A nuclear magnetic resonance imaging device has a magnetic structure which defines a cavity for housing at least part of the body of a patient with a magnetic field which permeates at least part of the cavity, an opening provides access to the imaging cavity from outside of the magnetic structure and electrically conductive and electrically grounded shielding provided at the opening is movable between a first inactive configuration wherein the opening is substantially open and an active configuration in which the opening is at least partially closed by the shielding.