Protons aligned against the main magnetic field are characterized as which energy state, and are they typically used for imaging?

• A. High energy protons
• B. Low energy protons
• C. Hydrogen nuclei
• D. Protons with random energy

Answer: (A)

In a magnetic field, protons have two energy states: those aligned with the field (lower energy) and those aligned against the field (higher energy). At body temperature, more protons sit in the lower-energy state, creating a small net magnetization along the main field. Protons aligned against the field are in the higher-energy state and are present in fewer numbers, so they’re not the basis for producing the MR image. The imaging signal comes from the net magnetization that arises mainly from the more populous, lower-energy state and its transverse component after RF excitation. So the protons aligned against the field are high-energy, and they are not typically used for imaging.