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CNS INFECTIONS
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MENINGITIS
The meninges consist of three layers: thedura mater; the arachnoid mater; and thepia mater.
However, for a better understanding ofmeningeal inflammation in neuroimaging, itis helpful to use the anatomicfunctionalclassification of lepto- and pachymeninges
The leptomeninges (skinny meninges)comprise the membranes of the arachnoidand the pia mater
The pachymeninges (thick meninges) consistof the dura mater and the outer layer of the
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INFECTIOUS MENINGITIS
Can be divided into acute pyogenic (bacterial), lymphocytic(viral), and chronic (TB) meningitis
Bacterial meningitis is the purulent infection of the cerebraland spinal leptomeninges
Infection may arise from hematogenous spread during
systemic infection, from a chronic suppurative focus, or percontinuitatem during acute or chronic infections of theparanasal sinus, the middle or inner ear, or the mastoid.
Another path of infection is brain injury with open orcovered disruption of the dura mater, especially when theparanasal sinus or the air-filled cavities of the temporal
bone are involved in the fracture In neonates, the most common pathogens are group-BStreptococcus and Escherichia coli, in infants, it isHaemophilus influenzae, Streptococcus pneumoniae, andNeisseria meningitidis. In adolescents and adults,Streptococcus pneumoniae and Neisseria meningitidis arefound most often.
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COMPLICATIONS
Extraventricular obstructivehydrocephalus (EVOH)
Ventriculitis, choroid plexitis
Abscess, empyema, effusion
Cerebrovascular (arteritis, infarct,venous
thrombosis) Cerebral edema, infarction
DWI useful in depicting perfusion
alterations
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CT FINDINGS
NECT o Normal study is most common o Mild ventricular enlargement, Subarachnoid space
enlargement, Basal cisterns effaced. Low density areasrelated to perfusion alterations
CECT o Enhancing exudate in sulci, cisterns may be seen CTA: Arterial narrowing, occlusion CT exam should be performed in all patients with
suspected bacterial meningitis prior to lumbarpuncture to rule out brain edema and potential
herniation. Parenchymal lesions are often difficult to distinguish;
however, CT is a simple and fast tool to rule out severecomplications of bacterial meningitis, such as resorptivehydrocephalus, swelling, venous thrombosis, or infarction.
CT also provides important information about probable
pathologies of the skull base, the temporal bone, and thenasal sinus, such as fractures, purulent sinusitis, otitis
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MR FINDINGS
FLAIR: Hyperintense signal in sulci,cisterns
DWI o Variable, may show restriction o Most useful for vascular complications Tl C+: Exudate typically enhances.
Abnormal enhancement of the leptomeninges is
usually asymmetrical and extends into thebasal cisterns
MRA: Arterial narrowing, occlusion When patients with bacterial meningitis
develop focal neurological symptoms orseizures, MRI is the tool of choice to
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Pneumococcal meningitis in a 5-year-old child. a AxialT1-weighted image after contrast administration. bSagittalT1-weighted image after contrast administration
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c Axial T2-weighted image. d Axial FLAIR image.Bifrontal subdural exudates (c,d) with relativelyhigh signal compared with the CSF (d), indicatingan elevated content of proteins
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Pyogenic meningitis. T2-weighted axial image (a) at the level of midbrainshows evidenceof dilated lateral and third ventricles. T1-weighted axial image (b) showsno other abnormality. MT T1-weighted axial image (c) shows hyperintense
thick meninges in the basal cisterns and along the Sylvian fissures andanterior interhemispheric fissure which show enhancement in postcontrastMT TI-weighted image (d).
