Magnetic Resonance features of pyogenic brain abscesses and differential diagnosis using morphological and functional imaging studies: a pictorial essay.

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JournalofNeuroradiology(2014)41,153—167

Availableonlineat

ScienceDirect

www.sciencedirect.com

REVIEW

Magnetic

resonance

features

of

pyogenic

brain

abscesses

and

differential

diagnosis

using

morphological

and

functional

imaging

studies:

A

pictorial

essay

Carmine

Franco

Muccio

a

,

Ferdinando

Caranci

b

,

Felice

D’Arco

b,∗

,

Alfonso

Cerase

c

,

Luca

De

Lipsis

d

,

Gennaro

Esposito

a

,

Enrico

Tedeschi

b

,

Cosma

Andreula

e

a_Unit_of_{Neuroradiology,}_Department_of_{Neuroscience,}_Hospital_{‘‘Gaetano}_Rummo’’,_Benevento,_Italy

b_Unit_of_{Neuroradiology,}_Department_of_Advanced_Biomedical_Sciences,_University_{‘‘Federico}_II’’,_Naples,

Italy

c_Unit_of_{Neuroradiology,}_Department_of_{Neuroscience,}_University_‘‘Santa_Maria_alle_{Scotte’’,}_Siena,_Italy

d_Department_of_Critical_Care_Medicine,_Hospital_{‘‘Fatebenefratelli’’,}_Benevento,_Italy

e_Unit_of_Radiology_and_{Neuroradiology,}_Anthea_Hospital,_Bari,_Italy

Availableonline21June2014

KEYWORDS Cerebralabscesses; Pyogenicabscesses; Magneticresonance; PWI; DWI; MR-spectroscopy

Summary Theaimofthispaperistoillustratethepotentialofmagneticresonanceimaging (MRI)indiagnosis,differentialdiagnosis,treatmentplanningandevaluationoftherapy effec-tivenessofpyogenicbrainabscesses,throughtheuseofmorphological(orconventional)and functional(oradvanced)sequences.ConventionalMRIstudyisusefulfortheidentiﬁcationof lesions,todeterminethelocationandmorphologyandallowsacorrecthypothesisofnature inthemosttypicalcases.However,thedifferentialdiagnosisfromotherbrainlesions,such as non-pyogenicabscesses or necrotictumors (high-grade gliomas andmetastases) isoften onlypossiblethroughtheuseoffunctionalsequences,asthemeasurementofdiffusionwith apparent diffusioncoefﬁcient(DWI-ADC),protonmagnetic resonancespectroscopy(1_H-MRS)

andperfusionweightedimaging (PWI),whichcomplementthemorphologicalsequences and provideessentialinformationonstructural,metabolicandhemodynamiccharacteristics allow-inggreaterneuroradiologicalconﬁdence.ModerndiagnosticMRIofpyogenic brainabscesses cannotbe separated fromknowledge, integrationandproperuse ofthemorphological and functionalsequences.

∗_{Corresponding}_author._Unit_of_{Neuroradiology}_Universty_{‘‘Federico}_II’’,_via_Pansini_5,₈₀₁₃₁_Naples,_Italy.

E-mailaddress:darcofel@gmail.com(F.D’Arco).

http://dx.doi.org/10.1016/j.neurad.2014.05.004

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Introduction

Cerebralabscessisalocalizedinfectionofthecentral ner-voussystem (CNS).Itisformed byacentralnecroticarea surroundedbyanexternalwall(collagen,granulationtissue, macrophages,gliosis).

Abscesses account for 1—2% of brain occupying space lesionsinwesterncountriesand8%indevelopingcountries [1].Theyarefrequentinadultswhileinonly15—30%ofthe cases,theyinvolveyoungpatients(<15years)[2—5].

Pyogenic brain abscesses are not common, accounting for one third of all the cerebral abscesses [6]. They are causedbycerebral disseminationofbacteriacoming from neighbouringinfections (sinusitis,otitis,mastotiitis)or by hematogenousspreadofaremoteinfection(sepsis); how-ever, in approximately 40% of cases, the primary site of infectionremainsunknown.

OthercausesofcerebralbacterialdisseminationinCNS arecranio-facialtrauma(withpenetrationofforeignbodies orbonefragments),meningitisandneurosurgery(iatrogenic infections).

Clinicalfeaturesaredifferentaccordingtothesiteofthe abscess.Diagnosisischallengingandimaginghasaprimary role in differentiating brain abscesses from other lesions

that can have similar clinical spectrum. MR is the imag-ingmodalityofchoicefordiagnosisandfollow-upofbrain abscesses.

