Which imaging modality uses magnetic fields and radio-frequency pulses to affect the body's hydrogen atoms?

• A. Scintigraphy
• B. MRI
• C. CT
• D. Ultrasound

Answer: (B)

The principle being tested is how MRI works with hydrogen nuclei. In a strong external magnetic field, the spins of hydrogen protons in the body's water and fat align. A radio-frequency pulse at the protons’ resonant (Larmor) frequency tips this net magnetization away from alignment. When the pulse stops, the protons relax back and emit radio waves that are detected by coils to create images. This process relies on magnetic fields and RF energy to affect hydrogen atoms, giving superb soft tissue contrast without ionizing radiation. Other imaging methods use different physics: scintigraphy involves radioactive tracers and gamma detection, CT uses X-ray attenuation, and ultrasound relies on sound wave echoes.