Scanner ostéo-articulaire à large système de détection : principes, techniques et

Le développement des scanners à large système de détection a permis la mise au point de

nouvelles applications avancées en scanner ostéo-articulaire : scanner dynamique 4D des

articulations et scanner de perfusion tumorale. Bien que ces techniques semblent jouer un rôle

important dans le diagnostic de nombreuses pathologies ostéo-articulaires, elles nécessitent la

répétition de multiples volumes d’acquisition, ce qui est à l’origine d’une augmentation

importante des doses délivrées. La maitrise des protocoles d’acquisition et des doses délivrées

est un facteur important pour permettre l’utilisation en pratique clinique de ces nouvelles

applications avancées.

Le but de cette mise au point était de fournir un guide pratique permettant d’utiliser ces

nouveaux outils en routine clinique.

Dans cette mise au point, nous montrons que l’utilisation d’un scanner à large système de

détection permet de réaliser des acquisitions volumiques qui ont pour avantage de supprimer

l’effet d’overrangingpar rapport à une acquisition hélicoïdale classique et donc de réduire les

doses délivrées. Par ailleurs, l’implantation des reconstructions itératives permet de réduire de

moitié les doses délivrées. En parallèle de ces évolutions technologiques, c’est aussi

l’optimisation des facteurs comportementaux qui reste indispensable. La limitation de la

couverture d’acquisition, la réduction du nombre de phases d’acquisition et l’utilisation d’une

acquisition intermittente plutôt que continue sont les principaux moyens comportementaux

qui permettent de limiter la dose d’irradiation et l’utilisation en pratique clinique courante de

ce type d’applications avancées.

Chapitre 4

Article 1 : Scanner basse dose pour la recherche d’une colique néphrétique : comment faire

en pratique clinique ?

Gervaise A, Gervaise-Henry C, Pernin M, Naulet P, Junca-Laplace C, Lapierre-Combes M. Low dose CT for

renal colic: how to do in clinical practice? Diagn Interv Imaging 2016; 97:393-400.

DiagnosticandInterventionalImaging(2016)97,393—400

REVIEW/Genito-urinaryimaging

How to perform low-dose computed

tomography for renal colic in clinical

practice

A. Gervaise^a,∗, C. Gervaise-Henry^b, M. Pernin^a,

P. Naulet^a, C. Junca-Laplace^a, M. Lapierre-Combes^a

a

Departmentofmedicalimaging,HIALegouest,57077Metz,France

b

Departmentofbiochemistry,hôpitalCentral,CHUdeNancy,54000Nancy,France

KEYWORDS

Computed

tomography(CT);

Dose;

Optimization;

Reduction;

Renalcolic

Abstract Computedtomography(CT)hasbecomethereferencetechniqueinmedicalimaging

forrenalcolic,todiagnose,plantreatmentandexploredifferentialdiagnosis.Itsmain

limita-tionistheradiationdose,especiallyasurinarystonediseasetendstorelapseandmainlyaffects

youngpeople.ItisthereforeessentialtoreducetheCTradiationdosewhenrenalcolicis

sus-pected.Thegoalofthisreviewwastwofold.First,wewantedtoshowhowtouselow-doseCT

inpatientswithsuspectedrenalcolicincurrentclinicalpractice.Second,wewishedtodiscuss

thedifferentwaysofreducingCTradiationdosebyconsideringbothbehavioraland

technolog-icalfactors.Amongthebehavioralfactors,limitingthescancoverageareaisastraightforward

andeffectivewaytoreducethedose.Improvementoftechnologicalfactorsreliesmainlyon

usingautomatictubecurrentmodulation,loweringthetubevoltageandcurrentaswellusing

iterativereconstruction.

©2015Éditionsfranc¸aisesderadiologie.PublishedbyElsevierMassonSAS.Allrightsreserved.

Sinceunenhanced(or plain)computedtomography(CT)wasintroducedin the1990s,it

hasbecomethereferencetoolforthediagnosisofrenalcolic[1—3].ThisisbecauseCT

hasmanyadvantages.Itisfast,doesnotrequireintravenousadministrationofiodinated

contrastmaterial,hashigh diagnosticcapabilities [2,4],helpsexcludeother conditions

that are clinically similar to renal colic [5—8], provides direct information relative to

thesizeandattenuationvalueofurinarystones[9]andhelpspredictspontaneousstone

passage[10].

∗

Correspondingauthorat:Departmentofmedicalimaging,HIALegouest,27,avenuedePlantières,BP90001,57077Metzcedex3,France.

