A patient is undergoing an MRI scan, which utilizes strong magnetic fields and radio waves. Which of the following fundamental physical principles BEST explains how the magnetic field affects the hydrogen nuclei in the patient's body?
- ACoulomb's Law
- BPhotoelectric Effect
- CFaraday's Law of Induction
- DNuclear Magnetic Resonance (NMR)
Show answer & explanationAnswer & explanation
Correct answer: D. Nuclear Magnetic Resonance (NMR)
MRI (Magnetic Resonance Imaging) is based on the principle of Nuclear Magnetic Resonance (NMR). This phenomenon describes how atomic nuclei with a net spin (like hydrogen nuclei, protons) align themselves in an external magnetic field. When radiofrequency pulses are applied, these aligned nuclei absorb energy and flip their spin. Upon relaxation, they emit radio signals that are detected and used to create detailed images of soft tissues.
Why the other options are wrong
- A. Coulomb's Law describes the electrostatic force between charged particles, not the interaction of nuclei with magnetic fields.
- B. The Photoelectric Effect describes the emission of electrons when light shines on a material, which is unrelated to MRI.
- C. Faraday's Law of Induction describes how a changing magnetic field induces an electromotive force, relevant for signal detection in MRI but not the primary interaction of nuclei with the static field.
Nuclear Magnetic Resonance (NMR)
Nuclear Magnetic Resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the form of radio waves) and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus.
- Requires nuclei with a net spin (e.g., ¹H, ¹³C).
- Nuclei align in an external magnetic field.
- Radiofrequency pulses cause nuclei to absorb energy and 'flip'.
- Relaxation emits detectable radio signals.
- Basis for MRI and NMR spectroscopy.
Memory trick: MRI uses NMR for clear pictures.