B-Scan Eye Ultrasound: Uses, Procedure & Results

Sometimes an eye specialist cannot see the back of the eye. A dense cataract, a bleed inside the eye or a scarred cornea blocks the view completely, and what needs checking is exactly what is hidden. A B-scan solves that problem with sound instead of light.

Key Takeaway

A B-scan eye ultrasound uses high-frequency sound waves to produce a cross-sectional picture of the inside of the eye. It is used mainly when the retina cannot be seen directly. The test is quick, painless and uses no radiation, and it can usually be done with your eyes closed.

What Is a B-Scan Eye Ultrasound?

A B-scan, short for brightness scan, and also called ocular echography or simply “echo” is an ultrasound of the eye. A small probe sends high-frequency sound waves into the eye, and the echoes bouncing back are assembled into a two-dimensional image of a thin slice of tissue.

Eye ultrasound uses a much higher frequency than the ultrasound used for a pregnancy scan, around 10 MHz. Higher frequency gives far finer detail, at the cost of penetrating less deeply. Since the eye is small and close to the surface, that trade works in its favour.

The result is a grey-scale image showing the shape of the eye and anything inside it. Denser tissue reflects more sound and appears brighter.

Why Is a B-Scan Eye Ultrasound Done?

When the doctor cannot see inside the eye

This is the main reason. A normal retinal examination shines light into the eye and looks at what comes back. That fails when something in between is not transparent:

• A dense or mature cataract
• Bleeding into the vitreous gel, which can occur in advanced diabetic eye disease
• A scarred or cloudy cornea after infection or injury
• Blood in the front of the eye after trauma
• A pupil that will not dilate adequately

Sound waves are not blocked by these. Where light cannot get through, ultrasound still can.

Before surgery on a dense cataract

This is a common reason for the test in India, where cataracts often present late and become very dense before surgery.

Before cataract surgery, the surgeon needs to know the retina is healthy. If the cataract is dense enough that the retina cannot be examined, a B-scan checks for a retinal detachment, a mass or other problem that would change the surgical plan or the expected outcome.

There is a second, related reason. Modern lens-power measurement uses light, and light-based measurement can fail through a very dense cataract — in which case ultrasound measurement is used instead. That is an A-scan rather than a B-scan, and the difference is explained below.

To confirm or characterise something already seen

Even when the view is clear, a B-scan can add information. It shows whether a lesion is solid or fluid-filled, how thick it is, and how it sits in relation to the layers of the eye — none of which is apparent from looking at the surface.

What a B-Scan Can Show

A B-scan is used to look for:

• Retinal detachment – a detached retina appears as a highly reflective, undulating membrane inside the eye
• Vitreous haemorrhage – blood in the gel filling the eye
• Posterior vitreous detachment – the gel separating from the retina
• Intraocular foreign bodies, including small metallic fragments after injury
• Intraocular tumours, and features that help characterise them
• Choroidal detachment
• Signs of infection inside the eye
• The position of an implanted lens, where this cannot be seen directly
• Optic nerve head abnormalities
• Changes in the shape of the eye wall

The interpretation always sits alongside your history, symptoms and examination. A B-scan image is read in context, not in isolation.

A-Scan vs B-Scan: What Is the Difference?

These two are constantly confused, partly because the names are so similar and partly because some patients have both.

In short: an A-scan measures, a B-scan looks.

If you are having cataract surgery you may have an A-scan for lens-power calculation. If your surgeon also cannot see your retina, you may have a B-scan as well. The two answer different questions and neither replaces the other.

One further point. Lens-power measurement is now usually done with light rather than sound, because it is more accurate. Ultrasound A-scan remains in use for eyes where a dense cataract blocks the light-based instrument.

Where ultrasound biomicroscopy fits

Ultrasound biomicroscopy, or UBM, is a third variant. It uses a much higher frequency — in the range of 20 to 50 MHz — which gives far finer detail but penetrates only a few millimetres.

That makes it unsuitable for the back of the eye and ideal for the front: the drainage angle, the area behind the iris, and the ciliary body. It is used in selected glaucoma and anterior segment cases, not as a general eye scan.

What Happens During a B-Scan

1. You sit or lie back comfortably. No injections and no dye are involved.
2. A clear gel is applied either to your closed eyelid or, if the probe is going directly on the eye, to the eye surface after anaesthetic drops.
3. The technician rests a small probe against the eye and moves it gently in sweeping motions.
4. You will be asked to look in different directions up, down, left, and right and to hold each position briefly. This is how different parts of the retina are brought into view.
5. Several short sweeps are taken, usually four around the eye plus one through the central area.
6. The gel is wiped off.
7. The same is repeated for the other eye if needed.