E-mailaddress:alban.gervaise@hotmail.fr(A.Gervaise).

http://dx.doi.org/10.1016/j.diii.2015.05.013

394 A.Gervaiseetal.

Itsmainlimitation,however,istheradiationdosegiven

to the patient, especially because urinary stone disease

tendsto relapseand mainlyto affect young people.Katz

etal. report that4% of the patients that undergoCT for

suspectedrenalcolic havehadatleastthreeCT

examina-tionsforthesameindication,withcumulateddosesranging

from20 to154mSv [11]. Considering theALARA principle

(AsLowAsReasonablyAchievable)andthepotentialrisksof

radiation-inducedcancercausedevenusinglowdosesof

X-rays[12,13],dosereductioninCTforsuspectedrenalcolic

ishenceessential.Inthiscontext,manystudieshaveshown

that itis possible to detect renalcolic with low-doseCT.

Dosesmaybereducedby75to90%comparedtostandard

acquisitiondoses,withoutmodifyingthediagnostic

perfor-mance[4,14—18].However,arecentstudy showedthatin

mostimagingcenterslow-doseCTprotocolswerenotused

todiagnoserenalcolic[19].

The goal of thisreview was twofold.First, we wanted

toshowhowtouselow-doseCTinpatientwithsuspected

renalcolicincurrentclinicalpractice.Secondwewishedto

discussthedifferentwaysofreducingtheCTradiationdose

byconsideringbothbehavioralandtechnologicalfactors.

Whatis low-doseCT?

Thedefinitionoflowdoseiscontroversial.Thetermrefers

toCTscanswhere,comparedtoa‘‘normal’’or‘‘standard’’

dosescan,theimagequalityhasbeendeliberatelymodified

toreducetheexposuredosewhilepreservingthediagnostic

performance[20].Renalcolicisparticularlyappropriatefor

low-doseCTbecauseoftheexcellentspontaneouscontrast

betweenmosturinarystonesthatarespontaneously

hyper-attenuating (between 200 and 2800 HU) [2] and the soft

tissuesthatsurroundthem.Thus,evenifthedosereduction

issubstantial,thenaturallyhighcontrastbetweenurinary

stonesandthesurroundingsofttissuespreventstoomuch

deteriorationofthecontrast-to-noiseratiowhilepreserving

gooddiagnosticperformance[9].

Datafromtheliteraturerevealthattheeffective‘‘low

dose’’todetectrenalcolic,isbetween1and3mSv[4,19].

The thresholdof 3mSv (i.e. a dose length product [DLP]

of 200mGy.cm) is arbitrary but has become the standard

thresholdforlow-doseCTwheninvestigatingrenalcolic[19]

becauseitcorresponds moreor lesstotheaverage

radia-tionofintravenousurographythatusedtobethereference

modalityinthepast [21].Ifweconsiderthattheaverage

dose of astandard abdomen andpelvic CTis between10

and12mSv[22,23],alow-dosescanoflessthan3mSv

cor-respondstoadosereductionofmorethan75%.

Despite this significant dose reduction,various studies

have shown that the diagnostic performance of low-dose

CTremainsexcellentcomparedtonormal-doseCT.A

meta-analysispublishedin2008showedanaveragesensitivityof

96.6%andanaverage specificityof94.9%[4].Atthesame

time,it wasshown thatlow-doseCTcouldexplore

differ-entialdiagnosis,justlikenormal-doseunenhancedCT[24]

(Fig.1) and alsothat therewas nosignificant difference

whendeterminingthesizeanddensityofthestones[17,25].

Recently,expertshavesuggestedusing‘‘ultra-low-dose’’

CT,belowthelevelof1mSvandclosetothedose usedto

Despite the recenttechnological advancesand theuse of

newverypowerfuliterativealgorithmsforreconstructions,

theseultra-low-doseprotocolsperformlesswellthan

low-dose protocols for detecting small urinary stones below

3mm[18,21].

How to perform low-doseCTto detect

renal colic?

The modalities to reduce dose in CT are based on the

radioprotection principles of CT dose justification and

optimization [26]. These modalities have already been

extensively described [27—33]. In this review, we discuss

themandconcentrateonhowtoreducethedoseof

abdomi-nalandpelvicCTwhenlookingforrenalcolic.Thedifferent

modalitiesdependbothonbehavioralfactors,independent

of the CTequipment, and technological factors, some of

which depend on how recent the CT equipment is. The

behavioralfactorsarethelevelofawarenessofthemedical

and paramedicalteams, theprinciplesof substitutionand

justification,aswellaslimitingthescancoveragearea.The

technologicalfactorsincludereductionofthetubecurrent

and voltage,automatic tube current modulationand

iter-ative reconstructions,aswell asoptimizationofthepitch

andslicethickness.

