chemical Shift: TheArtifact and ClinicalTool Revisited1

Maureen N. Hood, BSVincent B. Ho, MDJames G. Smirniotopoulos, MDJerzy Szumowski, PhD

The chemical shift phenomenon refers to the signal intensity alterationsseen in magnetic resonance (MR) imaging that result from the inherentdifferences in the resonant frequencies of precessing protons. Chemicalshift was first recognized as a misregistration artifact of image data. Morerecently, however, chemical shift has been recognized as a useful diag-nostic tool. By exploiting inherent differences in resonant frequencies oflipid and water, fatty elements within tissue can be confirmed with dedi-cated chemical shift MR pulse sequences. Alternatively, the recognitionof chemical shift on images obtained with standard MR pulse sequencesmay corroborate the diagnosis of lesions with substantial fatty elements.Chemical shift can aid in the diagnosis of lipid-containing lesions of thebrain (lipoma, dermoid, and teratoma) or the body (adrenal adenoma, fo-cal fat within the liver, and angiomyolipoma). In addition, chemical shiftcan be implemented to accentuate visceral margins (eg, kidney and liver).

Abbreviations: FMPSPGR = fast multiplanar spoiled gradient echo, TE = echo time

Index terms: Magnetic resonance (MR), artifacts • Magnetic resonance (MR), chemical shift • Magnetic resonance (MR),tissue characterization

RadioGraphics 1999; 19:357–371

1From the Department of Radiology and Nuclear Medicine, Uniformed Services University of the Health Sciences, 4301Jones Bridge Rd, Bethesda, MD 20814-4799 (M.N.H., V.B.H., J.G.S.) and the Department of Radiology, Oregon Health Sci-ences University, Portland (J.S.) Recipient of a Certificate of Merit award for a scientific exhibit at the 1997 RSNA scien-tific assembly. Received March 10, 1998; revision requested May 5 and received July 16; accepted July 16. Address re-print requests to M.N.H.

The opinions or assertions contained herein are the private views of the authors and are not to be construed as officialnor as reflecting the views of the Uniformed Services University of the Health Sciences or the Department of Defense.

©RSNA, 1999

LEARNINGOBJECTIVES

After reading this articleand taking the test, thereader will be able to:

• Describe the chemicalshift artifact and where itis typically seen on MRimages.

• Identify ways to min-imize or exaggerate thechemical shift artifact.

• Give examples of com-mon body and neurologicpathologic conditions forwhich the use of chemicalshift imaging can be advan-tageous.

358 ■■■■■ Scientific Exhibit Volume 19 Number 2

■■■■■ INTRODUCTIONPhenomena that may result in image artifactscan sometimes be used to assist in radiologicdiagnosis. Chemical shift is an example of sucha phenomenon in magnetic resonance (MR) im-aging. Chemical shift refers to the signal alter-ations that result from the inherent differencesin the resonant frequencies of precessing pro-tons. Clinically, the chemical shift phenomenonis most evident between the signals of waterand lipid. Chemical shift is a complicated phe-nomenon and is probably best known, or moreprecisely, notorious for its ability to cause mis-registration of MR image data. This effect, alsoreferred to as chemical shift artifact, results fromthe mismapping of water and lipid signals andis most commonly encountered when one im-ages fluid-filled structures such as the orbits orbladder, which are enveloped within fatty tissue(1–3).

Only recently, chemical shift has been rec-ognized as a diagnostic aid, possibly as a resultof the growing experience with faster gradient-echo pulse sequences or of the increased clini-cal use of MR imaging (4). When pulse sequencesare modified to be sensitive to chemical shifteffects, MR imaging can be used to confirm thepresence of fat within lesions, a finding that canresult in a definitive diagnosis. In addition, chemi-cal shift can be used to accentuate the fat-waterinterfaces that occur at many visceral margins.This technique can improve the evaluation ofperipheral tumors for possible extravisceral ex-tension or the visualization of visceral contours.

In this article, the basic principles of chemi-cal shift and their implications for clinical imag-ing are reviewed. Specific applications of chemi-cal shift as a diagnostic tool in neurologic andbody imaging are illustrated.

