Ring enhancing lesions

RING ENHANCING LESIONS

1. Neurocysticercosis

2. Tuberculoma

3. Toxoplasma

4. Crytococcus

5. Resolving hematoma (10-21 days)/ Subacute infarct/contusion

6. Abscess – ring is smooth and regular and usually thinner on the medial side

7. Primary brain tumor (glioblastoma)

8. Metastasis (especially post chemotherapy)

9. Multiple sclerosis

10. Radiation necrosis

11. Postoperative change

12. Aneurysm

HIV most common are toxoplasma, crytococcus, and TB (Toxoplasmosis is most common)

No single feature is pathognomonic, although a cystic lesion that markedly restricts centrally (the fluid component) on DWI should be considered an abscess until proven otherwise

Distinctive radiologic characteristics of ring-enhancing lesions

· Abscesses: thin, uniform ring, thinner on medial border, smoother outer margin; satellite lesions are often present

· Neoplasms: thicker, more irregular rims.

· Demyelinating disease: incomplete rings, "open-ring sign.

· Enhancing wall characteristics

o thick and nodular and irregular rims favours neoplasm

o thin and regular favours abscess

o incomplete ring favours demyelination (the "open ring" or "incomplete ring" sign)

o low T2 signal favours abscess

o restricted diffusion of enhancing wall favours GBM or demyelination

o Cerebrovascular disease: either partially cortical and partially deep - where the surface enhancement is serpentine in gray matter or, there is "ring enhancement" around basal ganglia (e.g. the caudate head).

· Surrounding edema

o extensive oedema relative to lesion size favours abscess

o increased perfusion favours neoplasm (metastases or primary cerebral malignancy)

o Demyelinating lesions usually do not have much mass effect

· Central fluid / content

o restricted diffusion (bright on DWI and low ADC values) favours abscess

o necrotic neoplasms have more liquid centers and are dark on DWI

· Number of lesions

o similar sized rounded lesions at grey white matter junction favours metastases or abscesses

o irregular mass with adjacent secondary lesions embedded in the same region of 'oedema' favours GBM

o Small (<1 to 2cm)> neurocysticercosis.

Diffusion-weighted MRI imaging (DWI)

DWI is based on the random movement of water known as Brownian motion.

Stationary water, unlike freely moving water, is depicted as high signal intensity on DWI, with a decreased signal on the corresponding apparent diffusion coefficient (ADC) maps. The more restricted the water motion is, the less is the value of the ADC.

· Abscesses: DWI bright/ADC dark

· Tumor cavities: DWI dark/ADC bright

· An abscess cavity usually demonstrates high signal on DWI with decreased ADC values, unlike necrotic tumor cavities, which demonstrate the opposite. The restricted diffusion is directly related to the presence of pus in the abscess cavity, likely associated with high cellularity and viscosity.

Metastases

• Marked vasogenic edema and mass effect
• Isointense to mildly hypointense on T1-weighted images
• Hyperintense on T2-weighted images or with FLAIR.
• Surrounding edema is relatively hypointense on FLAIR and on T1-weighted images; they are hyperintense on T2-weighted images.
• Hemorrhagic metastases or melanoma lesions are hyperintense on T1-weighted images.
• On T2-weighted images, mucinous adenocarcinoma may be hypointense, owing to calcification; hemorrhagic metastases may be hypointense, owing to the chronic breakdown of blood products.
• Following administration of a contrast agent, solid, nodular or irregular ring patterns of enhancement are seen. Nonenhancing lesions are less likely to be metastases.
• Contrast-enhanced MRI is the best method for detection of meningeal tumor seeding, which appears as abnormal dural enhancement. This is a nonspecific finding; however, in the correct clinical setting, it correlates with the presence of sheets of tumor cells affecting the meninges.

· On imaging, dural-based metastases may resemble meningioma.

· Leptomeningeal carcinomatosis may resemble chronic meningitis;

· Leptomeningeal enhancement may occur after the administration of radiation or following extra-axial hemorrhage; it may also occur below a craniotomy site.

· Single or multiple ring-enhancing lesions with edema may resemble infectious processes.

· Solitary lesions resemble primary brain tumors.

Abscess

MRI findings of brain abscess vary with time.

Early cerebritis stage

• Ill-defined subcortical hyperintense zone that can be noted on T2-weighted imaging.
• Hyperintense on DWI with ADC values of <0.9> 2 are more likely nonabscess cystic lesions.
• Contrast-enhanced T1W: poorly delineated enhancing areas within the isointense to mildly hypointense edematous region.

Late cerebritis stage

• Central necrotic area is hyperintense to brain tissue on proton-density and T2-weighted sequences.
• Thick & 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
• 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.
• DWI aids in depiction of specific features of a brain abscess. If a cerebral abscess ruptures into the ventricular system, DWI 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.

MRS 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.

Lymphoma

The classic appearance of CNS lymphoma on MRI

· T1-weighted: isointense to isointense-to-hypointense nodule or mass.

· T2-weighted: isointense-to-hyperintense mass.

· Gadolinium-enhanced T1-weighted: enhance intensely and diffusely.

In patients with AIDS-related immunosuppression, a ringlike enhancing pattern is seen most often. Often, little or no surrounding vasogenic edema is demonstrated.

