Figure 1: Ill-defined regions of mass-like signal abnormality in the right corona radiata and parietal subcortical white matter. Notice the T1 iso- to slightly hypointense signal (top row left) with corresponding FLAIR hyperintensity (top row right). There is faint, DWI hyperintense mildly reduced diffusivity (middle row) associated with the aforementioned lesions corresponding to areas of peripheral mildly nodular enhancement (bottom row left). The right corona radiata lesion that closely approximates the ependyma of the right lateral ventricle has a characteristic “eccentric target sign” where there is an eccentric enhancing nodule within cavity (bottom row left). While not a specific sign, it can help narrow your differential. Additionally, there is faint hypointense signal on the SWI image (bottom row right) representing focal hemorrhage. This can be helpful to distinguish from non-treated lymphoma. If further differentiation is needed between toxoplasmosis and lymphoma, a thallium nuclear medicine scan can be helpful.
- Opportunistic protozoan infection cause by Toxoplasma gondii
- Most common CNS opportunistic infection in AIDS patients
- Toxoplasma gondii is an obligate intracellular protozoan that exists in three forms
- Oocysts, tachyzoites, and bradyzoites
- Results in a necrotizing encephalitis
- Headache, malaise, fever, and possible seizures
- 20-70% of US population is seropositive
- 3-10% of AIDS patients have CNS toxoplasmosis
- More common when CD4 < 200 cell/uL
- Multiple lesions in different stages of evolution typically located at the corticomedullary junction, basal ganglia and thalami. Much less commonly involves the brainstem.
- Modality specific
- Multiple areas of hypoattenuation most frequently in the basal ganglia, thalamus and corticomedullary junction
- Peripheral or nodular enhancement
- Ill-defined hypointense lesions, rarely hyperintense
- Hypointense to isointense with surrounding vasogenic edema
- May see hypointense signal representing hemorrhage which is helpful when trying to distinguish from non-treated lymphoma
- Very similar to pyogenic abscess
- Reduced perfusion in the capsule
- Post contrast
- Nodular rim enhancement
- “Eccentric Target Sign” - enhancing nodule within enhancing rim, may be similar in appearance to neurocysticercosis
- Low specificity due to wide range of peaks
- Nuclear Medicine
- Thallium-201 SPECT and 18F-FDG PET
- May be helpful when trying to differentiate from lymphoma
- Toxoplasmosis will demonstrate low uptake on both modalities
- Imaging Recommendations
- MR with contrast. Consider SWI, PWI, and MRS.
- Thallium-201 SPECT and 18F-FDG PET may be helpful
- Often difficult to distinguish from lymphoma. Perfusion imaging can help differentiate (toxoplasmosis with normal or decreased perfusion and lymphoma with increased perfusion). Can use Thallium-201 SPECT to help differentiate as well.
For more information, please see the corresponding chapter in Radiopaedia, and the Toxoplasmosis chapter within the Cerebral Infectious Diseases sub-volume in the Neurosurgical Atlas.
Contributor: Sean Dodson, MD
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Kumar GG, et al. Eccentric Target Sign in Cerebral Toxoplasmosis – neuropathological correlate to the imaging feature. J Magn Reson Imaging. 2010; 31(6)1469-72.
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Ruiz A, et al. Use of Thallium-201 Brain SPECT to Differentiate Cerebral Lymphoma from Toxoplasma Encephalitis in AIDS Patients. AJNR. 1994; 15:1885-94.
Smith AB, et al. Central Nervous System Infections Associated with Human Immunodeficiency Virus Infection; Radiologic-Pathologic Correlation. Radiographics. 2008; 28(7):2033-58.
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