The test is quick it is one of the shorter eye investigations, and the whole thing is normally done in a single sitting. The exact time depends on how much needs to be examined.

Does the probe touch the eye?

This is what most people want to know, and the answer is reassuring.

A B-scan can be performed through your closed eyelid. The gel goes on the outside of the lid, the probe rests on the lid, and nothing touches your eye at all.

Where a more detailed image is needed, the probe may be placed directly on the surface of the eye but only after anaesthetic drops, so it is not painful. You will feel pressure and movement rather than pain.

If you are anxious, say so before the scan starts. The technician can often work through the lid.

Are the eyes dilated?

Dilating drops are not usually required for a B-scan, which makes it easier to fit around the rest of your day than many eye tests.

If dilation is planned for other parts of your examination, ask at reception how your appointment is sequenced, since dilating drops will blur your vision for several hours afterwards.

How to Prepare for a B-Scan

There is very little to do, which is worth saying plainly because people often assume otherwise.

• No fasting. Eat and drink normally.
• No need to stop your usual medicines, unless your doctor says otherwise.
• Remove contact lenses before the appointment.
• Bring your previous scans and reports. Comparison with an earlier image is often more informative than the new scan alone.
• Mention any recent eye injury or eye surgery when you check in. This matters see the next section.
• Leave eye make-up off on the day.
• Arrange transport if drops are planned for other parts of your visit.

Is a B-Scan Eye Ultrasound Safe?

Yes, for the great majority of patients. It uses sound waves, not radiation, involves no injection or dye, and has no recovery period. You can return to normal activity immediately.

There is one situation where care is required, and it explains a question you may be asked.

If there is any possibility of a penetrating injury to the eye an open globe ultrasound is performed only by an experienced operator, or deferred, because pressure applied to an eye whose wall is breached can cause further damage. This is why you will be asked about recent injury, and why a scan may be postponed or handled differently after trauma.

If the probe is placed directly on the eye rather than the lid, mild irritation afterwards is possible from the anaesthetic drops or the gel, and settles quickly. Tell the technician if you have an active eye infection, as this may change how the scan is done.

Understanding Your B-Scan Report

B-scan reports use unfamiliar words. Here is what the common ones mean.

The last one is the phrase most patients look up. It is a good result, and it means what it says nothing abnormal was seen. It does not mean every possible condition has been excluded, because a B-scan shows structure rather than function.

Ask your specialist to walk you through the images. They are easier to understand with a finger pointing at the screen than in text.

What a B-Scan Cannot Show

Being clear about limits is part of understanding any test.

• It does not measure vision. A structurally normal-looking eye can still see poorly.
• It cannot see behind dense tissue. Anything that reflects sound strongly casts a shadow, and whatever lies behind that shadow is not assessed.
• It does not show fine detail of the retinal layers. For that, a different scan is needed see below.
• It does not give a tissue diagnosis. Ultrasound can describe a lesion’s size, shape and density and suggest what it is likely to be, but it does not confirm the nature of a growth on its own.
• It is operator-dependent. The quality of a B-scan depends heavily on the skill of the person performing it, which is why these scans are usually done by trained ophthalmic technicians or specialists.

B-Scan and OCT Are Not the Same Test

Many patients have already had an OCT scan and reasonably ask why another scan is needed.

The difference comes down to what each one uses. OCT uses light. A B-scan uses sound.

OCT produces extremely fine detail of the retinal layers, far finer than ultrasound, and is the test of choice for conditions affecting the macula. But because it works with light, it needs a reasonably clear path into the eye. Through a dense cataract or a vitreous haemorrhage, it cannot get a usable image.

A B-scan gives a coarser picture but is not blocked by opacity. That is why the two are complementary: OCT for detail when the media are clear, ultrasound for structure when they are not.

If your specialist has ordered both, it is because they are answering different questions not duplication.

Conclusion

A B-scan answers a specific question: what is happening inside an eye that cannot be seen into. That situation is common a dense cataract, a bleed, a scarred cornea and until the answer is known, neither surgery nor treatment can be planned safely.

For the patient, the practical points are short. Nothing needs to be done to prepare. The test is quick and painless, and it can usually be done with your eyes closed. Mention any recent injury before you start, and bring your earlier reports, because comparison often tells your specialist more than a single scan.

If you have been advised to have one, it is worth keeping the appointment even if your eye feels unchanged — the whole reason for the test is that the problem, if there is one, cannot be seen from outside. If it has been more than a year since your last full eye examination, that is worth booking too.