Compliancewiththeindicationsand

substitution withanon-radiatingimaging

technique

Duetoitsexcellentdiagnosticperformance,CThasbecome

thereferenceinvestigationtodiagnoserenalcolic.In2014

the European Association of Urology has recommended

low-dose CT as the first-line imaging modality in case of

suspected renal colic (grade A recommendation) [34]. In

2008,theFrench-speakingSocietyofmedicalEmergencies

(Société Francophone d’Urgences Médicales) [35]

recom-mendedradiologiststoperformplainabdomenradiography

togetherwithanultrasoundoranunenhancedCTasa

first-lineexaminationforsuspectednon-complicatedrenalcolic.

However,CTshouldbefavoredifacomplicatedcaseis

sus-pected or in special situations (pregnancy, single kidney,

transplanted kidney,knownuropathyorrenalfailure)orif

there are signs of complications (signs of infection;

olig-uria, anuria or algesia) and in case of doubtful diagnosis.

In pregnant women, ultrasoundmust be usedas first-line

modalityand,incaseofdoubtfulultrasound,magnetic

res-onance imaging should be used as a second-line imaging

modalitybeforeCT[36].

Raisingtheawarenessandtrainingthe

medicalteams

Raisingtheawarenessandtrainingtheradiologistsand

clini-ciansisalsoessential[37].Cliniciansmustbeabletodetect

renalcolic andask explicitlytheradiologist tolook forit.

The radiologistmust usealow-doseCTprotocol with

pre-adjustedparameters.Itisalsoessentialthatcliniciansand

radiologistsagreetoseek,notthebestpossibleimage

qual-Howtoperformlow-dosecomputedtomographyforrenalcolicinclinicalpractice 395

Figure1. A28-year-oldwomanwasadmittedtotheemergencydepartmentforpelvicpainirradiatingtowardstheleftlumbarfossa.

UnenhancedabdominalandpelvicCT(100kVp,noiseindexat50,DLPof74mGy.cmandeffectivedoseof1.1mSv).Axialviews,1.25mm

centeredonthekidneys(a)andthepelvis(b).Low-doseunenhancedCTdoesnotshowanydilatationofthepelvicalycealsystem(arrows)

andnowedgedurinarystone,therebyexcludingthepresenceofrenalcolic.However,evenifthedosereductionhasbeensignificant,itis

possibletoevidenceintraperitonealperihepaticeffusion(asterisk)aswellasahyperattenuatingspontaneouseffusionintheDouglaspouch

(arrowhead)suggestinghemoperitoneum.FurtherenhancedCTconfirmedhemoperitoneumcausedbyleftovariancystrupture.

and operators tobe properlyaware of low-dose CT,they

must know the delivereddoses. Therefore,it is essential

that the dose (DLP) be displayed on the CT workstation

beforeanyacquisition.Currentlyallmanufacturers

system-atically provide this display. Awareness is also raised by

the software’s dose-recording system that allows

radiolo-gists to monitor the doses absorbed by the patients and

todetectcumulateddoses,sometimessubstantial[38,39].

Moregenerally,nationalandinternationaldoseregistersare

available.Forinstance,theCTDoseIndexRegistry[40]in

theUnitedStateshasmadeitpossibletoevidencethat

low-doseCTprotocolswerenotsufficientlyusedtodetectrenal

colic[19].

Limitingthescancoveragearea

Astraightforwardand effectiveway toreducedoses isto

reduce theacquisition length.Unenhanced image

acquisi-tionmustberestrictedtotheurinarytract,fromtheupper

pole of the kidneys to the base of the urinary bladder.

Besides reducing the CToverall dose by limiting thescan

coveragearea,thiscenteringpreventsradiosensitiveorgans

suchasgonadsinmenandbreastsinwomentobeexposed

toX-rays(Fig.2)[41].

Reducingthetubecurrent(mA)andtube

voltage(kV)

EffectsofmAandkV

Loweringthe tube current lowers thedose proportionally

but also causes an increase in image noise proportionally

tothereciprocalvalueofthesquarerootof themA[42].

Inpractice,reducingthetubecurrent byhalfreducesthe

doseby50%butincreasestheimagenoiseby41%.

LoweringthekVmayalsoreducethedose.However,this

willalsoincreasetheimagenoise[42].

