Which statement best explains why MRI can distinguish tissues?

• A. It relies on ionizing X-rays to form images
• B. It relies on acoustic waves to form images
• C. It relies on magnetic resonance of hydrogen nuclei
• D. It relies on optical light emission

Answer: (C)

MRI distinguishes tissues by exploiting the magnetic resonance properties of hydrogen protons. In a strong magnetic field, the nuclei of hydrogen atoms align with the field. When a radiofrequency pulse is applied, these protons are tipped and then release energy as they relax back to alignment. The rate at which they relax (called T1 and T2) and the density of protons in a tissue determine how strong the detected signal is. Different tissues have different hydrogen content and molecular environments, so they exhibit different relaxation times and signal levels, producing contrast in the images. This is why MRI can differentiate soft tissues so well, especially water- and fat-containing structures.

It doesn’t use ionizing X-rays, acoustic waves, or optical light emission.