sonography A-scan B-scan

A-scan and B-scan:In these techniques ultrasonic waves are used, and ultrasound probes can be placed directly on the eye or on the eyelid after using anesthetic drops.

 A-scan is part of the pre-operative cataract surgery, because before this surgery, the eye length must be precisely determined so that the intraocular lens capability can be accurately determined. 

The B-scan accurately specifies the anatomy and internal shape of the eye, so if there is a barrier to retinal evaluation, such as when there is severe cataract or vitreous hemorrhage, accurate information about the retina can be obtained. It can also be used to check the internal tumors of the eye.