Advanced MRI Brain Protocols: IAC, Trigeminal, Pituitary, Pineal, and Orbits

MRI of the Internal Auditory Canal (IAC)

• General Overview: The Internal Auditory Canal (IAC) is among the most frequently requested dedicated MRI protocols. Imaging typically focuses on the nerves and structures within the petrous portion of the temporal bone.

• Anatomical Structures of Interest:

  • Facial Nerve (VII): Composed of several branches including the Maxillary, Ophthalmic, and Mandibular branches (noting overlap in description with Trigeminal branches).
  • Vestibulocochlear Nerve (VIII): Divided into the Vestibular branch and the Cochlear branch.
  • Cerebellopontine Angle (CPA): The region between the cerebellum and the pons, a critical site for pathology.

• Key Indications for IAC MRI:

  • Tinnitus (ringing in the ears).
  • Vertigo or dizziness.
  • Hearing loss (specifically sensorineural).
  • CPA Masses:
    • Acoustic neuromas (account for $60\%$ to $75\%$ of all CPA masses).
    • Vestibular Schwannomas (referred to as acoustic neuromas because they originate from Schwann cells).
  • Cholesteatoma.
  • Glomus Jugulare.
  • Bell's Palsy.

• Technical Considerations:

  • Imaging planes of choice are high-resolution (HR) axial and coronal slices.
  • Multiplanar Reconstruction (MPR) is utilized for sagittal views of the nerves.
  • Fat Saturation Post-Resection: In cases of large tumor resection, surgeons may fill the empty space with fat tissue. Fat saturation must be applied post-contrast to differentiate bright fat signal from enhancing residual tumor.

• Standard IAC Protocol Sequences:

  • Routine Brain MRI: Typically performed alongside dedicated images.
  • Pre-Contrast:
    • T1 Coronal IAC (High Resolution).
    • T1 Axial IAC (High Resolution).
    • T2 Axial IAC (3D sequence for high-detail fluid/nerve contrast).
  • Gadolinium (Contrast) Administration: Required for masses or infections.
    • T1 Axial IAC.
    • T1 Coronal IAC.

• Slice Positioning and Parameters:

  • Coronal IAC:
    • Slices: $11$
    • Frequency FOV: $150\,mm$
    • Phase FOV: $150\,mm$
    • Slice Thickness: $3\,mm$
    • Matrix: $320 \times 320$
    • Orientation: Angled parallel to the IAC on the axial image. Coverage must span from the CPA posteriorly to the beginning of the sphenoid sinus anteriorly.
  • Axial IAC:
    • Slices: $11$
    • Frequency FOV: $150\,mm$
    • Slice Thickness: $3\,mm$
    • Orientation: Angled parallel to the IAC on the coronal image. Coverage must include the entire IAC and the pons.

MRI of the Trigeminal Nerve (V)

• Anatomy: The Trigeminal Nerve consists of the Trigeminal Ganglion and three main branches: the Ophthalmic branch ($V_1$), the Maxillary branch ($V_2$), and the Mandibular branch ($V_3$).

• Primary Indication: Trigeminal Neuralgia (TN). This is most commonly caused by vascular compression, where a blood vessel (artery) compresses the nerve near its exit from the brainstem.

• Protocol Details:

  • The protocol is very similar to the IAC protocol in terms of pulse sequences and slice positioning.
  • High-resolution (HR) Axial and Coronal images are necessary.
  • Axial Positioning:
    • $16$ slices, $3\,mm$ thickness, $0.5\,mm$ gap.
    • Coverage: From above the mesencephalon to below the pons.
  • Coronal Positioning:
    • $24$ slices, $3\,mm$ thickness, $0.5\,mm$ gap.
    • Coverage: From the posterior aspect of the pons to the posterior aspect of the orbits.

MRI of the Pituitary Gland (Sella Turcica)

• Anatomy of the Sella Region:

  • Hypophysis (Pituitary Gland).
  • Infundibulum (Pituitary Stalk).
  • Optic Chiasm (located superiorly to the gland).
  • Cavernous Sinus and Internal Carotid Artery (lateral to the gland).
  • Sphenoid Sinus (inferior to the gland).

• Normal MRI Appearance:

  • Anterior Pituitary: Isointense on both T1 and T2 weighted images.
  • Posterior Pituitary: Exhibits an intrinsic high T1 signal (Hyperintense) and hypointense T2 signal.
  • Posterior Pituitary Bright Spot (PPBS): This is the high T1 signal resulting from the storage of vasopressin (Antidiuretic Hormone/ADH), which has a T1-shortening effect.

