Mri findings in toxoplasmosis
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MRI Findings in Cerebral Toxoplasmosis
Cerebral toxoplasmosis is caused by reactivation of Toxoplasma gondii in immunocompromised patients - primarily HIV/AIDS with CD4 counts below 100 cells/mm³. MRI is the most sensitive imaging modality and is more useful than contrast-enhanced CT for detection.
Location / Distribution
- Predilection sites: basal ganglia, thalamus, and corticomedullary junction
- Lesions are typically multiple (multifocal), though solitary lesions occur
- Can involve periventricular white matter and corpus callosum
Signal Characteristics by Sequence
| MRI Sequence | Typical Finding |
|---|---|
| T1 (unenhanced) | Hypointense lesions; peripheral T1 shortening if hemorrhage present |
| T2 / FLAIR | Hyperintense lesions with surrounding hyperintense perilesional edema; variable hypo-isointense or mixed patterns also described |
| T1 + Gadolinium | Ring-enhancing lesions (most common); can be nodular; may be absent in severely immunocompromised patients |
| DWI | Hyperintense on DWI; the core of the abscess typically does not show restricted diffusion (ADC values relatively low, but less restricted than pyogenic abscess) |
| ADC map | Relatively low ADC, but overlaps with lymphoma |
(Grainger & Allison's Diagnostic Radiology, p. 1482)
Pathognomonic / Characteristic Signs
1. Eccentric Target Sign (post-contrast T1)
The most specific imaging sign - seen in up to ~30% of cases. It consists of:
- An innermost enhancing eccentric nodule (viable trophozoites)
- A surrounding hypointense zone (coagulative necrosis)
- A peripheral enhancing rim (inflammatory vascular reaction)
This sign is highly suggestive of toxoplasmosis over other lesions, including CNS lymphoma.
2. Concentric Target Sign (T2-weighted)
Three concentric rings on T2:
- Central hyperintensity (necrotic fluid core)
- Peripheral hypointensity (mural blood / hemosiderin)
- Outer hyperintense ring of perilesional edema
(Mahadevan et al., J Magn Reson Imaging 2013)
MR Images from Textbooks
Fig. 1 - Cerebral toxoplasmosis in a 50-year-old HIV-positive male: (A) Axial FLAIR showing hyperintense lesions in the right frontal periventricular region and splenium of the corpus callosum. (B) High signal on DWI trace diffusion imaging.
Fig. 2 - Cerebral toxoplasmosis in AIDS: Postgadolinium axial (A) and coronal (B) T1-weighted MRI showing multiple ring-enhancing lesions with mass effect.
Additional Advanced MRI Findings
MR Spectroscopy:
- Increased lipid-lactate peak (characteristic)
- Reduced Cho (choline), Cr (creatine), and NAA (N-acetylaspartate)
- Choline peak may occasionally also be elevated (overlaps with lymphoma)
MR Perfusion (DSC - Dynamic Susceptibility Contrast):
- Decreased perfusion with relative cerebral blood volume (rCBV) <1.5
- This contrasts with CNS lymphoma, which shows increased perfusion (rCBV >1.5)
- This distinction is the most useful advanced MRI feature to differentiate the two
(Radiopaedia - Neurotoxoplasmosis; AJR 2020)
Differential Diagnosis on MRI
| Feature | Toxoplasmosis | Primary CNS Lymphoma |
|---|---|---|
| Number | Multiple (usually) | Often solitary |
| Location | BG, corticomedullary junction | Periventricular, adjacent to lateral ventricles |
| DWI restriction | Variable (usually not restricted) | Restricted diffusion (hypercellular tumor) |
| Perfusion (rCBV) | Low (<1.5) | High (>1.5) |
| Thallium SPECT / FDG PET | Cold lesion (minimal uptake) | Hot lesion (high uptake) |
| Eccentric target sign | Present (~30%) | Absent |
| Response to empirical therapy | Lesions decrease in 10 days | No change |
(Grainger & Allison's Diagnostic Radiology, p. 1482; Tintinalli's Emergency Medicine, p. 1077)
Clinical Context
MRI is preferred over CT. In patients with HIV/AIDS, positive Toxoplasma IgG serology plus typical MRI findings justifies empirical treatment with pyrimethamine + sulfadiazine + leucovorin without waiting for biopsy. A decrease in lesion size within 10 days confirms the diagnosis. If no response, primary CNS lymphoma must be excluded by biopsy or Epstein-Barr virus PCR in CSF. - Bradley and Daroff's Neurology in Clinical Practice; Goldman-Cecil Medicine
Mri findings of brain in cmv and ebv
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https://pmc.ncbi.nlm.nih.gov/articles/PMC10494941
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MRI Findings of the Brain in CMV and EBV
Part 1: Cytomegalovirus (CMV) Encephalitis
CMV is a beta-herpesvirus. CNS infection almost exclusively occurs in severely immunocompromised patients - particularly HIV/AIDS with CD4 count <50 cells/µL. Two distinct CNS syndromes exist with different MRI patterns.
