The Cocktail Party Answer

You know, what these people do is really clever. They put little spies into molecules and send radio signals to them, and then they have to radio back what they are seeing

—Niels Bohr

I like this explanation, and not just because of the implication that MRI physicists are clever (and Neils Bohr was physicist, a Nobel prize winner and the father of quantum mechanics, so no intellectual slouch himself). This apparently glib quote hides a lot of truth. The “little spies” are hydrogen nuclei, and it is the hydrogen nuclei that are the source of the signal in MRI. We do send radio signals to these spies, and they absorb the energy of the radio wave. The spies then send some of this energy back to us, also as radio signals. The signals that we receive back depend on the microscopic environment that each hydrogen nucleus is seeing. However, this explanation is not very satisfying in that it doesn’t answer some other fairly obvious questions about MRI, such as

• Why do you need that big donut-shape thing?

• How do you know where the radio signals are coming from?

• Why do different tissues give different signals?

• Why’s the MRI so loud?

To answer those questions, we need to go into a bit more detail.