CT orbits (protocol)

CT orbits (computed tomography of the orbits) involves the visualization of bony and soft tissue structures of the orbits. This examination is most commonly performed as a non-contrast scan or reconstructed from other examinations such as a CT head/face. Contrast-enhanced scans are utilized depending on clinical indications, initial assessment, and the radiologist's consultation ¹.

Note: This article is a general guideline for CT imaging of the orbits.

A standard CT orbits protocol might follow as such:

Indications

A CT orbit exam aims to evaluate the following pathologies ²:

• congenital/pediatric orbital anomalies

• orbital trauma including fractures, penetrating or blunt trauma

• orbital inflammations or infections

• orbital neoplasms

• orbital mass or lesions

Purpose

CT is considered the optimal form of imaging for evaluating orbital pathologies in a trauma setting ³. The purpose of a non-contrast orbit CT predominantly includes the evaluation of bony structures for fractures and any blunt or penetrating trauma. Contrast-enhanced CT examinations are generally requested to better evaluate soft tissue structures in emergency imaging and in the circumstance the patient is unable to obtain a magnetic resonance imaging (MRI) orbits which is generally preferred ¹.

Technique

• patient preparation

  • remove all radiopaque jewelry and dental prosthetics

• patient position

  • head first

  • supine with arms by their side

• scout

  • anterior-posterior and lateral

  • C2 to vertex

• scan extent

  • hard palate to frontal sinus

• scan direction

  • caudocranial

• scan geometry

  • slice thickness: 0.6 mm

  • slice increment: 0.6 mm

• respiration

  • suspended

• contrast medium

  • non-ionic iodinated contrast agent as per site's protocol

• scan delay

  • contrast-enhanced CT: scan 70 - 80 seconds post injection

• multiplanar reconstructions

  • 2 mm axial, coronal and sagittal bone reformats

  • 3 mm axial, coronal and sagittal soft reformats

Cone beam CT

Although conventional multi-slice CT imaging is the preferred imaging method for visualizing orbits, dose optimization can be achieved through the use of cone beam computed tomography ⁴.  The tube current (mA) used in cone beam computed tomography is significantly lower in comparison to conventional CT ⁵. However, this can compromise image quality as there are fewer x-ray photons and thus contributing to greater noise and less contrast in the resultant image ⁵.

Practical points

• two scouts (anterior-posterior and lateral) for accurate dose modulation

• utilize the head holder for optimal position of the patient's head to reduce the dose delivered to the patient

• derive multiplanar reconstructions of the orbits through a CT head/face to avoid scanning the patient again

• access to multiplanar reconstructions facilitate surgical planning ⁴