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Limitations
CurrentlimitationsofMRIinstructuralimagingofchemosensorydysfunctionarerelatedtothesizeofthe
objectandthecontrastoftheobjectinrelationtoitssurroundings.Higherfieldstrengthsarepushingthe
limits,butcellularimaging,forexample,isonlypossibleusinglabeledcells.Imagingofsingleneurons
orevengroupsofcellsiscurrentlynotpossiblewithMRIwithoutlabeling.
Patient-relatedcontraindicationsofMRIarecontinuouslydecreasing.Whilecardiacpacemakershave
beenanabsolutecontraindication,someinstitutesarenowperformingMRIinpatientswithsuchdevices
under dedicatedsurveillance.Continuing contraindications are ferromagnetic foreignbodies inregions
wheretheirmovementmaydestroystructures,especiallyvesselsorneuralstructures.
The useofgadolinium-based contrastagents continues to be considered verysafe,eveninpatients
sufferingfromsevererenalinsufficiency(glomerularfiltrationrate<30mL/min/1.73m
2
).
TechnicalConsiderations:ComputedTomography
ImageAcquisition
Computed tomography (CT) imaging contrast is dependent on the electron density of the visualized
tissues.Therefore,softtissue contrast is low compared with MRI, andsome structures are noteasily
visualized (e.g., the OB). However, CT imaging of bone- and air-filled structures reveals a high
difference in electron density and can provide sufficient diagnostic information in some cases of
chemosensorydysfunction.Destructionofbonesrelatedtotraumaorinfiltrationandreplacementofairin
cranialcavitiesarethemainpathologiesvisualizedbyCT(Fig.17.4).Additionally,thevisualizationof
the foramina where the cranial nerves penetratethe skull base is a major indication for CT inchemo
sensory imaging.For thisindication,CTis onlylimitedbyspatial resolution(e.g.,theforaminaofthe
cribriform plate are not visualized in standard CT imaging). Typically, a 3D dataset of the skull is
acquiredandreformattedinthreeorthogonalimagingplanes.Dataacquisitionismainlyperformedinan
axial imaging plane, but also may be performed or supplemented in a coronal imaging plane. This
dependsontheclinical question:whiletraumaandtumorquestionsaremostoftenansweredwithaxial
imaging, pathology of the paranasal sinuses is mainly assessed using coronal imaging. Additionally,
artifactsrelatedtometal-based dental implants,which mayimpair imagequality,havetobetakeninto
considerationwhenplanningimageacquisition.
Fig.17.4Sagittalreformationofcomputedtomography oftheheadofapolytraumapatientin softtissuewindow.Displacedfractureofthe
frontal skullbase (arrow) indicating possiblelaterproblemswitholfactionduetoolfactorynerveand bulb injury. Thedarkareas resemble
intracranialaircollections.Additionally,intracranialfrontallobebleedingisdetected(bright,arrowhead).
Nowadays,CTisacquiredasatrue(isotropic)3Ddatasetrevealingavoxelsizebetween0.6mm×
0.6mm×0.6mm(voxelsize:0.216mm
3
)and1mm×1mm×1mm(voxelsize:1mm
3
),andthedesired
imagingplanesarereformattedoutofthisdataset.
ConsiderationsonDoseExposure
UnlikeMRI,CT is always associatedwith radiationexposure. In mostregions ofmany countries, the
natural annual radiationexposure ofthe human bodyreaches~2 to 3 mSv(millisievert)andismainly
relatedtotheinhalationof
222
radon,terrestrialradiation,andtotheingestionofradioactiveisotopessuch
as
40
potassium. OneCT ofthe headdeliversabout the same radiationdose ina fewseconds.In most
cases,thismaynotberelevantfortheindividualpatient,butitisrelatedtoadoseriskinadultsof~5%
persievertfortheinductionoflethal cancer.
3
Inotherwords:if1,000,000 peoplereceiveaCTofthe
head with a dose exposure of 2.5 mSv, 125 ofthemmay die from cancer induced by this diagnostic
procedure.Otherrisksofradiationexposuremaybehigher,forexample,theriskofdevelopingacataract.
For reasons of radiation protection, the risk factor of 5% per sievert was estimated in a pessimistic
fashion. The risk factor is expected to be lower, as the relation between dose and risk is not linear,
especiallyatlowradiationexposure,butdataforamorevalidriskapproximationarenotavailable.The
riskofdoseexposuremustbeconsideredbeforeaCToftheheadiscarriedout,butmustnotpreventthe
patientfromreceivinganadequatediagnosispriortotherapy.RadiologistsareabletoreduceCT-related
radiationburden,conservinghighsensitivityandspecificityofimages,iftheclinicalquestionisprecisely
specified.