■■■■■ BASIC PHYSICS OF CHEMICALSHIFT

●●●●● Physical BasisWhen an external magnetic field is applied totissue, its nuclei will resonate at a specific fre-quency that depends on the strength of that ex-ternal magnetic field (B

0) and its local microen-

vironment. The effects of the external magneticfield can be determined by the Larmor equation:ω

0 = γ · B

0, where ω is the resonant or preces-

sional frequency and γ is the gyromagnetic ratiofor a specific nuclear species (5). The effectsof the local environment on the individual pre-cessional frequency of each nucleus are furtherinfluenced by the electron shell interactions ofeach nucleus with those of surrounding mol-ecules. These additional electron shell interac-tions influence the local magnetic shielding ofthe protons, creating “effective” local magneticfield strengths (B

eff) that have the ultimate con-

trol over the “effective” precessional frequen-cies (ω

eff), as shown by the modified Larmor

equation: ωeff

= γ · Beff

(6,7).The molecular interactions that determine

Beff

form the basis for the chemical shift phe-nomenon (5). When the chemical environ-ments of adjacent nuclei are dramatically differ-ent in their electron (ie, magnetic) shielding, ashift in the precessional frequencies of the nu-clei can occur (6). If these precessional fre-quency shifts are large enough, a measurable

Figure 1. Equation illustrates the relationship betweenchemical shift and field strength, where ∆f

cs = resonant fre-

quency shift in parts per million (ppm), δ = relative chemicalshift frequency difference, and ω

0 = main magnetic field reso-

nant frequency. Given that human lipid and water have aresonant frequency difference of 3.5 ppm (ie, ∆f

cs= 3.5 ppm), the

chemical shift frequency change (δ) will be approximately224 Hz at a field strength of 1.5 T (ie, ω

0 = 64 MHz).

March-April 1999 Hood et al ■■■■■ RadioGraphics ■■■■■ 359

alteration in MR signal intensity will be ob-served. When imaging parameters (eg, in gradi-ent-echo sequences, the time at which theecho is sampled [ie, echo time or TE]) are ma-nipulated, the precessional frequency shifts, orchemical shift phenomenon, can be accentu-ated to generate visible differences betweenlipid and water protons in tissue (5).

In clinical imaging, hydrogen protons formthe basis for MR image signal and contrast. Thechemical shift phenomenon is most noticeablebetween the hydrogen protons of lipid and thoseof water because of their large relative differ-ences in magnetic shielding. The difference inresonant frequencies between lipid and waterprotons increases proportionately with the staticmagnetic field strength. The approximate chemi-cal shift between human lipid (methylene [CH

2]

n

of fatty acids) and water is 3.5 parts per million

(5,8–11). At a field strength of 1.5 T, this shifttranslates to approximately a 224-Hz separationbetween the resonant frequencies of fat andwater protons (Fig 1).

●●●●● Clinical Implications

Spatial Misregistration: The Artifact.—Theinherent differences in precessional frequen-cies between fat and water protons have sev-eral implications in clinical imaging. The oldestand best characterized effect of chemical shiftis that of spatial misregistration. Frequency andphase components of MR signal arising from atissue are used to encode the x- and y-axis spa-tial coordinates of the two-dimensional section(6). Because the frequency displacement causedby chemical shift cannot be differentiated fromintended spatial frequency encoding, misregis-tration of the resultant signal may occur alongthe frequency-encoding direction (also calledreadout axis) (6). If the MR imager is tuned orcentered to the frequency of water, fat will beshifted or spatially mismapped relative to itstrue spatial location. This mismapping occursthroughout the image, although it is most ap-parent between regions that are primarily fattyand those that are composed of water or fluid(ie, at lipid-water interfaces) (5,6). When thechemical shift misregistration is greater than orequal to the size of an individual pixel, a dark orbright band of signal intensity will occur at thelipid-water interface in the frequency-encodingdirection of the image (11). This pixel-by-pixelmisregistration along the frequency-encodingdirection visibly manifests itself as a bright ordark band running perpendicular to the frequency-encoding direction. Clinically, the chemical shiftartifact is most often recognized at the marginsof predominantly fluid-filled structures that areembedded in fatty regions, such as the bladder(Fig 2) and orbits (2,3,5,11). The misregistrationof signal along the frequency-encoding direction

