To peek inside a muscle, it is illuminated with radio waves in a strong magnetic field: that's how MRI works. Water in cells and water in blood respond differently, but a standard scanner doesn't tell them apart: it produces an averaged image where the signal from capillaries is superimposed on the signal from muscle fibers. Change the settings slightly, and the numbers on the map start to drift, like a reflection in water disturbed by a thrown pebble.
The authors made the scanner simultaneously measure how fast water molecules move (this is called diffusion) and how fast their signal decays (relaxation time). This combined approach is like a long-exposure photograph that separately captures clouds and an airplane against them. The data were run through six mathematical models, and those that accounted for both parameters no longer depended on echo time — the moment when the scanner listens to the tissue's response.
Simulations confirmed that the new approach measures the fraction of vascular water with a correlation of 0.95 to the true value and an error of only 0.03. This means that from a single scan, one can reliably assess muscle nutrition, recovery after exercise, or disease progression — without repeated scans or adjustments for the machine.
The relaxation time of muscle tissue turned out to be about 31–36 milliseconds, while that of blood in small vessels was 66–86 milliseconds. The two-fold difference allows them to be separated like two musical instruments in an orchestra.