Signals from Noise

by Scott Mattison (@FoolsPizza)

Imagine paying almost $20 to go to see the newest Marvel movie. The previews finally end and your movie starts, only parts of the screen are randomly dark. Likely, you would be upset and would either ask the theatre to fix the error or demand your money back. What if I told you the lasers that provide the light for imaging technologies like confocal microscopy and two-photon fluorescence microscopy had that exact problem?

Lasers enable scientists to easily capture incredibly detailed images of biological tissues and cells that were previously challenging, if not impossible, to achieve. One of the earliest challenges that had to be solved when using lasers for biological imaging was how to reduce an effect referred to as “speckle”.

If you have a laser pointer at home, you can observe speckle just by shining it at the wall; if you look at the spot made by the laser, you will see some areas that are bright and other areas that are dim. This is speckle!

Speckle is the result of two paths of light interacting with one another. In some cases, the two paths combine to make a brighter light whereas in other cases the two paths combine and cancel each other out. More specifically, this interaction is called interference. Interference causes speckles that appear as a grainy pattern of bright and dark spots.

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A simulated example of speckle originating from a laser beam illuminating a wall

As you can imagine, when you are trying to capture a detailed image of the inner workings of a cell, speckle is not desirable as this grainy pattern can degrade your image quality. In this regard, we consider speckle to be noise within our images. Luckily, scientists and engineers have worked over many years to find creative ways to reduce and remove speckle in imaging applications. As cool and interesting as a lot of these methods are, I am not  here to talk about how we can reduce speckle in our images, I am here to talk about how we can utilize it. However, before we can discuss how we can use speckle to our advantage, we need to know a bit more about it first.

When a beam of light interacts with a rough surface, light that bounces off this surface and the different paths will interfere with one another, creating a random speckle pattern. If neither the light source nor the rough surface move, the speckle pattern will remain constant. Any movement of the rough surface will cause corresponding changes to the speckle pattern that is generated. Now, we can start to see how we can utilize speckle to our advantage.

By tracking changes in speckle patterns, researchers can determine the movement of a sample over time. This technique has been used to monitor how tissue reacts when a specific force is applied. From this, properties related to the tissue such as its strength or how well it recovers after being changed can be determined. This approach has the potential to allow doctors to differentiate between healthy and cancerous tissues or identify unhealthy regions of blood vessels.

Tracking movement of tissues isn’t all speckle can do. By simply observing changes in speckle over time, researchers have demonstrated that we can actually tell the difference between the movements of liquids and solids. This has led to amazing techniques for mapping out small networks of blood vessels within the body, and has even allowed researchers to image blood flow in the brain!

To me, speckle is an awesome example of what makes research so powerful. We had this noise source in our images that was really slowing down progress in research. Instead of just finding a way to solve this problem (which we did), researchers have found a way to take that noise and make something useful out of it, a signal.

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