BOX 93 MRI Parameters for Orbital Study

Axial Tl-weighted: 3 mm with 100% gap Coronal Tl-weighted: 3 mm with 100% gap Coronal Tl-weighted: 3 mm with 10% gap Postcontrast fat-saturated Tl-weighted axial and coronals: 3 mm with 100% gap A routine brain scan is often included.

small lesions. Changes of the globe, such as retinal or choridal detachment and scleral compression, which may happen secondary to orbital tumors, also are properly evaluated with MRI. Furthermore, tissue characterization by MRI is useful in leading to a differential diagnosis of the space-occupying le-FIGURE 9.6. Sample MRI parameters. sions in the orbit.

FIGURE 9.7. Fat-saturated postgadolinium images: (A) coronal image through poste rior orbit, (B) axial image through optic nerve, and (C) coronal T2-weighted image through the optic chiasm. (D) Direct coronal T2-weighted image at midorbit.
FIGURE 9.8. (A) Coronal T1-weighted image of a relatively homogeneous mass in right orbit, isointense to the ocular muscles. (B) This T2-weighted image shows a multicompartment cystic mass with high signal indicating fluid. The lesion was a lymphangioma.

TABLE 9.4. MRI Artifacts.

Artifact

Bioinhomogeneity

Motion

Flow or pulsation Chemical shift

Partial volume

Wrong field of view

Magnetic field distortion by metal objects Patient motion during procedure Misregistration with resulting artifacts Usually related to fat protons'

resonating at a frequency different from water protons Related to two different tissues being measured in a single voxel, the resultant display is an average of the two tissues

In particular, cystic or vascular lesions are better characterized on MRI and its multisequenced parameters.5-7 For example, a lymphangioma of the orbit can be suggested with a high level of confidence based on the T1- and T2-weighted relaxation characteristics (Figure 9.8).6

Since the MRI does not use ionizing radiation, it does not pose any significant patient risk. Because MRI utilizes high-strength magnetic fields, however, injuries can occur. The deflection of aneurysm clips or other metallic devices can cause injury to the pa-

FIGURE 9.9. MRI artifacts. (A) Wraparound artifact due to wrong field of view. (B) Distortion because of the iron pigment in patient's mascara. (C) Surgical clip results in distortion or shielding of magnetic field. (D) Metal artifact (dental hardware). (E) Ghosting from orbital motion. (F) Distortion due to head motion.

FIGURE 9.9. MRI artifacts. (A) Wraparound artifact due to wrong field of view. (B) Distortion because of the iron pigment in patient's mascara. (C) Surgical clip results in distortion or shielding of magnetic field. (D) Metal artifact (dental hardware). (E) Ghosting from orbital motion. (F) Distortion due to head motion.

FIGURE 9.12. Proptosis with chemosis, congestion, and tortuosity of the conjunctival vessels in a carotid cavernous fistula.

FIGURE 9.10. Radiographs from views of a right internal carotid angiogram. (A) Anterior-posterior (AP) view shows a typical high-flow fistula from the right carotid artery to the cavernous sinus. (B) Lateral view: typically type A fistulas are treated endovascularly from the arterial side.

FIGURE 9.12. Proptosis with chemosis, congestion, and tortuosity of the conjunctival vessels in a carotid cavernous fistula.

injuries can occur with pacing or monitoring wires that form a loop adjacent to the skin.25

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