3D imaging speeds ear-canal scans

The inventor Douglas Hart, an MIT professor of mechanical engineering, plans to market the technology to hearing-aid manufacturers first, but believes it could also be useful to build fitted earphones for MP3 music players, or custom-fit earplugs for military personnel and other people who work in noisy environments.

The new technology is similar to a commercial 3D scanning system that Hart developed for dentistry, designed to replace the silicone moulds traditionally used to make impressions for dental crowns and bridges. While Hart was working on that imaging system, hearing-aid manufacturers approached him to see what he could do to improve their fitting process.

’Getting a precise 3D scan of the ear canal is the Holy Grail of the hearing-aid industry,’ said Scott Witt, head of research and development for hearing-aid manufacturer Phonak. ’Taking these impressions is still the messiest, least exact part of the process,’ he said.

Patients who need a hearing aid usually have to spend about an hour with an audiologist, who fills the patient’s ear canal with a gooey silicone substance. After about 15 minutes, the gel hardens into a mould that is removed from the ear and shipped to a hearing-aid manufacturer, who scans the mould and builds a custom-fit hearing aid using a 3D printer.

With this method, it can be difficult to achieve a tight seal between the hearing aid and the patient’s ear canal. A tight seal is necessary to prevent feedback between the microphone and receiver, which can produce squealing sounds annoying to the wearer and anyone standing nearby.

With the new MIT system a very stretchy, balloon-like membrane is inserted into the ear canal and inflated to take the shape of the canal. The membrane is filled with a fluorescent dye that can be imaged with a fibre-optic camera inside the balloon. Scanning the canal takes only a few seconds and the entire fitting process takes only a minute or two.

Because the camera captures 3D images so quickly, it can measure how much the surface of the ear canal deforms when the pressure changes, or how the canal shape changes when the wearer chews or talks. That could help hearing-aid manufacturers design devices that keep a tight seal in those situations. The higher accuracy of the digital scans could also eliminate the need for repeated impressions.

The researchers have built a prototype scanner to demonstrate the proof of concept and are now working on a handheld version of the device. Once it is ready, they plan to perform a study comparing the fit of hearing aids built with the new scanner to that of traditional hearing aids.