Integrated Photonics helps Optical Coherence Tomography to Set New Non-Invasive Standard

Optical Coherence Tomography (OCT) is a relatively new method of examining living tissue, which uses light instead of needles and incisions. Current OCT systems are rather cumbersome and involve a trolley laden with equipment being pushed through the hospital. This is expensive, unwieldy and unsuitable for a family doctor to carry out a quick check. The Academic Medical Centre of the University of Amsterdam (AMC), among others, is keen to miniaturise the trolley by using integrated optical circuitry.

OCT, the optical analogue of ultrasound imaging, provides high resolution imaging of living tissue to a depth of 1-3 millimetres. Because light is used, the depth information of the back-scattered light is obtained by low coherence interferometry. Currently, two methods are used, both based on the fact that the depth information from back-scattered light can be obtained by detection of the wavelength encoded interferometric signal. One method makes use of a broadband laser light source of which the back-scattered light is detected by a spectrograph equipped with a detector array. In the other method, the complexity of the system is the use of a fast scanning laser source, which bombards the tissue with light at successive wavelengths. At the detector, the wavelength information of the interferometric signal is decoded in the time, and is therefore smaller.

Bulky lab equipment

In both cases the lab equipment is particularly bulky. Classic optical building blocks, such as lenses, diffraction gratings, interferometers and mirrors expand the size of the equipment. Couldn’t it all be made smaller, more portable and less expensive? Couldn’t the equipment, in some cases, be reduced to the size of a pocket book or even a postage stamp?

Yes, that must be possible, according to the AMC, Twente University, the Technical University of Eindhoven and the companies Lionix, XiO Photonics and VTEC Eindhoven and 2M engineering. In roughly a year’s time they aim to demonstrate an OCT set-up approximately ten times smaller in all dimensions. This size reduction is enabled by the use of small, integrated, optical circuits, in which items such as optical wave-guides are incorporated. These photonic integrated chips manipulate light on a much smaller scale than lenses, mirrors and prisms.

The holy grail

The consortium was recently successful in integrating a spectrograph in a chip the size of 3-6 cm2. A further success was the integration of a laser source that emits light at various frequencies in quick succession. In fact, the consortium’s aim is to transport and manipulate the light as much as possible within the integrated optical circuitry itself. Glass fibre cables are only needed to transport light to and from the circuitry. The holy grail is a measuring device with optical chips that can be stuck onto tissue like a plaster, which would completely eliminate the need for glass fibre cables.

The consortium’s approach opens up a whole treasure trove of possibilities for alternative applications. With a spectrograph on a chip and the other building blocks currently in development, a technology such as fluorescence scanning literally loses mass. Fluorescence scanning is used to identify tumours and is an invaluable tool in crime scene investigations. In an ideal world, the technology would be so portable and inexpensive that a forensic detective could carry out blood spatter research and fingerprint analysis on site. The age of a fingerprint is currently determined through the use of substantial measuring equipment. In the future, perhaps, all that will be needed is a small device in the detective’s pocket.

Thinking outside the box

Miniaturisation would also relieve the burden on overstretched hospitals. In the near future, family doctors might be able to investigate skin blemishes themselves. And industries could also benefit from the technology; for example, in the detection of particle movements in liquids, for which bulk methods are currently employed.

And then there are numerous applications which no-one has yet imagined, e.g. measurements in places that are currently inaccessible to imaging equipment. Over the coming years, thinking outside the box will undoubtedly bring new possibilities to light, both figuratively and literally.

Source: Biomedical Engineering and Physics Academic Medical Center and RVO.nl