CMV Encephalitis (Hematogenous Spread)
Pathologically, CMV spreads hematogenously to cortical and subcortical gray matter, producing microglial nodules and cytomegalic cells.
| Sequence | Finding |
|---|---|
| T1 (unenhanced) | Decreased signal diffusely in parenchyma; ventriculomegaly |
| T2 / FLAIR | Increased signal intensity outlining the ventricles (periventricular); may be diffuse throughout the parenchyma |
| T1 + Gadolinium | Subependymal / periventricular enhancement - thin line of enhancement bordering the ventricles |
| CT equivalent | Ventriculomegaly; decreased attenuation diffusely |
Key feature: "Pencil-thin" T2/FLAIR periventricular hyperintensity with matching subependymal contrast enhancement - a pattern that distinguishes it from HIV encephalopathy (HAD) and PML. (Dartmouth CNS Infection Radiology; Harrison's Principles of Internal Medicine 22E)
CMV Ventriculoencephalitis (CSF Dissemination)
Occurs more acutely, often on a background of CMV retinitis. The MRI picture is more dramatic:
- Dilated ventricles (prominent ventriculomegaly) - a key distinguishing feature vs. CMV encephalitis
- Brainstem signal changes
- Ependymal enhancement lining the entire ventricular system
- Often associated with cerebellar T2/FLAIR abnormalities
(Bradley and Daroff's Neurology in Clinical Practice, p. 1639)
CMV - Summary of MRI Pattern
The hallmark is periventricular / subependymal disease, reflecting the virus's tropism for ependymal cells and periventricular tissue - the opposite of HIV encephalopathy which preferentially affects white matter.
- MRI is the method of choice (low sensitivity/specificity but best available imaging)
- May be normal in early disease
- Diagnosis confirmed by CSF CMV PCR (positive in <50% of encephalitis, better in ventriculoencephalitis)
Part 2: Epstein-Barr Virus (EBV) Brain Involvement
EBV causes CNS disease through three distinct mechanisms, each with a different MRI pattern:
2A. EBV Encephalitis (Immunocompetent, especially children)
EBV encephalitis is uncommon; MRI is abnormal in 27-80% of patients.
Distribution of involvement (from largest case series of 45 pediatric patients):
| Region | Frequency |
|---|---|
| Cortical / subcortical | 20% |
| White matter | 15.5% |
| Basal ganglia | 11% |
| Thalamus | 9% |
| Brainstem | 6.2% |
| Substantia nigra | 4.4% |
| Cerebellum | 4.4% |
| Spinal cord | 3% |
Signal characteristics:
- T2 / FLAIR: Hyperintense lesions in the above regions - often cortical/subcortical and basal ganglia
- DWI: Diffusion restriction seen in ~24% of cases
- SWI / GRE: Susceptibility changes (microhemorrhage) in ~15.5%
- Meningeal enhancement on post-contrast T1 in 5-22%
- Brainstem and cerebellar T2/FLAIR hyperintensity is notable in children
- Also in children/adolescents: prominent cerebellar involvement (Harrison's 22E)
(Vyas et al., as cited in PMC10494941)
2B. EBV-Associated Primary CNS Lymphoma (PCNSL) - Most Important CNS Manifestation
EBV is the causative driver of PCNSL in immunocompromised patients (HIV/AIDS, organ transplant recipients). In HIV-AIDS, virtually all PCNSL is EBV-driven.