The aim of this paper is to analyze the role of combined use of morphological and advanced magnetic resonance imaging (MRI) techniques (diffusion weighted imaging—DWI/ADC; perfusion weighted imaging—PWI; spectroscopy—1_H-MRS)_in_the_diagnosis,_differential

diagno-sis,treatmentplanningandfollow-upofpyogenicabscesses. Anextensivereviewoftheliteratureisalsoprovided.

Etiology

and

pathogenesis

Bacteriaresponsibleforbrainabscessescanbebothaerobic (mostfrequentlyStaphylococcus,Streptococcus and Pneu-mococcus)andanaerobic[7].Itisalsoimportanttohighlight thatoftentherearedifferentkindsofbacteriainvolvedin theformationofanabscess.

Themostcommonlocalizationofapyogenicabscessin the brain is the supratentorial region, in the subcortical whitematter,especially,iftheycomefromhematogeneous spreadofadistantinfection(Figs.1and4)[8,9].

Abscessessecondarytomiddle hear otitisaretypically locatedinthetemporallobeorinthecerebellum(Fig.2).

Figure 1 A—F: supratentorial pyogenic abscess in the gray-white junction: T1-weighted (A), T2-weighted (B), gadolinium-enhancedT1-weighted(Gd-MR)(C),DWI(b=1000)(D),ADC(E)andPWI mapofCBV(F)MRimages.Thecapsuleoftheabscess ishyperintenseinT1-weightedimage(A), partiallyhypointenseinT2-weightedimage (B)withsurroundingvasogenicoedema. Gd-MRimageshowsaring-enhancingmass.ThecentralcomponentofthelesionshowshighsignalintensityinDWIimage(D),and hypointensesignalinADCmap,ﬁndingsthatareconsistentwithrestricteddiffusion(ADC=0.440×10−3mm2_/s)._At_PWI,_the_CBV

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Magneticresonancefeaturesofpyogenicbrainabscessesanddifferentialdiagnosis 155

Figure2 A—F:pyogenicabscessintherightcerebellarhemisphere:T2-weighted(A),T1-weighted(B)gadolinium-enhanced T1-weighted(Gd-MR)(C)magneticresonanceimages,DWI(b=1000)(D),ADCmap(E)andPWImapofCBV(F).Thecapsuleofthe abscessishyperintenseinT2-weightedimage(A),partiallyhyperintenseinT1-weightedimage(B)andshowsenhancementinGd-MR image.ThecentralcomponentofthelesionishyperintenseinT2-weightedimage,hypointenseinT1-weightedimageandshows highsignalintensityinDWIimage(D)andhypointensesignalinADCmap(E),ﬁndingsthatareconsistentwithrestricteddiffusion (ADC=0.610×10−3mm2_/s)._At_PWI,_the_CBV_map_(F)_does_not_show_evidence_of_increased_perfusion_in_the_{gadolinium-enhancing}

rimofthelesion(rCBV=0.9).

Localizationofapyogenicabscessinthebasalgangliais pos-siblebutrare(Fig.3)[6].Furthermore,theabscessisoften anisolatedlesionbut,inimmunocompromised patients,it isfrequenttheﬁndingofmultiplelesions(Fig.4).

Theearlystageofpyogenicinfectioninthebrainiscalled cerebritis,alocalizedandblurredareaofbrain inﬂamma-tion[10,74].Cerebritisleadstoacerebralabscessinmost ofthecases;thisevolutionconsistsoffourstages:

• earlycerebritis(ﬁrststage),fromthe1sttothe3rdday, characterizedbyaninﬂammatoryresponsewith predom-inanceofpolymorphonuclearleukocytes;

• latecerebritis(secondstage),fromthe4thtothe9thday, inwhichtheimmuneresponseismediatedbylymphocytes andmacrophages;centralnecrosisformationstartswith peripheral neoangiogenesis andpresence of ﬁbroblasts; theneo-formedvesselshavenoblood-brainbarrier; • early encapsulation (third stage),from the 10thto the

13th day, in which there is formation of surrounding peripheralwallwithcentralnecrosis;

• latecapsulestage(forthstage):fromthe14thday, char-acterizedbya matureencapsulate brainabscesswitha widecentralnecroticarea.

Thepresenceofairintotheabscessisnotfrequentandis associatedwithClostridium,Klebsiella,Enterobacter, Pep-tococcusorPseudomonas[75,76].

