Ultrasonography of the Thyroid.pdf

Chapter6c.UltrasonographyoftheThyroid

INTRODUCTION

Amongtheseveralimagingtechniquesthatprovideclinicallyusefulanatomicin-

formationaboutthethyroidgland,sonographyhasbecomethemethodthatismost

commonlyemployed.Previously,imagingofthethyroidrequiredscintiscanningto

provideamapofthoseareasofthethyroidthataccumulateandprocessradioac-

tiveiodine.Although,scintiscanningremainsofprimaryimportanceinpatientswho

arehyperthyroidorfordetectionofiodineavidtissueafterthyroidectomyforthy-

roidcancer,sonographyhaslargelyreplaceditforthemajorityofpatientswhore-

quireagraphicrepresentationoftheregionalanatomybecauseofitshigherreso-

lution,superiorcorrelationoftruethyroiddimensionswiththeimage,smallerex-

pense,greatersimplicity,andlackofneedforradioisotopeadministration.Theother

imagingmethods,computerizedtomography(CT)andmagneticresonanceimaging

(MRI)aremorecostlythansonography,arenotasefﬁcientindetectingsmalllesions,

andarebestusedselectivelywhensonographyisinadequatetoelucidateaclinical

problem.[1,1A]

Aswithanytest,sonographyshouldbeusedtoreﬁneadifferentialdiagnosisonly

whenitisneededtoansweraspeciﬁcdiagnosticquestionthathasbeenraisedbythe

clinicalhistoryandphysicalexamination.[2]Theimagemustthenbeintegratedinto

patientmanagementandcorrelatedpreciselywiththeotherdata.

Althoughsonographycansupplycluesaboutthenatureofathyroidlesion,itdoes

notreliablydifferentiatebenignlesionsandcancer.Rather,sonographycan:

1.Depictaccuratelytheanatomyoftheneckinthyroidregion,

2.Helpthestudentandcliniciantolearnthyroidpalpation,

3.Elucidatecrypticﬁndingsonphysicalexamination,

4.Assessthecomparativesizeofnodules,lymphnodes,orgoitersinpatients

whoareunderobservationortherapy,

5.Detectanon-palpablethyroidlesioninapatientwhowasexposedtothera-

peuticirradiation,

6.Givecluesaboutthelikelihoodofmalignancy,

7.Identifythesolidcomponentofacomplexnodule,

8.Facilitateﬁneneedleaspirationbiopsyofanodule,

9.Evaluateforrecurrenceofathyroidmassaftersurgery,

10.Monitorthyroidcancerpatientsforearlyevidenceofreappearanceofmalig-

nancyinthethyroidbedorlymphadenopathy,

11.Identifypatientswhohaveultrasonicthyroidpatternsthatsuggestdiagnoses

suchasthyroiditis.

12.Reﬁneinthemanagementofpatientsontherapysuchasantithyroiddrugs,

13.Facilitatedeliveryofmedicationorphysicalhigh-energytherapypreciselyinto

alesionandsparethesurroundingtissue,

14.Monitorin-uterothefetalthyroidforsize,

15.Scrutinizetheneonatalthyroidforsizeandlocation,

16.Screenintheﬁeldthethyroidduringepidemiologicinvestigation.

Chapter6c.UltrasonographyoftheThyroid

TECHNICALASPECTS

Sonographydepictstheinternalstructureofthethyroidglandandtheregional

anatomyandpathologywithoutusingionizingradiationoriodinecontaining

contrastmedium.[3,4]Rather,highfrequencysoundwavesinthemegahertzrange

(ultrasound),areusedtoproduceanimage.Theprocedureissafe,doesnotcause

damagetotissueandislesscostlythananyotherimagingprocedure.Thepatient

remainscomfortableduringthetest,whichtakesonlyafewminutes,doesnot

requirediscontinuationofanymedication,orpreparationofthepatient.The

procedureisusuallydonewiththepatientrecliningwiththeneckhyperextended

butitcanbedoneintheseatedposition.Aprobethatcontainsapiezoelectriccrystal

calledatransducerisappliedtotheneckbutsinceairdoesnottransmitultrasound,

itmustbecoupledtotheskinwithaliquidmediumsuchagel.Thisinstrument

rapidlyalternatesasthegeneratoroftheultrasoundandthereceiverofthesignal

thathasbeenreﬂectedbyinternaltissues.Thesignalisorganizedelectronically

intonumerousshadesofgrayandisprocessedelectronicallytoproduceanimage

instantaneously(real-time).Althougheachimageisastaticpicture,rapidsequential

framesareprocessedelectronicallytodepictmotion.Two-dimentionalimages

havebeenstandardand3-dimentionalimagesareanimprovementincertain

circumstances.[4A]Thereisconsiderablepotentialforimprovingultrasound

imagesofthethyroidbyusingultrasoundcontrastagents.Thesematerialsare

gas-ﬁlledmicrobubbleswithameandiameterlessthanthatofaredbloodcorpuscle

andareinjectedintravenously.[5]

