Toxoplasmosis is the most common opportunistic CNS infection associated with AIDS. When infecting the brain parenchyma, toxoplasmosis can cause a necrotizing encephalitis. It is caused by the parasite Toxoplasma gondii, whose oocytes are commonly found in the feces of birds and mammals, and domestic cats in particular.

Humans become infected when they ingest the oocytes, usually via uncooked and contaminated foods. The infection is often indolent, and most affected patients are asymptomatic. However, headache is the most common presenting symptom. Patients are at greatest risk of developing opportunistic infections when the CD4+ cell count falls below 200 cells/mm3.

Imaging Features

Toxoplasmosis usually presents as multiple parenchymal lesions with surrounding edema and central necrosis. However, the lesions may be few or solitary (only 14% were solitary in one study), and the morphology may be nodular or mass-like. The basal ganglia, thalamus and grey-white matter junction are the most commonly affected structures. The brainstem may also be involved.

Toxoplasmosis (and other infectious processes that cause microabscesses) should be considered in the differential diagnosis for multiple enhancing lesions.

  • CT
    • In the acute phase, CNS toxoplasmosis presents as multiple low-density lesions.
    • In the chronic, post-treatment phase the lesions often calcify.
  • MR
    • T1-weighted images
      • Hypointense lesions
      • Often with ill-defined surrounding hypointense edema
      • ...