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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6022_Библиотеки_им_академика_М_И_Перельмана
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thicknessattainablewithconventional2Dspin-echoslices.However,thesignal
characteristicsofGREimagesdonotcorresponddirectlytoconventionalT1-or
T2-weightedspin-echoimaging(9).Inparticular,GREimagesofthespineare
inferiortoconventional T2-weightedorFSEimages,becausetheyconsistently
overestimate the hydration of the disks, overestimate the size of osteophytes
because of susceptibility artifact, and provide poor visualization of the neural
structures(9,10). Also,because of thelackof the180°refocusing pulse,GRE
scans are more prone to susceptibilityartifacts from tissue and magnetic field
inhomogeneityandfrommotion(11).
Inthespine,GREimagingcurrentlyhasitsmostwidespreadapplicationin
3D-volumetric thin-section axial images of the cervical spine, where the
relatively small size of the anatomic structures necessitates contiguous thinsectionimages(12,13).
FastSpin-Echo(FSEorRARE)Imaging
Developed in 1990 by Melki and Mulkern et al. (14,15) as a refinement of a
technique first proposed by Hennig et al. (16), the RARE sequence and
variationsthereuponhavebeenamajordevelopmentinfast-scanningandhave
beenparticularlyuseful inspine MRI.Thissequence,alsoknown asfast spinecho(FSE)orturbospin-echo,givesgoodT2-weightedcontrastinafractionof
the time required by true (conventional) spin-echo, thus resulting in better
resolutionandS/Nthanwaspreviouslypracticallyattainable(Figure6.2).(FSE
generallyrequiresonlyafewminutespersequence,ratherthanabout20minfor
anequivalentmultiexcitationtrueT2-weightedacquisition.)
Thissequenceplacesseveral(fromthreetosixteenormore)differentphaseencoding gradients followed by 180° radio frequency pulses after each 90°
pulse,ratherthanjustonephase-encodeandoneortwo180°pulses.Thisresults
inamuchmoreefficientuseofimagingtimeoverall,cuttingscanningtimebya
factorof2to6ormore.
FSEimagesdemonstratelessmagneticsusceptibilityartifactthantruespinechoimages(17,18),whichminimizesartifactsfrombonespursand,toacertain
degree,reduces artifact from surgicalhardware(19)(seeFigure 6.3). Possible
disadvantagesofFSEincludebrightsignalfromfatanddecreasedsensitivityto
hemosiderin(oldbloodproducts),aswellassomeoccasionalblurringofimages
(16–18,20).One major artifactonFSE imaging iscausedby CSF pulsationin
the rostral regions of the spine, which yields signal voids that could be
misinterpretedasarteriovenous(AV)fistulasorAVMsintheCSFdorsaltothe
cord, especially on FSE images (17,19) (Figure 6.4). Cardiac gating, flow
compensation, and other special pulse sequences may decrease artifacts from
pulsatileCSF(21,22),butoftensomeartifactspersist.
FSEsequenceshavedramaticallyimprovedspineimaging(19,23).GoodT2
weighting(extremelyusefulintheaxialaswellasthesagittalplane)(Figure6.5)
can be obtained in only a few (e.g., Figure 6.3) min, as opposed to about 10
minutesfor a true single-excitation T2-weighted sequence. As mentioned, this
markedlyfastersequencepermitshigherresolutionandmoreexcitations(better
S/N) to be achieved. T1- and T2-weighted images may be obtained in both
sagittal and axial planes with good resolution and S/N in very short imaging
times(about30mintotalexaminationtime,orless).
Recentdevelopmentof3DvolumetricFSEsequences(3DFSE-T2)provides
thin-section, contiguous, high-resolution T2-weighted images that may be
reconstructedinanyplanefromasingle,relativelyrapidacquisition(24)(Figure
6.6).
FIGURE 6.3 Titanium hardware minimizes artifact on MRI. Axial
FSE–T2-weighted image of vertebra at level of bilateral titanium
pedicle screws shows only minimal susceptibility artifact in the
vicinityof thescrews.The centralcanalis still clearlyvisualized, as
are the individual nerve roots. Stainless steel hardware would have
rendered this level uninterpretable. Use of FSE sequence also
contributestominimizingartifact.
