Ultrasonography utilizes sound waves which penetrate opaque ocular tissue in the front part of the eye (corneal scar, cataract, vitreous hemorrhage) and help determine the condition of the more posterior ocular structures and the need for any treatment–surgical or medical. B-scan ultrasound provides a picture analogous to a ‘slice’ of the eyeball at various depths in differing slants. A-scan ultrasound is performed to determine the length of the eyeball. It is an important test before cataract surgery, because the power of the intraocular lens is dependent upon it.