ProjectionRadiography
Intheclinicalsetting,projectionradiography(PRG)ismainlyusedforthevisualizationoftheparanasal
sinusesinsinusitis,butmanystandardizedprojectionsexistfortheevaluationofthecranialnervesasthey
pass through the foramina penetrating the base of the skull (Fig. 17.5). The aim of PRG is either to
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visualizeanenlargedordestroyedforamenthatindicatestumorextentalongtherespectivecranialnerve
or to demonstrate bony pathologies in trauma or inborn osteochondral diseases. With MRI and CT
available, PRG nowadays is a second-line diagnostic tool for certain infrequent indications. In the
diagnosticworkupoftheparanasalsinuses,PRGisfrequentlyofnovalue.
Fig. 17.5 Projection radiography of the skull with optimal visualization of the round foramina (arrows) letting the second branch of the
trigeminalnerve(cranialnerveV/II)passthroughthebaseoftheskull.
UltrasoundandFDG-PET/CT
Ultrasound and [18F]-fluorodeoxyglucose positron emission tomography–computed tomography (FDG-
PET/CT) may only supplement MRI and CT and play a minor role in chemosensory imaging. In
oncologicalimaging,ontheotherhand,bothmodalitiesareofconsiderableimportance.
OlfactoryImaging
Dedicated imaging, mainly using specific MRI techniques, is needed to optimize therapeutic
decisions.
Introduction
Theolfactorynerve(ON)isthesumoftheaxonsofallolfactoryreceptorneurons,whicharelocatedin
the nasal mucosa on both sides ofthenasal cavity.The ganglionofthe ONcollectsall thesesensory
afferents and constitutes the primary olfactory cortex. It is called the olfactory bulb. The ON passes
throughtheperforationsofthecribri-formplatetomakecontactwiththeOB.TheOBisalignedinastrict
ventrodorsalorientationandhasanaveragevolumeof125±17mm
3
(mean±standarddeviation).
4
The
olfactorysulcus(OS)ofthefrontallobeislocatedabovetheOBandtheOT,anditsmorphologyappears
tobedependentonthepresenceoftheOB.
4
TheOTrunsfromanteromedialtoposterolateral(perforate
substance) andits cross-sectional area is ~1/10 of the cross-sectional area ofthe OB.Adding up the
volumes ofOBandOTresults inanaverage of up to 160 mm
3
, butthis volume shows considerable
variationdependingonage.
5
EvaluationofthenasalmucosacontainingtheON(whichisinvisibleinhumansinvivo)ispreferably
performed by MRI usingT2-weighted sequences in a coronal imageorientation.
6
Inmany centers, the
nasal mucosaisinvestigated byCT,but MRIshouldbe consideredineverypatienttoreduceradiation
exposure.
7
AdequatevisualizationofthehumanOBandOTinvivoisonlypossibleusingMRI(seeFig.17.1).
However, owing to thesize and orientation of these structures, they are often notvisible on standard
clinicalMRIsofthebrainandskullusedtoexcludemalignantdisease.Pathologiesaffectingtheolfactory
systemincludeinborndiseasessuchasKallmannsyndrome,tumorssuchasmeningiomas,andmetastatic
disease.
ImagingoftheNasalMucosaContainingtheOlfactoryNerves
Inthenasalcavity,themostfrequentcausesofolfactorylossareinflammatorydisorderssuchassinonasal
disease with and without polyposis.
8
In addition, respiratory problems may prevent airflow to the
olfactory cleft andthus produce olfactory loss. More recently,localized inflammation of theolfactory
cleftwithconsecutivethickeningofthemucosahasbeendescribedasapossiblecauseofolfactoryloss.
9
ImagingoftheOlfactoryBulb
CongenitalAnosmia
Largely based onworkbyYousem andcolleagues
10
magnetic resonance–based imaging oftheOBhas
founditswayintoclinicalpractice(Fig.17.6).
Interestingly,flatteningoftheOSintheplaneoftheposteriortangentthroughtheeyeballscanbeused
asanadditionalcriteriontoseparatepatientswithisolatedcongenitalanosmiafromnormosmiccontrols.
4
ThismightbeimportantwhentheOBsaredifficulttoidentify(e.g.,fortechnicalreasons).
TumorsAffectingOlfactoryBulbFunction
Tumors ofthefrontalskullbasecommonlyinducealterationsoftheolfactorysense,such ashyposmia,
anosmia, or parosmia. Such distortions of olfaction only occur in bilateral damage. Most commonly
meningiomas(Fig.17.7) affect the OB and OT.
