CT scan: well-defined focal lesion, isodense or hyperdense to the gray matter
MRI (higher diagnostic yield): variable, isointense or hypointense lesions on T1-weighted images
Lesions that involve the corpus callosum, periventricular, or periependymal areas are more likely to be due to a<unk> 
Solitary lesions are often large (>4cm)
Lumbar puncture:
CSF cytology
CSF EBV PCR
Should also obtain toxoplasma serologies, most are treated empirically for toxoplasma while serology is pending
Stereotactic brain biopsy if necessary
Lesions that involve the corpus callosum, periventricular, or periependymal areas are more likely to be due to a<unk> 
Solitary lesions are often large (>4cm)
CSF cytology
CSF EBV PCR
Should also obtain toxoplasma serologies, most are treated empirically for toxoplasma while serology is pending
Primary CNS lymphoma
HIV neurologic complications
Drug induced aseptic meningitis
Toxoplasmosis
Neoplastic meningitis
Cryptococcosis
Limbic encephalitis
AIDS Dementia