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6
ImagingoftheSpinalCord
RolandR.LeeandBlaineL.Hart
INTRODUCTION
Imaging—in vivo visualization—of the spine and spinal cord is extremely
importantintheevaluationofpatientswithspinalcordpathology.Thischapter
discussesthetechniquesusedtoimagethespineandspinalcord,concentrating
on MRI. The contribution of imaging to the diagnosis of major categories of
spinalcordpathologyisthendiscussedandillustrated.
SPINALIMAGINGTECHNIQUES
Foroveracentury,plainx-rayshadbeentheimagingmodalityusedtoimagethe
spine. The spinal vertebrae and their alignment are well evaluated, and the
technique is simple and inexpensive, but the spinal cord proper cannot be
visualizedonx-rays.
Myelography, imaging of the spine by x-rays after introduction of contrast
material into the thecal sac, for the first time enabled visualization of the
silhouetteofthespinalcordwithincontrast-enhancedcerebrospinalfluid(CSF),
allowing diagnosis of epidural, intradural-extramedullary, and intramedullary
lesions.Thecontrastbetweenbone,spinalcord, and CSF is exquisite, but the
procedureisinvasive.
Computed tomography (CT) gives a good cross-sectional depiction of
primarilythebonyanatomyandtheparaspinalsofttissues.However,thespinal
cord itself, and the spinal nerve roots, are not well distinguished from the
surrounding intrathecal CSF. CT performed after myelography, however,does
giveexcellentvisualizationofthespinalcordandnerveroots.However,itagain
reliesontheinstillationofnonioniccontrastintothespinalcanal.Also,images
canonlybeobtainedintheaxialplane,althoughcomputerreconstructionsinthe
sagittalandcoronalplanesmaybeobtainedandmaybeofgoodqualityifthe
axialslicesarethin(3mmorthinner).Thewideavailabilityofmultislice,highresolution CT scanning now makes possible high-quality reconstruction of
imagesinsagittal,coronal,andobliqueplanes,andmarkedlydecreasesimaging
time.Thiscanbeespeciallyhelpfulinimagingtraumaticinjuries.
Spinalangiography is the best way to image the arteries and veins of the
spineandspinalcord,withgooddepictionofvascularmalformationsandtumor
vascularity.Thediagnosticangiogramisaprerequisiteenroutetoendovascular
treatment of these lesions. However, spinal angiography is invasive and
technicallydifficult;thus,itsuseisgenerallylimitedtodiagnosisandtreatment
ofspinalvascularmalformationsand,lesscommonly,vasculartumors.
Ultrasound is of limited utility in the diagnosis of spinal cord disease
becauseofthepresenceofthebonycanalsurroundingthethecaandcord,which
largelyblockstransmissionofsoundwaves.However,ultrasoundisaportable,
noninvasive, and relatively inexpensive modality that demonstrates good soft
tissue contrast. It is useful when thebonyspinal elements are absent, such as
duringintraoperativevisualizationofthespinalcordafterthebonylaminaehave
beensurgicallyremovedor,inearlyinfancy,whenspinalossificationislimited.
Nuclear medicine studies, including planar bone scans, PET, and SPECT,
enablefunctionaldetectionofinfection,cancer,ortrauma.Indium-labeledCSF
studiescanbeusedtoexaminethedistributionandflowofCSFinthespineand
head.
Ithasbeenlittlemorethan35yearssincetheintroductionofMRItospinal
imaging.Although MRI is more expensive than some ofthe techniqueslisted
above, because of its noninvasive nature (no ionizing radiation), multiplanar
imaging capabilities, superb soft tissue conspicuity, and ability to depict the
spinalneuralcontentsdirectly,ithasestablisheditselfastheimagingmodalityof
choicefor thespineand spinal cord(1).In certain situations, however,one or
moreoftheothermodalitiesdiscussedabovemaybemoreusefulthanMRI.
BasicConceptsofImaging
Three major parameters that characterize an imaging modality are spatial
resolution,signal-to-noise(S/N),andcontrastresolution(1).Anyimprovement
in image quality represents some combination of improved spatial resolution,
increasedS/N,andimprovedcontrastresolution.