11
Esthesioneuroblastomas typically involve the frontal
skullbaseandgrowalongthecranialnervesintoandoutoftheskullbase.Therefore,theON,OB,and
OTare commonlyaffected bysuch tumors (Fig.17.8). In rare cases, other tumors, such as olfactory-
ensheathingcelltumors
12
orprimitiveneuroectodermaltumorsinchildren(Fig.17.9),affecttheolfactory
system.
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Fig.17.6CoronalT1-weightedturbospin-echomagneticresonanceimaginginapatientwithcongenitalanosmia.Neithertheolfactorybulbs
(OBs)(arrows)northeolfactorytractsarevisibleandtheolfactorysulciaredeveloped,whicharefrequentfindingsinaplasticOBs.
Fig.17.7Axial T1-weightedturbospin-echomagneticresonance imaging after intravenousadministrationof contrastagent.As frequently
seen, thismeningioma(arrows) shows a marked contrastenhancement. Neither olfactory bulbs nor olfactory tracts are visible. Typically,
meningiomasshowashallowcontrastenhancementintheadjacentmeninges,referredtoasaduraltail.
TheVolumeoftheOlfactoryBulbmaybeModified
Morphological imaging in healthy, normosmic subjects showed that OB volume is highlyvariable.
5,13
Importantly, thereliability ofmeasures of OBvolume does not contribute tothisvariability,as a high
interobserverreliabilityhasbeenreportedinnumerousstudies.
4,14
Itseemslikelythat,onanindividual
level,changesoftheOBprovideameasureoftheprognosisofolfactorydysfunction.
OlfactoryImaging
Fig.17.8Axial T1-weightedturbo spin-echomagnetic resonanceimagingafterintravenousadministrationofa contrastagent.Thismassive
frontobasaltumor(esthesioneuroblastoma)showsinhomogeneous,partlyseptatedenhancement,cysticregions,andaseveremasseffect.
ThereasonforthehighplasticityoftheOBvolumecanbefoundinthecontinuingsynaptogenesisin
theOB,whichremainshighlyplasticthroughoutadultlife,reflecting,tosomedegree,thelevelofafferent
neural activity.
15,16
In patients with olfactory loss, OB volume correlates with decreased olfactory
sensitivity regardless ofthe cause ofolfactoryloss.
17
It varies withregard to olfactory function,and
decreaseswithdurationofolfactoryloss.
18
ImagingoftheOlfactoryTract
The OT has a comparativelysmall cross-sectional area and an oblique course along the frontobasis.
ImagingisthereforedemandingandonlyMRIcansuccessfullyvisualizetheOT.AswiththeOB,tumors
(e.g.,meningiomasand metastases)andtraumaarethe maincausesofdamagetotheOT.Inrarecases,
largeaneurysmsoftheanteriorcerebralarterymayalsocompresstheOT(Fig.17.10).
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Fig. 17.9 Coronal contrast-enhanced T1-weighted turbo spin-echo magnetic resonance imaging in a child suffering from a primitive
neuroectodermal tumor with cystic components. The olfactory nerve, olfactory bulb, olfactory tract, and parts of the frontal lobes are
destroyed.
Fig. 17.10 Axial contrast-enhanced computed tomography of the brain. Emanating from the anterior cerebral artery, a large contrast-
enhancinganeurysm(arrow)withapartlycalcifiedmarginhascompressedbotholfactorytracts,leadingtoanosmiaasafirstsymptominthis
patient.
ImagingofOlfactoryEloquentBrainStructures
Severalbrainstructuresarehighlysignificantinprocessingolfactoryinformation.Thesearedividedinto
the primary olfactory cortex and secondary or tertiary olfactory projection areas.
19
Among other
processes, tumors, infarctions, hemorrhage, superficial siderosis, or changes in blood perfusion may
affectolfactoryfunction.
Mesialtumors(e.g.,oftheamygdala)seemtoplayanimportantroleinthegenesisofolfactoryauras.
20
Furthermore,inpatientswhohaveundergonebrainsurgeryithasbeenshownthatthetemporallobeand
theorbitofrontalcortexplayaroleintheexecutionofvariousolfactoryfunctionsandodormemory.
21,22
Conclusions
Inconclusion,olfactorydysfunctionmaybecausedatvariouslevelsduringtheprocessing ofolfactory
information.
23
Interestingly, olfactory function appears to be assessable through measurement of the
volume of the OB. Itcanbe expectedthat techniques with higher resolution will provide even better
insightsintothestructuralandfunctionalorganizationoftheolfactorysystem.
Aswiththeentirechemosensorysystem,olfactoryfunctioncanbeaffectedbybrainlesionsevenwhen
theyare not directlyassociatedwith theOB or OT. In turn,when using MRIas a diagnostic tool, the
wholebrainofpatientswitholfactorydysfunctionneedstobethoroughlyinvestigated.