In Immunocompetent Patients (EBV-negative or EBV-positive PCNSL):
The classic, "textbook" pattern:
| Feature | Finding |
|---|---|
| T1 | Hypointense to isointense |
| T2 | Iso- to hypointense (dense cellularity - unlike most tumors which are T2 bright) |
| T1 + Gadolinium | Vivid homogeneous enhancement (contact of intact blood-brain barrier in immunocompetent patients) |
| DWI | Restricted diffusion (hypercellular tumor - ADC low) |
| Location | Periventricular deep white matter, corpus callosum, basal ganglia; may show subependymal extension |
| Perfusion | Lower CBV than glioblastoma; lower peak height on perfusion curve |
In Immunocompromised Patients (HIV-associated EBV-driven PCNSL):
The pattern is different and more heterogeneous - mimics toxoplasmosis:
| Feature | Finding |
|---|---|
| Enhancement | Ring-enhancement (not homogeneous) - due to necrosis |
| T2 | Hyperintense necrotic core with peripheral iso/hypointense rim |
| SWI | Hypointense rim (hemorrhage/hemosiderin) |
| DWI | Peripheral restriction (ring pattern) |
| Necrosis & hemorrhage | More common than in immunocompetent PCNSL |
| Location | Deep, periventricular, often multiple |
| Size | Typically ≥4 cm (larger than toxoplasmosis lesions which are usually <4 cm) |
EBV-positive vs EBV-negative PCNSL (Lee et al.): Tumor necrosis, hemorrhage, and irregular/ring enhancement are significantly more common in EBV-positive PCNSL (P <0.001). (PMC10494941)
2C. EBV Smooth Muscle Tumors (EBV-SMT) - Rare
Seen in severely immunocompromised patients:
- CT: Iso- to hyperdense dural-based extra-axial lesions with substantial enhancement
- MRI T1: Hypo- to isointense
- T2: Prolongation (bright)
- Enhancement: Variable
- DWI: NO restriction (distinguishes from PCNSL)
- Resemble meningiomas or dural metastases
Comparison Table: CMV vs EBV Brain MRI
| Feature | CMV Encephalitis | EBV Encephalitis | EBV-PCNSL (Immunocompetent) | EBV-PCNSL (Immunocompromised) |
|---|---|---|---|---|
| Pattern | Periventricular | Cortical/BG/WM | Periventricular deep WM | Deep periventricular |
| T2/FLAIR | Periventricular rim | Focal hyperintensity | Iso/hypointense | Hyperintense (necrotic) |
| Enhancement | Subependymal (thin) | Meningeal ± parenchymal | Vivid homogeneous | Ring enhancement |
| DWI | Not prominently restricted | +/- restriction (24%) | Restricted | Peripheral restriction |
| Hemorrhage | Rare | SWI changes ~15% | Rare | Common |
| Mass effect | Minimal | Variable | Present | Present |
| Ventriculomegaly | Yes (especially ventriculoencephalitis) | No | No | No |
| Host | CD4 <50, HIV/AIDS | Children, immunocompetent | Immunocompetent adults | HIV/AIDS, transplant |
| Key biomarker | CSF CMV PCR | CSF EBV PCR / serology | CSF cytology / biopsy | CSF EBV PCR (nearly 100% sensitive in HIV-PCNSL) |
Sources: Bradley and Daroff's Neurology in Clinical Practice (p. 1639-1641); Harrison's Principles of Internal Medicine 22E (p. 1471); Kaplan & Sadock's Comprehensive Textbook of Psychiatry (p. 1868); Dartmouth CNS Infection Radiology Review; Unpacking CNS Manifestations of EBV - PMC
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