Clinical

features

and

treatment

Pyogenic abscesses have non-speciﬁc symptoms. In early stages,headacheandfeverarethemostcommonsymptoms butthey arenot always present. Notably only 30—55% of patientshave bodytemperature above38.5◦C [11].Focal neurological signs are appreciable in 40—60% of patients depending on the site of the lesion [11]. Complications, notfrequentbutserious,areintraventricularruptureofthe abscessthatleadtodisseminationoftheinfectionandacute hydrocephalus(Fig.5),andduralsinusthrombosis.

There are two possible therapeutic choices in case of pyogenicabscess:medicalorsurgical.

The useofmedicaltherapyalonecan beconsidered in casesofanabscesswith<2.5cmmaximumdiameter, local-izationinnotcriticalareasforthebrainfunction, concomi-tantmeningitis/ependymitisandmultipleabscesses[11].

However, the medical treatment alone is often not enough to eradicate the abscess being necessary the

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Figure3 A—N:pyogenicabscessinvolvingtherightnucleo-capsularregion,andfollow-upafterstereotacticneurosurgery aspi-rationandcontinuedantibiotictheraphy.Atclinicalpresentation,theabscessshowsringenhancementongadoliniumT1-weighted image(A),highsignalintensityonDWIimageofthecentralcomponent(B)andlowvaluesonADCmap(0.434×10−3mm2_/s)

con-sistentwithrestricteddiffusion(C).Follow-up,afterstereotacticaspiration,showsreductioninthesizeoftheabscesscavityon gadolinium-enhancedT1-weightedimages(D),dishomogeneuspatternhyper-andhypointenseonDWI(E)withincreasedofADC values(1.03×10−3mm2_/s)_(F)._One_month_(G,_H,_I)_and₃_months_(L,_M,_N)_follow-up_studies,_during_antibiotic_therapy,_show_further

decreaseinsizeoftheabscessonT1-weightedimages(G,L),lowsignalintensityonDWI(H,M)andhighvaluesonADCmap(I,N), suggestingresolutionoftheabscess(ADCvaluesatthelastfollow-up:1.51×10−3mm2_/s).

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Figure4 A—F:multiplepyogenic abscessesinimmunocompromisedpatient locatedatthegraymatterjunction:T2-weighted (A),T1-weighted(B)andgadolinium-enhancedT1-weighted(Gd-MR)(C)magneticresonanceimages,DWI(b=1000)(D),ADCmap (E)and1_H-MRS_(TE₌₁₄₄_ms)._The_central_component_of_the_abscesses_shows_{dishomogeneous}_{hyperintensity}_signal_in_T2-weighted

image(A).AtGd-MRimage,(C)therearesmalladjacentsatellitelesionsbyabscesses(arrows)andathickenedcapsuleonthegray matter(smallarrow).TheabscessesshowhyperintensesignalinDWI(D)lowsignalinADCmap(E),ﬁndingsthatareconsistentwith restricteddiffusion.The1_H-MRS_(F)_from_the_abscess_cavity_shows_resonance_representing_amino_acids_(AA),_{lipids/lactate}_(Lip/L)

andacetate(Ac).

drainage of the necroticpart or the totalexcision of the mass[73].

Magnetic

resonance

imaging

MRI shows pyogenic abscessas a round/oval-shapedmass witha central areaof suppurative necrosis anda periph-eral capsule. The central necroticarea is hypointense to thecerebralwhitematteronT1-weighted(w)and hyperin-tenseonT2-wimages(Fig.6).However,thesefeaturesare notalwayspresent,beingpossible,insomecases,avariable T2signalintensity(Fig.4).

The external capsule appears typically as a complete hypointenseonT2-wandhyperintenseonT1-wimagesrim (Fig.6)[8,12,13].TheshortT1andT2oftherimseemto beduerespectivelytothepresenceofcollagen ﬁbresand tomacrophagesreleasingfreeradicalswithaparamagnetic effect[13].Itisalsopossiblethattheexternalcapsuleshows a non-typicalsignal, being hyperintense onT2-w and iso-hypointenseonT1-wimages(Fig.2)[8,12,13].Conversely, thesurroundingvasogenicoedemaappearshypointenseon T1-wandhyperintenseonT2-wimages(Fig.6).After intra-venousinjection ofcontrastmediun(gadolinium),abscess showsatypicalperipheralthinandregular‘‘ring enhance-ment’’thatcorrespondstotheT2hypointenserim(Fig.6)

[12]. It is important to note that the peripheral contrast enhancementcanbestrongerintheportionoftheabscess closesttothegreymatter,becauseofthestronger inﬂam-matoryreactionduetothebiggervascularizationofthegrey matter[13].