Source: From Lee RR. Recent advances in spinal MRI. In: Lee RR, ed. Spinal Imaging.
Philadelphia,PA:Hanley&Belfus;1995:45–60.
FIGURE 6.4 Consecutive sagittal FSE–T2-weighted images of the
thoracicspineshowmultiplefocalsignalvoidsintheCSFposteriorto
thespinalcord.Thesearecommonartifactsonthispulsesequenceand
mustnotbemisinterpretedasavascularmalformation.
CSF,cerebrospinalfluid;FSE,fastspin-echo.
OtherFast-ImagingTechniques
Fast fluid-attenuated inversion recovery (FLAIR) is an effective technique in
evaluatingT2-brightsignalinthebrain,becauseconfoundingbrightCSFsignal
issuppressedbytheappropriatechoiceofinversiontime(25).However,itsuse
in evaluating the spinal cord is controversial, and some authors find that
conspicuity of spinal cord lesions with FLAIR is inferior to that with
conventionalT2-weightedimages(26).
FIGURE 6.5 Axial FSE T2 image through a lumbar disc shows a
myelogram effect with exquisite delineation of nerve roots in the
thecalsac.Thedisc,bones,andfacetjointsaswellastheparavertebral
softtissuesarewellvisualized.
Source: From Lee RR. Recent advances in spinal MRI. In: Lee RR, ed. Spinal Imaging.
Philadelphia,PA:Hanley&Belfus;1995:45–60.
FIGURE6.63DFSE-T2ofcervicalspine.(A)1.2-mm-thicksagittal
FSE-–T2-weighted image was obtained as part of a 3D volume
acquisition with 48 slices in 7.5 minutes. Note the clear delineation
betweenthevertebrae,CSF,andcordinthis36-year-oldwomanwith
prior C4–C5 fusion. (B) 0.9-mm-thick axial T2-weighted imagewas
reconstructedfromthesagittaldataacquiredandshowninA.Noaxial
images were directly acquired. Hence, both sagittal and axial views
can be obtained in a single 7.5-minute acquisition with a 1.2-mm
resolution. Note the clear delineation of the spinal cord and neural
foraminaonthisreconstructedimage.
CSF,cerebrospinalfluid;FSE,Fastspin-echo.
Source: From Lee RR. Recent advances in spinal MRI. In: Lee RR, ed. Spinal Imaging.
Philadelphia,PA:Hanley&Belfus;1995:45–60.
Fast short inversion–time inversion–recovery (STIR) images have an
appearancesimilartothatofstandardT2-weightedimages,withbrightCSFand
dark neural tissue. However,normal fatty bone marrow also appears dark on
STIRimagescomparedwithitsrelativelybrightappearanceonFSE-T2(Figure
6.7). A comparative study concluded that fast STIR images more sensitively
detectspinalcordlesionsthanFSE-T2orfastFLAIRimages(26).
All of these fast-imaging techniques continue to improve. There is a
multitude of variables that may be optimized (27). For example, in FSE,
variablesincludeechotrainlength,partitionofthevariousphase-encodingsteps
in k-space, number of echoes obtained, and interecho spacing, just to name a
few.
Fast-ImagingTechniquesintheSpine
ThemyelographiceffectonT2-weightedimages,wherebyCSFisbrightwhite,
contrastingsharplywithdarknerverootsanddarkvertebraeanddiscs,is very
valuableinthediagnosisofspinalpathology.Evenmoreimportant,T2-weighted
imagesdirectlydemonstrateedemaorgliosiswithinthespinalcordasbrightT2signal,contrastedwithT2-darknormaltissue.
Allthefast-imaging techniquesdiscussedgiveagoodmyelographiceffect,
exceptforthefastFLAIRsequence.Thebestsequencesforevaluatingintrinsic
spinalcordlesionsareFSE-T2andfastSTIR,buttheformeralsogivesthebest
depictionoftheintervertebraldiscandothersofttissues.Inourexperience,FSE
proton-densityimagesare a usefuladjunct to T2-weightedimagesin detecting
spinal cord lesions; they corroborate lesions suspected on FSE-T2, often with
increasedconspicuity(Figure6.7).
