MRI of the Orbit and Sella Turcica

MRI of the Orbit and Sella Turcica

• Anatomical Overview: The orbits are bilateral, symmetrical bony cavities in the head, enclosing the eyeball and associated structures. They are pyramidal, with the apex pointing posteriorly and the base situated anteriorly. The orbit's boundaries are formed by seven bones.

• Bony Orbit Structures: The bones comprising the bony orbit include the frontal, sphenoid, lacrimal, ethmoid, maxilla, and zygomatic bones. These are illustrated in Figures 1, 4, 5.

• Sella Turcica (Pituitary Region): The Sella Turcica, a saddle-shaped depression in the sphenoid bone, houses the pituitary gland, a vital part of the endocrine system. It's bounded by the tuberculum sellae (anterior portion), dorsum sellae (posterior portion), and the hypophyseal fossa. A protective layer, the diaphragm sellae, covers the pituitary gland. Figure 3 and 17 illustrate the structure.

Indications (MRI of Orbit and Sella Turcica)

• Retro-orbital Lesions: Lesions behind the eye.
• Optic Disc Distortion or Pallor: Damage to optic nerve axons, occurring in various disorders.
• Orbital Cellulitis: Infection of orbital fat and muscles.
• Intra-ocular Lesions: Lesions within the eye.
• Retinoblastoma: A type of eye cancer.

MRI Procedure (Orbit)

• Patient Position: The patient is positioned supine (head first). Remove all eye makeup. Align the chin using the Orbital-meatal line (+15 to table top), ensuring the optic nerve is perpendicular to the table. Turn on alignment lights. Place the sagittal light on the mid-sagittal line of the patient's head and the axial line to pass through.

• Coils and Immobilization: Position coils near the eye without touching the patient. Immobilize using sponges and straps. Ask patients to close their eyes during acquisition to limit motion artifacts.

• Scout Slice Placement: Obtain axial slices using a coronal localizer. Align the slices parallel to a line drawn through the inferior orbital margins.. Obtain coverage from superior to inferior (inferior orbital margin to superior), lateral to medial (zygoma to zygoma) and posterior to anterior (pons to anterior globes).

• Axial Localizer for Coronal Slices: Use an axial localizer for coronal slices. Align the scan parallel to a line connecting posterior orbital margins and perpendicular to the cribriform plate. Obtain coverage as per axial scans.

MRI Procedure (Sella Turcica/Pituitary)

• Patient Position: Similar to orbits protocol, ensure the patient is supine (head first).

• Scout slice placement: Use a coronal localizer to obtain sagittal slices; position parallel to falx cerebri.

• Coverage: Obtain coverage from floor of sphenoid sinus to genu of corpus callosum; laterally, from cavernous sinus to cavernous sinus; posteriorly, from ventral aspect of pons to anterior clinoid.

• Sagittal Localizer for Coronal Slices: Use a sagittal localizer to get a coronal slice with alignment directly perpendicular to the floor of the sella on sagittal images. Obtain coverage as per sagittal plane.

MRI Sequences

• Orbit: Included sequences include axial (T2 Flair), axial (T1 Flair) (FS), axial (T1) (Flair) (FS) (post GAD), and coronal (T1) (Flair) (FS) (post GAD). Specific parameters like TR, TE, FA, ETL, and slice thickness are detailed in the provided table.

• Pituitary: Included sequences include sagittal (T1WI) SE, coronal (T2WI) FSE, coronal (T1WI) SE, coronal (T1WI) SE (post GAD), sagittal (T1WI) SE (post GAD), and coronal [(T1WI)-Dyn study (post GAD)]. Table with specific parameters is provided..