Brain Abses Imaging

7/28/2019 Brain Abses Imaging

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Overview

The introduction of infectious agents results in various responses fromthe central nervous system (CNS). In the earliest stage of purulentbacterial brain infection, the generalized initial reaction is cerebritis.

Within the background of cellular response to the infection, cerebritisevolves into a localized abscess in a predictable series of stages.Neuroimaging of these stages reflects the underlying pathophysiologyof abscess formation. Variations in the brain's reaction at differentlocations and similarities in the brain's reaction to certain agents and inthe appearances of aggressive neoplasms all require correlation ofmedical history, neuroimaging, and results of microbiologic analysis.

Early and improved diagnostic imaging techniques have allowed thediscovery of brain abscess at a much earlier stage. (See the images

below.)

Brain abscess. Axial CT scan in a patient who presented with a headache, fever, anda history of a recent pneumonia demonstrates a poorly defined area of posteriorparietal brain edema (arrows). Early cerebritis may not outline a focal mass clearly.

Brain abscess. Axial nonenhanced cranial CT scan in a patient who presented withfever, headache, and a previous paranasal sinus infection demonstrates a poorly

defined pattern of mass effect and low attenuation in the left temporal lobe. The

pattern is consistent with possible early cerebritis; however, glioma and infarct may

have similar presentations.

Brain abscess. Three-dimensional surface model of a cranial CT scan in a patient

with a postcraniotomy abscess. The large deformity in the skull indicates the route ofabscess spread.

Brain abscess. Axial T2-weighted MRI in a patient with a right frontal abscess. Note

the mass effect and surrounding edema. The wall of the abscess is relatively thin

(black arrows).

Preferred examination

The preferred initial examination of the patient in whom brain abscess issuspected is MRI with and without gadolinium enhancement. Similardiagnostic results can be expected from cranial CT scans without andwith the intravenous administration of iodinated contrast medium. Bothimaging techniques help detect the mass effect of the abscess;however, findings in MRI with a diffusion protocol are more specific indifferentiating cerebral tumor, stroke, and abscess. In particular,examination of the metabolite peaks with MR spectroscopy can help tospecifically differentiate tumor, radiation necrosis, and abscess byidentifying their different spectral profiles.[1, 2]

Perfusion MRI has also been used to differentiate these lesions byevaluating vascularity with blood flow analysis with dynamic intravenous
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gadolinium contrast injection studies.

Occasionally, distinguishing brain abscess from neoplasm orpostoperative changes from infection is difficult. In these patients, anuclear agent can be used to tag white blood cells or antibodies to help

differentiation.

Gadolinium-based contrast agents (gadopentetate dimeglumine[Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide[Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance])have been linked to the development of nephrogenic systemic fibrosis(NSF) or nephrogenic fibrosing dermopathy (NFD). For moreinformation, see the Medscape Reference topicNephrogenic SystemicFibrosis.The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast

agent to enhance MRI or MRA scans.

NSF/NFD is a debilitating and sometimes fatal disease. Characteristicsinclude red or dark patches on the skin; burning, itching, swelling,hardening, and tightening of the skin; yellow spots on the whites of theeyes; joint stiffness with trouble moving or straightening the arms,hands, legs, or feet; pain deep in the hip bones or ribs; and muscleweakness. For more information, seeMedscape.

Limitations of techniques

Plain radiographs of the paranasal sinuses can only suggest a possibleetiology for cerebral abscess. Early findings of CT examinations are notspecific for cerebral abscess. The edema pattern and moderate masseffect cannot be differentiated from tumor or stroke in some patients.MRI findings in patients with cerebritis may resemble findings in stroke,while findings in the infarcts that result from vasculitis and cerebritismay resemble those of embolic strokes. Nuclear medicine single photonemission computed tomographic (SPECT) findings are not specific forbrain abscess unless a white cell tag is used.

Follow-up scans for certain infectious agents, such as M tuberculosis,may be necessary because infection by these organisms may not followa predictable response to treatment. Tuberculosis-related brainabscesses that retain positive results to culture and smears following 4weeks of treatment may not represent treatment failure. In addition,treatment of fungal infections may require many weeks of treatment withinterval follow-up imaging studies. Follow-up imaging during thetreatment for toxoplasmosis is important in avoiding brain biopsy.

Intervention

Intervention in patients with cerebral abscess is most commonly limitedto biopsy and aspiration of infectious material that may represent the
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origin of a CNS infection.

