eMedicine Specialties > Radiology > Brain/Spine

Brain, Abscess: Imaging

Author: Lennard A Nadalo, MD, Clinical Professor, Department of Radiology, University of Texas Southwestern Medical School; Consulting Staff, Envision Imaging of Allen and Radiological Consultants Association
Coauthor(s): Leigh K Hunter, MD, FACP, Clinical Professor, Infectious Diseases Division, University of Texas Southwestern Medical School; Director, Internal Medicine Residency Program, Methodist Medical Center of Dallas
Contributor Information and Disclosures

Updated: Jan 24, 2007

Radiography

Findings

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

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

Occasionally, foreign bodies (eg, in gunshot wounds) or osteomyelitis of the maxillary bone may indicate a probable source for an intracranial abscess. Bone destruction of the roof, floor, or lateral wall of the sinuses may indicate an aggressive osteomyelitis with extension into the 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 cranial vault strongly suggest abscess formation.

False Positives/Negatives

Patients with established intracranial abscesses may develop fluid retention within the mastoid and paranasal sinuses secondary to endotracheal intubation and chronic disability. Most patients with osteomyelitis of the mandible or maxilla do not develop intracranial abscesses.

Computed Tomography

Findings

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 meningeal surfaces is a nonspecific and inconsistent finding in patients with meningitis.

• During early cerebritis, nonenhanced CT scans may demonstrate normal 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, with an irregular enhancing rim that surrounds a central low-attenuating area.
• Scans obtained with a time delay following contrast enhancement in cerebritis may show contrast "filling in" the central low-attenuating region. A formed abscess will not "fill in" the central portion of the abscess.
• Peripheral edema results in considerable mass effect with sulcal obliteration.
• The early capsule stage is characterized by a distinct collagenous capsule.
• A relatively thin well-delineated capsule marks the final stage of a fully formed abscess.

Ring-enhancing lesions are commonly seen in various disease conditions. Besides abscess, metastatic brain tumors, some primary brain tumors (particularly grade 4 astrocytomas), granulomas, resolving hematomas, and infarctions are associated with a ringlike enhancement pattern. The cystic pattern is a particularly prominent feature of cysticercosis, due to the infestation of the larva of Taenia solium. In most pyogenic abscesses, the ring is smooth and thin walled ( <5 mm). The medial margin is often thinner along the medial margin, which may reflect the variation of cerebral perfusion of gray and white matter. The wall of a cystic neoplasm is generally thick and irregular, frondlike, or lobulated.

Degree of Confidence

The moderate vasogenic edema that is seen in the early stages of cerebritis and abscess formation must be interpreted in the context of the clinical presentation. The presence of fever, known infection, and immunosuppression supports the probable diagnosis of early abscess formation; however, cerebrovascular accidents (CVAs) and tumors must be included in the differential diagnosis. Later, the well-formed abscess wall 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 Positives/Negatives

False-negative CT scans may occur if intravenous contrast enhancement is not adequate or if imaging of the brain is performed too soon 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 alternative causes of ringlike lesions of the brain for an abscess. Ring-enhancing lesions must be placed into the differential diagnosis, which includes some primary brain tumors (eg, anaplastic astrocytoma), metastatic brain tumors, abscess, granuloma, resolving hematoma, brain infarct, thrombosed vascular malformation, demyelinating disease (eg, multiple sclerosis), thrombosed aneurysm, and other primary brain tumors, particularly primary CNS lymphoma in patients with AIDS.

Magnetic Resonance Imaging

Findings

MRI findings of brain abscess vary with time.

• 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 with apparent-diffusion-coefficient (ADC) values of <0.9 are most commonly brain abscess, whereas hypointense lesions on diffusion-weighted imaging with ADC values > 2 are more likely nonabscess cystic lesions.
  • Contrast-enhanced T1-weighted studies demonstrate poorly delineated enhancing areas within the isointense to mildly hypointense edematous region.
• Late cerebritis stage
  • During the late cerebritis stage, the central necrotic area is hyperintense to brain tissue on proton-density and T2-weighted sequences.
  • The thick somewhat irregularly marginated rim appears isointense to mildly hyperintense on spin-echo T1-weighted images and isointense to relatively hypointense on proton-density and T2-weighted scans.
  • Peripheral edema is common. The rim enhances intensely following contrast administration.
  • Satellite lesions may be demonstrated.
• Early and late capsule stages
  • During the early and late capsule stages, the collagenous abscess capsule 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 a brain abscess. If a cerebral abscess ruptures into the ventricular system, diffusion-weighted images demonstrate specific patterns.
  • Purulent material within the ventricle appears similar to that of the central abscess cavity, with a strongly hyperintense signal on diffusion-weighted images.

