When the heart contracts, it pushes blood into the brain, and the arteries inside the head briefly swell, like a garden hose under water pressure. This motion is transmitted to the brain tissue, and the brain pulsates ever so slightly with each heartbeat. The displacements are tiny, about the thickness of a hair, but they can influence how the brain clears waste and how drugs spread through it.
Scientists wanted to find out what affects this pulsation most: the bends in the vessels, the thickness of their walls, or the softness of the brain itself. They built a computer model based on MRI scans of a healthy woman and validated it against data on brain deformation during the cardiac cycle. It turned out that winding arteries, especially the internal carotid, deform the brain more, while thick walls, on the contrary, damp the motion, like a pillow softening a blow.
Previously it was unclear why the brain pulsates in this particular way, but now it's clear: the geometry of the vessels is to blame. This knowledge will be useful to surgeons operating on vessels with aneurysms or narrowings, and will also help predict how a drug will spread through the brain.
With every heartbeat, the brain shifts by roughly 0.1–0.3 mm, comparable to the thickness of a human hair, yet this deformation is enough to influence substance transport in perivascular spaces.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.