Brain Infection, AIDS and Demyelinating Diseases

CT

There is central low attenuation pus or necrotic debris but rarely gas (unless there has been a surgical intervention or a gas-forming organism is present) ▸ following IV contrast medium administration the ring of enhancement corresponds to the abscess capsule (and is surrounded by low attenuation vasogenic oedema)

• The enhancing rim typically has a smooth inner margin with thinning of its medial aspect (as the white matter is perfused less than the grey matter)

• The abscess centre never enhances on delayed images (cf. cerebritis)

• The degree of enhancement is diminished in immunocompromised patients

MRI

A similar pattern of rim enhancement as with CT

• Abscess centre: high DWI/low ADC (pus) ▸ restricted diffusion techniques are unable to distinguish an abscess from a tumour

• Susceptibility-weighted imaging: very low SI rim ▸ ‘dual rim' sign: concentric outer hypointensity and inner hyperintensity

• Dynamic contrast-enhanced perfusion MRI: abscesses have a lower relative cerebral blood volume within their enhancing rim than a glioma

• Resolution post treatment: this is indicated by resolution of any rim enhancement or disappearance of the low SI rim (T2WI) ▸ a low SI on DWI correlates with a good clinical response (increasing SI implies pus reaccumulation)

Other intracranial infections

• Intracranial epidural abscess:

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    A collection of pus between the skull inner table and skull endosteum ▸ usually direct spread from contiguous infection (e.g. sinusitis) ▸ slow growing

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    CT / MRI: lentiform collection of fluid constrained by the dura at the sutures ▸ can cross the midline (cf. subdural empyemas which do not) ▸ the dura at the deep margin shows thick and irregular enhancement ▸ internal pus can show restricted diffusion

• Subdural empyema:

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    A collection of pus in the potential space between the inner layer of the dura mater and the arachnoid mater ▸ most common predisposing causes: sinusitis/otogenic infection

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    CT/ MRI: crescentic fluid collection overlying the cerebral convexity or in the interhemispheric fissure along the falx cerebri (irregular and scalloped margins as a result of loculation) ▸ contrast enhancement at the deep margin (subtle or absent in the early stages) ▸ adjacent brain can show oedema ± enhancement

• Ventriculitis:

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    Uncommon ▸ causes: trauma/intraventricular abscess rupture/shunt infection/haematogenous infection spread to the ependymal or choroid plexus

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    CT/MRI: intraventricular debris (slightly hyperattenuating with restricted diffusion) ▸ periventricular and subependymal high SI ▸ enhancement of ventricular margins

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    Cerebritis: a focal infection without a capsule or pus formation ▸ it is usually pyogenic in origin and can resolve or develop into a frank abscess

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      CT: an ill defined area of low attenuation with thick ring enhancement that may progress centrally on delayed images (cf. no central enhancement with an abscess) ▸ there may be haemorrhagic transformation

CT

Abnormalities usually visible within 3 days ▸ this is followed by low attenuation within the anteromedial temporal lobe (± involvement of the insula or the orbital surface of the frontal lobe) ▸ haemorrhage is not usually prominent and is a late feature ▸ there can be patchy or gyriform enhancement ▸ initially unilateral, progressing to bilateral

• Perfusion CT: this is increased during the acute phase

MRI

T2WI/FLAIR: there is high SI within the antero-medial temporal lobe within 2 days of onset ▸ the abnormal SI is mainly cortical (with secondary subjacent white matter involvement) ▸ it is more sensitive than CT for detecting haemorrhagic foci (particularly with T2* or SWI)

• DWI: cortical high SI

CT

Patchy white matter oedema ▸ cortical areas of increased density (which are not limited to the temporal lobes) ▸ there can be lesion progression to a multicystic encephalomalacia

MRI

T2WI: low SI regions

CT

This is usually normal in uncomplicated pyogenic meningitis ▸ CT is useful for detecting any complications (e.g. hydrocephalus, a subdural empyema, an abscess or a cerebral infarction)

MRI

FLAIR: high SI (this is non-specific and can also be seen with SAH and leptomeningeal metastases) ▸ FLAIR + Gad: meningeal enhancement (this may be more sensitive than T1WI + Gad)

CT

There is obliteration of the basal cisterns by isodense or slightly hyperdense exudates ▸ there is avid enhancement of the basal meninges extending into the ambient, sylvian, pontine and chiasmatic cisterns

• This meningeal exudate obstructs CSF resorption and causes a communicating hydrocephalus ▸ meningeal calcification is rarely seen with healing

• An arteritis of the penetrating arteries at the base of brain can lead to infarctions of the basal ganglia and internal capsule

MRI

This is more sensitive than CT for the above signs

• Differential: fungal meningitis ▸ neurosarcoid ▸ carcinomatous meningitis

CT

A small rounded lesion which is isodense or hypodense to brain ▸ there is variable surrounding oedema ▸ there is homogeneous enhancement (with solid lesions) or rim enhancement (with central caseation or liquefaction) ▸ lesions rarely calcify with healing ▸ brainstem involvement is uncommon

• The ‘target sign’ of central high attenuation with rim enhancement is not pathognomonic for a tuberculoma

MRI

T1WI: low SI ▸ T2WI: high SI (but low SI with caseation) ▸ T1WI + Gad: solid lesions demonstrate homogeneous enhancement ▸ ring enhancement is seen with caseation