Dynamicinformationsuchasbloodﬂowcanbeaddedtothesignalbyemployinga

physicsprinciplecalledtheDopplereffect.TheDopplersignals,whicharesuperim-

posedonrealtimegrayscaleimages,areextremelybrightinblackandwhiteimages

andmaybecolorcodedtorevealthevelocity(frequencyshift)anddirectionofblood

ﬂow(phaseshift)aswellasthedegreeofvascularityofanorgan.[6,7]Flowinone

directionismaderedandintheoppositedirection,blue.Theshadeandintensityof

colorcancorrelatewiththevelocityofﬂow.Thus,ingeneralterms,venousandarte-

rialﬂowcanbedepictedbyassumingthatﬂowinthesetwokindsofbloodvesselsis

parallel,butinoppositedirections.Sinceportionsofbloodvesselsmaybetortuous,

modifyingorientationtotheprobe,differentcolorsaredisplayedwithinthesame

vesselevenifthetruedirectionofbloodﬂowinthatvesselhasnotchanged.Thus,

ananalysisofﬂowcharacteristicsrequirescarefulobservationsandcautiousinter-

pretations.Theabsenceofﬂowinaﬂuid-ﬁlledstructurecandifferentiateacystic

structureandabloodvessel.

Theultrasoundistreateddifferentlybythevarioustissues.[1,4]Theair-ﬁlledtra-

cheadoesnottransmittheultrasound.Calciﬁedtissuessuchasboneandsometimes

cartilageandcalciﬁcdepositsinotheranatomicstructuresblockthepassageofultra-

soundresultinginaverybrightsignalandalinearecho-freeshadowdistally.Most

tissuestransmittheultrasoundtovaryingdegreesandinterfacesbetweentissuesre-

ﬂectportionsofthesoundwaves.Fluid-ﬁlledstructureshaveauniformecho-free

appearancewhereasﬂeshystructuresandorganshaveagroundglassappearance

thatmaybeuniformorheterogeneousdependingonthecharacteristicsofthestruc-

ture.

Thedepthpenetrationandresolvingpowerofultrasounddependsgreatlyonfre-

quency.[3]Depthpenetrationisinverselyrelatedandspatialresolutionisdirectly

relatedtothefrequencyoftheultrasound.Forthyroid,afrequencyof7.5to10or14

megahertzisgenerallyoptimalforallbutthelargestgoiters.Usingthesefrequencies,

nodulesassmallastwotothreemillimeterscanbeidentiﬁed.

Routineprotocolsforsonographyarenotadequate.Althoughsometechnologists

becomeextremelyproﬁcientafterspeciﬁctrainingandexperience,supervisionand

participationbyaknowledgeableandinterestedphysician-sonographerisusually

requiredtoobtainapreciseandpertinentanswertoaspeciﬁcproblemthathasbeen

posedbytheclinician.Standardsonographicreportsmayprovideconsiderablein-

formationabouttheanatomy,butaresuboptimalunlessthespeciﬁcclinicalconcern

isexploredandanswered.Indeed,becausesomeradiologistscannotaddresstheclin-

Chapter6c.UltrasonographyoftheThyroid

icalissueadequately,andforconvenience,numerousthyroidologistsperformtheir

ownultrasoundexaminations,inwhichcaseitisessentialthattheyhavestate-of-the-

artequipment(thatmightnotbecost-effective)andthattheyarewillingtoexpend

aconsiderableamountoftimeforacompletestudy.Technicalingenuity,electronic

enhancementssuchasDopplercapability,andevenartistryarefrequentlyrequired.

Specialmaneuvers,variousdegreesofhyperextensionoftheneck,swallowingto

thefacilitateelevationofthelowerportionsofthethyroidglandabovetheclavicles,

swallowingwatertoidentifytheesophagus,andaValsalvamaneuvertodistendthe

jugularveinsmayenhancethevalueofdata.Nevertheless,sonographyisratherdifﬁ-

culttointerpretintheupperportioninofthejugularregionandintheareasadjacent

tothetrachea.Sonographyisgenerallynotusefulbelowtheclavicles.

Itisinformativefororientationtosurveytheentirethyroidglandwithalow-energy

transducerbeforeproceedingto10-14megahertzequipmenttodelineatetheﬁne

anatomy.Protocolshavebeendevisedtoassembleamontageofimagestoencom-

passanunusuallylargelobeorgoiter.Foranoverview,panoramicultrasound,which

isavariationofconventionalultrasoundhasbeenreportedtoproduceimageswith

alargeanatomicﬁeldofview,displayingbothlobesofthethyroidglandonasingle

image.[5A]

