Technical Articles

Image Processing 4D-tomography with MHz-OCT: Optics goes big data

Thanks to a new laser technology, OCT imaging speed can be increased by more than one order of magnitude. This enables real-time video-rate investigation of three-dimensional sample structure.

Optical coherence tomography (OCT) is a non-contact, non-destructive optical imaging technique. OCT scans the inner structure of many different samples, similar to ultrasound, but with a much higher resolution of a few micrometers. Thus, considerably finer structures can be visualized compared to ultrasound. Moreover, no potentially harmful radiation, such as is the case for computed ­tomography (CT), is needed. The achievable image quality can be comparable to cross-sections in histology, but is obtained entirely non-invasively. Since its first description in 1991 [1], and with the stated advantages, OCT has now become an established optical imaging technique, and is standard, for instance, in ophthalmology [2].

Methods such as triangulation or chromatic-confocal sensors are frequently also called “3D imaging”, as they measure the sample’s surface shape. However, they do not provide data from inside the sample, so the full 3D structure of the sample cannot be resolved. OCT is cheaper, more compact, and is faster than both CT and magnetic resonance imaging (MRI). Due to its lower penetration depth, OCT is not a direct competitor for these techniques, but it does have the advantage of no side-effects. Compared to these other approaches, OCT has a unique combination of resolution and imaging depth. For instance, stacked data from confocal microscopes suffers from limited imaging depth. Thus, OCT occupies a hitherto inaccessible area in the resolution vs. image penetration plane ( figure 1, left).

Technology and performance parameters

The optical setup of an OCT system is similar to the setup of a confocal scanning-laser microscope – the probe is successively raster-scanned by a laser beam, and at every sample location a depth profile is recorded via the back-scattered signal‘s time-of-flight. Light from deeper sample locations needs more time than light which is reflected from the surface. As the speed of light is orders of magnitude larger than speed of sound, time-of-flight differences cannot be measured directly. Therefore, OCT measures the interferometric superimposition between back-scattered light from the sample and a reference signal from a mirror. However, whereas for confocal scanning the depth resolution is only supplied by an appropriately positioned spatial filter, in OCT it is the interference of the light itself that provides a route for assigning depth information to the acquired signal.

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