Effectofpatient’s bodymass

Because of the high natural contrast between most

uri-nary stones and surrounding soft tissues, several experts

have recommendedlow-doseCTprotocolswith

significan-tly lowered tube current, by 10 to 100milliamperes per

second (Fig. 3) [5,24,43—46]. Many studies have shown

excellent diagnostic performance for low-dose CT,

equiv-alent tothe one of a standard-doseCT [4]. Hamm etal.

[44] and Polettiet al. [24] have, however, observed that

low-doseCTperformedlesswellinobesepatientswhohad

aBodyMassIndex(BMI)>30kg/m

2

.This wasassociatedto

the constant mA used for all the patients, resulting in a

significant loss of image quality in obese patients. Based

onthis, some experts have suggested not using low-dose

CT for obese patients (>30kg/m) [24,44,45] while others

haverecommendedtailoringthemAtothesepatients[5].

After these studies were published, automatic tube

cur-rent modulation during acquisition was introduced. This

has allowed radiologists to adapt the mA and the image

quality to the patient’s body mass while reducing the

dose by about 43 to 66% [47,48]. Mulkens et al.

con-firmed that low-dose CT with automatic tube current

modulationprovidesexcellentdiagnosticperformanceinall

patients with suspected renal colic, includingoverweight

and obese patients [49]. However, in order to preserve

an acceptable image quality in overweight patients, the

automatictubecurrentmodulationincreasestheCTdose.

Moreover, it has been shown that automatic tube

cur-rent modulation provides better scores of image quality

and diagnostic performance for overweight patients with

a BMI_≥25kg/m

2

than for patients with a BMI<25kg/m

2

[16]. These results may seem inconsistent, but they can

be explained by the fact that, with an equivalent level

of image noise, it is easier to diagnose renal colic in a

patientwho hasa lot of intra-abdominal and intra-pelvic

fat [50].Indeed,fat mayhelp delineate theureters from

surroundingstructures,evenifthe imagenoise is high.It

alsoseemseasier todetect secondary signsof renalcolic

such as perirenal stranding and the ‘‘rim sign’’ in

over-weightpatients.Thisiswhydiagnosiserrorsaremoreoften

observed in thin patients who have a BMI<25kg/m

2

, in

whomitisdifficulttodistinguishsmallstonesinthelower

ureterfrompelvic phleboliths,evenwithnormal-dose CT

[16,49].

AsfarasthekVisconcerned,beam-hardeningartifacts

havebeenobservedinoverweightpatientsifthekVhasbeen

toomuchreduced.So,whileitispossibletoreducethetube

voltageto80kVpinapatientwithstandardmorphotype,it

396 A.Gervaiseetal.

Figure2. 41-year-oldwomanwithsuspectedleftrenalcolic.Low-doseunenhancedCTfollowedbystandard-doseabdominalandpelvic

enhancedCT(sincerenalcolicwasexcluded).Scoutview(a)showsthebordersoftheunenhanced(redlines)andenhanced(bluelines)

acquisitionsandfirstandlastimagesinaxialviewwithout(bandc)andafterinjection(dande).Notethelow-doseCTcenteredfrom

theupperpoleofthekidneystothemidpubicsymphysismakingitpossibletoreduceby20%thescancoverageareacomparedtothe

standardabdominalandpelvicimages(35.1cmversus43.7cm).Alsonotethepresenceofmammarytissue(arrow)onthefirstsectionof

thestandardacquisition,absentinthelow-doseseriesofimages.

Figure3. 30-year-oldmanmonitoredfora4-mmurinarystoneintheleftkidney(arrow).Normal-doseunenhancedabdominalandpelvic

CT(120kVp,noiseindexat21.4,DLPat1189mGy.cmandeffectivedoseof17.8mSv)and(b)follow-upCTwithourlow-doseprotocol

(100kVp,noiseindexof50,DLPof80mGy.cmandeffectivedoseof1.2mSv).Evenwitha93.5%reductionofthedose,low-doseCTperfectly

showstheleftrenalstone(arrow).

mustbekeptto100kVpinoverweightpatients(Figs.4and5)

[14,16].