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Breast, lung mostcommon distantprimary tumors Primary CNStumors include:GBM,medulloblastoma,pineal tumors,
choroid plexustumors Primary tumoroften known
Lacy leptomeningeal
enhancement typical May haveventricular, duralbased enhancingmasses
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BRAIN ABSCESS
Focal pyogenic infection of the brainparenchyma, typically bacterial; fungal orparasitic less common
Four pathologic stages: Early cerebritis, late
cerebritis, early capsule, late capsule Early cerebritis: +/- Mild patchy enhancement o Late cerebritis: Irregular peripheral rim
enhancement o Early capsule: Low density center with thin,
distinct enhancing capsule. Deep part ofcapsule is thinnest; thickest near cortex
o Late capsule: Cavity shrinks, capsulethickens. May be multiloculated and have"daughter abscesses
Hemorrhage common in fungal cerebritis
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(Left) Axial T2WI MR shows an ill-defined hyperintense mass in the rightfrontal and parietal lobes of this 40year old male with severe headachesand elevated ESR Early cerebritis.(Right) Coronal T1 C+ MR shows
patchy enhancement, edema andmass effect.
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(Left) Axial T2WI MR shows the typical hypointenserim with surrounding hyperintense edema in this 38 yearold male with early capsule formation. (Right)Axial OWlMR shows characteristic restricted diffusion of the earlycapsule stage. The ADC map showed markedly decreased
signal centrally within the abscess.
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Thick, nodularenhancing walltypical Low signal on DWI(occasionally high,can mimic abscess)
History of traumaor vascular lesion Blood productspresent
Enhancementoften incompletering Characteristiclesions elsewherein brain
Small amount ofmass effect forsize of lesion
Usually no diffusion restrictiH/o primary tumor
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VENTRICULITIS
Ventricular ependyma infection relatedto meningitis, ruptured brain abscess, orventricular catheter
Ventriculomegaly with debris level,enhancing ependyma, periventricular T2hyperintensity
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Ventriculitis as a complication of meningoencephalitis. A 79-year-old man,severe meningoencephalitis (streptococci). a Axial T2-weighted image. bAxial DWI. Pathological substrate with intermediate to hypointense signal on
T2-weighted images (a) and strongly hyperintense signal on DWI due torestricted diffusion (b) in the lateral ventricles, suspicious for pus.
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Ependymalenhancement,typically nodular Parenchymaldisease usually
present
Primary brain tumors:GBM,medulloblastoma,pinealtumors,
ependymoma, choroidplexus tumors Metastatic tumorfrom extracranialprimary (Le.,breast)
History of trauma;other sequelae seen Ventricles typicallynot enlarged
Prominentependymal veins Vascularmalformations: AVM,DVA, cav malf
Abnormal venousdrainage (Le., Sturge-Weber)
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EMPYEMA
Loculated collection of pus in subdural orepidural space, or both
Extra-axial collection with rim
enhancement Location: Supratentorial typical SDE: Convexity in > 50%, parafalcine in
20% EDE: Often adjacent to frontal sinus SDE: Crescentic typical; may be
lentiform on coronal images
EDE: Biconvex, lens-shaped (lentiform)
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(Left) Axial T1 C+ MR shows a left frontal subdural empyemawith extension along the falx (arrow), typical of SDE. Noteenhancement along the deep margin. Adult patient with frontalsinusitis. (Right) AxialT1 C+ MR shows a right frontal subduralempyema with loculation and enhancement (arrow). Note the left
frontal subdural effusion with no Enhancement
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MR shows bloodproducts, may beloculated Often enhancesalong edge, typicallythinner than SDE May beindistinguishable;
history may help
Sterile, CSF-likecollection associatedwith meningitis Follows CSF on allMR sequences Usuallynonenhancing; mayenhance mildly
Frontal andtemporal regionscommon, oftenbilateral
Nonenhancing CSFcollection, oftentrauma history
Primary tumor oftenknown, typicallybreast, prostate Often diffuse,nodularenhancement May have
associated bonemetastases
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TUBERCULOSIS
Most TB CNS infections are secondary result of hematogenous spread (oftenpulmonary)
Meningitis is most common manifestation. Meningitis + parenchymal lesionshighly suggestive
TBM: Basal meningitis o Tuberculomas: Typically parenchymal, supratentorial (often parietal lobes)
Infratentorial lesions are less common, can involve brainstem (up to 8%) Dural tuberculomas may occur NECT o TBM: May be normal early (10-15%) Isodense to hyperdense exudate effaces CSF spaces, fills basal cisterns,
sulci
o Tuberculoma: Hypodense to hyperdense round or lobulated nodule/masswith moderate to marked edema
Ca++ uncommon (approximately 20%) CECT o TBM: Intense basilar meningeal enhancement o Tuberculoma: Solid or ring-enhancing "Target sign": Central Ca++ or
enhancement surrounded by enhancing rim (not pathognomonic for TB)
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MRI
Increased signal in sulcal spaces, predominantly in basalcisterns on FLAIR due to exudates
The MR features of the individual tuberculoma depend onwhether the granuloma is noncaseating or caseating with asolid center, or caseating with a liquid center. Thenoncaseating granuloma is usually iso/hypointense on T1-and hyperintense on T2-weighted images. Thesegranulomas show homogeneous enhancement afterinjection of paramagnetic contrast agent.
