Final answer:
Gradient coils are used for the spatial encoding of an MR image in conjunction with the main superconducting cylindrical coil, allowing the MR system to create detailed images of the body's internal structures.
Step-by-step explanation:
The coils used in spatial encoding of an MR image are known as gradient coils. These coils are responsible for varying the magnetic field slightly over the volume being imaged. This variation allows the resonant frequency of protons to vary with position within the body. By sweeping radio frequencies over a range, protons in a specific magnetic field strength absorb and reemit them, which the imaging receiver then uses to create a tissue map. The combination of gradient coils, together with the main superconducting cylindrical coil, allows for the production of detailed cross-sectional images of the body. These imaging techniques are powerful for diagnosing conditions without exposing the patient to ionizing radiation.