QUIZ: No. 6, 25/02/2026

Diagnosis: Neurotoxoplasmosis

Findings: MRI brain demonstrates multiple bilateral supratentorial lesions predominantly involving the basal ganglia, thalami, and corticomedullary junction. On T1-weighted images, the lesions appear hypointense. On T2-weighted and FLAIR images, the lesions are hyperintense with surrounding vasogenic edema. Post-contrast images show multiple ring-enhancing lesions. Some lesions demonstrate an eccentric enhancing mural nodule along the wall (eccentric target sign). Diffusion-weighted imaging show faint peripheral diffusion restriction. No significant midline shift or hydrocephalus is noted.

Discussion

Ø  Neurotoxoplasmosis is caused by reactivation of latent infection by Toxoplasma gondii, most commonly seen in immunocompromised patients, particularly those with HIV infection and CD4 count <100 cells/mm³.

Ø  The basal ganglia are commonly involved due to hematogenous spread.

Ø  The imaging hallmark is multiple ring-enhancing lesions with surrounding edema. The eccentric target sign (enhancing mural nodule within a ring lesion) is highly suggestive of toxoplasmosis in the appropriate clinical setting.

Ø  On T2-weighted images, some lesions demonstrate a concentric alternating zone of hypo-/hyper-/isointense signal, also known as the “concentric target sign.”

Ø  Management: Treatment consists of sulfadiazine with pyrimethamine.

Teaching pearl:

• Eccentric target sign – Ring-enhancing lesion with an enhancing mural nodule on post-contrast T1; highly suggestive of toxoplasmosis.

• Concentric target sign – Concentric alternating hypo-/iso-/hyperintense rings on T2; relatively specific for neurotoxoplasmosis.

Multiple basal ganglia lesions with these signs in an immunocompromised patient strongly favor neurotoxoplasmosis.