Computed tomography technology is widely available and provides rapid acquisition of images. The scanners generate cross-sectional images of the body as an x-ray tube continuously rotates around the patient. Current-generation scanners can image body slices as thin as 0.5 mm, which may be reformatted in multiple anatomical planes. In addition, the acquired sections can be reconstructed in varying thicknesses from the source data, depending on the study and anatomical region examined. Studies are conducted with or without intravenous contrast material enhancement depending on the clinical situation. Although contrast-enhanced studies can increase the sensitivity and specificity of CT scans in disease diagnosis, contrast is not always required. Table 17-1 presents some of the advantages and disadvantages of CT, as well as contraindications. CT scans are very useful for identifying acute intracranial/orbital hemorrhage and osseous abnormalities, where the ease of CT and rapidity in obtaining images make it the method of choice for evaluating trauma involving the face and orbit.
Generally, for evaluation of orbital conditions, thin-section (ie, high-resolution) studies are critical to delineate the small anatomical structures of the orbit (Fig 17-1):
Axial scans are always performed during orbital studies. However, coronal reformations, which provide optimal evaluation of the orbital roof and floor, should also be a standard part of these examinations. Sagittal reformations may be added to help further characterize and localize lesions.
Further, CT is an excellent modality for evaluating the vascular system. CT angiography (CTA) combines intravenous contrast enhancement with high-resolution imaging to produce high-quality, noninvasive scans of arterial and venous pathologies. Three-dimensional reformations that mimic catheter angiography are routinely produced and can detect cerebral aneurysms measuring 3–5 mm with high sensitivity and specificity. Additional series acquired at later times (CT venography) can be used to evaluate the cerebral venous system, especially in suspected cases of venous thrombosis or obstruction.
When additional diagnostic information is needed, CT scans can be combined with nuclear medicine imaging, as in single-photon emission computed tomography (SPECT) and positron emission tomography (PET-CT). These modalities use radiolabeled molecules to help evaluate metabolic activity in a wide range of diseases. SPECT is commonly used to evaluate myocardial perfusion and brain function, whereas PET-CT scans are typically used to diagnose and stage tumors, as well as to diagnose degenerative diseases of the brain. In ophthalmology, PET-CT has been used to assess ocular adnexal lymphoma and cortical blindness. In addition, PET-CT scans of the body are utilized in evaluation of patients with sarcoidosis and for metastatic screening of patients with uveal melanoma.
Table 17-1 Comparison of Magnetic Resonance Imaging and Computed Tomography
Figure 17-1 Computed tomography (CT) scans. A, Axial orbital view of a healthy subject. Note the orbital and intracanalicular portions of the optic nerve. B, Coronal orbital view of a healthy subject.
(Courtesy of Rod Foroozan, MD.)
Betts AM, O’Brien WT, Davies BW, Youssef OH. A systematic approach to CT evaluation of orbital trauma. Emerg Radiol. 2014;21(5):511–531.
Yang ZL, Ni QQ, Schoepf UJ, et al. Small intracranial aneurysms: diagnostic accuracy of CT angiography. Radiology. 2017;285(3):941–952.
Although CT is a transformational noninvasive technology for evaluating orbital and central nervous system diseases, ophthalmologists should be aware of the limitations of this imaging modality and safety concerns. CT scans employ ionizing radiation, a potential concern especially in pediatric cases and pregnant patients. In general, CT scans expose the patient to higher doses of radiation than conventional x-ray studies do. CT scans of the brain are typically oriented to avoid imaging of the globe, which is a more radiosensitive organ. In the International System of Units, millisievert (mSv) is the unit used to determine the amount of tissue damage expected from the absorbed dose of ionizing radiation. Millisievert is technically different from milligray (mGy), which refers to the total dose of ionizing radiation delivered to a tissue during a particular scan sequence. The National Science Foundation has estimated that 10 mSv of radiation may cause an additional case of cancer in 1/1000 patients. However, the impact of a single (or even serial) CT scan(s) of the brain in relation to the risk of cancer development is typically outweighed by the clinical need for diagnostic information; nonetheless, ordering clinicians should be aware of this safety consideration when ordering a CT scan, particularly in children.
While CT scans are very useful in studying bony structures, visibility of the posterior fossa may be reduced because of streak artifact from the skull base. Further, in evaluation of the central nervous system, CT scans provide lower spatial resolution than does MRI, although the intravenous administration of iodinated contrast material improves the soft-tissue imaging capabilities of CT.
Iodinated contrast agents pose another potential safety concern for patients under-going CT, mostly related to allergic reactions and potential nephrotoxicity in those with underlying renal insufficiency. Allergic reactions have ranged from 1% to 12%, depending on the type of contrast material used, with symptoms ranging from relatively mild (eg, pruritus, nausea, and vomiting) to severe (eg, anaphylaxis). The rate of severe allergic reactions has been reduced to less than 0.1% with the use of newer low osmolar contrast agents. Nephrotoxicity has been reported in 2%–7% of patients receiving contrast media, with higher rates in those with preexisting kidney disease and/or diabetes mellitus. The American College of Radiology (ACR) currently recommends limiting intravenous contrast agent administration in patients with an estimated glomerular filtration rate less than 30 mL/min/1.73 m2, and considering alternative imaging methods (eg, MRI) or hydrating before the examination. Because recommendations for the use of contrast agents vary by institution, consultation with a diagnostic radiologist is advised before ordering contrast-enhanced CT examinations in at-risk patients.
American College of Radiology, ACR Committee on Drugs and Contrast Media. ACR Manual on Contrast Media. Version 10.3; 2018. www.acr.org/-/media/ACR/Files/Clinical-Resources/Contrast_Media.pdf. Accessed February 25, 2019.
Meinel FG, De Cecco CN, Schoepf UJ, Katzberg R. Contrast-induced acute kidney injury: definition, epidemiology, and outcome. Biomed Res Int. 2014;2014:859328.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.