[Infection or Autoimmunity: Key Imaging Points for Differential Diagnosis]

Differentiating infectious encephalitis from autoimmune encephalitis remains a major diagnostic challenge in clinical practice because their magnetic resonance imaging (MRI) findings frequently overlap. Moreover, prior treatment with antimicrobial agents or corticosteroids may alter imaging appearances, obscure typical radiological patterns and further complicate interpretation. This review summarizes the key MRI features of infectious and autoimmune encephalitis, with emphasis on imaging patterns before treatment. Infectious encephalitis typically demonstrates meningeal enhancement, diffusion restriction, hemorrhagic or vascular changes, and infection-related complications. In contrast, autoimmune encephalitis more often presents with relatively mild or nonspecific signal abnormalities and limited contrast enhancement. Particular discriminating features include lesion distribution, symmetry, and temporal evolution on follow-up imaging. We also discuss important diagnostic pitfalls, including radiological lag after antiviral therapy, secondary autoimmune encephalitis following herpes simplex encephalitis, steroid-responsive lesions that mimic inflammatory disease, and reversible postictal changes. Rather than relying on imaging alone to establish a binary diagnosis, radiologists should integrate MRI findings with clinical and laboratory data while recognizing both the strengths and limitations of imaging. Understanding typical patterns and their variations supports more accurate diagnostic reasoning and treatment strategies in patients with suspected encephalitis.