Aspiration and biopsy of small lesions is performed best using a CT-guided computer-assisted technique or with the aid of an externalframe, which (with the aid of CT data) directs the placement of the

aspiration needle. More recently, fully computer-aided virtual imagingprograms have provided greater flexibility in the application of both CTand MRI sets during craniotomy procedures and in the aspiration ofselected lesions. Intraoperative ultrasonography may aid in thedetection and treatment of relatively large superficial abscesscollections.

Recent studies

In a study by Chiang et al, diffusion, perfusion-weighted, andspectroscopic MRIs were able to differentiate cerebral abscesses fromnecrotic tumors. The authors found that the mean apparent diffusioncoefficient value at the central cavities of the cerebral abscesses weresignificantly lower; the mean relative cerebral blood volume values ofthe necrotic tumor walls were significantly higher; and amino acids werepresent only in the cerebral abscesses.[3]

Sepahdari et al found that in 9 cases of orbital cellulitis, including 6cases of pyogenic abscess, diffusion-weighted imaging confirmedabscess in a majority of cases without contrast-enhanced imaging.According to the authors, this may be of particular importance in

patients in whom the use of contrast is contraindicated. Diffusion-weighted imaging improved diagnostic confidence in virtually all thepatients with orbital abscess when it was used along with contrast-enhanced imaging.[4]

For excellent patient education resources, visit eMedicineHealth'sBrainand Nervous System Center. Also, see eMedicineHealth's patienteducation articleBrain Infection.

Radiography

Radiographic findings usually are limited to paranasal or mastoid sinusopacification; however, gas bubbles or air-fluid levels within the craniummay indicate a gas-producing organism or a communication with theparanasal sinuses or the nose.

Direct evidence of osteomyelitis of the skull is generally a mixed patternof lucency with a destruction of the outer or inner tables of the skull.

Occasionally, foreign bodies (eg, in gunshot wounds) or osteomyelitis ofthe maxillary bone may indicate a probable source for an intracranial

abscess. Bone destruction of the roof, floor, or lateral wall of the sinusesmay indicate an aggressive osteomyelitis with extension into the
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intracranial space.

Degree of confidence

Clouding of the sinuses is not a direct indication of an intracranial

abscess, merely a possible etiology. Air-fluid levels within the cranialvault strongly suggest abscess formation.

False positives/negatives

Patients with established intracranial abscesses may develop fluidretention within the mastoid and paranasal sinuses secondary toendotracheal intubation and chronic disability. Most patients withosteomyelitis of the mandible or maxilla do not develop intracranialabscesses.

Computed TomographyOn nonenhanced CT, Toxoplasma encephalitis appears as areas ofisointense or hypodense mass effect. The basal ganglia and thecorticomedullary junction are most commonly affected. Contrast-enhanced CT demonstrates a ring or nodular enhancement pattern withlesions of 1-3 cm in diameter. The enhancement is greatest within theintermediate zone where inflammation is the greatest. (See the imagesbelow.)

Brain abscess. Axial CT scan with intravenous (IV) contrast enhancement in apatient who presented with headache and fever. Initial CT scan demonstrated masseffect and edema within the left temporal lobe. Since the edema and mass pattern

were poorly defined, a biopsy of the left temporal lobe was performed to exclude a

tumor. Following resection of the temporal lobe abscess, extracranial, subdural, andintracerebral abscesses developed (arrows).

Brain abscess. Coronal multiplanar reformatted CT scan in a patient who developed

temporal brain abscesses (yellow arrows) and a left-sided extracranial abscess(white arrow) following surgery of the left temporal skull.

Brain abscess. Axial contrast-enhanced CT scan in a patient who was treatedsurgically for a depressed skull fracture. The left parietal cranial injury has become

complicated by an abscess of the subgaleal space (SGA), of the epidural space(EDA), and within the left cerebral hemisphere (CA). Edema related to the abscess isindicated by the yellow arrow. The cerebral abscess wall enhances (white arrow).

Brain abscess. Axial CT scan with intravenous (IV) contrast enhancement in a

patient with fever and diplopia demonstrates an enhancing mass arising from within

the ethmoid air cells, with expansion into the medial right orbit (black arrow). Theoptic nerve is in contact with the mass (blue arrow).

