Comparison of CT and MRI
Although both CT and MRI are important modalities for the detection and characterization of orbital and ocular diseases, CT is currently the primary and most useful orbital imaging technique. Compared with MRI, it is faster, less expensive, and less sensitive to motion artifact. In general, CT provides better spatial resolution, allowing precise localization of a lesion. MRI generally provides better tissue contrast than CT; however, in most orbital disorders, the orbital fat provides sufficient natural tissue contrast to allow ready visualization of orbital tumors on CT. Each of the techniques has advantages in specific situations, some of which are discussed in the following text and in Table 2-2.
Table 2-2 Comparison of CT and MRI in Orbital Disease
MRI offers advantages over CT in some situations. It allows the direct display of anatomic information in multiple planes (sagittal, axial, coronal, and any oblique plane). MRI provides better soft-tissue definition than does CT, a capability that is especially helpful in the evaluation of demyelination and in vascular and hemorrhagic lesions (Fig 2-7). As with CT, contrast agents are available to improve MRI detail.
Compared with CT, MRI also provides better tissue contrast of structures in the orbital apex, intracanalicular portion of the optic nerve, structures in periorbital spaces, and orbitocranial tumors, because there is no artifact from the skull-base bones. Bone and calcification produce low signal on MRI. Bony structures may be evaluated by visualization of the signal void left by the bone. However, this is not possible when the bone is adjacent to structures that also create a signal void, such as air, rapidly flowing blood, calcification, and dura mater. Thus, CT is superior to MRI for the evaluation of fractures, bone destruction, and tissue calcification.
MRI is contraindicated in patients who have ferromagnetic metallic foreign bodies in the orbit or periorbital soft tissue, ferromagnetic vascular clips from previous surgery, magnetic intravascular filters, or electronic devices in the body such as cardiac pacemakers. If necessary, the presence of such foreign material can be ruled out with plain film x-ray or CT. Certain types of eye makeup can produce artifacts and should be removed prior to MRI. Dental amalgam is not a ferromagnetic substance and is not a contraindication to MRI, but this material produces artifacts and degrades the images on both MRI and CT.
Because CT and MRI yield different types of images, it is not unusual for both techniques to be employed in the evaluation of an orbital disorder. The use of these modalities should be based on the specific patient’s condition. In most cases, CT is the more effective and economical choice (see Table 2-2). MRI is the better primary technique for imaging the orbitocranial junction and brain, but CT scanning may enhance the assessment by providing better bone images. When the orbitocranial junction or brain is involved, CT scanning and MRI may be complementary; and in some cases, both may be required to evaluate complex lesions.
Figure 2-7 Acute proptosis. A, CT image of a patient with acute right proptosis resulting from spontaneous orbital hemorrhage. The hematoma exhibits discrete margins, homogeneous consistency, and a radiodensity similar to that of blood vessels and muscle. B, T1-weighted MR scan obtained 4 days after the hemorrhage demonstrates the transient bull’s-eye pattern characteristic of a hematoma beginning to undergo physical changes and biochemical hemoglobin degradation. C, T2-weighted MR scan obtained the same day as the image in part B shows a characteristic ring pattern. D, T1-weighted MR scan performed 3 months later shows that the hematoma has decreased in size, and there is layering of the degraded blood components.
CT imaging should be obtained judiciously in children, as they are much more radiosensitive than adults. Extrapolation of lifetime risk of malignancy development from atomic bomb survivors led to a consensus statement that low-level radiation may have a small risk of causing cancer. When possible, MRI should be considered first for children, with CT reserved for select clinical scenarios.
Brody AS, Frush DP, Huda W, Brent RL; American Academy of Pediatrics Section on Radiology. Radiation risk to children from computed tomography. Pediatrics. 2007; 120(3):677–682.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.