Adding milk to an optical imaging system can help recover details obscured by a thin layer of biological tissue, researchers have found, in an experiment that used a slice of mouse brain to test a new way of reducing image noise.
Scientists from the International Centre for Translational Eye Research (ICTER) and the Institute of Physical Chemistry of the Polish Academy of Sciences used ordinary milk to reduce interference that can obscure fine details in optical coherence tomography (OCT), a technique used to create cross-sectional images of tissues, including the retina. Their findings were published in Scientific Reports.
The method exploits a phenomenon known as laser speckle, a grainy pattern of light and dark spots produced when light waves travelling along different paths overlap. Although OCT relies on the interference of light waves to measure structures with high precision, the resulting speckle can obscure details. Imperfections in optical systems and biological tissue can further distort the light, degrading image quality.
Rather than trying to eliminate these effects in a single measurement, the researchers collected multiple images with slightly different interference patterns and averaged them. Because the target remained stationary while the speckle and some of the distortions changed between measurements, combining the images helped suppress unwanted noise.
The team used milk as a dynamic scattering layer. Its microscopic components are constantly moving, causing light passing through it to scatter differently over time. The researchers placed a flat test target featuring increasingly fine lines in the milk and performed 100 spatial OCT scans, selecting images corresponding to the depth at which the target was located.
They combined the images using two methods. The first relied mainly on signal brightness, while the second also used phase information, which describes the relative alignment of light waves. The phase-based method removed background noise more effectively while better preserving contrast, but it required precise alignment of successive measurements.
Without an obstruction, however, adding milk made the images worse. Contrast fell from 0.87 to between 0.81 and 0.84, depending on the method used, while lateral resolution deteriorated from 4.6 micrometres to approximately 7.5–7.7 micrometres. Reducing speckle therefore came at the cost of some sharpness.
The advantage emerged when the researchers placed an obstacle between the imaging system and the target. In the first test, adhesive tape distorted the light passing through it, making the target's lines difficult to distinguish in a single image. When the researchers added milk and averaged multiple measurements, the pattern became much clearer.
They then replaced the tape with a fixed slice of mouse brain measuring 250 micrometres thick. The target's known pattern allowed them to identify which details disappeared as light passed through the tissue and which became visible again after the measurements were combined.
When the procedure was repeated with milk, some of the previously obscured lines reappeared. The results showed that controlled scattering could help mitigate interference caused by a thin layer in the light's path.
However, the experiment does not establish that the technique can produce more accurate images of biological structures themselves or improve scans of a living eye.
Nor can averaging recover information that has been completely lost. If individual images are too blurred, combining them will not restore the missing details. The tests also involved a stationary target and fixed tissue, with 60 seconds between successive scans, rather than a moving biological structure.
Calculations suggested that a new speckle pattern could form in as little as 0.4 seconds. Even so, capturing and combining enough images quickly enough to image a moving eye remains a challenge.
One possible next step would be to replace the milk with a small chamber containing a rapidly moving scattering medium attached to the microscope objective. Whether such a system could operate quickly enough for clinical use remains to be established.
The study suggests that introducing controlled scattering could help recover details obscured by biological tissue under certain conditions. But any improvement in visibility comes with a trade-off in sharpness, and further work is needed to determine whether the approach could be useful in practical medical imaging.
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