Cerebral Toxoplasmosis

Axial T1W MR image post contrast shows a ring enhancing mass with "target appearance" in the left lentiform nucleus with marked surrounding edema, resulting in midline shift in an AIDS patient.

Facts: Cerebral Toxoplasmosis

• Infection with parasite Toxoplasma gondii
• Manifested clinically as acute, symptomatic infection or reactivation of latent infection in immunocompromised hosts. It can be seen as a self-limited infection in immunocompetent hosts, or congenital infection
• Etiology: ingestion of raw or undercooked meat, food or water containing tissue cysts or oocytes, transplacental, blood transfusion, solid organ transplantation
• In immunocompromised hosts, 50% presents with CNS disease (encephalitis, meningoencephalitis, seizures, mental status change, etc)

MR Imaging Appearance

• Ring enhancing lesions with marked surrounding edema
• Most common location = basal ganglia and gray-white matter junction of cerebral hemisphere
• Low signal on T1WI, intermediate/low/high signal on T2WI. May be hemorrhagic

Distinguishing Features from Lymphoma

• Toxoplasmosis and lymphoma are two most common enhancing intracranial masses in AIDS patients
• Toxoplasmosis tends to be multiple, and lesions are usually smaller than lymphoma
• On conventional MRI, they may look similar.
• On DWI/ADC, toxoplasmosis shows less restriction than in lymphoma.
• On MR spectroscopy, toxoplasmosis shows increased lactic and lipid peak (opposite in lymphoma)
• Toxoplasmosis is metabolically inactive on FDG-PET
• After 2-4 weeks of antitoxoplasma treatment, some healing should be seen on imaging of toxoplasmosis

Reference:

1. Lin EC, et al. Practical differential diagnosis for CT and MRI, 2008.
2. Domino FJ. 5-minute Clinical Consult 2007.
3. Castillo M. Neuroradiology companion: methods, guidelines, and imaging fundamentals, 3rd edition, 2006.

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