They chose a technique known as in vivo molecular imaging. In a process similar to magnetic resonance imaging (MRI), a living organism such as a human, or in this study's case, a mouse, is placed into a chamber and is exposed to a variety of invisible rays. But unlike an MRI, which uses magnetic fields and radio waves, this form of imaging uses no emissions. Instead, it only detects molecules with the capability of emitting natural light, a phenomenon known as luminescence. The result is a dark outline of the individual with bright spots visible on the inside.