There are different types of ultrasound test that can be applied to the eye. Broadly speaking these include A-scans which produce a graph of echogenicity along one direction, and B-scans which are graphical representations of several A-scans which form a dynamic black and white picture of the eye (sort of like a baby scan of the eyeball).
Some reasons to do ophthalmic ultrasound include:
- To measure the length of the eye (A-scan biometry) in order to calculate what power of lens implant is required in cataract surgery. This has now been superseded by partial coherence interferometry which is more accurate – but A-scan ultrasound is still occasionally required
- To verify the health of the retina where it cannot be viewed due to opaque media (eg. dense cataract, vitreous haemorrhage)
- To measure or monitor the thickness of an abnormality at the back of the eye (eg. a choroidal naevus or melanoma)
- To look for optic nerve drusen (collections of calcium on the optic nerve)
- To look for posterior scleritis which is inflammation of the wall of the eye (evidenced by fluid in sub-Tenon’s space around the optic nerve)
- To look at the ciliary body in detail (UBM – a special machine and probe are required)