Scientists have worked for many years to improve microscopes for clearer views of brain activity, reaching not just the outer layer but deeper areas like the hippocampus, which is a brain region important for memory. This helps in both research and medical fields by showing how cells work inside living tissues. In a recent study, researchers from MIT created a new microscope that looks very deep into brain samples to spot activity in single cells using sound waves.
The main improvement is imaging at the level of individual cells much deeper than before. The researchers showed the system could detect NAD(P)H, a molecule linked to cell metabolism, and especially to electrical signals in neurons (brain cells that send messages). This was done in a 1.1 millimeter thick cerebral organoid, which is a small 3D model of brain tissue grown from human stem cells, and in a 0.7 millimeter slice of mouse brain.
The microscope went as deep as the samples allowed, and researchers believe it can go even further in larger tissues. This depth is over five times better than other methods for seeing NAD(P)H in thick brain tissue. The system works without adding any labels, meaning no extra chemicals or genetic changes to make things glow.
How the system achieves depth
It uses several advanced methods together. Instead of regular light, it sends very short bursts of light at a longer wavelength to excite NAD(P)H through three-photon excitation, where three light particles combine to activate the molecule. This longer light scatters less in tissue, like how fog lights cut through mist. The excitation causes a tiny heat expansion in the cell, creating sound waves that travel easily through tissue. A sensitive microphone picks up these waves, and software turns them into sharp images, a process called photoacoustic imaging.
The researchers combined this with third-harmonic generation imaging, which shows cell structures clearly. The method could also detect other molecules, like those used to track neuron firing.
Looking ahead, this label-free multiphoton photoacoustic microscopy might help study diseases like Alzheimer's, where NAD(P)H levels change, or guide surgeries. The next goal is testing in living animals, where the setup must adjust for the microphone placement.