A deep tissue, high-speed, 3D, time-resolved holographic microscope developed by a team from the Center for Molecular Spectroscopy and Dynamics at the Institute for Basic Science (Daejeon, South Korea) allows researchers to view a mouse’s neural network under its intact skull, which is about .2 mm thick.
The team’s microscope builds upon technology they created several years ago to allow them to see through the intact skull of a live zebrafish to observe its neural network (see video).
Their new holographic microscope eliminates the multiple scattering and aberration that occurs in living tissue when light hits the cells, which makes it difficult to obtain sharp images. Correcting light’s wavefront distortion reflected from the target being observed is necessary (because multiple scattering hinders it) to enable high-resolution, deep-tissue imaging. Removing the multiple scattered waves and increasing the ratio of single scattered waves can also improve this type of imaging (see Fig. 1).
“We can solve inverse scattering problems at the microscopic level of scattering events and identify the time-dependent modes of the scattering media,” says researcher Wonshik Choi, director of the Center for Molecular Spectroscopy and Dynamics. “In doing so, we can achieve super deep imaging at microscopic resolution and deliver light energy that is two orders of magnitude, or more, larger than existing methods.”
Their holographic microscope also measures the amplitude and phase of light, and selectively acquires optical signals at a specific depth via a light source with a very short interference length of about 10 µm.
The researchers used a wave correction algorithm to allow them to select single scattered waves. This algorithm also analyzes the eigenmode of the medium—a normal mode of vibration of an oscillating system, which features a unique wave that delivers light energy into the medium—to find a resonance mode that maximizes constructive interference, which happens when waves of the same phase overlap between wavefronts.
Choi says this allowed their microscope to focus more than 80 times of light energy, while simultaneously removing unnecessary signals. So the ratio of single vs. multiple scattered waves increased by several orders of magnitude. The team achieved in-depth imaging of the mouse’s neural network in the visible wavelength range without altering its skull or using fluorescent labels (see Fig. 2). This isn’t possible with conventional techniques.
“From basic principles to practical application of observing the neural network beneath a mouse’s skull, we’ve opened a new way for brain neuroimaging convergent technology,” he says. “For a long time, we’ve developed super deep imaging technology based on physical principles. It is expected that our present finding will greatly contribute to the development of biomedical interdisciplinary research, including neuroscience and the industry of precision metrology.”