The presence of a satellite abscess(closely associated withaprimaryabscess)thatgivesapolylobateappearance tothemassisalsopossible(Fig.4)[14].

After surgical and/or medical therapy, the degree of masseffect,surroundingedema,andcontrastenhancement decreases slightlybut does notchange substantially com-paredwiththepre-treatmentstudies[15].

Advanced

MR

techniques

DWI-ADC, 1_H-MRS _and _PWI, _together _with _{morphological}

MRI,cangiveinformationabout structural,metabolicand hemodynamicfeaturesof abscesseshelpingindifferential diagnosis.

Diffusionweightedimaging

DWI gives a critical support in the diagnosis of cerebral abscesses.Apyogenic abscess is typicallyhyperintense in

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Figure5 A—F: hydrocephalus followingintraventricular ruptureofpyogenic abscess: T2-weightedcoronal (A) andaxial (B), gadolinium-enhancedT1-weightedaxial(C)andcoronal(D)magneticresonanceimages,DWI(b=1000)(E),ADCmap(F).In T2-weightedimages(A,B),thelateralventriclesaredilatatedandthediluteintraventricularpusshowshypointesesignal(whitearrow) thatislessintenseinbrightnesscomparedwiththatofthecerebrospinalﬂuid(CSF)(blackarrows).Thediluteintraventricular pusisisointenseinT1-weightedimage(shortwhitearrow)(C),hyperintenseinDWI(shortblackarrows)(E)andlowvalueofADC (0.570×10−3mm2_/s)_on_the_map._{Gadolinium-enhanced}_T1-weighted_coronal_image_(D)_shows_ependymitis_following_rupture_of

abscess.

DWI,withlowvaluesonADCmaps(Figs.1,2and4)because ofthewaterrestrictioninthecentralnecroticarea contain-ingproteins,bacterialandcellulardebris[15—19].

ADCvaluesofthenecroticareainpyogenicabscessesare reportedinliteraturebetween0.28and0.73×10−3mm2_/s

[8,20].However,thedifferentconcentrationofbacteriaand whitecells,thedifferentkindofbacteriainvolvedandthe immuneresponseofthehost canleadtoawiderrangeof

ADCvaluesinthecentralnecroticarea[21];indeed,there aresomecasesofpyogenicabscesseswithhighADCvalues inthepurulentarea[22].

DWIis alsouseful toimprovethediagnosisof an intra-ventricular rupture of a pyogenic abscess. The purulent ﬂuid appears hyperintensein DWI withlow valueson the ADC maps in comparison with cerebrospinalﬂuid (Fig. 7) [23].

Figure6 A—C:cerebellarpyogenicabscess:T2-weighted(A),T1-weighted(B)andgadolinium-enhancedT1-weightedMRimages. InT2-weightedimage,(A)thecoreoftheabscessishyperintense(asterisk)andthehypointensesignalofthecapsule(whitearrow) iseaslydistinguishablefromperipheral vasogenicedemathatappears hyperintense(blackarrow). In T1-weightedimage after administratione.g.ofgadolinium(C)image,theabscessshowsanenhancedregularandthincapsule.

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Figure7 A—F:intraventricular ruptureofpyogenic abscess:T1-weighted (A), T2-weighted(B)andgadolinium-enhanced T1-weighted(C)images,PWImapofCBV(D),DWI(b=1,000)(E),ADCmap(F)MRimages.AtPWI,theCBVmap(D)doesnotshow evidenceofincreasedperfusioninthegadolinium-enhancingrimofthelesion.Thecentralcoreoftheabscessshowshighsignal intensityinDWIimage(E),andhypointensesignalinADCmap(F),ﬁndingsthatareconsistentwithrestricteddiffusionthatextends intotrigoneofthelateralventricle(arrow),signofintraventricularruptureoftheabscess.

SomepaperssuggesttheusefulnessofDWIinthe follow-upofapyogenicabscessduringtreatment[15,24,25].The progressiveincreaseofADCvaluescouldbeanearlierindex of medical response to antibiotics earlier than the size reduction of the lesion and/or the presence of a gap in theperipheralrimenhancementthatarelatesignsofdrug response.HigherADCvaluesinthecentralareaarealso vis-ibleafter neurosurgical aspirationof the central necrotic area(Fig.3).Ontheother hand,constantlowADCvalues overtimewouldbeduetoanineffectivetherapy,andthe recurrenceoflowADCvaluessuggestsrecurrenceofactive disease.