The caseating solid granulomas appear relativelyiso/hypointense on T1-weighted images withiso/hyperintense rim and isointense to hypointense on T2-
weighted images. The rim may be inseparable from edemawhen present on T2-weighted images or appearshyperintense in the absence of edema
The granulomas with central liquefaction of the caseousmaterial appear centrally hypointense on T1- andhyperintense on T2-weighted images with a variable
peripheral hypointense rim on T2-weighted imagesdepending on the amount of liquefaction of the solid
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COMPLICATIONS
Hydrocephalus: It is usually of the communicating type, secondaryto the blockage of the basal cisterns by the inflammatory exudatebut occasionally may be the obstructive type secondary to a focalparenchymal lesion with mass effect or due to entrapment of theventricle by granulomatous ependymitis
Choroid plexitis: It is usually associated with intense enhancement
of the ventricular lining (ventriculitis) and meningitis Vasculitis: Because of the predominant basal distribution ofexudate, a majority of the infarcts are in the basal ganglia andinternal capsule region owing to the involvement of thelenticulostriate arteries. Brainstem and major territory infarcts areuncommon
Cranial neuropathies: On contrast-enhanced MRI, the proximal
portion of the root at the nerve entry/exit zone is mostly affectedand may be associated with enhancement. Cranial nerveinvolvement has also been shown by the mass effect on the nervein the subarachnoid space as well as by nuclear and supranuclearinvolvement of the cranial nerve nuclei in the brainstem due toinfarct, focal cerebritis, or tuberculoma
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(Left) Axial CECT shows an irregular, peripherally enhancing mass in the leftfrontal lobe with central necrosis and surrounding edema. Solitary tuberculomamimics tumor (Courtesy R.Ramakantan, MO). (Right)Axial CECT shows an intracranial empyema(arrows) and a subgaleal
abscess in this young adult with a history of IV drug use, TB. Bone windowsshowed osteomyelitis
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(Left) Axial T2WI MR shows a hypointense cerebellarmass (arrow), typical of caseating tuberculoma. Note thesurrounding edema,mass effect and hydrocephalus. T2 hypointensity can helpindiagnosis of TB. (Right) AxialT1 C+ MR in same case
shows peripheral
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Tuberculous ependymitis (ventriculitis) in a 53-yearoldpatient with tuberculous meningitis. Coronal T1-weightedimage after contrast administration. Ependymal
enhancement(arrow).
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Fig 5.1ah. A 24-year-old man who suffered fromfever, headacheand weight loss for several weeks and came to admissionafter a seizure. a Non-enhanced CT. b Axial DWI. c
SagittalT1-weighted image after contrast administration. dCoronalT1-weighted image after contrast administration. e,f AxialT1-weighted image after contrast administration. g AxialT2-
weighted image. h Axial DWI (follow-up). (a) Enlargedventricles.On DWI (b) focal hyperintensities are seen at thelevel ofthe right thalamus and the basal ganglia (arrow) withreduced
ADC values, reflecting acute infarcts. Key to the diagnosisisthe severe basal meningeal enhancement as seen on thesagittaland coronal reconstruction of 3D T1-weighted images(c,d).
