Fig. 13. Intravitreal gas. B-scan longitudinal view shows probable meniscus of gas (arrowheads). No structures are visible behind the gas bubble because of extensive shadowing.

MIRAgel implant can present as an orbital tumor.46 The epibulbar location of the implant, density consistent with a scleral buckle, and orbital shadowing allow for differentiation from an orbital mass. The extensive swelling of the implant can necessitate removal of the buckle, which often is complicated by fragmentation of the implant on removal.47

On ultrasound, the MIRAgel implant causes intrusion of the retina, choroid, and sclera into the vitreous cavity, similar to all scleral buckles. Although MIRAgel implants have lower reflectivity than a regular buckle, they still cause shadowing behind the implant (Fig. 12). The implant also may extrude through the sclera into the vitreous cavity.

Gas/Air Bubbles

Intraocular gases are used commonly to assist in the repair of RD. The high surface tension present

between gas and liquid functions to tamponade the retina and prevent the flow of fluid into the subretinal space from the vitreous cavity. Secondly, the buoyancy of the gas bubble exerts a force on the retina and holds it against the pigment epithelium. Various gases including sulfur hexafluoride and perfluoropropane are preferred over air, because they maintain therapeutic size for a longer duration of time.

No sound penetration is possible through a gas bubble that completely fills the vitreous cavity. If the bubble is small enough, however, it can be moved by head positioning to allow ultrasonographic evaluation of the posterior segment (Fig. 13).

SILICONE OIL

Silicone oil tamponade is used in lieu of gas/air bubbles in cases of severe RD caused by proliferative diabetic retinopathy, proliferative vitreoretinopathy, or giant retinal tears, in repeat operations for retinal detachment, and if the patient is unable to comply with positioning requirements of gas/air bubbles.48 Silicone oil has a lower specific gravity than water and will rise to the top of the vitreous cavity when the patient is upright; therefore it is suited best for cases where the detachment/tear is located superiorly. Once stable attachment of the retina has been achieved, the silicone oil is removed, usually between 6 weeks and 3 months postoperatively.

Silicone oil has a significantly lower sound velocity than the vitreous, resulting in significant reductions in penetration of the ultrasound signal and limiting observation of the posterior ocular wall (Fig. 14A). The lower sound velocity also causes a 50% echographic elongation of the