From the Guidelines
To assess trigeminal nerve roots, I recommend ordering a high-resolution MRI of the brain with thin-slice sequences through the posterior fossa, specifically focusing on the brainstem and cerebellopontine angle, as this approach is supported by the most recent guidelines from the American College of Radiology 1. The protocol should include T1-weighted images with and without gadolinium contrast, T2-weighted images, and FIESTA/CISS sequences (constructive interference in steady state) to provide detailed visualization of the trigeminal nerve from its origin at the pons through its course in the cerebellopontine angle and into Meckel's cave. Some key points to consider when ordering this MRI include:
- The use of thin-slice sequences (typically 1mm or less) to provide the anatomical detail needed to evaluate the nerve roots properly 1.
- The inclusion of contrast enhancement to identify any abnormal enhancement or masses affecting the nerve 1.
- The value of FIESTA/CISS sequences in visualizing the cisternal segment of the trigeminal nerve where compression often occurs 1. This imaging approach is essential for diagnosing conditions like trigeminal neuralgia, where vascular compression of the nerve root is a common finding, as well as identifying tumors, demyelinating plaques, or other pathologies affecting the trigeminal nerve 1. It's also important to note that advanced MRI techniques, such as diffusion tensor imaging (DTI) and functional MRI, may be useful in researching the pathophysiology of trigeminal neuralgia, but are not typically part of the initial imaging protocol 1.
From the Research
MRI Sequences for Assessing Trigeminal Nerve Roots
To assess the trigeminal nerve roots, the following MRI sequences can be ordered:
- 3D constructive interference in steady-state (CISS) sequence, which is useful for delineating the trigeminal root 2, 3
- 3D Fourier transform constructive interference in steady-state (CISS) imaging, which can display sensory, superior motor, and inferior motor roots of the trigeminal nerve and their relationships to the superior cerebellar artery (SCA) and anterior inferior cerebellar artery (AICA) 3
- 3D fast imaging employing steady-state acquisition (3D FIESTA) and coregistered 3D time of flight MR angiography (3D TOF MRA) sequences, which can be used to investigate neurovascular compression of the trigeminal nerve 4
- 3D T2-weighted driven equilibrium (DRIVE), 3D time-of-flight (TOF) magnetic resonance angiography (MRA), and 3D T1-weighted gadolinium-enhanced sequences, which can be used to predict fine trigeminal neurovascular anatomy and detect features of neurovascular compression (NVC) 5
Key Findings
- The anatomy of the trigeminal root presents considerable inter- and intra-individual variability, which can influence the symptoms of trigeminal neurovascular compression 2
- The SCA and AICA often contact the sensory root and motor rootlets of the trigeminal nerve, which can cause neurovascular compression 3, 4
- 3D CISS MRI can be used to diagnose trigeminal neuralgia and assess the degree of neurovascular conflict responsible for the condition 6
- High-resolution 3T MRI can predict fine trigeminal neurovascular anatomy and detect features of NVC, particularly regarding the degree of compression exerted on the root 5