CT manifestations of an intracranial abscess depend on the stage of the

abscess formation. The earliest phase may be related to meningitis,with no findings on unenhanced CT studies. Enhancement of the


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meningeal surfaces is a nonspecific and inconsistent finding in patientswith meningitis.[5]

During early cerebritis, nonenhanced CT scans may demonstratenormal findings or may show only poorly marginated subcortical

hypodense areas. Contrast-enhanced CT studies demonstrate an ill-defined contrast-enhancing area within the edematous region.

During the early stage of a formed abscess, the lesion coalesces, withan irregular enhancing rim that surrounds a central low-attenuatingarea.

Scans obtained with a time delay following contrast enhancement incerebritis may show contrast "filling in" the central low-attenuatingregion. A formed abscess will not "fill in" the central portion of the

abscess.

Peripheral edema results in considerable mass effect with sulcalobliteration.

The early capsule stage is characterized by a distinct collagenouscapsule, while a relatively thin, well-delineated capsule marks the finalstage of a fully formed abscess.

Ring-enhancing lesions are commonly seen in various diseaseconditions. Besides abscess, metastatic brain tumors, some primarybrain tumors (particularly grade 4 astrocytomas), granulomas, resolvinghematomas, and infarctions are associated with a ringlike enhancementpattern. The cystic pattern is a particularly prominent feature ofcysticercosis, due to the infestation of the larva ofTaenia solium. Inmost pyogenic abscesses, the ring is smooth and thin walled (< 5 mm).The medial margin is often thinner along the medial margin, which mayreflect the variation of cerebral perfusion of gray and white matter. Thewall of a cystic neoplasm is generally thick and irregular, frondlike, orlobulated. (See the images below.)

Brain abscess. Axial CT scan obtained with intravenous (IV) contrast enhancement

in a patient with fever and headaches. Because a definite diagnosis of abscess is

difficult to determine in some patients in whom ring enhancement is not associatedwith an apparent source of infection, stereotactic biopsy and culture of a walled

abscess may be necessary.

Brain abscess. Surface 3-dimensional model of a craniofacial CT scan in a patient

with headache, orbital swelling, and diplopia of 48 hours' duration. Note theremarkable degree of right orbital swelling, which has resulted in the right lid being

closed.

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The moderate vasogenic edema that is seen in the early stages ofcerebritis and abscess formation must be interpreted in the context ofthe clinical presentation. The presence of fever, known infection, andimmunosuppression supports the probable diagnosis of early abscessformation; however, cerebrovascular accidents (CVAs) and tumors mustbe included in the differential diagnosis. Later, the well-formed abscesswall must be inspected within the context of other known malignancies,which may be a source for cerebral metastatic disease, glioma,lymphoma, and multiple sclerosis.


False-negative CT scans may occur if intravenous contrastenhancement is not adequate or if imaging of the brain is performed toosoon after contrast administration, which can happen easily when a

rapid CT (eg, multisection) scanner is used.

False-positive results primarily are the result of mistaking alternativecauses of ringlike lesions of the brain for an abscess. Ring-enhancinglesions must be placed into the differential diagnosis, which includessome primary brain tumors (eg, anaplastic astrocytoma), metastaticbrain tumors, abscess, granuloma, resolving hematoma, brain infarct,thrombosed vascular malformation, demyelinating disease (eg, multiplesclerosis), thrombosed aneurysm, and other primary brain tumors,particularly primary CNS lymphoma in patients with AIDS.

Magnetic Resonance Imaging

MRI of the brain without and with intravenous gadolinium contrastenhancement is the most sensitive test forToxoplasma encephalitis.Lesions with contrast may be hyperintense compared with normal braintissue and may be difficult to identify compared to the edema patternotherwise seen in the surrounding brain. The ring enhancement, whichis best seen on T1-weighted gadolinium-enhanced studies, representsthe enhancement within the most active area of the infection. Followingtreatment with pyrimethamine and sulfadiazine or clindamycin, the

lesions become reduced in size with resolution of the ring ofenhancement. (See the images below.)

Brain abscess. Coronal T1-weighted postgadolinium-enhanced MRI of the brain in a

patient with fever following head trauma. Osteomyelitis of the skull developed in this

patient following cranial trauma. Bilateral subdural abscesses (yellow arrow)developed by direct extension of the infection beyond the skull. The leading edge ofthe cerebritis is marked by the pattern of enhancement within the deeper margins of

the left parietal lobe (white arrow).