TrigeminalImaging
Thesensorypartsofthetrigeminalnerve(TN,cranialnerveV)transfersensationsofpain,temperature
differences, and chemical stimulationin the nasal and oral mucosa to central processing, andthereby
modifyandenhanceolfactory andgustatorychemosensation.
24
The mostprominent chemicalcompound
stimulating trigeminal receptors inthe mouthandnose is probablycapsaicin, whichcanbe found,for
example,inpepperoni. Thethreebranches oftheTNleave the skull throughthreedifferentperipheral
foramina (branch I [NV/I], supraorbital foramen; branch II [NV/II], infraorbital foramen; branch III
[NV/III], mental foramen—touch these trigeminal trigger points synchronously for mind melding) and
threedifferentcentralcanals(NV/I,superiororbitalfissure;NV/II,roundforamen[seeFig.17.5];NV/III,
ovalforamen).AllthesecanalscanbevisualizedbyPRG,butdestructionofbonystructuressurrounding
thenervesisbettervisualizedonCT(Fig.17.11).Axial,T1-weighted,fat-suppressed,contrast-enhanced
MRIsequencesallowdirectvisualizationandsuperiordetectionofschwannomasormetastaticordirect
tumorspreadalongthenerves.ThesearesupplementedbyaxialTIRMimaging(CSFisbrightandfatis
dark),especiallyforextracraniallesiondetection.Squamouscellcarcinomasoftheheadandneckregion
mayinducedamageoftheperipheralbranchesoftheTN,butonlyinrarecasescanolfactionorgustation
bealteredbysuchtumors.Themajorganglionofthetrigeminalnervethatcollectsallsensoryandmost
motorafferentsconstitutestheprimarytrigeminalcortex,andiscalledtheganglionofGasser(GG).The
GGis located bilaterallywithinthe Meckel cave andis best visualizedbyheavily T2-weighted MRI
(CISS,Fig.17.12).ThesesequencesprovidehighcontrastbetweenthebrightCSFandthedarknerves,
andarealsousedtoinvestigatethecisternalsegmentofthenerverootsoftheTNbetweentheGGandthe
brainstem(Fig.17.13). In this region, the major pathologies affecting thetrigeminal systemare tumors
(vestibularschwannomas compressingthecisternalsegmentoftheTN,meningiomas,epidermoidcysts,
and,lesscommonly,trigeminalschwannomas,neurofibromatosistypeII,andmeningealcarcinomatosis),
congenital (arachnoid cysts, aplasia), inflammatory or infectious (multiple sclerosis, sarcoidosis,
tuberculosis,bacterialandviralinfections),andvascular(contactorcompressionoftheTNrootcausing
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focal demyelination,which is themost frequentdetectable cause of classic trigeminal neuralgia). The
sensory(major)nerverootisfarlargerthanthemotornerveroot,butseparationonimagingisfrequently
difficult.Sensoryinputfromthenasalandoralmucosaisprocessedintwo(nucleuspontinusandnucleus
spinalis)ofthefournucleilocatedinthebrainstem(Fig.17.14)andconnectedtotheTN.Thesenuclei
are connected to the reticular formation, the olfactoryandgustatorycenters, and otherdeep structures
(especially the nucleus accumbens) for information analysis. Imaging of the brain and brainstem is
preferably performed using sagittal T2-weighted and axial T1-weighted sequences after intravenous
injectionofcontrastmaterialor,withhighersensitivity,byapplyingfluid-attenuated inversionrecovery
(FLAIR)imagingaftercontrastenhancement.Majorpathologiesofthebrainstemareagaindivisibleinto
vascular lesions (strokecaused by occlusionof the posterior inferior cerebellar artery or the basilar
artery;hemorrhagecausedbyhypertension,arteriovenousmalformationsorcavernousangiomas;trauma),
inflammation or infection (multiple sclerosis, acute demyelinating encephalomyelitis, human
immunodeficiency virus encephalitis, andothers), andtumors(especiallypontine gliomas, lymphomas,
hemangioblastomas,andmetastases).
Fig.17.11 Axial computedtomography oftheskullbase,bonewindow. Thebonymedialskullbaseisdestroyedbya large,squamous cell
carcinomaofthenasopharynx(asterisk).Therightandleftovalforaminaarealsodepicted(arrows).
Fig. 17.12 Axial image from isotropic 3D CISS magnetic resonance imaging. Within the cerebrospinal fluid–filled (bright) Meckel cave
(arrowhead), the ganglionof Gasser (dark)isvisualized. More posteriorly, the cisternal segmentof the trigeminal nerve root is visualized
(arrow).
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