Light scattered in tissue usually ceases to be a useful source of information. But what if, instead of forcing it to travel along a single path, we allowed it to wander a little differently each time? Scientists from ICTER tested this seemingly risky idea and showed that a series of measurements that differ from one another can produce a clearer image than a single frame.
The new method was developed by scientists from the International Centre for Translational Eye Research (ICTER), part of the Institute of Physical Chemistry of the Polish Academy of Sciences. The study's first author is Klaudia Nowacka-Pieszak, MSc Eng. She worked with Piotr Kasprzycki, PhD; Patricio Espinoza Guevara, MSc; Karol Karnowski, PhD; and Dawid Borycki, PhD, DSc.
The team combined a mathematical model, computer simulations, and experiments using optical coherence tomography (OCT). The researchers wanted to determine whether controlled changes in the way light is scattered could reduce speckle noise and make images less sensitive to selected optical aberrations. The findings are described in the article "Reducing spatial coherence via dynamic scattering media enables aberration and speckle suppression in optical imaging" , published in "Scientific Reports".
The same object, different distortions
Light travelling through tissue encounters cell membranes, fibres, blood vessels and boundaries between structures with different optical properties. Some rays change direction, while the phase of the light wave is disrupted. As a result, the image may lose sharpness, become distorted or be covered by a grainy texture.
This texture is known as speckle noise. It arises when waves scattered by different parts of a sample overlap. In some places they reinforce one another, while in others they cancel one another out. The resulting image contains a random pattern of bright and dark spots that can obscure fine structures.
Speckle is not ordinary electronic noise; it arises from the properties of the light being used (in this case, coherent laser light). Speckle is intrinsically linked to the use of spatially coherent light. Without coherence, it disappears, but so does the ability to perform interferometric imaging such as OCT, in which coherence is used to select photons that carry structural information.
The method developed by the ICTER team does not try to provide identical conditions for every measurement. It does the opposite: it deliberately changes the way light reaches a stationary object. Each frame therefore contains a slightly different arrangement of speckle and distortions. If successive images are sufficiently independent of one another, they can be averaged. The fixed structure of the object is repeated and preserved. Random artefacts shift position, so they gradually weaken as the data are combined.
"Scattering is usually seen as the enemy of high-resolution imaging. We show that, when it is carefully controlled and made dynamic, it can actually become a useful tool: by changing the way light reaches the sample from one measurement to the next, we can suppress speckle and reduce image distortions. This opens a new way of improving the robustness of optical imaging without relying on increasingly complex optical hardware," said Klaudia Nowacka-Pieszak, first author of the paper.
A Layer That Sets the Image in Motion
The diversity of measurements was provided by a dynamic scattering medium (DSM), which produced small, time-varying changes in the direction and phase of the light. Each successive measurement therefore shows the same object illuminated by a slightly different light wave. The differences cannot be too large. Excessive scattering would destroy the useful signal and severely blur the image. A balance is needed: the variation must be sufficient to generate independent speckle patterns, but it must not remove information about the object.
The dynamic scattering medium reduces the effective spatial coherence of the light. High spatial coherence is necessary for interferometric measurements, but it also promotes persistent speckle. The dynamic layer changes the phase relationships between successive measurements, allowing the random component of the image to be weakened through averaging.