Figure 2. Coronal T1-weighted spin-echo image(repetition time msec/TE msec = 666/10) of a 52-year-old woman with an anteverted uterus demon-strates a chemical shift artifact around the uterus. Thesuperior border of the uterus (U) has a bright band(black arrow), whereas the inferior border of the blad-der has a dark band (white arrow). The severe lipid-water interface between the uterus and the pelvic fatproduces a strong shift of the protons at the marginof the bladder.


results from the erroneous mapping of the sig-nal of the fatty elements relative to that of thefluid-filled structures. This misregistration re-sults in the production of dark or bright bandsat the lipid-water interface (Fig 3). The darkbands result from the shifting of the lipid pro-ton signals to a lower frequency, away fromthe actual lipid-water interface, which causes asignal void (Fig 4) (5,6,12). The bright bands re-sult from the overlapping of water signal with“shifted” lipid signal on the high-frequency sideof the interface (Fig 4). The bright bands, althoughpresent, may be more difficult to appreciate whenthe object being imaged has a curved surface(6,12).

There are several ways to minimize the spatialmisregistration caused by the chemical shift ar-tifact, including choice of frequency-encodingdirection, field of view, and receiver bandwidthand use of fat-suppression techniques. The size(width of dark and bright bands) and locationof the chemical shift artifact are influenced bya number of factors. Most important is the se-lection of the frequency-encoding direction. Theclinical significance of the chemical shift arti-fact can be reduced by choosing the frequency-encoding direction in the plane with the nar-rowest lipid-water interface or by selecting adirection that minimizes the chemical shift ef-fect over the area of primary interest (Fig 5).

The imaging field of view is an operator vari-able that can easily be controlled and has a stronginfluence on the size of the chemical shift arti-fact seen in an image. The artifact can be mini-mized by decreasing the field of view (5,9).

Receiver bandwidth is another imaging pa-rameter that may affect the amount of chemi-cal shift artifact seen in routine MR images. Theartifact is usually not readily apparent in spin-echo images unless the receiver bandwidth is

Figure 3. Coronal T1-weighted fast multiplanarspoiled gradient-echo (FMPSPGR) image (repetitiontime msec/TE msec, flip angle = 103/5.6, 80°) ob-tained with right-to-left frequency encoding of a 61-year-old man demonstrates chemical shift artifact inmany of the abdominal lipid-water interfaces. Darkbands (arrowheads) are seen along the lateral rightpsoas muscle, lateral right renal, medial splenic, andmedial left renal margins. Bright bands are visualizedat the opposing lipid-water interfaces of the lateralleft psoas muscle, medial right renal, lateral splenic,and lateral left renal margins.

Figure 4. Schematic depicts chemical shift alongthe frequency-encoding direction. Lipid signal isshifted to a lower-frequency position when it is sur-rounded by water, which leaves a signal void (darkband) on the high-frequency side of the lipid struc-ture and an increase in signal (bright band) on thelow-frequency side.


narrowed to less than the typical ±16 kHz. Whena narrow bandwidth is used, the strength of thegradients along the frequency-encoding direc-tion is reduced (5,9,10). The readout (frequency)window must remain open proportionally longerto preserve the field of view; thus, the appar-ent shift in resonant frequencies between lipidand water protons becomes more widely sepa-rated spatially (Figs 6, 7) (9,10). The effect ofbandwidth on chemical shift is directly propor-tional to the static magnetic field strength. Whenusing MR imaging units with 1.5 T (or greater)magnetic field strength, the operator must bevery careful when narrowing the bandwidthto image anatomic areas with lipid-water inter-faces because this action will increase the de-gree of chemical shift artifact in the resultantimages (Fig 8) (2).

a. b.

Figure 5. T1-weighted FMPSPGR images (51/2.1, 90°) of a phantom with inner water (w) and outer oil (o) com-ponents illustrate the chemical shift artifact, which is perpendicular to the frequency-encoding direction. (a) Onthe image obtained with left-to-right frequency encoding (open arrow), the dark band of the chemical shift ar-tifact is noted along the superior-to-inferior water-oil interfaces (solid arrow). (b) On this image, the frequencyencoding is switched to inferior to superior (open arrow), and the dark band of the artifact is noted along theright-left water-oil interfaces (solid arrow). The bright band is poorly visualized in both images because it is su-perimposed over the already bright signal of the surrounding oil.

6.

7.

