CNS toxoplasmosis is seen in immunosuppressed patients and is one of the most common CNS infections in AIDS patients. The classic locations are the basal ganglia and thalami, but it can occur anywhere along the gray-white junction (see Film 3 of a different patient). Lesions are typically ring enhancing with surrounding edema and mass effect. Nodular enhancement or lack of enhancement, however, is not uncommon. Rarely, the lesions are hemorrhagic. Unlike intrauterine toxoplasmosis, calcification is rare. The differential diagnosis includes other CNS infections, e.g. CMV and lymphoma. In AIDS patients, it is not uncommon for toxoplasmosis, other infections and/or lymphoma to occur in the same individual and occasionally in the same location.

In yet another case, 2 scans (Film 4) were obtained in a 47-year-old homosexual male with AIDS. The proton-density MR (right) scans show a large high signal intensity lesion clearly seen in the left lenticular nucleus. Smaller lesions are seen in the left thalamus and at the left temporoparietal junction. The changes were much less dramatic on the postcontrast CT scan (left). The only abnormality is an ill-defined low-density area in the left lenticular nucleus. Note also the frontal horns dilatation. Stereotactic biopsy of the left basal ganglia lesion revealed toxoplasmosis.