Imaging Techniques

3.2.1 Computed Tomography

Modern CT scanners produce high-resolution images that can be reformatted into multiple planes. Intravenously administered nonionic iodinated contrast is primarily used in the setting of infection, inflammatory disease, or tumor. When contrast-enhanced images are required for assessment of infection or tumor, images are typically acquired following a 60-second delay after administration of iodinated contrast. To best delineate the arterial anatomy, a CT angiogram (CTA) can be performed. CTAs are typically acquired following a power injection of intravenous iodinated contrast. Timing of contrast enhancement is usually made following a test bolus injection or using Hounsfield unit timing off the aortic arch. This technique allows for optimized arterial opacification. Typically, images are acquired around 15 to 20 seconds following onset of contrast injection. Both contrast-enhanced CT and CTAs are acquired with submillimeter slice thickness, which can be reformatted into thicker section imaging for easier viewing or maximum intensity projection (MIP) images. This can be helpful when following the course of the ophthalmic artery.

3.2.2 Magnetic Resonance

High-resolution MR imaging of the orbits can be performed when better soft-tissue detail and delineation of surrounding critical anatomic structures is required preoperatively. Images can be obtained at both 1.5 and 3 T and are typically acquired at 2- to 3-mm slice thickness.

Coronal and axial T1-weighted images without fat suppression provide helpful anatomic images delineating the extraocular muscles (EOM), and major neurovascular structures including the course of the optic nerve. Orbital fat is high signal on T1 and provides excellent soft-tissue contrast to help define these small structures.

Coronal short-tau inversion recovery (STIR) and fat-suppressed postcontrast T1-weighted images are helpful for delineating underlying pathology as most abnormalities will appear bright on STIR images and typically demonstrate some degree of contrast enhancement. STIR images or T2-weighted images are also helpful in demonstrating the cerebrospinal fluid (CSF) surrounding the optic nerve as fluid has high signal on these sequences. In addition, signal change in the optic nerve due to mass effect, edema, or injury can best be appreciated on coronal STIR images.

Diffusion-weighted imaging can also be performed in the orbit. Although diffusion-weighted imaging can be distorted at the skull base because of susceptibility artifact due to the presence of multiple soft tissue, bone, and air interfaces, this technique can sometimes be helpful to assess the cellularity of orbital neoplasms. ¹ Because hypercellular lesions result in less free movement of water, these lesions typically manifest as dark on the calculated apparent diffusion coefficient (ADC) images and bright on the corresponding diffusion-weighted images. This pattern is commonly referred to as restricted diffusion.

Noncontrast time of flight MR angiogram (MRA) of the circle of Willis can delineate the course of the ophthalmic artery in patients with poor renal function or in young patients to avoid radiation. Occasionally, due to the tortuosity and size of the ophthalmic artery, the anatomy through the optic canal and distally within the orbit may be suboptimal.

3.2.3 Dacryocystogram

Fluoroscopic dacryocystogram can be used to delineate the nasolacrimal drainage system. Following intracanalicular administration of nonionic iodinated contrast, the course and caliber of the nasolacrimal duct is followed to the inferior meatus. CT performed shortly after contrast administration can also be acquired to provide adjunctive cross-sectional data.

3.3 Imaging of Orbital Anatomy

The orbit is readily divided into multiple compartments: preseptal, postseptal, intraconal, and extraconal. The pre- and postseptal orbit is divided by the thin fibrous orbital septum, which provides a strong barrier to spread of disease. The orbital septum can be seen on high-resolution CT and MR images running parallel to the thicker more superficial orbicularis oculi muscle ( ▶ Fig. 3.1).

The EOM form the boundary of the intra- and extraconal compartments of the orbit ( ▶ Fig. 3.2). The extraconal soft tissues contain mostly fat and a few small vessels. The frontal nerve of the V1 division of the trigeminal nerve lies within the extraconal superior orbit above the levator palpebrae muscle. An important landmark to assist in endoscopic planning is the medial rectus muscle separating the medial intra- and extraconal spaces. ² In the medial extraconal space, there is a medial ethmoidal vein and small ethmoidal vasculature ( ▶ Fig. 3.2). The intraconal orbit contains critical structures including the optic nerve, ophthalmic artery and branches as well as the superior ophthalmic vein ( ▶ Fig. 3.2). Smaller vessels and nerves such as the long ciliary artery and nerve, and medial ophthalmic vein are not consistently visualized on standard MR or CT imaging. The ophthalmic artery passes through the optic canal and courses superior and lateral to the optic nerve before giving rise to the lateral lacrimal branch and turning medially and branching into the anterior and posterior ethmoidal branches, which can occasionally be identified ( ▶ Fig. 3.3, ▶ Fig. 3.4). Occasionally, the ophthalmic artery passes inferior and medial to the optic nerve before coursing superiorly and anteriorly in medial intraconal segment of the orbit, an important variation to note prior to endoscopic surgery. ² Posteriorly, the intraconal compartment is bounded by the annulus of Zinn, the common tendinous ring from which arise the four rectus EOM. The annulus of Zinn is not readily visible but can be approximated by following the course of the rectus muscles posteriorly to the point at which the rectus muscles become indistinct from the optic nerve ( ▶ Fig. 3.5). This point is also often located at the boundary of the sphenoid and ethmoid sinuses. The bony orbit posteriorly has three openings: the superior and inferior orbital fissures and the optic canal. These are readily visible on both CT and MR ( ▶ Fig. 3.5). The cranial nerves III, V1, and V2 are visible on MR with thin section non-fat-suppressed T1-weighted images. These cranial nerves can be identified in the cavernous sinuses and followed into and along the orbit. Cranial nerves IV and VI are less readily visible on conventional MR sequences; however, following the expected course of these cranial nerves is helpful for delineating pathology. Cranial nerves are more readily visible when abnormal, for example, when a schwannoma or perineural tumor spread is present.

The lacrimal gland is readily visible on both CT and MR as an ovoid soft-tissue structure along the superolateral extraconal orbit. On MR, the lacrimal gland is slightly brighter on T1 compared with adjacent EOM ( ▶ Fig. 3.6), intermediate in signal on T2, and enhances homogeneously. The levator aponeurosis, a T1 hypointense curvilinear band, extends through the lacrimal gland and separates the more superior orbital lobe from the inferior palpebral lobe ( ▶ Fig. 3.6). The lacrimal outflow apparatus consists of the superior and inferior canaliculi, the common canaliculus, lacrimal sac, and nasolacrimal duct. The nasolacrimal duct passes through the valve of Hasner before draining into the inferior meatus of the nasal cavity ( ▶ Fig. 3.6). MR is a helpful adjunct to differentiate pathology from surrounding sinonasal secretions and mucosal thickening ( ▶ Fig. 3.7, ▶ Fig. 3.8, ▶ Fig. 3.9, ▶ Fig. 3.10).

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