OneuseofGREimagesisindetectingsmallamountsofhemosiderin(such
asinspinalcordtrauma)orcalcium,whichcouldbemissedonFSEimagesand
mightbemissedonSET2-weightedimages.
In summary, the screening MRI examination of the cervical spine should
includea sagittal T1-weightedsequence;a sagittal flow-compensated, cardiacgatedconventional,orpreferablyFSE–T2-weighted,sequence;andanaxial3D
GRE sequence. Alternatively, a sagittal 3D FSE sequence, with axial
reformation,mayreplacetheseparateaxialandsagittalT2-weightedsequences
(Figure 6.6) (24). For imaging of acute traumatic injuries, one or more T2weighted sequences with fat suppression (either STIR or fat saturation) are
helpfultodistinguishedemainthesofttissuesfromfat(seelater).
Thoracic spine images should include sagittal T1-weighted and FSE–T2weighted sequences with axial images obtained only through regions of
abnormality.
FIGURE6.7SagittalMRimagesofa58-year-oldmanwithmultiple
sclerosis. Left. FSE proton-density image clearly demonstrates a
small, ovoid, bright plaque in the distal conus. Middle. FSE–T2weightedimageonlysubtlydemonstratesthelesion.Right.FastSTIR
imagesubtlydemonstratestheplaqueslightlybetterthanFSE-T2,but
notaswellasFSEproton-density.Notethedarkappearanceofallthe
bonyandsofttissuestructuresonthefastSTIRimagewithonlyCSF
andlesionhyperintense.
CSF,cerebrospinalfluid;FSE,Fastspin-echo;STIR,shortinversion–timeinversion–recovery.
Inthelumbarspine,T1andmultiechoFSE–T2-weightedimagesshouldbe
obtainedinthesagittalplane,followedbyaxialT1andFSE–T2images.
Asdiscussed,thebesttechnique forevaluatingintrinsicspinalcord lesions
includes sagittal FSE proton-density and T2-weighted images, supplemented
with sagittal fast STIR images. T1-weighted images after intravenous
administration of Gd-gadolinum DTPA (Gd-DTPA) are necessary to evaluate
intradural-extramedullaryorintramedullarylesions.
FatSaturation
Fatisbright onT1-weighted imagesandfairlybrighton FSE-T2images.This
canbeadvantageous,providingnaturalcontrastbetweentheT1-darkCSFinthe
thecalsac,nerverootsintheneuralforamina,andthesurroundingepiduralfat.
However,therearetwomajorcircumstancesinwhichthepresenceofbrightfat
can mask pathology: (a) enhancing structures on postgadolinium T1-weighted
sequencesmaybemaskedbysurroundingT1-brightfat,and(b)intrinsicallyT2brightlesionsinthevertebralmarrow,such as metastases, may be masked by
T2-brightfatonFSEimages.
In both of these situations, suppression of the bright fat signal by special
pulsesequencesenablestheenhancementortheT2-brightsignaloflesionstobe
visualized;this mayprovide better conspicuityof inflammatoryandneoplastic
lesionsinthe spine(20,28,29). STIRis anothermethodoffatsuppression that
may be especially useful in visualizing vertebral metastases (30). Fat
suppression may be applied in combination with FSE sequences, as well as
conventionalspin-echoimages, to improve lesion conspicuity in marrow (31),
althoughT1-weightedimagesperhapsremainmostsensitive(31).
Another valuable application of spectroscopic fat suppression is in the
identification or diagnosis of fat-containing lesions. Because subacute blood,
proteinaceous fluid, and occasionally calcium (32) can also be T1-bright, the
selectivesaturation of T1-bright fatcanbe diagnostic in distinguishingamong
thesepossibilities(Figure6.8).(Thepresenceofchemicalshiftartifactisalsoa
cluetothepresenceoffat,evenwithouttheuseoffatsaturation.)