MR-spectroscopy

MR-spectroscopy(1_H-MRS)_is_a_non-invasive_analytical

tech-nique that has been used to study metabolic changes in normal and pathological tissues. Typically, the central necroticareaofpyogenicabscessesshowspeaks correspond-ing to lipids (0.8—1.2ppm), lactate (1.3ppm) and amino acids (0.9ppm), without peaks corresponding to normal spectrumofnervoustissue(suchasN-acetyl-aspartateand choline)[26,27].

Moreover,itispossibletoﬁndpeaksofalanine(1.5ppm), acetate (1.9ppm) and succinate (2.4ppm) [26,27]. The

absence of choline is related to the lack of normal cell membranes,theabsenceofN-acetyl-aspartateisduetothe factthattherearenotfunctioningneuronsintotheabscess (Fig.4)[26].Presenceoflactate,acetateandsuccinateis probablyrelated to bacterial glycolysisand fermentation [27,28].Aminoacidsareproducedfromtheproteolysisdue to enzymes released by neutrophils [20,28—30] and they aretypicalin pyogenic abscesses butnot always present. Inacaseseriesof194patientswithpyogenicabscesses,Pal etal.foundthepresenceofaminoacidpeak(aloneor asso-ciatedwithotherpeaksdescribedabove)in80%ofpatients [31].

1_H-MRS_could_be_useful_in_{distinguishing}_between

differ-entbacteriaresponsiblefortheabscessandinchoosingan appropriatetherapy[32].Therearethreedifferentspectra toconsider:

• type A: presence of lactate, amino acids, alanine, acetate,succinateandlipidsrelatedtothepresenceof obligateanaerobeswithorwithoutfacultativeanaerobes; • typeB:presenceoflactate,aminoacidsandoccasionally lipidsrelatedtoobligateaerobesandfacultative anaer-obes;

• typeC:presenceoflactatealone,associatedwith Strep-tococcusandwithtreatedabscesses.

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After medical therapy, abscesses show a non-speciﬁc peakof lipids and lactate that are also present in cystic tumors[33,34]. Forthis reason,it is criticalto use spec-troscopybeforethemedicaltreatment[33].

Perfusionweightedimaging

Perfusionweighted imaging(PWI)is anMR techniquethat givesinformationaboutcerebral hemodynamics.Injection ofgadoliniumleadstoareductionofT2andT2*signalinthe tissue(becauseofamagneticsusceptibilityeffect)related tothevasculardensity[72].

Therearefew studiesabout PWIinpyogenic abscesses demonstratinglow perfusion(incomparisontothenormal white matter) in the abscess capsule (Figs. 1, 2 and 7). Erdoganetal.[35]describedfourpyogenicabscesseswith a mean cerebral blood volume (rCBV) in the capsule of 0.76±0.12.Muccioetal.[36]describedﬁveabscesseswith ameanrCBVof0.72±0.08.Holmesetal.[37]studiedfour abscesseswithameanrCBVof0.79±0.18.Chanetal.[38] found a mean rCBV of 0.45±0.11. Finally, Chiang etal., usinga3Tscan,foundmeanvaluesofrCBVof0.94±0.09 in20pyogenicabscesses[51].

In only one paper, Harris at al [39] found rCBV values higherintheabscesscapsulethaninthenormalwhite mat-ter.

Itispossiblethatperfusionvaluesarerelatedtothestage ofthecapsule.Inearlystage,thevascularizationishigher thaninthelatestageinwhichtheﬁbroblastsaredominant, soexplainingthedifferenceinrCBVvaluesbetweenauthors [40].

Differential

diagnosis

Thedifferentialdiagnosisbetweenpyogenicabscessesand otherbrain lesionswitha‘‘ring-like’’ enhancement,such as other kind of abscesses and necrotic tumors, is not always possible with MRI only. The features of the cap-sule,hypointenseonT2-wandhyperintenseonT1-wimages, witharegularcontrastenhancementareoften enough to distinguish an abscess from a necrotic tumor that show an irregular central cavity and ill-deﬁned margins with nodules. However, the T2 hypointensity of the capsule and the regular contrast enhancement can be present in non-pyogenicabscesses(Mycobacteriumtubercolaris, Toxo-plasma,Aspergillus),insomecasesofnecrotictumorsandin lymphomas [8,12—14,41—45]. The spontaneous peripheral T1hyperintensityistypicalofpyogenicabscessesbutcanbe alsopresentinother lesions,suchasmasswithperipheral hemorrhagiccomponentor withcalciﬁcwall(granulomas, multiple sclerosis plaques, primary brain tumors, metas-tases,non-pyogenicabscesses)[45,46].