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

Diffusion-weighted MR may be useful in differentiating abscess from necrotic tumor. Diffusion-weighted echo planar images demonstrate an abscess as a high signal intensity with a corresponding reduction in the apparent diffusion coefficient. The brightness on DWI is related to the cellularity and viscosity of the contents within the abscess cavity. Tumors with central necrosis have marked hypointensity on diffusion-weighted images and much higher apparent diffusion coefficient values. The pattern described above for an abscess has also been noted for acute cerebral infarction.

Degree of Confidence

In patients with ring-enhancing cerebral mass lesions, restricted diffusion is characteristic but is not pathognomonic for abscess. Low apparent diffusion coefficient values also may be found in brain metastases. Diffusion imaging techniques should be corrected for T2 brightness contribution. Corrected diffusion maps more accurately reflect the relative diffusion within a large or complex lesion. Diffusion imaging is more sensitive than conventional MRI alone in detection of changes due to infections and ischemic lesions.

Single-voxel proton MR spectroscopy is useful in differentiating ringlike enhanced lesions that cannot be diagnosed correctly using enhanced MRI alone. MR spectroscopy can help to specifically differentiate tumor, radiation necrosis, or abscess by identifying their different spectral profiles. Perfusion MRI has also been used to differentiate these lesions by evaluating their degree of vascularity through dynamic blood flow analysis studies.

False Positives/Negatives

Diffusion MRI does not help in differentiating brain abscess formation from focal brain infarcts related to venous thrombosis, although superior imaging of the anatomic distribution of lesions proves useful. Restricted diffusion within ring enhancement is not pathognomonic for brain abscess.

Ultrasonography

Findings

On sonograms, cerebral abscess is depicted as a complex cystic pattern with an echogenic wall and a sonographically hypoechoic or mildly hyperechoic central zone of necrosis. Cerebral ultrasonography is rarely used in the evaluation of cerebral abscess in the 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 sonographic images obtained through the anterior fontanelle. Brain sonograms can reveal the size and number of abscesses but provide only a limited suggestion of a possible origin for the infection. Ultrasonography-guided aspiration of brain abscesses through a single burr hole has been performed with excellent overall results.

Degree of Confidence

Sonography cannot help differentiate a cystic neoplasm from an abscess. When seen in the neonate, periventricular and arachnoid cysts commonly are not abscesses.

False Positives/Negatives

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

Nuclear Imaging

Findings

Brain SPECT imaging by using thallous chloride Tl 201 (thallium-201;²⁰¹ Tl)can help detect and differentiate infectious processes from lymphoma and other primary brain neoplasms. Brain abscess may be evaluated using gallium Ga 67 (gallium-67;⁶⁷ Ga) citrate and technetium-99mm hexamethylpropyleneamine oxime (HMPAO)–labeled leukocytes. In patients with an active abscess, nuclear agents collect in the wall of the abscess. 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), metastatic brain tumors, abscess, granuloma, resolving hematoma, brain infarct, thrombosed vascular malformation, demyelinating disease, thrombosed aneurysm, and primary CNS lymphoma in patients with AIDS.

Degree of Confidence

²⁰¹ Tl brain SPECT imaging appears to be unreliable for differentiating primary lymphoma from nonmalignant brain lesions in patients with AIDS. Follow-up scans showing improvement may help further differentiate the lesions, but brain biopsy is necessary to establish a definitive diagnosis in questionable cases.

False Positives/Negatives

False-positive²⁰¹ Tl SPECT imaging in brain abscess may indicate focally increased intracranial²⁰¹ Tl uptake; however, such activity may be an abscess if positive tumor activity is reported. Single lesions demonstrated on MRI scans with focal accumulation of²⁰¹ Tl strongly suggest lymphoma. Multiple lesions demonstrated on MRIs with²⁰¹ Tl SPECT uptake ratios >2.9 also suggest lymphoma; however, uptake ratios <2.1 do not aid in discrimination.

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

Angiography

Findings

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

Degree of Confidence

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

False Positives/Negatives

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

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