Theremaybeconsiderabledifferencesbetweensonologistsinestimatingthesizeof

largegoitersornodules.Oneinvestigationhasreportedthatcurved-arraytransduc-

ersavoidsigniﬁcantinter-observervariationthatmayoccurwhenlinear-arrayequip-

mentisemployed,especiallywhentheglandisenlarged.[5B]Theinter-observer

variationmaybealmost50%amongexperiencedultrasonographersforthedetermi-

nationofthevolumeofthyroidnodules,becauseitisdifﬁculttoreproduceatwo-

dimensionalimageplaneformultiplestudies.[5C]Accuracyinvolumeestimation

becomesmostimportantwhenoneusesultrasoundmeasurementstocalculatean

isotopedoseortocomparechangesovertimeinthesizeofanoduleoragoiter.Us-

ingplanimetryfromthree-dimensionalimagesreportedlyhaslowerintra-observer

variability(3.4%)andhigherrepeatability(96.5%)thanthestandardellipsoidmodel

fornodulesandlobes,with14.4%variabilityand84.8%repeatability(p<0.001).[5D]

SONOGRAPHYOFTHENORMALTHYROIDANDITSREGION

Theanteriorneckisdepictedratherwellwithstandardgrayscalesonography.(FIG-

URE1)Thethyroidglandisslightlymoreecho-densethantheadjacentstructures

becauseofitsiodinecontent.Ithasahomogenousgroundglassappearance.Each

lobehasasmoothglobular-shapedcontourandisnomorethan3-4centimetersin

height,1-1.5cminwidth,and1centimeterindepth.Theisthmusisidentiﬁed,ante-

riortothetracheaasauniformstructurethatisapproximately0.5cminheightand2

-3mmindepth.Thepyramidallobeisnotseenunlessitissigniﬁcantlyenlarged.In

thefemale,theupperpoleofeachthyroidlobemaybeseenatthelevelofthethyroid

cartilage,lowerinthemale.Thesurroundingmusclesareoflowerechogenicitythan

thethyroidandtissueplanesbetweenmusclesareusuallyidentiﬁable.Theair-ﬁlled

tracheadoesnottransmittheultrasoundandonlytheanteriorportionofthecar-

tilaginousringisrepresentedbydense,brightechoes.Thecarotidarteryandother

bloodvesselsareecho-freeunlesstheyarecalciﬁed.Thejugularveinisusuallyina

collapsedconditionanditdistendswithaValsalvamaneuver.Therearefrequently

1-2mmecho-freezonesonthesurfaceandwithinthethyroidglandthatrepresent

bloodvessels.Thevascularnatureofalloftheseecholessareascanbedemonstrated

bycolorDopplerimagingtodifferentiatethemfromcysticstructures.[6,7]Lymph

nodesmaybeobservedandnervesaregenerallynotseen.Theparathyroidglands

areobservedonlywhentheyareenlargedandarelessdenseultrasonicallythanthy-

roidtissuebecauseoftheabsenceofiodine.Theesophagusmaybedemonstrated

behindthemedialpartoftheleftthyroidlobe,especiallyifitisdistendedbyasipof

water.(FIGURE2)

Chapter6c.UltrasonographyoftheThyroid

Figure1.Sonogramoftheneckinthetransverseplaneshowinganormalrightthy-

roidlobeandisthmus.L=smallthyroidlobeinapatientwhoistakingsuppressive

amountsofL-thyroxine,I=isthmus,T=trachealring(densewhitearciscalciﬁca-

tion,distaltoitisartefact),C=carotidartery(notetheenhancedechoesdeeptothe

ﬂuid-ﬁlledbloodvessel),J=jugularvein,S=Sternocleidomastoidmuscle,m=strap

muscle.

Chapter6c.UltrasonographyoftheThyroid

Figure2.Sonogramoftheleftlobeofthethyroidglandinthetransverseplane

showingaroundedlobeofagoiter.L=enlargedlobe,I=widenedisthmus,

T=trachea,C=carotidartery(notetheenhancedechoesdeeptotheﬂuid-ﬁlled

bloodvessel),J=jugularvein,S=Sternocleidomastoidmuscle,m=strapmuscles,

E=esophagus.

Thyroidsonographyplayslittleornousefulroleinthemanagementofpatients

whohaveanormalthyroidexaminationandtheprocedureisnotcosteffectiveas

ascreeningtest.[1]However,thyroidsonographycanbeusedselectivelytosupple-

mentorconﬁrmaphysicalexaminationwhenclinicalperceptionisconfusedbyobe-

sity,greatmuscularity,distortionbyabnormaladjacentstructures,tortuousregional

bloodvessels,aprominentthyroidcartilage,metastatictumor,lymphadenopathy,or

priorsurgery.Inpractice,theproceduremaybeusedtosupplementanexamination

whenthereisuncertaintyaboutthepalpation.Intheacademicsituation,sonography

isusefultoteachpalpationofthethyroidgland.

SONOGRAPHYINTHEPATIENTWITHANENLARGEDTHYROID

GLAND(GOITER)

Thyroidsonographyprobablyisnotcosteffectiveinevaluatingtheaveragepatient

withthyroidenlargement.Sincethyroidgoitersarecommonandrarelyassociated

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