· Tumor lesions may cross the midline and may appear as a butterfly tumor involving both cerebral hemispheres.
· In 30% of patients, leptomeningeal involvement is encountered, usually in secondary systemic lymphoma; in such cases, meningeal involvement is typical.
· Involvement of the perivascular spaces with contrast enhancement is strongly suggestive of CNS lymphoma (in such cases, lymphoma must be differentiated from sarcoidosis and CNS tuberculosis);
· Involvement of the corpus callosum is also strongly suggestive of CNS lymphoma (in such cases, lymphoma must be differentiated from glioma and metastatic neoplasm).
· Contrast-enhancing, thickened ependyma may be seen (cytomegalovirus ependymitis in AIDS or metastatic neoplasm such as carcinoma of lung or breast, and ependymal spread of anaplastic glioma must be differentiated).
· Contrast-enhancing, thickened ependyma may be seen. If such findings are seen in patients with AIDS, lymphoma must be differentiated from cytomegalovirus ependymitis; if such findings are encountered in patients who do not have AIDS, lymphoma must be differentiated from metastatic neoplasm, such as carcinoma of the lung or breast.
· In addition, in patients with these findings, lymphoma must be differentiated from ependymal spread of anaplastic glioma.

Meningioma

· Homogeneous, extraaxial mass, it may show meningeal cysts, ring enhancement, fatty transformation, and en plaque morphology
· Nonenhanced T1-weighted: most are isointense. Fibromatous meningiomas may be more hypointense than the cerebral cortex.
· T2-weighted: hyperintense, also show the extent of edema.
· Multiple meningioma occurred between 5-40%
· Intense enhancement is seen in 85% of tumors, A ring appearance may represent a capsule
· Dural tail: collar of thickened, enhancing tissue that surrounds their dural attachment, represents thickened dura which may be either reactive or neoplastic. A dural tail occurs in approximately 65% of meningiomas and 15% of other peripheral tumors; not specific for meningiomas, it is highly suggestive of the diagnosis.

Hyperintensity on T2-weighted images indicates soft tumor consistency and microhypervascularity. This is seen more often in aggressive, angioblastic, or meningothelial tumors. T2-weighted signal intensity is best correlated with both the histology and consistency of the meningioma. Generally, low-intensity portions of the tumor on T2-weighted images indicate a more fibrous and harder character (eg, fibroblastic meningiomas), whereas higher-intensity portions indicate a softer character (eg, angioblastic tumor).

Malignant meningioma may invade the calvarium and cerebral parenchyma 1%

MRS reveals lactate in embolized areas of the meningioma immediately after embolization. Lipids are not observed before the 3rd day after embolization and are always associated with avascular and soft tissue at the time of surgery.

Tuberculosis

Contrast-enhanced CT scanning of the brain

· Prominent leptomeningeal and basal cistern enhancement

· With ependymitis, linear periventricular enhancement is present.

· Ventricular dilatation (eg, dilatation of the third and fourth ventricles) due to hydrocephalus is usually seen.

· Often, low-attenuating focal infarcts are seen in the deep gray-matter nuclei, deep white matter, and pons; these infarcts result from associated vasculitis.

· Caseating granulomas are rim enhancing; if these have a central calcific focus, they may form a targetlike lesion.

· All lesions are surrounded by hypoattenuating edema.

Primary differential diagnoses are fungal meningitis, bacterial meningitis, carcinomatous meningitis, and neurosarcoidosis.

MRI

· Gadolinium-enhanced T1-weighted images

o Prominent leptomeningeal and basal cistern enhancement

o With ependymitis, linear periventricular enhancement is present.

· Ventricular dilatation due to hydrocephalus is usually seen.

· Deep gray-matter nuclei, deep white matter, and pontine infarctions resulting from vasculitis are hyperintense on T2-weighted images.

· Diffusion-weighted MRI is especially sensitive in depicting early ischemic lesions when findings on the T2-weighted MRIs are normal.

Parenchymal cerebritis may show hyperintensity with little or no enhancement on T2-weighted images.

Parenchymal tuberculomas demonstrate various patterns

· Typically hypointense on T2-weighted images, but they may be hyperintense as well.

· Hypointense walls or rims on T2-weighted MRIs.

· Noncaseating granulomas are homogeneously enhancing lesions.

· Caseating granulomas are rim enhancing.

· Granulomas may also form a miliary pattern with multiple tiny, enhancing nodules scattered throughout the brain.

· Lesions are typically surrounded by hyperintense edema on T2-weighted images.

MRS: To differentiate from neoplasms

· Elevated fatty-acid spectra that are best seen by using the stimulated-echo acquisition mode technique and a short echo time.

· Necrosis of the waxy walls of mycobacteria within the granuloma is believed to cause the elevation of fatty-acid peaks.

· Lactate peak is caused by anaerobic glycolysis and is found in inflammatory, ischemic, and neoplastic lesions of the brain; this finding is nonspecific

MULTIPLE ENHANCING LESIONS

Hematogenous:

1. Metastases

2. Lymphoma

3. AIDS Disseminated infection (multiple abscesses)

4. Multifocal infarction

5. Inflammatory/Unknown Etiology:

6. Multiple Sclerosis (white matter lesions)

7. Vasculitis

8. Hypertensive Crisis/Ecclampsia

Inherited Mass Lesions/Neoplasms:

1. Hemangioblastoma (von Hippel-Lindau)

2. Arteriovenous malformations (cavernous hemangioma >> AVM)

3. Meningiomas – 4% are multiple (some with NF-2, most without)

4. Multicentric gliomas – 5% of all gliomas

5. Tuberous sclerosis

6. Neurofibromatosis (both types – NF1 (von Recklinhausen) and NF2 (MISME)