What are the typical intracerebral lesions of toxoplasmosis, including their imaging appearance and common locations?

Typical Brain Lesions of Toxoplasmosis

The classic imaging appearance of cerebral toxoplasmosis consists of multiple bilateral ring-enhancing lesions predominantly located in the basal ganglia and cerebral corticomedullary junction. 1

Imaging Characteristics

Location

• The basal ganglia and cerebral corticomedullary junction are the most common sites, though cerebellar and infratentorial involvement is well-documented 2, 1
• Lesions can be corticosubcortical, with approximately 60% having infratentorial locations in some series 3
• Multiple bilateral distribution is typical, distinguishing toxoplasmosis from other infectious processes 2, 1

Signal Characteristics on MRI

• T1-weighted sequences typically show iso- or hypointense signal intensity 3
• T2-weighted sequences demonstrate variable signal intensity with surrounding hyperintensity 3
• MRI is more sensitive than CT for detecting lesions and should be the preferred imaging modality 2, 1

Enhancement Pattern

• Ring enhancement with contrast occurs in virtually 100% of lesions 2, 3
• The "eccentric target sign" is considered pathognomonic when present - this appears as a small enhancing nodule eccentrically located within the ring-enhancing lesion 4, 5, 6
• The pathological correlate of the eccentric target sign is a leash of inflamed vessels extending through the sulcus, surrounded by concentric zones of necrosis and a wall of histiocytes with proliferating blood vessels 6
• Both eccentric and concentric target signs can rarely appear in the same lesion 4

Associated Features

• Perilesional edema is present in 100% of cases 3
• Mass effect may be present depending on lesion size and edema 2
• Meningeal enhancement can occur but is less frequent than ring enhancement 3

Distinguishing Features from Other Pathologies

Versus Primary CNS Lymphoma

• Lymphoma typically presents as more homogeneous enhancing masses rather than ring-enhancing lesions 2
• The eccentric target sign favors toxoplasmosis over lymphoma 5, 6

Versus Bacterial Abscess

• Bacterial abscesses (particularly staphylococcal) present more acutely with fever and systemic signs, are usually complications of endocarditis or trauma, and are less commonly multiple and bilateral 1

Versus Tuberculomas

• Tuberculomas tend to be more numerous and smaller, often associated with basilar meningitis, and present more indolently 1

Congenital Toxoplasmosis

• In neonates, CT or MRI may reveal hydrocephalus and intracranial calcifications 2
• Ventriculomegaly is a common finding 2

Clinical Context

A critical caveat: even single lesions with iso- or hypointense T1 signal, perilesional edema, and ring enhancement should raise suspicion for cerebral toxoplasmosis in the appropriate clinical context (immunocompromised patients, particularly those with HIV and CD4 counts <200 cells/mm³) 3, 1