Nowadays plain radiograph is often used to screen foreign bodies in orbital regions, especially to rule out metallic foreign bodies before MR when patient has a pertinent history.

Computed tomography (CT) is one of main modalities used not only to assess the presence of orbital pathology, but also to help characterize the process. It is a preferred tool in facial-orbital trauma or other urgent indications, as CT is easily accessible compared to MR, with a quicker acquisition time especially using modern multidetector technology and enabling multiplanar reconstruction in any plane deemed necessary. Additionally, CT is useful in evaluating integrity or lesion involvement of the bony orbital wall. Relatively large amounts of inherent background fat content in orbital space aids in detection of soft tissue mass in orbital space by inherently providing contrast from default background hypodensity.

At our institution, CT of the orbit is obtained from the top of the frontal sinus down to maxilla level. It is usually acquired in bone and soft tissue algorithm with 1.25 mm or 1.5 mm slice thickness and multiplanar reconstruction images are often made in 0.625 mm slice thickness, balancing spatial resolution and signal-to-noise ratio.

Oftentimes, orbital–periorbital pathology can be of vascular etiology or closely associated with adjacent vascular structures, necessitating vascular imaging for further investigation. For example, vascular imaging is helpful in detection and characterization of orbital cavernous malformation, hemangiopericytoma, venous varix (with a Valsalva maneuver), arteriovenous fistula/carotid-cavernous fistula, other various types of vascular malformations, and ophthalmic artery aneurysm. In addition, vascular imaging may be useful in assessing tumor vascularity, identifying tumor feeding arteries as well as draining veins, and locating landmark vessels for preoperative vascular mapping when surgical treatment is planned for known orbital tumors.

The gold standard diagnostic tool is digital subtraction angiography (DSA) which can accurately depict vascular anatomy and temporal vascular flow dynamics. It can also provide endovascular treatment as indicated. However, this requires arterial puncture harboring small but nonnegligible risk of bleeding, thrombus, iatrogenic dissection, pseudoaneurysm or arteriovenous fistula formation. In this context, CT based vascular imaging is a good noninvasive alternative option for baseline diagnostic purpose or follow-up of known or treated lesions, unless the patient has a contraindication to iodinated contrast media.

CT arteriography is focused on arterial structure assessment, in which imaging at the peak arterial phase following intravenous contrast administration is essential before venous contamination dominates. To achieve this, most institutions use a bolus tracking technique.

On the other hand, CT venogram (CTV) aims at venous structure assessment by obtaining images in the venous phase. Additional postprocessed images are typically provided with bone subtraction and 3-dimensional (3D) multiplanar reconstruction, which are helpful in conjunction with raw image interpretation. Variation in CTV protocol can be made when orbital venous varix is suspected. In this case, noncontrast orbit CT in resting status can be first obtained, followed by CTV with provocative maneuvers—commonly Valsalva maneuver—that cause elevated venous pressure resulting in characteristic dynamic appearance of venous varix on imaging ( Fig. 4 ).

Patient with history of lymphoma with new orbital mass reported on surveillance neck CT. ( A ) Noncontrast orbit CT in resting status shows minimal soft tissue fullness in right orbit near the orbital apex ( arrow ). ( B ) Postcontrast orbit CT with Valsalva maneuver shows enlarged enhancing mass in right orbit which has similar density as the vessels (dashed circle). This dynamic appearance is most consistent with orbital venous varix and imaging with a provocative maneuver helps clenching the diagnosis.

Cone beam CT is a relatively new technique which is being explored for its clinical utilities. Images are generated by cone-shaped radiograph beam detected by 2-dimensional detectors, while conventional CT is acquired by fan shaped radiograph beam detected by 1-dimensional detector. This allows reduction in number of gantry rotations during image acquisition, which in turn saves imaging time. It is also advantageous in that patient radiation dose is lower than that of conventional CT. High spatial resolution and 3D volume rendering capacity are other strengths of cone beam CT.

However, there are drawbacks of cone beam CT mostly derived inherently from the above-mentioned imaging acquisition principle. These include under sampling error, imaging distortion resulted from unequal degree of radiograph attenuation between the center and periphery of the patient, increased scatter radiation with decreased signal to noise ratio and decreased soft tissue contrast resolution, which limits evaluation for soft tissue structures.

Cone beam CT has been clinically applied in dentistry, breast imaging, and imaging guidance for interventional radiology procedures, taking advantage of its accuracy and 3D reconstruction ability in relatively short acquisition time.

In terms of orbital imaging, cone beam CT has also been attempted experimentally, for example, in bony orbital wall integrity assessment in trauma, and relationship of radio-dense graft and bony wall evaluation following orbital reconstruction surgery. Its potential utilities in quantitative measurement of orbital volume and evaluation of bony orbit anatomic landmarks, such as the infraorbital canal have also been reported. In some of these reported cases, cone beam orbit CT produced acceptable quality images for osseous structure evaluation with decreased radiation dose, which has merit over conventional CT, considering the risk of cataract development following repetitive cumulative radiation to the ocular lens. In addition, Raz and colleagues reported that the central retinal artery could be detected on cone beam CT in the orbit region obtained with DSA suggesting its potential role in delicate orbital vessel evaluation, although the study was performed in only small number of patients. However, it needs to be kept in mind that cone beam CT is inferior in the evaluation of soft tissue anatomic structures, characterization of soft tissue mass, and orbital hematoma. Therefore, further validation studies in larger number of cases and careful scrutinization of indication for cone beam CT will be necessary before universal clinical application.

This new emerging CT technique is becoming popular due to its superior spatial and contrast resolution with decreased noise allowing for sharper crisp images optimal to assess minute delicate structures. There are additional benefits of reduced radiation dose and decreased iodine contrast dose needed. These advantages over conventional CT were facilitated by key differences in radiograph detectors. Unlike conventional CT with a scintillator where radiograph beams are converted to light then to electronic signals, photon counting CT uses a semiconductor with voltage across, converting incoming radiograph photons directly to an electronic signal; by skipping the middle light conversion step, septa-related dead space related problems are resolved while achieving smaller pixel size. Another powerful advantage of photon counting CT is multi-energy spectral imaging which has potential applications such as in material composition evaluation or in vascular imaging by reducing calcification related artifacts or with virtual noncalcium mapping. ^, There is a reported potential utility of photon counting CT in orbital arterial imaging, although there remain technical limitations and data in orbital imaging application in general is sparse, requiring further elucidation.