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Findings: Image #1: T2 weighted axial imaging of the brain demonstrates an area of abnormally high signal within the right basal ganglia and thalamus.
Image # 2: Following the intravenous administration of contrast, T1 weighted coronal images demonstrate enhancement in this region.
Image # 3: T2 weighted axial image through the brain five months later demonstrates a small amount of residual high signal within the right thalamus following treatment.
Diagnosis: CNS toxoplasmosis.
Discussion: CNS toxoplasmosis encephalitis can present with fever, headache, seizures as well as a change in mental status. It is one of the more frequent opportunistic infections in AIDS patients. Radiographically, toxoplasmosis has a predilection for the basal ganglia as well as corticomedullary junction. CT examination often demonstrates an isodense mass or masses with ring-like enhancement. On MR, T2 weighted images demonstrate abnormally high signal reflective of the underlying mass and edema. On T1 weighted images the lesion is often isodense to gray matter with ring enhancement following the intravenous administration of contrast. Following proper treatment, an effective response is characterized by a decrease in the size of the lesions.
Primary CNS lymphoma is included in the differential diagnosis for ring enhancing lesions in AIDS patient. The distinction between the two diseases is important because treatment is markedly different. Lymphoma also presents as a focal mass lesion, however, it has a predilection for the corpus callosum and periventricular white matter. Otherwise, these two entities may be radiographically indistinguishable from one another.
References:
Osborn AG. Diagnostic Neuroradiology. Mosby, St. Louis; 1994:696-701.
Orvira MJ, et al. Central Nervous System Infections in HIV Positive Persons.
In: Neuroimaging Clinics of North America 1991;1(1):179-199.
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