Figures 6, 7. (6) Equation demonstrates the rela-tionship between chemical shift, frequency shift, fieldof view (FOV), and bandwidth. (7) Pulse sequencediagram shows the differences in readout (frequency)window length with a change in the receiver band-width. The readout window must be open longer topreserve the field of view, which in turn, increasesthe chemical shift artifact.


Chemical shift artifact can also be reducedby minimizing the signal contribution from lip-ids with the use of fat-suppression techniquessuch as spectral saturation.

Phase Cancellation.—The effects of chemicalshift are most prominent in images obtainedwith gradient-echo pulse sequences, such asFMPSPGR. Because gradient-echo sequences arefaster than spin-echo sequences, they can beused for dynamic imaging and applications thatrequire breath-holding techniques. These imag-ing strategies have contributed to the rise inpopularity of gradient-echo imaging of the body.

Traditional spin-echo imaging uses a 180° re-focusing radio-frequency pulse applied at thecenter of the TE (1/2 TE). This pulse redirectsthe fat and water proton precessions or spinsback into phase with respect to one other bythe actual readout or TE (13). In contrast, gradi-ent-echo sequences do not employ a 180° radio-frequency pulse to refocus the image signal.The lack of a 180° refocusing radio-frequencypulse in gradient-echo sequences results in thecycling of fat and water proton signals in andout of phase with respect to each other overtime (5,14). Unlike repetition time, the TE (ie,the time the echo is sampled) plays a significantrole in determining the relative degree of phase

differences between fat and water protons (ie,degree of chemical shift) occurring in the im-age and the amount of signal decay that occursover time (5,9,14). At 1.5 T, fat and water sig-nals precess in phase approximately every 4.4msec; at 1.0 T, every 7.0 msec; and at 0.5 T, ev-ery 13.2 msec (5). At in-phase TE selections (eg,TE of 4.2 msec on a 1.5-T MR imager), the sig-nal contributions of lipid and water protons areadditive in their contributions to the resultantimage. During other TEs, they become progres-sively more out of phase, with the peak occur-ring at the intermediate times of about 2.1 msecand 6.3 msec on a 1.5-T MR imager. In terms ofimage signal, a time component called T2* alsocomes into play as the TE lengthens. T2* is theamount of dephasing that arises from magneticfield inhomogeneities and causes the signal todecay over time (5). Use of longer TEs will re-sult in less signal contributing to the image thanif shorter TEs were used.

If the echo is sampled at a time (TE) whenthe fat and water signals are out of phase (Fig 9),only the difference in fat and water proton sig-nals within the voxel contributes to the resultantimage. If considerable fat signal is present, thetissue will manifest diminished signal intensity.This mixing of lipid and water signals within thevoxel accounts for the improved visualizationof visceral margins seen in out-of-phase gradi-ent-echo images (Fig 10).

a. b.

Figure 8. T1-weighted FMPSPGR images (51/2.1, 90°) of a phantom with inner water and outer oil componentsillustrate the effect of bandwidth on the degree of chemical shift seen in images. (a) This image, obtained witha bandwidth of 7.8 kHz, shows a dark band on the inferior border (arrow) and a bright band on the superior bor-der (arrowhead) of the water portion of the phantom. (b) On this image, obtained with a 31.2-kHz bandwidth,the dark and bright chemical shift bands are barely noticeable.


a. b.

Figure 9. In-phase FMPSPGR image (90/4.2, 90°) of a 57-year-old man with focal scarring of his right kidneymore accurately demonstrates enhancement (a), compared with the out-of-phase FMPSPGR image (90/2.2, 90°)(b), which better delineates the renal contours. The out-of-phase gradient-echo image better delineates the focalscar in the middle of the right kidney (arrow).

Figure 10. Schematic shows lipid and water protons precessing inand out of phase with respect to each other over time at 1.5 T. Whenlipid and water are in phase with each other, the net signal is additive,as shown at time points A, C, and E. Out-of-phase lipid and water sig-nals nearly cancel each other out, as shown by the drop in the sinecurve at time points B and D. The sine wave depicts the signal intensitywithin a voxel over time as the water and lipid protons oscillate be-tween in phase and out of phase after the radio-frequency excitationpulse is delivered. The curve also drops in overall signal intensity dueto T2* decay. (Modified and reprinted, with permission, from refer-ence 5.)


