OCT imaging of the inner ear promising for new hearing loss therapies

Employing optical coherence tomography (OCT), which generates high-resolution, three-dimensional (3D) images, Brian Applegate, associate professor in the Department of Biomedical Engineering at Texas A&M University (College Station, TX), and colleagues from Stanford University (Stanford, CA) are mapping the tissues within the cochlea, the portion of the inner ear responsible for hearing. Their research could lead to breakthroughs in understanding cochlear function, Applegate says, including new therapies for hearing loss.

Related: FD-OCT overcomes hurdles in inner ear imaging

Although the hearing process hinges on what takes place inside the cochlea, that important area of the inner ear has been traditionally difficult to study, Applegate says. Its small size and the fact that in humans its tissues are encapsulated—and therefore obscured—by dense bone create significant access issues. Researchers, he explains, can't drill into the bone without risking damage to the tissues within or, at the very least, altering the mechanics of the delicate system.

However, the OCT system developed by Applegate and his colleagues is capable of rendering detailed images of tissues within an intact cochlea. The images are produced from measurements of the inner ear's structure and the incredibly small vibrations within the cochlea, Applegate says.

Though the technology has been primarily used in animal models to date, it's already resulted in the first vibration measurements from the apex of an unopened mouse cochlea, allowing researchers to image the portion of the cochlea responsible for low frequencies. Since mammalian hearing is similar across species, the model allows the researchers to use the technology on a hearing system similar to the one in humans, Applegate notes.

Among the team's findings is evidence suggesting different areas of the cochlea are responsible for different things, Applegate says. Specifically, gain (the amplification of sound) and narrowing of the frequency band (which enables a person to zero-in a specific sound) take place in distinct areas, he explains. Up until now, these functions have been thought to be closely linked, possibly taking place in the same location, he notes.

"What we've found is that while the outer hair cells inside the cochlea generate gain, some of the nearby supporting cells are largely responsible for the narrowing of the frequency band," Applegate says. "This information contributes to our understanding of the morphology of the inner ear as well as its mechanical functions so that therapies might be developed in the future—but before that can happen, we have to understand what it is that we’re trying to fix."

Towards that goal, the OCT technology employed by Applegate generates huge amounts of data about the cochlea. Thousands of measurements are taken from myriad points throughout the cochlea, resulting in gigabytes—and sometimes terabytes—of information that must be processed and interpreted in order to produce images, he explains.

"All of this requires a fair amount of math to generate and interpret the results," Applegate says. "The signal is digitized and passed to something called a field-programmable gated array (FPGA), where initial processing is done before sending it to the CPU. We collect two channels of data, but end up only passing one to the CPU because of the data reduction on the FPGA."

Data reduction, Applegate explains, is an important time-saving part of the process made possible by equipment produced by National Instruments, which provides test, measurement, and embedded systems for engineers and scientists. With the aid of the equipment, the data generated from measurements within the cochlea is processed in a couple of minutes, Applegate says.

Encouraged by their results, Applegate and his team have developed a prototype device for use on humans. The device, a hand-held instrument, enables a researcher or physician to pass a probe through the ear canal and tympanic membrane to shine a laser through a thin membrane located on the cochlea, where they can then image the inner ear tissues with the same technology.

Full details of the team's work appear in the Journal of Neurophysiology; for more information, please visit http://dx.doi.org/10.1152/jn.00306.2014.

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