Toxoplasmosis

Open Table of Contents: Toxoplasmosis

Figure 1: Ill-defined regions of masslike signal abnormality in the right corona radiata and parietal subcortical white matter. Notice the T1 isointense to slightly hypointense signal (top left) with corresponding FLAIR hyperintensity (top right). There is faint, DWI-hyperintense, mildly reduced diffusivity (middle left and middle right) associated with the aforementioned lesions corresponding to areas of peripheral mildly nodular enhancement (bottom left). (Bottom Left) The right corona radiata lesion that closely approximates the ependyma of the right lateral ventricle has a characteristic “eccentric target sign,” an eccentric enhancing nodule within the cavity. Although not a specific sign, it can help narrow the differential. (Bottom Right) In addition, there is faint hypointense signal on SWI representing focal hemorrhage, which can be helpful in distinguishing toxoplasmosis from nontreated lymphoma. If further differentiation between toxoplasmosis and lymphoma is needed, a thallium nuclear medicine scan can be helpful.

Description

Opportunistic protozoan infection cause by Toxoplasma gondii
Most common CNS opportunistic infection in patients with AIDS

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Pathology

Toxoplasma gondii is an obligate intracellular protozoan that exists in 3 forms
- Oocysts
- Tachyzoites
- Bradyzoites

Results in necrotizing encephalitis

Clinical Features

Symptoms
- Headache, malaise, fever, and possible seizures
Demographics
- 20% to 70% of US population is seropositive
- 3% to 10% of patients with AIDS have CNS toxoplasmosis
  - More common when CD4 cell count is <200 cells/µL

Imaging

General
- 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
- CT
  - Multiple areas of hypoattenuation, most frequently in the basal ganglia, thalamus, and corticomedullary junction
  - Peripheral or nodular enhancement
- MRI
  - T1WI
    - Ill-defined hypointense lesions, rarely hyperintense
  - T2WI
    - Hypointense to isointense with surrounding vasogenic edema
  - DWI
    - Highly variable
  - SWI
    - May see hypointense signal representing hemorrhage, which is helpful when trying to distinguish toxoplasmosis from nontreated lymphoma
  - PWI
    - Very similar to pyogenic abscess
      - Reduced perfusion in the capsule
  - Postcontrast
    - Nodular rim enhancement
    - Eccentric sarget sign (enhancing nodule within enhancing rim, may be similar in appearance to neurocysticercosis)
  - MRS
    - Low specificity due to wide range of peaks
- Nuclear medicine
  - Thallium-201 SPECT and ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET)
    - May be helpful when trying to differentiate from lymphoma
    - Toxoplasmosis will demonstrate low uptake on both modalities
Imaging recommendations
- MRI with contrast; consider SWI, PWI, and MRS
- Thallium-201 SPECT and ¹⁸F-FDG PET may be helpful
Mimic
- Often difficult to distinguish from lymphoma, but perfusion imaging can help differentiate between toxoplasmosis with normal or decreased perfusion and lymphoma with increased perfusion; thallium-201 SPECT can also be used to help differentiate between them

For more information, please see the corresponding chapter in Radiopaedia and the Toxoplasmosis chapter within the Cerebral Infectious Diseases subvolume in The Neurosurgical Atlas.

Contributor: Sean Dodson, MD

DOI: https://doi.org/10.18791/nsatlas.v1.03.02.25

References

Camacho DL, Smith JK, Castillo M. Differentiation of toxoplasmosis and lymphoma in aids patients by using apparent diffusion coefficients. AJNR Am J Neuroradiol 2003;24:633–637.

Kessler LS, Ruiz A, Donovan Post MJ, et al. Thallium-201 brain SPECT of lymphoma in AIDS patients: pitfalls and technique optimization. AJNR Am J Neuroradiol 1998;19:1105–1109.

Kumar GG, Mahadevan A, Guruprasad AS, et al. Eccentric target sign in cerebral toxoplasmosis: neuropathological correlate to the imaging feature. J Magn Reson Imaging 2010;31:1469–1472. doi.org/10.1002/jmri.22192

Lee GT, Antelo F, Mlikotic AA. Best cases from the AFIP: cerebral toxoplasmosis. Radiographics 2009;29:1200–1205. doi.org/10.1148/rg.294085205

Rabelo NN, Filho LJS, da Silva BNB, et al. Differential diagnosis between neoplastic and non-neoplastic brain lesions in radiology. Arq Bras Neurocir 2016. doi.org/10.1055/s-0035-1570362

Ruiz A, Ganz WI, Post MJ, et al. Use of thallium-201 brain SPECT to differentiate cerebral lymphoma from toxoplasma encephalitis in AIDS patients. AJNR Am J Neuroradiol 1994;15:1885–1894.

Smith AB, Smirniotopoulos JG, Rushing EJ. Central nervous system infections associated with human immunodeficiency virus infection; radiologic-pathologic correlation. Radiographics. 2008;28:2033–2058. doi.org/10.1148/rg.287085135

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