Removing Noise in OCT

Seeing small objects more clearly, with less speckle.

Optical coherence tomography (OCT) is a non-invasive three-dimensional clinical imaging method that uses a coherent light source, a laser, to image different depths into tissue. However, speckle noise – an imaging artifact due to scattering and interference of coherent light – has been unavoidable and has limited the capabilities of OCT. Now, at team led by Adam de la Zerda at Stanford University has implemented a clever trick to remove speckle noise, opening up the full diagnostic potential of OCT.

Depth imaging with OCT is enabled by a low-coherence, broadband, laser source; half of the laser light is sent into the tissue and is then scattered back out, while the other half of the light travels the same physical distance but not in tissue. Then, similar to holography, the two beams are combined. The short temporal coherence of the laser requires that the two different paths must be the same distance if they are to interfere and form an image, giving depth resolution. The challenge is that biological tissue is turbid and surfaces are rough which gives rise to speckle noise – interference that causes some points within an image to appear dark and others bright.

To take control of the speckle noise, the team used a moving piece of ground glass to modulate the distance that each light ray traveled both before entering and after being scattered by the tissue. By moving the glass at the correct speed, the researchers changed the speckle noise pattern randomly between each sequential image acquisition. The speckle noise was then removed by averaging the images, leaving a low-noise, high-resolution, three-dimensional OCT image.

OCT is already a widely-used clinical diagnostic imaging tool for retinal diseases and skin cancers. By removing speckle noise, researchers have imaged small features which are generally unresolvable, as exemplified by human fingertip sweat ducts shown above. As an easily implementable addition to current OCT systems, perhaps you may see this in your doctor’s office one day soon.

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Reference & Image Credit

Liba, O. et al. Speckle-Modulating Optical Coherence Tomography. Nature Communications http://dx.doi.org/10.1038/ncomms15845 (2017)

More about coherence and phase.

Fun fact: speckle makes stars twinkle.