Posts from: April 2015
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”.
Recently, there has been a lot of interest in the application of optical coherence tomography (OCT) to retinal surgery. While OCT is already established as a tool for diagnosis and pre-surgical planning, the idea of imaging during the surgery itself hasn’t found much traction. Initially, this was partly due to the lack of commercial OCT systems that were well-integrated with ophthalmic microscopes. This meant that the surgery had to be halted, the ophthalmic microscope removed, and the OCT slid into place every time an OCT image was wanted. More recently, Carl Zeiss and Haag-Steit have begun marketing devices where the OCT is integrated into the surgical microscope, so that both can be used simultaneously. The OCT images can then be displayed to the surgeon in the microscope view. However, the authors of a recent paper in Biomedical Optics Express claim that these integrated OCT systems are still not ideal. Instead, they propose an OCT scanner which is built into the surgical instrument itself.