Brain abscess. Axial T2-weighted MRI in a patient with a right frontal abscess. Notethe mass effect and surrounding edema. The wall of the abscess is relatively thin
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(black arrows).

Brain abscess. Gadolinium-enhanced coronal T1-weighted MRI in a patient who

presented with headache, fever, and diplopia. The right frontal lobe of the brain is

shifted across the midline (double arrow) by an intracranial abscess (single black

arrow) that has extended upward from the medial right orbit and medial ethmoid aircells (curved dotted arrow). Aspergillus organisms were recovered from the sinuses

and brain tissue.

Brain abscess. Coronal T1-weighted gadolinium-enhanced MRI in a patient with

sudden onset of diplopia, fever, and right orbital swelling. Note the enhancementwithin the right ethmoid sinuses from which the infection arose. The medial superiorright maxillary sinus has been destroyed (yellow arrow).

Brain abscess. Coronal T1-weighted spin-echo gadolinium-enhanced MRI

demonstrates a central zone of enhancement within the abscess, with a zone of

decreased brightness (edema, white arrow). Nocardia organisms were cultured fromwithin the abscess cavity.

Brain abscess. Axial fluid-attenuated inversion recovery (FLAIR) MRI of a left

occipital-parietal brain abscess. The edema pattern (white arrows) surrounds thecentral abscess (A). A secondary (daughter) abscess is noted anterior to the primary

abscess cavity.

MRI findings of brain abscess vary with time.[6, 7, 8, 9, 10, 11, 12]

Early cerebritis stage

The early cerebritis stage presents as an ill-defined subcortical

hyperintense zone that can be noted on T2-weighted imaging.

Lesions appearing hyperintense on diffusion-weighted imaging withapparent-diffusion-coefficient (ADC) values of < 0.9 are most commonlybrain abscess, whereas hypointense lesions on diffusion-weightedimaging with ADC values > 2 are more likely nonabscess cystic lesions.

Contrast-enhanced T1-weighted studies demonstrate poorly delineatedenhancing areas within the isointense to mildly hypointense edematousregion.

Late cerebritis stage

During the late cerebritis stage, the central necrotic area is hyperintenseto brain tissue on proton-density and T2-weighted sequences.

The thick, somewhat irregularly marginated rim appears isointense tomildly hyperintense on spin-echo T1-weighted images and isointense torelatively hypointense on proton-density and T2-weighted scans.

Peripheral edema is common. The rim enhances intensely following

contrast administration. Satellite lesions may be demonstrated.


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Early and late capsule stages

During the early and late capsule stages, the collagenous abscesscapsule is visible prior to contrast as a comparatively thin-walled,isointense to slightly hyperintense ring that becomes hypointense on

T2-weighted MRIs.

Diffusion-weighted imaging aids in depiction of specific features of abrain abscess. If a cerebral abscess ruptures into the ventricularsystem, diffusion-weighted images demonstrate specific patterns.

Purulent material within the ventricle appears similar to that of thecentral abscess cavity, with a strongly hyperintense signal on diffusion-weighted images.

Magnetic resonance spectroscopy

Magnetic resonance (MR) spectroscopy may be helpful in thedifferential diagnosis of toxoplasmosis versus CNS lymphoma. CNSlymphoma generally shows a mild pattern of elevated lipid and lactatepeaks, with a prominent choline peak with some other normalmetabolites. In toxoplasmosis, there are elevated lipid and lactatepeaks, while other normal brain metabolites are nearly absent.

Diffusion-weighted MRI

Diffusion-weighted MRI may be useful in differentiating abscess from

necrotic tumor. Diffusion-weighted echo planar images demonstrate anabscess as a high signal intensity with a corresponding reduction in theapparent diffusion coefficient. The brightness on diffusion-weightedimaging (DWI) is related to the cellularity and viscosity of the contentswithin the abscess cavity. Tumors with central necrosis have markedhypointensity on diffusion-weighted images and much higher apparentdiffusion coefficient values. The pattern described above for an abscesshas also been noted for acute cerebral infarction.


In patients with ring-enhancing cerebral mass lesions, restricteddiffusion is characteristic but is not pathognomonic for abscess. Lowapparent diffusion coefficient values also may be found in brainmetastases. Diffusion imaging techniques should be corrected for T2brightness contribution. Corrected diffusion maps more accuratelyreflect the relative diffusion within a large or complex lesion. Diffusionimaging is more sensitive than conventional MRI alone in detection ofchanges due to infections and ischemic lesions.

