MHz OCT: Why faster is better

When we first used optical coherence tomography (OCT) in the early 90s, images were pixelated and even the retina borders sometimes difficult to distinguish. Technology was time domain, enabling approximately 600 single A-Scans per second. Generating retina maps at this speed required considerable interpolations. Nevertheless, this technology allowed novel insights into the retina and thus started to be implemented into clinical routine.

By about 2003, spectral domain OCTs started to improve scan speed to 20000–50000 A-scans per second. This enabled denser sampling in the X–Y plane and features such as 3D presentation of data cubes. Especially when combined with eye-tracking, this spectral domain technology allowed for clearly improved repeatability and follow-up examinations. Therefore, this technology has become standard for clinical evaluation and follow-up today.

Shortcomings remain

Another relevant point appears to be fundus image quality: When obtained by simple infrared camera imaging, image quality is usually not very good, while adding modalities like a scanning laser ophthalmoscope (SLO) or photo adds to system complexity and system costs.

There are several reasons, why in the future ophthalmic OCT technology may move from Spectral Domain OCT to systems based on wavelength swept lasers (swept source OCT: SS-OCT). One argument is the dramatically higher imaging speed that can be achieved with SS-OCT, especially using so called FDML lasers.

FDML lasers

Fourier Domain Locking (FDML) of lasers for swept source was first published in 2006 by Dr R. Huber at J.G. Fujimoto's lab and he continued to work on this new type of laser. One of the major advantages of this technology is that scan speed in the megahertz range is feasible today.

Leading a laser and imaging group at the faculty of physics (Institute for Biomolecular Optics, BMO) of Ludwig-Maximilians-University in Munich, Dr Huber developed several OCT applications based on this swept source FDML approach. In collaboration with the department of ophthalmology, we jointly defined a system capable of imaging up to 60° × 60° by OCT without pupil dilation. Given the current scan speed of ~1.68 MHz, this system is >40 times faster than usual spectral domain OCT systems today.