InatypicalcasesinwhichtheabscesscapsuleshowsT2 hyperintensityandT1hypo/isointensity,differential diagno-siswithotherlesionsisverydifﬁcultbyusingmorphological MRI,anditcanbeimprovedwithadvancedMRItechniques.

Necroticbraintumors

Necroticbraintumorscanbehigh-gradegliomas(HGGs)and metastases.TheyusuallyhaveaT2hypointenseperipheral

rimbutitisoftennotcompleteasinthepyogenicabscesses. After gadolinium injection, HGGs show non-homogeneous enhancementwithsomenodularareasofenhancement pro-truding in the central cavity (Fig.8). Metastases, on the other hand, canhave a regularrim of enhancement simi-larlytopyogenicabscesses(Fig.9).Asarule,necroticbrain tumorsarehypointense,in thecentralarea,in DWI(with highADCvalues)(Figs.8and9).However,DWI hyperinten-sityandlowADChavebeendescribedinnecroticmetastases from lung, breast, colorectal and testicular cancer, and in bladder transitional cell carcinoma; nevertheless, this seemstobeaveryrareﬁnding[44,47—49].

Furthermore,an intratumoralhemorrhagecan increase theDWIsignal[50]butthedifferentialdiagnosisiseasyon themorphologicalimagesinwhichbloodhastypicalsignal changesovertime.

On1_H-MRS,_presence_of_amino_acids,_acetate_and

succi-nateistypicalforcerebralabscess[8,33].

Moreover, HGGs and metastases typically show an increaseofneoangiogenesiswithhighervaluesofrCBVthan abscesses in the periphery of the mass (Figs. 8 and 9) [35—39,51,52].

Finally, using susceptibility weighted imaging (SWI) (a new MR sequence enhancing the paramagnetic propriety of brain tissue), Toh et al. described a ‘‘double rim’’ hypointensitysign presentinmost oftheabscessesandin noneofthenecroticgliomas[70];thiscanbeanother use-fulsigninthedifferentialdiagnosisbetweenabscessesand necrotictumors.

Fungalabscesses

Fungalabscessesarequiterare[54]andtheycanshow,like pyogenicabscesses,a‘‘ring-like’’enhancementanda cen-tralhyperintenseareaonDWIwithlowADCvalues[53,55] (Fig.10).

Some fungal infections (i.e. aspergillosis) can invade brain vessels leading to hemorrhagic strokes that are extremely rarein pyogenic infections [57].The ﬁnding of hemorrhagic strokes in basalganglia and/or in thalami in a patient with a primary aspergillosis (lungs, paranasal sinuses,etc.)isstronglysuggestiveofcerebralaspergillosis. Sometimesthecentralcavityoffungalabscesseshas het-erogeneoussignalonT2-wimagesbecauseofthepresence ofhypointenseareaswithrestricteddiffusionandwithout contrastenhancement,representingzonesofactivefungal proliferations[8].

At1_H-MRS,_fungal_abscesses_show_peaks_of_lipids,_lactate

andaminoacidssimilarlytopyogenicabscesses.

Tuberculomaandtubercularabscesses

Tuberculoma typically shows a hypointense T2 peripheral rim and a ‘‘ring-like’’ contrast enhancement after injec-tion of gadolinium [58]. The centre of the lesion shows iso/hypointensity in T1-w images and variable T2 signal relatedtothepresenceofcaseousorliquefactivenecrosis [58,59].

Three different kind of tuberculomas have been describedaccordingtoT2andADCfeatures[60]:

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Figure8 A—F:glioblastomas:T2-weighted(A),T1-weighted(B)andgadolinium-enhancedT1-weighted(Gd-MR)(A)images,DWI (b=1000)(D),ADCmap(E) andmapofCBV(F).TheimagesshowalesionwithhyperintenseinT2(A)andisointenseinT1(B) capsule.AtGd-MRimage(C),thelesionshows arim-enhancing masswithirregularmarginssurroundingvasogenicedema.The centralcomponentofthelesionshowsiso-hyperintensityinDWI(D)andhighsignalintensityintheADCmap(E),ﬁndingsthatare consistentwithincreaseddiffusion.AtPWI,theCBVmap(F)showsincreasedperfusionintheGd-enhancingrimofthelesion.