Single-voxel proton MR spectroscopy is useful in differentiating ringlike

enhanced lesions that cannot be diagnosed correctly using enhancedMRI alone. MR spectroscopy can help to specifically differentiate tumor,


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radiation necrosis, or abscess by identifying their different spectralprofiles. Perfusion MRI has also been used to differentiate these lesionsby evaluating their degree of vascularity through dynamic blood flowanalysis studies.


Diffusion MRI does not help in differentiating brain abscess formationfrom focal brain infarcts related to venous thrombosis, although superiorimaging of the anatomic distribution of lesions proves useful. Restricteddiffusion within ring enhancement is not pathognomonic for brainabscess.

Ultrasonography

On ultrasonograms, cerebral abscess is depicted as a complex cystic

pattern with an echogenic wall and an ultrasonographically hypoechoicor mildly hyperechoic central zone of necrosis. Cerebralultrasonography is rarely used in the evaluation of cerebral abscess inthe adult, except for intraoperative guidance for aspiration procedures,because the intact skull is a barrier to the procedure.

In the neonate, abscess can be diagnosed by using ultrasonographicimages obtained through the anterior fontanelle. Brain ultrasonogramscan reveal the size and number of abscesses but provide only a limitedsuggestion of a possible origin for the infection. Ultrasonography-guided

aspiration of brain abscesses through a single burr hole has beenperformed with excellent overall results.

Ultrasonography cannot help to differentiate a cystic neoplasm from anabscess. When seen in the neonate, periventricular and arachnoid cystscommonly are not abscesses.

Porencephalic cysts may suggest thin-walled abscesses ifcommunication with the ventricle is not depicted clearly. Arachnoidcysts have thin walls with a marked, hypoechoic pattern.

Nuclear Imaging

Brain SPECT imaging by using thallous chloride Tl 201 (thallium-201;201Tl)can help detect and differentiate infectious processes from lymphomaand other primary brain neoplasms. Brain abscess may be evaluatedusing gallium Ga 67 (gallium-67;67 Ga) citrate and technetium-99mmhexamethylpropyleneamine oxime (HMPAO)labeled leukocytes. Inpatients with an active abscess, nuclear agents collect in the wall of theabscess. Similar findings occur within high-grade brain tumors (glioma).Differential considerations of rounded (ring) lesions of the brain include

some primary brain tumors (eg, anaplastic astrocytoma), metastaticbrain tumors, abscess, granuloma, resolving hematoma, brain infarct,


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thrombosed vascular malformation, demyelinating disease, thrombosedaneurysm, and primary CNS lymphoma in patients with AIDS.

Degree of confidence201

Tl brain SPECT imaging appears to be unreliable for differentiatingprimary lymphoma from nonmalignant brain lesions in patients withAIDS. Follow-up scans showing improvement may help furtherdifferentiate the lesions, but brain biopsy is necessary to establish adefinitive diagnosis in questionable cases.


False-positive201 Tl SPECT imaging in brain abscess may indicatefocally increased intracranial201 Tl uptake; however, such activity may bean abscess if positive tumor activity is reported. Single lesions

demonstrated on MRI scans with focal accumulation of201

Tl stronglysuggest lymphoma. Multiple lesions demonstrated on MRIs with201 TlSPECT uptake ratios 2.9 also suggest lymphoma; however, uptakeratios < 2.1 do not aid in discrimination.

Differentiation of toxoplasmosis abscess from primary brain lymphomarequires a difficult combination of clinical history, laboratory findings,and radiographic considerations. A trial period of treatment against thetoxoplasmosis organism with follow-up imaging is necessary in somepatients before excluding the possibility of CNS lymphoma.

Angiography

Cerebral angiography is rarely performed to define an abscess;however, mycotic cerebral aneurysms may occur related to aninfectious vasculitis. These may rupture, resulting in a cerebralhematoma. If the hematoma is evacuated without adequate antibiotictreatment, the bed of the hematoma near the site of the mycoticaneurysm may become infected, later forming an abscess.


Cerebral angiography is the best means with which to detect vasculitisor mycotic aneurysms. The mass effect caused by an abscess can belocalized using angiographic criteria.


The beaded appearance of the blood vessels affected by activevasculitis may be mistaken for movement on the part of the patient.

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Brain Abses Imaging