This question is about nuclear magnetic resonance (NMR).

In nuclear magnetic resonance imaging, the patient is placed in a large uniform magnetic field. In addition, the part of the patient under investigation is subject to a weaker non-uniform (gradient) field.

Explain the role of these two fields in the imaging process.

strong field causes protons (in the body) to precess/align with the field;
gradient magnetic field applied so that magnetic field at every point in the body is different;
radio frequency radiation directed at the body excites protons (to a higher energy state);
frequency emitted (by de-exciting protons) depends on magnetic field;
frequency determines point of emission (since different parts of the body correspond to different frequency);
intensity of radiation/relaxation times depend on type of tissue (so that contrast can be achieved);