MRI Brain with Contrast of the Skull Base: Adequacy for Viewing Brainstem, Facial Nerve, and Parotid
An MRI brain with contrast focused on the skull base will adequately visualize the brainstem but will provide incomplete evaluation of the facial nerve and will not adequately image the parotid gland.
Brainstem Visualization
The brainstem will be excellently visualized with this protocol. 1
- MRI head allows for direct visualization and excellent assessment of the brainstem, particularly when focused on the posterior fossa and posterior skull base 1
- T1-weighted images maximize brainstem-CSF contrast and are useful for demonstrating external brainstem anatomy 2
- T2-weighted images show internal brainstem anatomy and highlight pathological conditions 2
Facial Nerve (CN VII) Visualization
The facial nerve will only be partially visualized—the intracranial and intratemporal portions may be seen, but the extracranial course through the parotid will not be adequately imaged. 3, 4
What Will Be Visible:
- The facial nerve nucleus in the brainstem 4
- The cisternal segment in the cerebellopontine angle 3, 4
- The labyrinthine and tympanic portions within the temporal bone 4
- The geniculate ganglion 4
Critical Limitation:
- Standard MRI brain protocols do not evaluate the extracranial facial nerve course through the parotid gland 3, 4
- The facial nerve branches within the parotid bed require dedicated high-resolution imaging with surface coils, which are not part of standard brain MRI protocols 5, 4
- To visualize the main trunk and divisions of the facial nerve in the parotid, you would need MRI orbits, face, and neck with specialized sequences 5
Parotid Gland Visualization
The parotid gland will not be adequately imaged with an MRI brain protocol focused on the skull base. 5, 4
- Standard MRI brain imaging does not extend sufficiently inferiorly to include the parotid gland in its entirety 4
- Parotid evaluation requires dedicated neck imaging, typically achieved with MRI orbits, face, and neck 5
- High-resolution imaging with surface coils is necessary for optimal parotid visualization and assessment of its relationship to the facial nerve 5
Recommended Alternative Protocol
If comprehensive evaluation of all three structures is clinically necessary, order MRI orbits, face, and neck with contrast, which can be performed simultaneously with or instead of MRI brain. 1
- This combined approach provides complete visualization from the brainstem through the extracranial facial nerve course and includes the parotid gland 1
- Pre- and postcontrast imaging provides the best opportunity to identify and characterize lesions along the entire facial nerve course 1
- Thin-cut high-resolution techniques focused on the posterior fossa, posterior skull base, and neck should be specified 1
Clinical Pitfall to Avoid
Do not assume that "skull base" imaging automatically includes adequate parotid or extracranial facial nerve visualization. The skull base focus typically ends at the level of the skull base foramina and does not extend inferiorly enough to capture the parotid gland or the extracranial facial nerve branches 3, 5, 4. If your clinical question involves facial nerve pathology that could affect its extracranial course or parotid pathology, explicitly request MRI orbits, face, and neck rather than relying on brain imaging alone 1.