What is the recommended imaging study for diagnosing encephalitis?

MRI is the Recommended Imaging Study for Diagnosing Encephalitis

Magnetic Resonance Imaging (MRI) is the preferred imaging modality for diagnosing encephalitis and should be performed as soon as possible on all patients with suspected encephalitis, ideally within 24 hours of hospital admission, but certainly within 48 hours. 1

Rationale for MRI as First-Line Imaging

• MRI is significantly more sensitive than CT in detecting the early cerebral changes of viral encephalitis 1
• MRI obtained within 48 hours of hospital admission is abnormal in approximately 90% of patients with herpes simplex encephalitis (HSV), while early CT may be normal or show only subtle abnormalities 1
• MRI can identify alternative, often treatable diagnoses in patients with conditions mimicking encephalitis 1, 2

Specific MRI Sequences and Findings

• Diffusion-weighted imaging (DWI) is especially sensitive for early changes in encephalitis and should be included in the protocol 1, 3
• T2-weighted and T2 fluid attenuated inversion recovery (FLAIR) images are crucial for detecting the high signal intensity changes characteristic of encephalitis 1, 2
• Early MRI changes in HSV encephalitis typically occur in the cingulate gyrus and medial temporal lobe, showing gyral edema on T1-weighted images 1

Characteristic MRI Patterns by Etiology

• HSV encephalitis: Limbic encephalitis pattern with unilateral or bilateral involvement of the medial temporal lobes; bilateral temporal lobe involvement is nearly pathognomonic 1, 2
• VZV-related CNS disease: Ischemic or hemorrhagic infarcts and intracranial arterial abnormalities 1
• Autoimmune encephalitis: Variable patterns depending on antibody type; NMDA receptor encephalitis often shows normal MRI or subtle non-specific white matter lesions, while LGI1/CASPR2/GAD antibody encephalitis typically shows T2/FLAIR hyperintensities in the medial temporal lobes 4
• Rhombencephalitis (brainstem encephalitis): Characteristic involvement of cerebellum, pons, and medulla; suggestive of tuberculosis and listeriosis 2, 5

Role of CT Scanning

• If MRI is unavailable or impractical (e.g., in acutely ill, comatosed, or confused patients), urgent CT scanning should be performed 1
• CT can exclude structural causes of raised intracranial pressure or reveal alternative diagnoses 1
• CT should be performed before lumbar puncture to rule out brain lesions with mass effect that would contraindicate the procedure 2

Special Considerations

• In pregnant women, especially in the first trimester, MRI is not usually performed unless there is no clear alternative; however, the benefits likely outweigh the risks in suspected encephalitis 1
• For immunocompromised patients, additional testing should be considered alongside imaging, including CMV PCR, HHV6/7 PCR, HIV PCR, Toxoplasma testing, and fungal testing 1

Other Imaging Modalities

• The role of MR spectroscopy is uncertain in encephalitis diagnosis 1
• Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are not indicated in the assessment of suspected acute viral encephalitis 1

Clinical Correlation

• Imaging findings should always be interpreted in conjunction with clinical presentation, cerebrospinal fluid analysis, and other laboratory tests 1, 6
• Follow-up MRI may be useful for evaluation of evolving necrosis or demyelination and to assess treatment response 1, 4