Protons parallel with the main magnetic field are low energy protons. Are they used for imaging?

• A. High energy protons
• B. Protons along field only
• C. Low energy protons
• D. Protons that are neutral

Answer: (C)

In MRI, imaging relies on the net magnetization created by hydrogen protons in a strong external magnetic field. More protons sit in the lower-energy state, which is the parallel orientation to the main field, than in the higher-energy anti-parallel state. This population difference produces a net magnetic moment along the field that we can manipulate and detect. When a radiofrequency pulse is applied, we tip that net magnetization into the transverse plane, and as the spins relax back they emit signals used to form the image. So those low-energy protons are the ones we use for imaging because they establish the detectable magnetization that the scanner reads. High-energy protons are much less populated and contribute little to the signal; protons along the field alone wouldn’t capture the necessary transverse signal; and neutered or neutral protons aren’t the players in MR imaging.