After stimulation, the protons relax and energy is captured by the receiver antenna, resulting in the MRI signal. Which statement best describes the source of the MRI signal?

• A. Protons relax and emit a signal captured by the receiver antenna
• B. Electrons relax and emit a signal captured by the receiver antenna
• C. Neutrons decay releasing gamma rays captured by the receiver antenna
• D. Protons remain in excited state and no signal is produced

Answer: (B)

The MRI signal comes from hydrogen protons. In the strong magnetic field, protons are excited by a radiofrequency pulse and tipped into the transverse plane. As they relax back to alignment with the main field, they emit radiofrequency energy. The receiver coil detects this emitted energy, which is the MR signal. Electrons don’t produce the MRI signal in this context, and gamma rays from neutron decay aren’t involved in MRI. If protons stayed in an excited state, no detectable signal would be produced.