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applications optical coherence tomography

to almost perfectly synchronise the filter with the optical round trip time.’ But as Huber points out, there are many

more challenges in this area of research than the technology alone. While the researchers have demonstrated unprecedented performance, it is debated in the industry as to whether this kind of speed is actually required. ‘The higher the speed, the less signal and the more complex the system,’ says Huber. ‘We have to demonstrate the benefits of high speed OCT clearly for different applications to enable a range of new commercial applications and markets.’

Ultra high speed OCT imaging has distinct advantages over lower speed systems. It allows transient dynamics to be resolved in real time and enables dense sampling of in vivo samples where OCT acquisition times are usually restricted to a few seconds due to motion. Using slower systems, OCT images are restricted to small areas or dense sampling is not possible. Dense sampling not only increases the amount of recorded information, but also allows for advanced averaging methods in post-processing. This is a considerable advantage over other averaging methods that require fixing of the averaging area prior to data acquisition. ‘We believe that FDML lasers offer better speed performance in the 1050nm, 1300nm and 1550nm wavelength range than line scan cameras or current commercially available short cavity swept lasers,’ says Huber. 1050nm light can be used for retinal imaging, because of the moderate water absorption in this wavelength range. 1300nm is the wavelength of choice for imaging in highly scattering tissue and is used for intravascular imaging. 1550nm light has potential in industrial OCT applications, because it is very eye safe.

Huber and his colleagues used the FDML laser to obtain the first dense three dimensional ultra- wide-field OCT images of human retina in vivo. In ophthalmology special cameras, called fundus cameras, are used to image the interior surface of the eye. Scanning laser ophthalmoscopes can also be used. In a single shot, the Munich group’s setup achieved a 70° field of view. From the 3D OCT data, an en face fundus view can be reconstructed, which exceeds the quality of fundus cameras and is approaching that of scanning laser ophthalmoscopes. ‘Compared to our setup, commercial OCT systems capture fewer than 5 per cent of the interior surface of the eye,’ says Huber. ‘This increased coverage of the investigated area of our system will lead to improved diagnosis for the patient in the future, because the probability of missing pathologies is reduced.’

excitation. The degree to which a particular wavelength of light is absorbed depends on the type of tissue and, in the case of blood, on whether it is oxygenated or deoxygenated. PAT can be used in conjunction with a contrast agent, but in blood haemoglobin acts as a natural contrast agent. Drexler and his colleagues were inspired to combine PAT with OCT when they heard about a unique transparent ultrasound sensor that has been developed by Professor Paul Beard and his group at Imperial College London in the UK. ‘Researchers at Imperial had developed a Fabry-Perot polymer film ultrasound sensor, which is highly transmissive between 600nm and 1200nm, but highly reflective between 1500 and 1700nm,’ explains Drexler. ‘This makes it possible to send two laser beams

through the sensor – one for PAT and one for OCT. While PAT gives information about blood vessels, OCT gives information about tissue morphology. This makes the two techniques a perfect combination.’

Using this technique the Vienna and Imperial teams have obtained amazing 3D in vivo images of mouse skin and human skin. The images show the vasculature and the micro-morphology of the skin. Potential applications include characterising skin conditions such as tumours, vascular lesions, soft tissue damage such as burns and wounds, inflammatory conditions such as dermatitis and other superficial tissue abnormalities. While PAT and OCT are complementary, combining them in one system was a technological challenge.

combining oct with photoacoustic tomography to image skin (in this case mouse skin) can give information about the structure of the skin as well as the blood vessels beneath it. Images courtesy of Medical University Vienna

Huber and his colleagues are currently investigating the benefits of the high OCT imaging speed offered by FDML technology. They are also developing a comprehensive physical model of the FDML laser, which is as yet not theoretically fully understood.

Working with sound and light Meanwhile, just across the German border in Austria, Wolfgang Drexler and his colleagues at the Medical University in Vienna are developing a system that combines photoacoustic tomography (PAT) with OCT.

While OCT is sensitive to scattering, PAT is sensitive to absorption. In PAT, a laser beam is used to produce ultrasound waves in a biological sample through absorption and localised thermal

14 electro optics l December 2011/January 2012

‘This technique might also be applied endoscopically in the future,’ says Drexler. ‘But that is a huge challenge. Just as the other research groups working as part of the FUN OCT project, our aim is for this combined technique to give additional information and aid in the diagnosis and treatment of a variety of conditions. For example, it could be used to perform optical biopsies, giving an answer within minutes rather than days with performance levels approaching those of histopathology.’ While the FUN OCT project is scheduled to come to an end in April, the research into functional OCT is ongoing. With the OCT ophthalmology market now mature and the cardiology market beginning to form, OCT has already overcome a major hurdle in the tough medical market – acceptance. But for many applications, OCT on its own is not enough. This is why many see combining OCT with other complementary technologies as the answer to its survival. l

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