Misexcitation.—Most of the structures ofthe human body have complex and irregularshapes. Smith et al (12) explored the effect ofshape on chemical shift artifact. Using a coro-nal, oil-water interface, they were able to ac-quire oblique axial sections through the lipidslab in specific angles. They determined thatchemical shift artifact was still apparent onoblique images with an angulation range of−20° to +45° from the lipid-water interface(12). This effect is postulated to occur becauseof chemical shift misregistration, as describedearlier, and because of misexcitation occurringalong the slice-selection direction. Misexcita-tion is the chemical shift that occurs during theslice selection of the pulse sequence. Misexci-tation occurs because a frequency excitationpulse is applied while the slice-selection en-coding gradient is being applied. The lipid re-gion is chemically shifted along the slice-selec-tion direction, which causes misexcitation ofthe lipid components and eventually results asa chemical shift artifact similar to misregistra-tion (Fig 11) (5,12,15).

■■■■■ CLINICAL USE OF CHEMICAL SHIFTEFFECT

●●●●● Tissue Characterization

Neurologic Imaging.—In the head, fatty sub-stances are mainly found in the orbits, bonemarrow, and subcutaneous tissues of the skull.Lipid-containing intracranial masses, althoughrare, can be well demonstrated as hyperintenselesions on T1-weighted MR images (16,17).However, a number of other substances (in-cluding methemoglobin, protein, calcifications,and melanin) can also appear hyperintense onT1-weighted images (18–20). The coexistenceof a chemical shift banding artifact around ahigh-signal-intensity focus on a T1-weighted im-age can help confirm the presence of lipidwithin the lesion. Because of the sharp lipid–cerebrospinal fluid or lipid-brain interface,chemical shift artifact can be noted in most in-tracranial lipomas (Fig 12) (10).

Teratomas and dermoids are other lipid-con-taining lesions that can occur inside the calva-ria (21,22). Teratomas contain varying amountsof tissues from more than a single germ layer,

usually in a heterogeneous multiloculated massthat corresponds to the heterogeneous signalintensity depicted on MR images (21). Dermoidscan show lipid material floating on top of theheavier water or proteinaceous material (21).

Body Imaging.—In gradient-echo MR imagingof the body, the selection of TE can be used tomanipulate the relative contributions of fat andwater signals for diagnostic purposes. For ex-ample, images obtained with out-of-phase TEwill demonstrate visible or measurable decreasesin signal intensity in tissues with considerablelipid, compared with the signal intensity onequivalent in-phase images. This use of “out-of-phase” gradient-echo sequences has become apopular technique for aiding in the diagnosis ofboth fatty infiltration and focal fat within theliver (23–27). A visible decrease in signal inten-sity will be apparent on out-of-phase gradient-echo images of these benign conditions (Fig13). Because MR imaging can demonstrate sig-nal intensity changes (due to fatty infiltration)quantitatively even at levels as low as 10% (28),

Figure 11. Schematic illustrates chemical shiftmisexcitation. During the slice-selection radio-fre-quency excitation pulse, protons are subjected toa range of frequencies or bandwidths (BW). Thechemical shift between lipid and water actuallyshifts the lipid protons along the slice-selection axis.The lipid protons are excited in a position that isshifted slightly lower along the slice-selection axisthan the water protons in the section, which causesa misexcitation to occur in the slice-selection direc-tion. (Modified and reprinted, with permission,from reference 5.)


a.

Figure 12. (a) Computed tomographic (CT) scan of a 1-year-old boywith a left external ear deformity shows an intracranial lipoma as a hy-poattenuating region (*). (b) Sagittal T1-weighted image (617/20) dis-plays the fatty nature of the intracranial lipoma (L) as a homogeneouslyhigh-signal-intensity structure within the brain. (c) Axial proton-density–weighted image (3,500/20) obtained with inferior-to-superior frequencyencoding shows a small amount of chemical shift artifact (arrow) occur-ring between the lipoma and the cerebrospinal fluid in the foramenmagnum. (d) T2-weighted image (3,500/100) obtained with inferior-to-superior frequency encoding shows a much larger chemical shiftartifact.

b. c. d.

a. b.