Spatialresolution,essentiallythesmallestsizedetectablebythetechnique,is
determined by slice thickness, field of view (FOV), and the size of the
acquisitionanddisplaymatrices.InadigitalsystemsuchasMRI,theunitofinplane spatial resolution (defined as a pixel)is theFOVdivided bythematrix
size. Thus, using a FOV of 48 cm (which is roughly the length of the spinal
cord)anda512×512matrixsizeyieldsanominalin-planespatialresolutionof
0.94 mm, certainly adequate for imaging spinal structures. Ideally, to achieve
goodspatialresolution,thinsections(3mm)withalargematrix(256×256,or
upto512×512)shouldbeused,butunfortunately,asdiscussedinthefollowing
paragraph, there is a trade-off between improving spatial resolution and
worseningS/N.
Signal-to-noise(S/N),asthenameimplies,istheratioofthedesiredsignal
tobemeasured,dividedbytheinevitablenoisethatdegradesandcontaminates
thesignal.BecausetheMRIsignal isproportionalto thenumberofprotonsin
the imaged volume element (voxel), increasing the volume sampled (i.e.,
increasing voxel and pixel size) increases the signal (more than the noise).
However,increasingpixelsizemeansdecreasingspatialresolution.
However,anintrinsicincreaseinS/N,suchasfromincreasingMRImagnet
strengthorfrom improvements in coil technology,isextremely beneficial, not
onlybecauseS/Nisincreased,butalsobecauseitallowsforsmallerpixelsize,
andhenceimprovedspatialresolution.
Contrast resolution is the ability to discriminate between different tissues.
MRI,whichdistinguishesmatternotonlybyitsdensity(asdoesx-rayandCT),
but also by its different T1 and T2 relaxation parameters and diffusivity (i.e.,
diffusion-weightedimaging),isofunparalleledvalueindifferentiatingbetween
softtissues,suchasgrayandwhitematter,andnormalversusedematoustissue.
ImagingSpeed
Imaging speed is another important parameter in addition to the three listed
previously. Unlike the prior three parameters of image quality, which can be
measured or calculated directly from the image itself, the time required to
produce the image cannot (although it is measurable by other means). The
imagingspeed (i.e., the time required to obtain the image)is oneof themost
important benchmarks of an imaging system, and this is where many of the
improvementsinMRIandCT(andindeedallmedicalimaging)arebeingmade.
Some portions of the human anatomy impose stringent requirements on
imaging speed, most notably the beating heart or respiratory motion of the
thorax, but the spine in a supine patient is relatively motion-free and
considerably less demanding in this regard. However, both in the interests of
patientcomfortandtoavoiddegradationbypatientmotion(especiallyinthose
patients with severe back pain), it is desirable to minimize imaging time.
Moreover,significantincreasesinimagingspeedcanyieldmajorimprovements
inS/N,asdiscussedintheSectiononFastSpin-Echo(FSE),alsoknownasthe
rapidacquisitionrelaxationenhanced(RARE)sequence.
As has historically occurred with all imaging modalities, including CT,
improvementsinimagequalityhavebeenaccompaniedbyincreasesinimaging
speed. Faster and more powerful computers, better reconstruction algorithms,
andhardwareimprovementshaveallresultedinthesegains.
Phased-ArraySurfaceCoilsandMultichannel
ParallelImaging
Improved spatial resolution in spine imaging has long beenachieved withthe
useofsurfacecoils,withtheirhigherS/Nratio,butwithapenaltyofasmaller
FOV and a limited penetration into the body (2,3). Fortunately, the spinal
structures lie close to the posterior skin surface, so modern spine MRI uses
posteriorsurfacecoilstoreceivethemagneticresonancesignals.
The development of the phased-array surface coil solved the problem of
limited longitudinal FOV by coupling many (four to six) such coils in a
longitudinal array (4), mounted as a single long unit. This allows nearly the
entirespine(48–50cm)tobeimagedinasingleacquisitionusingalargeFOV,
with good resolution (using a 512 × 512 or 512 × 384 matrix), rather than
requiringseparate,time-consumingimagingofthecervical,thoracic,andlumbar
regionsindividually(Figure6.1).Variouscombinationsofthephased-arraycoils
may be electronically activated to view only the cervical, thoracic, or lumbar
regions, or a combination thereof, without physically moving the coil or the
patient.
