FEATURE ENDOSCOPY
➤ about the tissue they scatter through, enabling an image of the tissue to be produced after the photons are detected by a sensitive GaAsP photomultiplier. Groot’s team is currently using an optical parametric oscillator pumped by a Coherent titanium-sapphire laser to produce the photons required for second and third-harmonic generation. The laser operates using 200-250 femtosecond pulses at a wavelength of 1,200nm to generate photons with sufficient energy to carry out the process. However, the laser is currently too large and expensive to be used for endoscopy, according to Groot. ‘In order to make this handheld endoscope commercially available, the laser needs to be made smaller, cheaper and easier to handle.’ This is where a compromise will have to be made in order to reduce the size of the photonic components. ‘Ideally, we would like to use a fibre laser to produce the desired wavelength; however, they are not currently available at 1,200nm,’ Groot continued. ‘One option is to use a fibre laser that produces photons at 1,560nm. There are at least two manufacturers that make these lasers, which are far smaller than our current laser.’ Groot’s team has applied for a Horizon 2020 grant with the European Union, which if successful, will be used to start developing the endoscope using a 1,560nm laser. ‘However, we are hoping a company will produce a 1,200nm fibre laser, as these are the ideal wavelength photons that we would like to use in this technique,’ Groot noted. ‘1,200nm provides the optimal penetration depth. It’s a cut off between tissue penetration and the amount of scattering.’ In order to penetrate the tissue deep enough to scatter photons off the target area, Groot’s team is attaching a needle to the handheld device. ‘A needle is the ideal way of miniaturising this set up, as we require a rigid channel to deliver the photons, not only to the surface of the tissue, but deep inside the tissue if the diameter is small enough,’ Groot explained. Currently, the table-top system can quickly scan a 0.5 x 0.5cm excised piece of tissue using an ex-vivo microscope. The system produces images in seconds, with a quality comparable to standard pathology techniques that can take several hours. ‘When I showed these images to the pathologists that we work with, they were amazed,’ Groot remarked.
In attaching the needle, however, a further
compromise will have to be made in order to miniaturise the optical set-up of this system. ‘The
16 ELECTRO OPTICS l JUNE 2016
A microlens assembly from Qioptiq
We are hoping a
company will produce a 1,200nm fibre laser… 1,200nm provides the optimal penetration depth
drawback of this needle is that the field of view of the images produced is always limited. The field of view from this needle could never be larger than 300 x 300µm.’ Despite this limitation, this endoscope could be an invaluable addition to the operating room, allowing surgeons to identify tumour borders during surgery. A graded-index (GRIN) lens is used within the device to focus incoming photons along the needle so that harmonic generation can occur at the target area. A certain fraction
of photons is scattered back within the same GRIN objective, back along the needle. Groot’s team recently described their work with the table-top system in Biomedical Optics Express. ‘In the publication we show that the GRIN lens can not only be used to focus the photons strongly enough to generate a signal, but it is also efficient enough to collect the signal back in that direction,’ Groot said.
Her team has used optical components with specific material properties to produce this endoscope. Conventional optical fibres cannot be used in the device to deliver photons to the end of the needle, as third-harmonic generation will occur within the fibres themselves. Fibres with a very high threshold for these non-linear effects, so must be used in order to prevent them. ‘GRIN lenses are used as they also do not produce these non-linear effects, they are able to focus the light down the needle without unwanted third- harmonic generation,’ Groot explained. In the publication in Biomedical Optics Express, Groot’s team used a needle of 7.5mm; however they are now experimenting with lengths up to 180mm. With this optical technique working for a variety of length needles, the handheld endoscope will be adaptable to whatever needle is required to deliver the photons to the required depth. Groot and her team could start developing the endoscope in 2017, should their application be accepted. ‘We think that the device would be ready for commercial development in 2021,’ Groot said. l
@electrooptics |
www.electrooptics.com
Qioptiq, an Excelitas Technologies Company
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