Active shielding can be defined as:

Active shielding refers specifically to a technique used in magnetic resonance imaging to minimize magnetic field leakage that occurs outside the designated scanning area. This is particularly important as it helps to reduce the influence of stray magnetic fields on nearby equipment and ensures a safe environment for both patients and staff.

The presence of superconducting windings within the MR scanner is essential for creating the magnetic field required for imaging, but they can also be configured to actively generate a counteracting magnetic field to cancel out the stray magnetic fields that extend beyond the desired area. This method effectively 'shields' the surrounding environment from the magnetic effects that could interfere with nearby electronic systems, improve safety, and enhance overall imaging quality.

In contrast, options that mention copper or steel lining pertain to passive shielding methods, which involve physical materials to absorb or redirect the magnetic fields but do not actively manipulate those fields. Additionally, performing prescan measurements focuses on preparing the imaging system for optimal performance but does not directly involve the mechanism of shielding. Therefore, the description involving superconducting windings accurately captures the definition of active shielding in the context of MRI technology.