Ultrasound isn't just for baby pictures on a screen. It can scan muscles and tendons, but there's a catch: bones. To sound waves, they act like mirrors to light. The reflection is so strong that nothing behind them is visible.

The new approach is like tuning an old radio. First, you hunt for a station by turning the dial roughly. Same here: the first stage, a fast method using traveling waves, gives a rough image. Then comes fine-tuning: the full-waveform inversion algorithm (which reconstructs tissue properties from how sound passed through it) fills in the details. Before, without a good starting guess, this precise method simply didn't work, like trying to tune in a distant station by starting in the middle of the dial. Now, thanks to the rough guide, it works even in tricky areas full of bones.

The authors tested the method on computer models, tissue samples in test tubes, and living humans. In every case, the image was just as good as with perfect initial conditions, but without extra computation.