Imaging

Encephalitis

Preliminary Diagnosis: Encephalitis

I. What imaging technique is first-line for this diagnosis?

MR imaging with intravenous contrast of the brain.

II. Describe the advantages and disadvantages of this technique for diagnosing encephalitis.

Advantages

  • Diagnostic with positive pertinent findings in the correct clinical setting, including focal parenchymal inflammation secondary to a viral infection.

  • Able to locate the focal area of encephalitis and predict any neurological abnormalities.

  • Able to evaluate for any secondary complication from primary infectious or inflammatory encephalitis, such as an intra-axial or extra-axial abscess, meningitis, ventriculitis, or petechial hemorrhages.

  • Does not make use of ionizing radiation.

  • May identify lesions secondary to encephalitis earlier than other MR sequences.

Disadvantages

  • Expensive and time consuming imaging modality.

  • Requires significant patient cooperation to limit motion artifact.

  • Requires that patients have good renal function with GFR > 60. Administration of intravenous contrast is decided on a case-by-case basis, with 30< GFR < 60.

  • Findings may be nonspecific and may mimic other etiologies.

III. What are the contraindications for the first-line imaging technique?

Contraindicated in patients with non-magnetic resonance compatible hardware and foreign bodies.

IV. What alternative imaging techniques are available?

  • Non-enhanced and enhanced (intravenous contrast) CT imaging of the brain.

  • Nuclear medicine brain imaging, utilizing the following radiopharmaceuticals: Tc 99m DTPA, I-123 Iodoamphetamine, or Tc 99m HMPAO.

V. Describe the advantages and disadvantages of the alternative techniques for diagnosing encephalitis.

Non-enhanced and enhanced CT imaging of the brain

Advantages

  • Better able to detect some secondary complications from focal encephalitis, such as petechial hemorrhage, which is seen in Japanese encephalitis.

  • May be performed much faster than MR imaging.

  • Requires less patient cooperation, and is associated with less motion artifact, compared to MR imaging.

Disadvantages

  • Nonspecific and may be negative within the first three days.

  • Exposes the patient to a large amount of ionizing radiation.

  • Has less anatomic detail when compared to MR imaging and may not be able to detect secondary inflammation of a cranial nerve.

Nuclear medicine brain imaging

Advantages

  • Exposes the patient to less ionizing radiation compared to CT imaging.

Disadvantages

  • Results are very nonspecific and only demonstrate increased areas of brain activity and vascularity from the breakdown of the blood-brain barrier, which may have many etiologies, such as infection, trauma, tumor, infarction, and vascular lesions.

  • Results contain very little anatomic information and detail.

VI. What are the contraindications for the alternative imaging techniques?

Non-enhanced and enhanced CT imaging of the brain

Contraindicated in pregnant patients, especially during the first two trimesters.

Nuclear medicine brain imaging

There are no significant contraindications to nuclear medicine brain imaging.

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