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21 May 2007
A new laser source has enabled US and German researchers to build an OCT scanner that is ten times faster than commercial versions.
Optical coherence tomography (OCT) has become a powerful technique for imaging the surface of the retina, but today's commercial instruments cannot scan the eye fast enough to generate dense 3D datasets. This new device, unveiled at this year's CLEO conference, can scan the eye some 10 times faster and so could pave the way for high-speed 3D retinal OCT scanners.
OCT is an optical inteferometric technique that enables biological samples to be imaged non-invasively with high spatial resolution. The technique's ability to obtain structural information without damaging the tissue has already made it a firm favorite for retinal imaging.
The latest breed of OCT scanners exploits a photodetector in combination with a tunable or swept light source. Such lasers usually exploit a broadband gain medium together with a tunable optical band-pass filter inside the cavity. But even though the filter characteristics can be changed extremely rapidly, the rate at which the frequency can be tuned is limited by the time taken to build up laser activity inside the cavity.
As a result, most commercial OCT scanners scan at speeds of about 25 kHz. Most people cannot keep their eyes still for much more than one second, which means that existing ophthalmic OCT scanners are not fast enough to generate densely sampled 3D images.
In contrast, the new instrument, developed by researchers at MIT and the Ludwig Maximilian University (LMU) in Germany, can achieve a speed of 250 kHz, which should be fast enough produce a highly accurate 3D image of part of the retina.
At the heart of the MIT-LMU instrument is a new Fourier Domain Mode Locked (FDML) laser source. As with other tunable lasers, the filter of the FDML laser has a period over which different wavelengths are transmitted in a specific order.
"The uniqueness of the FDML laser is that the optical round-trip time is exactly synchronized to the filter tuning period," LMU's Robert Huber told optics.org. "We have an extremely long cavity (typically several km), and the wavelength that is transmitted through the filter at any given time travels through the length of the entire cavity and returns to the filter exactly after one tuning period has elapsed. Lasing does not have to build up repeatedly like in conventional lasers and thus FDML removes limitations on the achievable tuning rate."
The first FDML laser, which operated at 1300 nm, was demonstrated by Huber in 2006. For retinal imaging purposes, the device had to be modified to work over a wavelength range of 1040-1100 nm.
At CLEO, Huber showed that the current prototype can scan a 4x4 mm region of the retina in just 0.87 seconds, and with an average depth resolution of between 10-15 µm. But much more work is needed to commercialize the technology. "Clinical studies need to be conducted to show if the image quality is sufficient and potential advantages for diagnosis have to be identified," explained Huber.
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