Every time an MRI scanner measures water movement in tissues, it sends a magnetic pulse in a specific direction. One pulse gives one data point. To understand how water moves in all directions at once, you need many such pulses with different rotations. Previously, rotations were chosen almost at random, just to cover all directions evenly. But the signal has a hidden symmetry: if you rotate the pulse by 180 degrees, the signal doesn't change. It's like a square tile: no matter how you turn it, the pattern stays the same.
The authors of the new study used this symmetry to choose rotations not randomly but according to a strict rule. They called their method "geometric filter optimization" — essentially, it's a sieve that sifts out redundant rotations. With such a sieve, the average signal becomes more accurate even if you take fewer measurements. The method works especially well for elongated and flattened shapes of the magnetic field used in modern scanners.
As a result, scanning time can be reduced while obtaining more reliable diffusion maps. This matters in diagnostics, where every minute counts, and accuracy determines whether a doctor notices early changes in tissues.
Rotating the magnetic pulse by 180 degrees does not change the diffusion signal — just as a square tile looks the same after a half-turn.