Figure 13. (a) Axial T1-weighted image (400/10) of a 48-year-old woman, in whom CT had revealed a focalhypoattenuating mass in the left lateral hepatic lobe, shows the lesion as a homogeneous, mildly high-signal-in-tensity area (arrow) without mass effect, findings suggestive of focal fat. The axial in-phase gradient-echo im-age (not shown) also depicted the lesion as hyperintense compared with the rest of the liver. (b) Axial out-of-phase FMPSPGR image (34/2.3, 90°) helps confirm that the mass is focal fat by demonstrating the same area asbeing very dark (arrow). Because of destructive interaction between fat and water signals on out-of-phase im-ages, fatty lesions appear as a signal void.


the selection of TE for dynamic imaging is im-portant. An inappropriate TE selection may oc-casionally result in benign fatty infiltration ofthe liver and focal fat being mistaken for a ma-lignancy or may mask the identity of the lesion(Fig 14) (24–27).

Chemical shift MR imaging has become a popu-lar technique for diagnosing adrenal adenomas(29-42). Benign adrenal adenomas, which typi-cally are composed of approximately 16% lipidbased on in vitro studies (38), can demonstratemeasurable differences in signal intensity whentheir appearance on in-phase gradient-echo im-ages is compared with that on the out-of-phasecounterparts (Fig 15). A decrease in signal in-tensity of greater than 20% within an adrenal masson out-of-phase images helps confirm the diag-nosis of an adrenal adenoma (40,42). Adrenalmetastases (Fig 16), on the other hand, typicallydo not contain any significant lipid elements and

will not demonstrate an appreciable change insignal intensity with in-phase and out-of-phasechemical shift imaging (32,33,37,42). Some re-searchers have reported that chemical shift MRimaging is even more reliable than the measure-ment of Hounsfield units on nonenhanced CTscans for the confirmation of an adrenal adenoma(39). However, radiologists should be cautiousabout their conclusions in cases of subtle, het-erogeneous signal changes and carefully inspectthe mass for internal signal changes. In a recentreport by Shifrin et al (43), a benign adrenal ad-enoma demonstrated apparent positive lipidsignal changes on chemical shift images butalso some heterogeneous signal changes due toa small focus of metastatic adenocarcinoma.

The speed of gradient-echo imaging has al-lowed for dynamic body MR imaging. Gradient-echo sequences that can be acquired during abrief breath hold provide the temporal resolu-tion needed to evaluate tissue enhancementfollowing administration of a contrast agent(Fig 17) (13,28). In an effort to decrease the

a. b.

Figure 14. (a) Axial in-phase FMPSPGR T1-weighted, dynamic gadolinium-enhanced image (89.5/4.2, 80°) ofa 46-year-old woman with diffuse fatty liver and focal nodular hyperplasia shows an enhancing mass, the focalnodular hyperplasia (arrow), in the right lobe of the liver (the mass was suspected from appearances on previ-ous T1- and T2-weighted images). (b) Corresponding out-of-phase FMPSPGR image (78.3/2.3, 80°) poorly illus-trates the mass (arrow). Because of suppression of the signal within surrounding fatty liver, the enhancing masswas less apparent on the out-of-phase image (b) than on the in-phase image (a). A small second lesion, a he-mangioma (arrowhead), is incidentally noted just posterior to the focal nodular hyperplasia.


15a. 15b.

16a. 16b.

Figures 15, 16. (15) Axial in-phase FMPSPGR (89/4.2, 90°) (a) and out-of-phase FMPSPGR (89/3.1, 90°) (b)images of a 62-year-old woman show a 3-cm left adrenal mass, which had lower signal intensity (arrow) with theout-of-phase pulse sequence. Region-of-interest measurements of the mass were performed for both images. Asubstantial difference in mean signal intensity was measured between the images (in phase = 115, out of phase= 66; 42% decrease in signal intensity), which helped confirm the presence of fat within the lesion and the di-agnosis of benign adrenal adenoma. (16) Axial in-phase FMPSPGR (74/4.2, 60°) (a) and out-of-phase (74/2.3, 60°)(b) images of a 76-year-old man with metastatic non-small cell and squamous cell carcinoma of the lung demon-strate a left adrenal mass (*). No differences in signal are seen on the two images. Metastases generally do notcontain fat and do not exhibit the signal loss caused by chemical shift effects.


time needed for the breath hold, gradient-echosequences are commonly performed with theshortest possible TE, which frequently is anout-of-phase TE (4,41,44). Use of only out-of-

phase images, however, may mask lesions inthe liver (Fig 14). Mitchell et al (44) recentlydescribed a similar phenomenon in the kidney.They reported paradoxical suppression of sig-nal intensity in angiomyolipomas (Fig 18) onout-of-phase gradient-echo images obtained af-ter administration of a gadolinium-based con-

a. b.

