What is the first imaging test recommended for a suspected case of encephalitis?

First Imaging Test for Suspected Encephalitis

MRI is the preferred first imaging test for suspected encephalitis and should be performed as soon as possible, ideally within 24 hours of hospital admission but certainly within 48 hours. 1

Primary Imaging Recommendation

MRI with diffusion-weighted imaging (DWI) is significantly superior to CT scanning for detecting early cerebral changes in viral encephalitis, showing abnormalities in approximately 90% of patients with HSV encephalitis within 48 hours of admission, compared to only 25% sensitivity with initial CT scans. 1, 2

Key MRI Advantages:

• Detects early changes in the cingulate gyrus and medial temporal lobe, showing gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR sequences 1, 2
• Diffusion-weighted imaging is especially sensitive for detecting early pathological changes that may be missed on conventional sequences 1, 2
• Highly specific (87.5%) for PCR-confirmed HSV encephalitis when characteristic limbic patterns are present 2
• Identifies alternative treatable diagnoses that mimic encephalitis, including vascular, tumoral, or inflammatory causes 1, 3

When CT Should Be Used Instead

CT scanning is appropriate as the initial imaging modality in specific urgent situations: 1

• When MRI is not immediately available and imaging cannot be delayed 1
• Before lumbar puncture in patients with contraindications, including:
  • Focal neurological signs suggesting mass effect 1
  • Papilledema on examination 1
  • Severe immunocompromise (e.g., advanced HIV) 1
  • Depressed level of consciousness (though the exact Glasgow Coma Scale threshold remains debated—some use <12, others <9) 1
  • New-onset seizures 1

Critical CT Limitations:

• Only 25-80% of HSV encephalitis cases show abnormalities on initial CT, and even when positive, findings are often subtle 1, 2
• CT's primary role is excluding structural causes of raised intracranial pressure that would contraindicate lumbar puncture, not diagnosing encephalitis 1, 3

Important Clinical Caveats

Never delay empiric acyclovir treatment while waiting for imaging results. 1, 2 If encephalitis is suspected clinically, intravenous acyclovir (10 mg/kg three times daily) should be started immediately, as untreated HSV encephalitis carries significant mortality and severe neurological morbidity. 2

A normal MRI does not exclude encephalitis—approximately 10% of HSV encephalitis cases have normal initial imaging, and treatment decisions should never be delayed based on negative imaging. 2

Timing matters critically: The median time to lumbar puncture is significantly longer when CT is performed first (18.5 hours versus 6 hours), causing unnecessary delays in diagnosis and treatment for the majority of patients who have no contraindications to immediate LP. 1

Practical Algorithm

1. If MRI is immediately available: Proceed directly to MRI with DWI, T2-weighted, FLAIR, and T1-weighted sequences 1

2. If MRI unavailable or patient too unstable: Perform urgent CT to exclude mass effect, then proceed to lumbar puncture if safe 1

3. Start empiric acyclovir immediately regardless of imaging availability or results if clinical suspicion is present 1, 2

4. Obtain MRI within 48 hours even if initial CT was performed, as it provides superior diagnostic and prognostic information 1