Figure9 A—F:metastasisfromcoloncancer:T2-weigheted(A),T1-weighted(B)andgadolinium-enhancedT1-weighted(Gd-MR) (A)images,DWI(b=1000) (D),ADCmap(E)andmapofCBV(F).TheimagesshowalesionwithaT2-hyperintense(A)and T1-isointense(B)capsule.AtGd-MRimage(C),themetastaticlesionshowsarim-enhancingmasswithregularmargins.Thecentral componentofthelesionshowshypointensityatDWI(D)andhighsignalintensityintheADCmap(E),ﬁndingsthatareconsistent withincreaseddiffusion.AtPWI,theCBVmap(F)showsincreasedperfusionintheGd-enhancingrimofthelesion.

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Figure10 A—D:multiplefungalabscessesinimmunocompromisedpatient:T2-weighted(A),Gadolinium-enhancedT1-weighted (Gd-RM)(B)images,DWI(b=1000)(C),ADCmap(D).AtGd-MRIimage(B),thelesionsshowrim-enhancingmasses.Thecentral componentofthelesions(arrows)showshighsignalintensityinDWIimage(C),andhypointensesignalinADCmap(D),ﬁndingsthat areconsistentwithrestricteddiffusion.

• typeI:hypointensecoreinT2(caseousnecrosis)withhigh ADCvalues(1.24±0.32×10−3mm2_/s);

• typeII:slightlyhyperintensecoreinT2,withintermediate ADCvalues(0.80±0.08×10−3mm2_/s);

• typeIII:stronglyhyperintensecoreinT2,withlowerADC values(0.74±0.13×10−3mm2_/s),_consistent_with

lique-factivenecrosis.

Furthermore, tubercolumas are often associated with tubercularmeningitisthattypicallyinvolvesbasalmeninges [59](Fig.11).

Tubercularabscessisrare,morecommonly in immuno-compromisedpatients[60,61].Itappearssimilartopyogenic abscessatMRI,andcanbesingleormultipleandoften mul-tiloculatewiththinandregularborders[8].Thecentralarea showslowADCvaluesasinpyogenicabscesses[8,58,60].1

H-MRSshowsonlypeaksoflipidswithoutanyotherpeak[8];

thisﬁndingseemstobeusefulinthedifferentialdiagnosis amongthesetwoentities.

PWIcandemonstratehighvaluesofrCBVinthe periph-ery ofthe mass,more similartothevaluesof HGGs than pyogenicabscesses[62].

Cerebraltoxoplasmosis

Cerebral toxoplasmosis is typical of immunocompromised patients.

MRIdemonstratesmultiplelesionsindifferentstagesof evolution(Fig.12),typicallylocatedinthesubcorticaland periventricularwhitematter,inthebasalgangliaand thal-ami (rarely in cerebellum) [63—65]. The presence of an eccentric area of contrast enhancement (‘‘eccentric tar-get sign’’) is a typical feature of cerebral toxoplasmosis (Fig.12CandF).The‘‘concentrictargetsign’’isarecently

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Figure11 A—E:tuberculoma associatedwithtuberculum meningitis: T2-weighted(A), DWI(b=1000s/mm2₎_(B),

gadolinium-enhanced T1-weighted coronal (Gd-MR) (C) andaxial (D), ADC map (E) magnetic resonance images. The central part ofthe tuberculomas(groupII)showsmildlyhyperintensityinT2-weightedimage(whitearrowhead)(A),isointensityinDWI(B)andADC map(whitearrows)(E)withintermediateADCvalue(0.81×10−3mm2_/sec)._At_Gd-MRI_images_(C,_D),_the_tuberculomas_appears

asrim-enhancingmasswiththickmargins.Itisassociatedwithtuberculousmeningitisthattypicallyaffectsthebasalmeninges, andependymitisofthethirdventricle;therearealsotuberculousgranuloma(blackarrow)andothertuberculomasadjacenttothe basalsylvianﬁssure.

