RF pulses deposit energy into the patient to excite hydrogen nuclei. Which modality uses this principle?

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

Answer: (C)

RF pulses excite hydrogen nuclei in a strong magnetic field, which is the hallmark of MRI. The pulse tips the net magnetization away from alignment with the main field, depositing energy into the tissue. After the pulse, the protons relax back to their resting state and emit signals that are captured by coils. The rate at which they relax (T1 and T2) varies among tissues, and MRI uses different sequences and gradient fields to convert these relaxation properties into images with contrast. This mechanism—using radiofrequency energy to manipulate and detect hydrogen nuclei—sets MRI apart from other imaging modalities: ultrasound relies on sound waves, CT on X-ray attenuation, and PET on radioactive tracer decay. So the principle described is used by MRI.