When thermal equilibrium is reached, there are more hydrogen protons in which energy state?

• A. High Energy State
• B. Low Energy State
• C. Neutral State
• D. Equally Distributed

Answer: (B)

In magnetic resonance, the populations of the two spin states at thermal equilibrium follow the Boltzmann distribution in the presence of a magnetic field. The energy difference between the states (the Zeeman splitting) makes the lower-energy orientation—the one aligned with the field—more populated than the higher-energy orientation. Although the difference is small under physiological conditions, there are more protons in the low-energy state, creating a net longitudinal magnetization that MRI detects. The neutral state or equal distribution would only occur if there were no field or at infinitely high temperature, which isn’t the case here. The high-energy state is less populated than the low-energy state. Therefore, there are more hydrogen protons in the low energy state.