Figure12 A—F:multipleToxoplasmagondiiabscesses: T2-weighted(A,D),T1-weighted(B,E) andgadolinium-enhanced T1-weighted resonancemagnetic (Gg-MR) (C,F) images. Multiple supratentorial abscesses in various stages ofdevelopment with ‘‘starrysky’’appearanceandenhancingasymmetricnodules(‘‘eccentrictargetsign’’)(arrows)inGd-MRimages(C,F).Notealso theconcentricalternatingzonesofhypo-andhyperintensitiesinT2-weightedimages:‘‘concentrictargetsign’’(D)(arrowheads).

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Figure13 A—F:Toxoplasmagondiiabscessinimmunocompromisedpatient:T2-weighted(A),T1-weighted(B)and gadolinium-enhancedT1-weightedmagneticresonance(Gd-MR)(C)images,DWI(b=1000)(D),ADC(E)andmapofcerebralbloodvolume(CBV) perfusionweightedimaging(PWI)(F).Thelesionshowsringenhancementwithextensiveperilesionalvasogenicoedema.Thecentre ofthelesionshowsisointensesignalinDWI(D),hyperintensesignalintheADCmap(E)reﬂectingincreaseddiffusion.AtPWI,the CBVmap(F)doesnotshowevidenceofincreasedperfusioninthecapsuleoftheabscess.

Figure14 A—F: neurocystercosis in colloidal-vesicular stage:T2-weighted axial (A) and coronal (B), T1-weighted axial (C), gadolinium-enhancedT1-weighted axial (Gd-MR) (D) and coronal (E) MR images, DWI (F). At Gd-MRimages (D, E) the lesion showsarim-enhancingmassandinT2-weightedimages thecapsuleishypointense(A,B).The MRimagesshowthescolexasa hypointenseeccentricnoduleinT2(blackarrows)andenhancednoduleinGd-MR(whitearrows).Thecentralpartofthelesion appearshypointenseinDWI(E),ﬁndingthatisconsistentwithincreaseddiffusion.

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Magneticresonancefeaturesofpyogenicbrainabscessesanddifferentialdiagnosis 165 describedMRIsignonT2-weightedimagingwhichconsistsof

concentricalternatingzonesofhypo-andhyperintensities (Fig.12D).The‘‘concentrictargetsign’’seemstobemore speciﬁc thanthe ‘‘eccentric targetsign’’ in thediagnosis of cerebraltoxoplasmosisespeciallyin thecontext ofAIDS [56].

DWI canbe usefulfor differentialdiagnosis [36,58,67], since the centre of the toxoplasma abscess has a hypo/isointensesignal inDWIwithADCvalueshigherthan in pyogenic abscesses(Fig.13).On the contrary,PWI and

1_H-MRS_seem_to_be_not_useful_in_{distinguishing}

toxoplasmo-sis from a pyogenic abscess: PWI shows in both pyogenic abscessesandincerebraltoxoplasmosisareducedperfusion inthecapsule(Fig.13)withareportedrCBVmeanvalueof 0.84±0.07[66];1_H-MRS_has_low_speciﬁcity_because_of_the

widerangeofpossiblepeaks[67,68].

Neurocysticercosis

MRfeatures ofneurocysticercosischange accordingtothe stage of cyst evolution. Cysts in the colloidal-vescicular stageshowhypointensecoreonT1-wandhyperintenseon T2-w images, with a T2 hypointense pseudo-capsule and peripheralenhancement,withasurroundingoedemaasin caseofpyogenicabscesses.Thescolexappearsasan eccen-trichypointensenoduleonT2-wimagesthatshowsintense contrast enhancement (Fig. 14). In contrast to pyogenic abscess,thecystincaseofneurocysticercosishashighADC values(Fig.14)[69].

PWI shows a reduction of rCBVand thus, is not useful in differentialdiagnosis withpyogenicinfection[77]. Itis importanttomentiontheintraventricularformof neurocys-ticercosisthathasarapidlyprogressivecourseanditisseen inmorethan54%ofpatientwithintracranialcysticercosis [69,71].

Conclusion

MRI is a useful tool for diagnosis, differential diagno-sis, treatment planning and follow-up of pyogenic brain abscesses. Particularly the combined use of morphologi-calandadvancedtechniquestogether,usingthedatafrom literature, is now critical in the diagnostic approach to abscess-likemassesinthebrain.

Disclosure

of

interest

Carmine F. Muccio, Ferdinando Caranci, Felice D’Arco, Alfonso Cerase, Luca De Lipsis, Gennaro Esposito, Enrico Tedeschi and Cosma Andreula declare that they have no conﬂictsofinterestconcerningthisarticle.

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