MRI in Diagnosing Encephalitis

Rationale for MRI as First-Line Imaging

MRI is significantly more sensitive than CT in detecting the early cerebral changes of viral encephalitis, particularly in patients with suspected encephalitis, and should be performed as soon as possible, ideally within 24 hours of hospital admission, but certainly within 48 hours, as recommended by the Infectious Diseases Society of America 1, 2
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, highlighting the importance of early MRI in diagnosis 1

Diffusion-weighted imaging (DWI) is especially sensitive for early changes in encephalitis and should be included in the protocol, as it can help identify areas of restricted diffusion, which are characteristic of encephalitis 1
Early MRI changes in HSV encephalitis typically occur in the cingulate gyrus and medial temporal lobe, showing gyral edema on T1-weighted images, which can aid in diagnosis and treatment planning 1

HSV encephalitis is characterized by a limbic encephalitis pattern with unilateral or bilateral involvement of the medial temporal lobes, with bilateral temporal lobe involvement being nearly pathognomonic, as noted by the American Academy of Neurology 2
VZV-related CNS disease is associated with ischemic or hemorrhagic infarcts and intracranial arterial abnormalities, which can be detected on MRI, as recommended by the Infectious Diseases Society of America 1

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, according to the American College of Obstetricians and Gynecologists 1
For immunocompromised patients, additional testing should be considered alongside imaging, including CMV PCR, HHV6/7 PCR, HIV PCR, Toxoplasma testing, and fungal testing, as recommended by the Infectious Diseases Society of America 3, 4

Imaging findings should always be interpreted in conjunction with clinical presentation, cerebrospinal fluid analysis, and other laboratory tests, as emphasized by the American Academy of Neurology 3
Follow-up MRI may be useful for evaluation of evolving necrosis or demyelination and to assess treatment response, particularly in patients with HSV encephalitis, as noted by the American Academy of Neurology 2