How is active shielding accomplished?

Active shielding in magnetic resonance imaging (MRI) is achieved by utilizing a coil of wire in the MRI unit, through which an electric current passes. This current generates a magnetic field that is designed to counterbalance the main magnetic field (B0) produced by the MRI system. The result is that the magnetic field generated by the coil effectively reduces the fringe field's strength outside the MRI unit, which is crucial for minimizing the potential impact on nearby objects and enhancing safety around the MRI environment.

By actively generating a magnetic field that opposes the primary field, this method ensures that the overall magnetic influence outside the MRI system is kept within safe limits. This process enhances the containment of the magnetic field, making it safer for patients and staff in the vicinity of the MRI scanner.

The other methods of shielding, such as using magnetically conductive materials or stacking bricks of permanent magnets, do not involve the dynamic response of a current-generating coil and thus do not qualify as active shielding. Instead, they are considered passive shielding techniques. Passive shielding typically relies on static materials to redirect or absorb magnetic fields, whereas active shielding actively manages the magnetic environment through the application of an adjustable magnetic field.