Posts with the tag: coherence
Optical microscopy can only penetrate a few hundred microns into thick tissue, a limit imposed by scattering. High resolution imaging requires single-scattering events, so when we have multiple-scattering from particles above and below the focal plane, the resolution and signal to noise ratio quickly degrade. The thicker the tissue (i.e. the deeper the plane of interest) the more the multiple-scattering events dominate over single scattering. Techniques such as optical coherence tomography (OCT) enhance the penetration depth by rejecting multiple-scattered light using what is effectively a time-of-flight measurement. This works because light that has been scattered multiple times will tend to have travelled further than light that has been scattered only once. However, even with this technique, the penetration depth seldom exceeds 1-2 mm, as some multiple-scattered photons will (by chance) have a time of flight close to that of the single scattered photons. As we try to go deeper, these events will begin to dominate again. Now, a group mainly from Korea University in Seoul have suggested an additional method of discriminating between single and multiple-scattered photons, using a technique they call “collective accumulation of single-scattered waves”.
Optical Coherence Tomography (OCT) has a lot of advantages over confocal microscopy, especially for applications where it’s useful to have a large working distance between the probe and the tissue. But a big limitation is that it can only detect reflected light, and so can’t be used with fluorescent stains. Fluorescence is often preferred in conventional microscopy because it allows us to visualise structures that we can’t easily identify in reflectance images. So the race is on to find a way to make OCT work with fluorescent emission as well as reflected light.