Your back hurts, and the doctor asks you to bend forward and backward to see how your vertebrae move. Usually, this means a series of X-rays: they show motion, but only in a plane, like a shadow on a wall. And volumetric methods like CT scans can't keep up with movement.
Engineers proposed a different approach: a robot sweeps an ultrasound probe over your back while you bend. Ultrasound only sees bone surfaces, but that's enough to match the image with a pre-taken 3D scan of your spine in a neutral position. Then a computer model, like a jointed doll, mimics the motion, adjusting to the sensor data.
Tests on a flexible plastic spine model showed the error stays within a couple of millimeters. For a doctor, that means seeing how vertebrae shift during bending and spotting instability — the very reason your back "shoots" with a sudden move. And all of this without any X-ray radiation.
Ultrasound is so safe it's used to monitor a baby in the womb, but it's less suited for bones: sound waves are almost completely reflected off their surface.
In the movie Total Recall, the hero gets a full-body scan in seconds: it seems the future is heading toward seeing inside the body in motion without any harm.