Iterativereconstructions

ReducingmA andkVislimited bytheuse ofconventional

FilteredBack Projection (FBP) reconstructions becauseof

the significant increase in image noise when doses have

been too reduced [51]. The recent introduction of

iter-ative reconstruction algorithms has significantly reduced

[14—18,52—56]. So, when doses are lowered by mA and

kVreductions,iterativereconstructionscompensateforthe

decreasedimagequality.Onstandardabdominalandpelvic

CT, iterative reconstructions have allowed radiologists to

reducedosesbyatleast50%[57].Kulkarnietal.haveshown

that,forsuspectedrenalcolic,itwaspossibletomaintain

excellentdiagnosticperformanceequivalenttotheoneof

standard-doseCTbyusingautomaticmAmodulation,

adap-tivestatisticaliterativereconstruction(ASIR)andakVfixed

at80kVpforpatientsweighinglessthan90kg[14].Iterative

Howtoperformlow-dosecomputedtomographyforrenalcolicinclinicalpractice 397

Figure4. 58-year-oldwoman(weight,64kg; BMI,22.7kg/m

2

)withleftrenalcoliccausedbya3.5-mmurinarystonewedgedinthe

ureterovesicalmeatus.Unenhancedlow-doseCTwithtubevoltageat80kVpandnoiseindexat50foraDLPof105mGy.cm.Axialplane,

1.25mmsection(a)and3-mm(b).Despitethesignificantreductionoftubevoltage,theurinarystoneisperfectlyvisible.However,we

observebeam-hardeningartifacts(a)partiallyreducedbythethickeningofthesections(b).

Figure5. 32-year-oldman,obese(BMI,35.7kg/m

2

),withsuspectedrightrenalcolic.Low-doseunenhancedCT(100kVp,noiseindex

at50,DLPof325mGy.cm,effectivedoseof4.8mSv)axialview,1.25-mmsectionscenteredonthekidney(a)andtheurinarybladder(b)

and5-mmcoronalMIPreformation(c).Forthisobesepatient,automatictubecurrentmodulationmakesitpossibletomaintainagood

qualityofimagewithouttheneedtoincreasetheCTdose.Notehowwellitispossibletovisualizetheinfiltrationaroundtherightkidney

(asterisk)andthesmalldilatationontherightsideofthepelvicalycealsystem(arrow)proximaltoa2-mmurinarystonewedgedinthe

rightureterovesicalmeatus(arrowhead).Thestoneiswelldetectedbythe5-mmcoronalMIP(c).

overweight patients [16] while using low kV (Fig.5) and,

inaddition,hastheadvantageofreducingbeam-hardening

artifacts,includingatthepelvis[57].

Pitcheffect

Some experts have recommended increasing the pitch in

low-doseCTprotocolsforpatientswithsuspectedrenalcolic

[58].Nowadays,pitchdoesnotaffectdoseanymore,since

mostCThaveautomatictubecurrentmodulationsoftware

[59].However,ahighpitch,about1to1.5,isbetter,because

itreducesacquisitiontimeand,thereby,movementartifacts

bythepatient.

Adaptingtheslicethickness

To obtain high spatial resolution, images should always

be acquired using thin sections (1 to 1.25mm). Thin

sectionswithisotropicvoxelsenhancethequalityof

three-dimensionalmultiplanarreformationsandvolumerendering

[60].However,thinsectionsalsocausesignificantincrease

inimagenoise,especiallyifthemAandkVhavebeen

con-siderably reduced, as happens in low-dose CT. So, after

using thinsections for image acquisition, it is possible to

reconstructthickersectionsduringimagereviewattheCT

workstation[61].Withthickened3-mmsectionsitispossible

to reduce image noise while preserving good

detectabil-ity and characterization of all radiodense urinary stones,

includingthosebelow3mm(Fig.4)[62,63].Other

abdom-inal structuresare alsobetter visualized. However,5-mm

thickenedsectionsmaycausepartial-volumeartifactsand

reducethedetectabilityof smallstones below3mm[64].

Smallstonesandspontaneouslydensestonesarealsomore

readilydetectedwiththickenedsectionsinmaximum

inten-sityprojection(MIP)andlowerimagenoise.Intheirstudy,

Corwinetal.haveconfirmedthaturinarystonesandtheir

densityaremoreaccuratelymeasuredon5-mmcoronalMIP

images(Fig.5)[65].

Inroutinepractice

Acquisition must be centeredfrom the upperpole of the

kidneystothemiddleofthepubicsymphysis. ThekVmay

bereducedto100kVp,even80kVpinpatientsthatarenot

overweight,and the level of noiseof the automatic tube

currentmodulation maybeincreasedinordertoobtaina

75%reduction of dose comparedtoa standard abdominal

Dans le document Optimisation et réduction de la dose d’irradiation au scanner : aspects techniques et impact en pratique clinique courante (Page 144-200)