MR-Physics and Safety

Question 1

What happens to the spins of hydrogen atoms in an external magnetic field?

Answer 1

• each proton’s axis of spin will tend to take one of two states: either aligned along (parallel to) or against (antiparallel to) the magnetic field
• more spins will enter the parallel state (quantum mechanics effect), resulting in a net magnetization parallel to the scanner’s magnetic field

Question 2

What is the Larmor frequency?

Answer 2

• spinning protons also rotate around the axis of the external magnetic field (precession)
• frequency of precession of the spins (= Lamor frequency) is proportional to the magnetic field strength B0
• omega0 = gamma * B0
• gamma = gyromagnetic ratio = 42 MHz/T

Question 3

What is the net magnetization?

Answer 3

• M = sum(mu)
• M scales with B0 by factor of Chi0

Question 4

How strong is the magnetic field in a MRI scanner and how strong is it with respect to the Earth’s magnetic field?

Answer 4

3 Tesla = 60,000 * Earth’s magnetic field

Question 5

Why are even non-magnetic metals (e.g. metallic implants) dangerous in a MRI scanner?

Answer 5

• magnetic field induces a electric current in the metal leading to heating
• danger of localized burns due to metallic implants