Marked contrast enhancement of the right seventh and
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TUBERCULOSIS - MRS
Reveal lipid resonances at 0.9, 1.3, 2.0and 2.8 ppm and phosphoserine at 3.7ppm.
Reflects the high lipid content of thecasseous material
Normal or decreased choline, Cr, NAA.
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TUBERCULAR ABSCESS
FOCAL OR DIFFUSE
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FOCAL OR DIFFUSEPACHYMENINGITIS
Isolated involvement of the dura mater isconsidered as an unusual presentation ofCNS tuberculosis. Pachymeningeal
tuberculosis is an entity distinct from theinflammation of the dura mater adjacent toan intraparenchymal tuberculoma that hasbeen shown on histopathology to be present
in almost 50% of all tuberculomas. Pachymeningeal tuberculosis consists of
either isolated dural involvement or apredominantly dural-based lesion with
secondary pial or parenchymal involvement
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Diffuse pachymeningitis.T2-weighted axial image (a)at the level of fourth ventricle shows a focalhypointense lesion in the fourth ventricle.Postcontrast T1-weighted axial (b) and coronal (c)images show enhancement of the intraventricularlesion and diffuse enhancement of the dura mater.
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NCC
Intracranial parasitic infection caused by the pork tapeworm, Taenia solium Four pathologic stages: Vesicular, colloidal vesicular, granular nodular,
nodular calcified Best diagnostic clue: Cyst with "dot" inside
Location Convexity subarachnoid spaces most common
May involve cisterns> parenchyma> ventricles Parenchymal cysts often hemispheric, at gray-white junction Intraventricular cysts are often isolated: Fourth ventricle is most common Basal cistern cysts may be racemose (grape-like) NECT Vesicular stage (viable larva): Smooth, thin-walled cyst, isodense to CSF, no
edema. Hyperdense "dot" within cyst = proto scolex Colloidal vesicular stage (degenerating larva): Hyperdense cyst fluid with
surrounding edema Granular nodular (healing) stage: Mild edema Nodular calcified (healed) stage: Small, Ca++ nodule
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NCC
CECT Vesicular stage: No (or mild) wall enhancement Colloidal vesicular stage: Thicker ring-enhancing fibrous capsule Granular nodular stage: Involuting enhancing nodule Nodular calcified stage: Shrunken, calcified nodule Subarachnoid lesions: Multiple isodense cysts without scolex, may cause
meningitis Intraventricular cysts not well seen on CT, may see hydrocephalus
MRI:
Vesicular stage: cytic lesion with eccentric scolex
Colloid vesicular: Cyst mildly hyperintense to CSF on T1/T2W images. Mild tomarked perilesional edema appears
Granular nodular stage: Cyst wall retracted and thickened. Edema decreases
Nodular calcified: Calcified nodule. Edema disappears
MRS:
Few reports show increased lactate, alanine, succinate, choline and decreasedNAA and Cr
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Vesicular neurocysticercosisMR images obtained in a 45-year-old woman with
headaches show vesicular neurocysticercosis in the
subarach-noid space of the temporal lobe (large arrows
in ac, arrow in d). The cyst contents appear similar to
CSF on the T1- and T2-weighted images
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DW AND FLAIR
and do not demonstrate fluid restriction on the diffusion-weighted image.
There is no surrounding edema on the fluid-attenuated inversion recovery
image or enhancement after the intravenous administration of contrast
material. The round structure within the cyst represents the scolex (small
arrows in ac). (e) Follow-up nonenhanced CT scan obtained after
antihelminthic therapy shows calcification of the nodule (arrowhead).
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Multiple neurocysticercosis cysts in a 25-year-old man
with headache, nausea, and vomiting. a Axial T2-weighted
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(Left) Axial CECT shows right external capsule cyst withcentral "dot" representing a scolex. No edema orenhancement is seen, NCC vesicular stage. Calcified leftputamen nodule, granular nodular stage. (Right) Axial T2WIMR shows cysts in subarachnoid spaces (arrows), filling the
Sylvian fissure and causing mild mass effect on the cerebralpeduncle. Note the lack of a scolex, typical of cisternal NCC.