This allows screening studies of the entire spine to be made in one
acquisition, rather than treating the spine as three separate units (cervical,
thoracic, and lumbar). It is particularly valuable in screening for metastatic
disease;screening3-mmseriesofT1-weightedsagittalimagesofessentiallythe
entirespinemaybeperformedinasingle3-to7-minutesequence,andselected
axial images (T1- or FSE-T2) should be obtained only through the levels of
abnormality, ratherthanthrough the entirespine. AlargeFOV-FSE (discussed
below)T2-weightedsagittalimagemaybeobtainedinabout3to6minutesto
clearly delineate the regions of cord impingement (Figure 6.2). The total
imagingtimeforthisstudyisthusonlyabout20minutes,whichcangenerally
betoleratedbyalmostanypatient.Whenclinicalindicationspointtoaspecific
regionof thespine,as lumbar radiculopathy, thentargeted,morelimitedspine
imagingisappropriate.
FIGURE 6.1 Sagittal T1 large FOV (48 cm, effective 512 × 512
matrix using rectangular FOV) using a phased-arraycoilthat covers
theentirespineofthisnormal11-year-oldgirlfromponsthroughL4
in only 4.5 minutes. Spatial resolution is 0.9 mm. Note the superb
detailanduniformcoverageofallanatomy.
Source: From Lee RR. Recent advances in spinal MRI. In: Lee RR, ed. Spinal Imaging.
Philadelphia,PA:Hanley&Belfus;1995:45–60.
FOV,fieldofview
FIGURE 6.2 FSE-T2 used with phased-array spinal coil. This
screening FSE-T2 scan covered in only 6.5 min 48 cm (skull base
throughL3)inthiskyphoticpatientwithmetastaticthyroidcarcinoma.
Note the superb detail (0.9 mm resolution) and uniformity of signal
throughout the length of the spine. T2-weighting yields anexquisite
myelogrameffectclearlydelineatingtheretropulsedtumormassatT2
thatisposteriorlydisplacingandcompressingthespinalcord,aswell
asthespondyloticchangesinthecervicalregion.
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.
Also,screeningforintrathecaldiseasethroughoutthespinecanbeperformed
with a single postgadolinium sagittal sequence in about 5 minutes, with a
resolutionof0.9mm(48cmFOV/512matrix),usingselectedaxialimagesonly
throughtheregionsofabnormalenhancement.
With recent advances in coil technology and reconstruction algorithms,
multichannelphased-arrayradiofrequencycoilscanbeusedinparallel-imaging
modetomarkedlyshortenscantimesbyfactorsoftwoormore.Parallelimaging
is becoming widely used in brain MRI and is especially valuable at field
strengthsof3 Tesla andhigher(5).This technologywill beemployedtogreat
advantageinspinalimagingaswell(6).
FAST-SCANNINGTECHNIQUES:RATIONALE
ToimprovetheS/Nratio,onemayincreasethesignalbyincreasingthenumber
ofacquisitions(i.e.,thenumberoftimestheimagedataisacquired—ineffect,
scanningthepatientmultiple times), but this obviously significantly lengthens
the exam and increases the risk of image degradation from patient motion.
However,ifanimagingsequenceisintrinsically veryfast,doublingortripling
the (very short) imaging time is a feasible option, resulting in significant
improvementinS/Nandconsequentlyallowingsmallerpixels(i.e.,betterspatial
resolution).Hence,recentdevelopmentshavefocusedondecreasingscantime—
techniquesknownasfast-scanning.
Gradient-EchoImaging
Many fast-scanning methods have been developed, of which gradient-echo
imaging(GRE)wasoneofthefirst.Thistechniquereliesongradientstorefocus
thespins,ratherthan180°pulses,asareusedinconventionalspin-echoimaging.
Images are produced rapidly because of small flip angles and short repetition
times(TRs)(7,8).GRE’smajoradvantage,besidesspeed,istheverythinslices
obtainable(<1-mmthick),representingmultiplethinpartitionsofalargethreedimensional(3D)imagingvolumeincontrasttotheusual2-mmminimumslice
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