CSFFlow:MRIMeasurements
MRIcanqualitativelyandquantitativelymeasureCSFflowbecausethephaseof
theprecessingnucleiinthemagneticfieldgradientsisverysensitivetomotion
within the field. In particular,the oscillatory motion of CSF in the brain and
spinalcanal(especiallyinthemorerostralspine),drivenbybrainexpansionand
contractionduringthecardiaccycleandbymotionofthebrainandcorditself,
may be directly imaged by phase-contrast cine technique (20,33–36). This
techniquecanassessthedegreeofblockageofCSFflowwithinthespinalcanal
—for example, from congenital stenosis at the cervicomedullary junction, as
seen in achondroplasia. Other possible usesincludeevaluating lack of normal
cord motion in tethered cords and distinguishing between a intradural
subarachnoid cyst versus a normally widened spinal canal, although the
interpretationofsuchstudieshasyettobeperfected(20,34).
FIGURE 6.8 Use of fat saturation to diagnose intradural spinal
lipoma. Left. Sagittal T1 shows a very large, well-circumscribed
intradural-extramedullaryT1-brightmassposteriortoandcompressing
the upper thoracic cord. Right. Fat-saturated post-gadolinium T1
imageshowscompletesuppressionoftheT1-brightsignal,provingthe
presence of fat rather than T1-bright blood. Hence, the diagnosis of
lipomarather thanhematoma wasmadeand confirmedatsurgery. A
subtlethinrimofenhancementisnotedaroundthetumorcapsule.
Source: From Lee RR. Recent advances in spinal MRI. In: Lee RR, ed. Spinal Imaging.
Philadelphia,PA:Hanley&Belfus;1995:45–60.
Diffusion-WeightedImagingandDiffusion-Tensor
Imaging
Diffusion-weightedimaging(DWI)anddiffusion-tensorimaging(DTI)areMRI
techniques that measure the diffusivity of protons in tissue, providing another
MRI parameter (besides T1, T2, proton density, and flow) to differentially
characterize tissues and identify pathology. DWI can improve specificity of
tissue characterization, for example in distinguishing between abscess and
necrotictumor,or betweenepidermoid tumorandarachnoid cyst(37).DWI is
widelyusedinMRIofthebraintodetectacuteinfarctionandotherpathologies
withincreased sensitivity andspecificity;acutely infarcted tissue demonstrates
restricted diffusion compared with normal tissue as early as 30 minutes after
arterialocclusion(38). DTIprovides a3Dmapofthewhitematter fibertracts
(39).
DWIandDTIarenowbeingappliedtothespineandspinalcord,aswellas
thebrain.Itistechnicallymoredifficulttoapplythesetechniquesinthespine,
duetothesmalleraxialdimensionsofthespinalcordcomparedwiththebrain,
artifacts from CSF pulsation in the spinal canal, and the presence of
susceptibilitydifferencesbetweenthespinalcordandbonycanal(40).However,
among other applications, DWI has been successfully used to diagnose spinal
cord ischemia (41), to evaluate cervical spondylotic myelopathy (42), and to
distinguishbetweenspinalepidermoidtumorandarachnoidcyst(43)(seeFigure
6.9). Some applications of DTI include spinal cord trauma (40), spondylotic
myelopathy(42),andmultiplesclerosis(44).Inthefuture,technicalrefinements
will enable these techniques, as well as other techniques such as MR
spectroscopyandDKI(diffusionkurtosisimaging),tobecomemorewidelyused
inthespine(45).
TitaniumSpinalHardware
Imagingthe postoperative spinebyCT orMRI after implantationoffusion or
stabilizationmetallicinstrumentationisoftenfrustratinganddifficultbecauseof
excessive beam-hardening artifacts in CT or very extensive ferromagnetic
susceptibilityartifacts inMRI(Figure6.10).Titaniumis a strongmetal that is
lessdensethanstainlesssteelandisnonferromagneticwhencomparedtosteel.
Consequently,theimagingoftitaniumimplantsbothonCTandMRI,although
farfromperfect,islessfraughtwithartifactsthanimagingsteelhardware.Often
the relevant structures can be adequately evaluated by a postoperative CT or
MRI after titanium implants, whereas steel implants would have resultedin a
nondiagnosticstudysecondarytoextensiveartifact(Figures6.3and6.11(46).
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