Figure 17. (a) Coronal in-phase FMPSPGR image (57/4.2, 80°) of a 70-year-old woman obtained af-ter intravenous administration of a gadolinium-chelate contrast agent reveals several nonenhancingsimple renal cysts in the left kidney. (b) Coronal out-of-phase image (57.4/2.4, 80°) better delineatesthe renal margins and relative locations of the cysts.

a. b.

Figure 18. (a) Coronal gadolinium-enhanced in-phase FMPSPGR image (90/4.2, 90°) of a 44-year-old womandemonstrates an enhancing renal mass (arrow). (Previous CT scan revealed a 2-cm complex low-attenuation le-sion in the inferior pole of the left kidney that was not consistent with a simple renal cyst.) (b) Coronal out-of-phase image (90/2.3, 90°) obtained less than 30 seconds after a reveals a dramatic reduction in the signal in-tensity of the mass, despite the gadolinium enhancement.


trast agent. For these reasons, one should be ju-dicious in the selection of the TE for dynamiccontrast material–enhanced imaging, especiallyif an assessment of signal intensity is desired. Agood example that illustrates the importance ofusing an in-phase TE to evaluate contrast mediaenhancement patterns is the case in Figure 14,in which the patient had both focal nodular hy-perplasia and diffuse fatty infiltration of the liver.The mass of focal nodular hyperplasia was muchmore conspicuous on the in-phase images thanon the out-of-phase images, possibly because ofthe signal suppression in adjacent enhancingfatty liver. Because out-of-phase images can beused to confirm contrast material enhancementwithin lipomatous tumors such as angiomyoli-pomas (Fig 18), they should be included, albeitat the end of the dynamic imaging examination.

The out-of-phase images also can be very help-ful because they afford improved anatomic de-lineation of peripheral lesions that border onlipid-water interfaces (Fig 17).

●●●●● Visceral DelineationChemical shift can be used as a tool for delin-eating structures that are surrounded by fat. Out-of-phase images can aid in the demarcation ofthe renal contour (Fig 9), the margin of the ad-renal glands (Fig 19), and the liver edge (Fig 20).Gradient-echo images that use out-of-phase chemi-cal shift demonstrate dark lines around organsembedded in fat. These dark lines are createdby the phase cancellation of the fat and watersignals that exist within the voxels of the lipid-water interface. The width of the dark lines canbe accentuated by increasing the field of view.

19a. 19b.

Figures 19, 20. (19a) Axial CT scan of a 69-year-oldwoman shows a mass in the left adrenal gland that ap-pears to invade the left kidney (arrow). (19b) On anout-of-phase FMPSPGR image (90/6.3, 90°), a blackboundary surrounding the adrenal gland and the kid-ney can be seen. On a subsequent follow-up MR image(not shown), the left adrenal mass had decreased insize, a finding consistent with an adrenal hemorrhage.(20) Sagittal out-of-phase FMPSPGR image (50/6, 90°)of a 57-year-old man with known colon cancer shows amass (m) invading through the Gerota fascia and into theanterior aspect of the lower pole of the right kidney(arrow), as evidenced by the interruption of the darkcortical line that demarcates the renal margin. The masshas also invaded the liver, as shown by the absence ofthe dark line that should be seen along the inferior mar-gin of the liver.

20.


Use of a narrower bandwidth will also increasethe chemical shift banding seen on the images(2,5). However, narrowing the bandwidth mayincrease the TE of the sequence or limit thenumber of sections that can be acquired.

■■■■■ CONCLUSIONSChemical shift principle can be applied to helpcharacterize and delineate tissues in the headand body. The acquisition of both in- and out-of-phase images, requiring only 1–2 minutes ofimaging time, can be added to almost any MRimaging protocol. The information from thesequick sequences can often aid in the diagnosis,either by delineating visceral contours or by dem-onstrating the presence of lipid elements. Thisquick imaging technique is a valuable adjunctto a radiologist’s bag of tricks.

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