Specific Imaging Findings

In immunocompromised hosts, lesions due to cerebral toxoplasmosis will appear on unenhanced CT as multiple areas of low attenuation, most commonly in the deep gray matter and at the corticomedullary junction. Lesions exhibit nodular or ring enhancement on post-contrast CT and T1-weighted images. Ringenhancing lesions in about one-third of cases show an internal eccentric nodule, referred to as the “eccentric target sign”, which is considered highly specific for toxoplasmosis. The enhancing portions of the lesions will typically show iso- to hypointense T2 signal with surrounding hyperintense vasogenic edema. Hemorrhage is occasionally present. Butterfly lesions crossing the corpus callosum have also been described. The lesions of toxoplasmosis demonstrate central high signal intensity on ADC maps, consistent with increased diffusivity, and decreased rCBV on perfusion imaging. MR spectroscopy tends to show moderately reduced choline peaks.

Pertinent Clinical Information

Cerebral toxoplasmosis is the most common opportunistic CNS infection in AIDS patients, who are most susceptible to develop an active disease when their CD4 count falls below 100 cells/μl. The most common presenting symptom is headache, frequently accompanied by fever and altered mental status. Seizures, visual field defects, cranial neuropathies, and other focal neurologic deficits may also be present. Diagnosis usually relies on direct detection of the organism (usually through PCR amplification) in CSF, blood, or urine. Thallium-201 SPECT and FDG-PET may reliably differentiate CNS lymphoma from toxoplasmosis in lesions over 2 cm in size, as toxoplasmosis is not radiotracer-avid.