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(Left) Axial CECT shows a ring-enhancing mass with surrounding edema in thefrontal lobe. Patient with headaches and seizures. Ventricular lesion is not wellseen (arrow). NCe, colloidal vesicular stage. (Right) Axial TlWI MR shows thefrontal lesion and the intraventricular cyst. Note cyst wall (open arrow) and thehyperintense scolex (arrow). TlWI and FLAIR MR are helpful to identify
ventricular lesions
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OTHER PARASITIC
INFECTIONS
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AMEBIC ENCHEPHALITIS
Meningoencephalitis; single ormultiple focal,
nodular or ring-enhancingmasses
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ECHINOCOCCOSIS
Large uni- ormultilocular cyst+/- detached
germinalmembrane,daughter cysts,no edema
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Large, well-defined rounded hyperintense area with hypointense rim on T2-weighted image (a). The lesion appears hypointense on non-enhanced T1-weighted image (b). Particulate wall-adherent substrate within the cyst thatprobably represents daughter scolices and hydatid sand
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SCHISTOSOMIASIS
Causes non specificgranulomatousinfmammation.
On MRI the lesions
are T2hyperintense andshow acharacteristicpattern of contrast
enhancement onT1-weightedsequences, which isa centrally linearenhancement
surrounded by
(Left) Axial T7 C+ MRshowsposterior fossa masses withpunctate enhancement
(arrows
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The left parietal lesion shows punctate contrast enhancement
(b) surrounded by edema as vizualized best on FLAIRimaging (a).
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CRYPTOCOCCOSIS
Cryptococcus is the third most commoncause ofCNS infection in AIDS patients,ranking behind HIV and Toxoplasma
The imaging findings may consist of
meningoencephalitis, intraventricular orintraparenchymal cryptococcomas,gelatinous pseudocysts, or hydrocephalus
Dilated perivascularlar spaces resulting fromthe presence of gelatinous pseudocysts are a
frequent finding Meningoencephalitis results in T2
hyperintensity within the region ofinvolvement, and meningeal enhancementmay be seen
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Axial T2-weighted image showsmultiple foci of high signal intensitywithin thebilateral basal ganglia, findingsconsistent with gelatinouspseudocysts in dilated perivascularspaces(arrows). Enlargement of the
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ASPERGILLOSIS
A fumigatus is the mostcommon causativeagent
Three imaging patterns: (a) multiple corticaland subcortical regions of low attenuation on
CT images, with T2 hyperintensity seen incorresponding areas on MR images; (b)multiple ring-enhancing lesions; and (c)dural enhancementadjacent to enhancing
lesions of the paranasal sinuses or calvaria. The presence of hemorrhage associated
with the lesions and intraparenchymalhemorrhage in an immunocompromised
patient should cause one to consider
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Disseminated aspergillosis in a 39-year-old AIDSpatient. (a) Axial T2-weighted FLAIR image showstwo well-circumscribed foci of hyperintensity within thecentrum semiovale. There is nosignificant surrounding edema. (b) Axial postcontrastT1-weighted image demonstrates low-
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Aspergillosis in a 36-year-old woman with HIV infection whoexperienced a rapid decline in consciousness. (a) Axial unenhancedCT image reveals a large parenchymal hemorrhage involving
the left hemisphere that caused mass effect and midline shift. (b)hoto ra h o the ross s ecimen demonstrates the lar e area
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[
(a) Sagittal unenhanced T1-weighted MR image shows a 3-cm mass (*) withheterogeneous signal intensity in the right frontal lobe. The mass has a thin,
hyperintense rim (arrowheads), a finding that is consistent with methemoglobin. Arrows
indicate a large surrounding area of hypointensity, a finding that is consistent with
vasogenic edema. (b) On a sagittal contrast materialenhanced T1-weighted MR
image, the mass shows only slight rim enhancement (arrows). (c) Coronal