• Pituitary Gland Size by Demographic:

  • Children ($< 12$ years): Approximately $6\,mm$.
  • Puberty: Approximately $10\,mm$.
  • Young male adults: Approximately $8\,mm$.
  • Young female adults: Approximately $9\,mm$.
  • Pregnancy: Grows to approximately $12\,mm$.
  • Older adults ($> 50$ years): Gradually decreases in size.

• Indications:

  • Pituitary Adenomas: Microadenomas ($< 10\,mm$) or Macroadenomas ($> 10\,mm$).
  • Craniopharyngioma.
  • Empty Sella (Rathke’s Cleft Cyst).
  • Prolactinoma.
  • Systemic Diseases: Cushing’s disease, Acromegaly, Hypopituitarism, Diabetes insipidus, Amenorrhea, and Hypo/Hyperthyroidism.
  • Visual field defects (due to compression of the optic chiasm).

• Imaging Protocol and Parameters:

  • Dynamic enhancement is crucial. Post-contrast scanning must occur immediately or via dynamic sequences to catch microadenomas before they become isointense with the enhancing normal gland.
  • Slices (Coronal/Sagittal): $11$ slices, $2.5\,mm$ thickness, $0$ gap, $140\,mm$ FOV.
  • Coronal Alignment: Perpendicular to the longitudinal fissure; angled parallel to the infundibulum.
  • Sagittal Alignment: Parallel to the longitudinal fissure; covering from right to left.

MRI of the Pineal Gland

• General Protocol: Routine brain MRI plus Sagittal and Coronal high-resolution (HR) images (Pre and Post Contrast).

• Tumor Classifications:

  • Germ Cell Tumors: Germinomas, Embryonal cell carcinoma, Teratomas.
  • Pineal Cell Tumors: Pineocytoma, Pineoblastoma.

• Positioning:

  • Coronal HR: $11$ slices, $3\,mm$ thickness, perpendicular to the longitudinal fissure, covering posterior to anterior.
  • Sagittal HR: $11$ slices, $3\,mm$ thickness, parallel to the longitudinal fissure, covering right to left.

MRI of the Orbits

• General Overview: MRI of the orbits (Brain Routine + Orbits/Face/Neck protocol) is used to evaluate the eyes and associated neural/muscular structures.

• Orbital Anatomy:

  • Muscles: Superior rectus, Inferior rectus, Medial rectus, Lateral rectus, and Superior Oblique (Trochlear nerve).
  • Nerves: Optic Nerve (CN II), Optic Chiasm.
  • Bony/Fluid structures: Eyeball, Ethmoid and Maxillary sinuses.

• Technical Considerations:

  • Fat Saturation: Vital to eliminate signal from retro-orbital fat.
  • STIR (T2 Fat Sat): Excellent for detecting edema within the optic nerve.
  • Contrast (T1 Fat Sat): Essential for detecting pathological processes post-gadolinium.
  • Motion Control: Patients must be instructed not to move their eyes.
  • Artifact Suppression: Remove eyeliner and makeup, as they often contain metallic components that cause susceptibility artifacts.

• Indications:

  • Vision loss and Papilledema (optic disc swelling).
  • Optic Neuritis (associated with Multiple Sclerosis).
  • Proptosis (forward displacement of the eye).
  • Optic Gliomas and Retro-orbital masses.
  • Diplopia (double vision).
  • Palsy of Cranial Nerves III, IV, or VI.

• Parameters:

  • Axial Slices: $14$ slices, $3\,mm$ thickness, aligned parallel to the optic nerve.
  • Coronal Slices: $24$ slices, $3\,mm$ thickness, perpendicular to the optic nerve. Coverage: anterior eyeball to posterior optic chiasm.

Lab and Course Requirements

• Performance Metrics:

  • $10$ weekly lab simulations are required.
  • A score of $> 70\%$ is necessary for a simulation to count.
  • Weekly Grade Calculation:
    • $10$ procedures: $100\%$
    • $9$ procedures: $90\%$
    • $5$ procedures: $50\%$
    • $0$ procedures: $0\%$

• Lab Scheduling:

  • Sandbox Mode: Open Wednesday to Friday, $3\,pm$ for practice (does not count toward weekly $10$).
  • Official Simulations: Open Friday $3\,pm$ to Monday $11:59\,pm$.
  • Note: There is no score preview for official simulation attempts.