A trailer for housing a mobile MRI system has magnetic shielding in the form of a plurality of sheets of magnetic material which are supported within the trailer, by a rigid supporting structure or frame. The layers are arranged in a stacked or laminar configuration with the laterally longest layer disposed inward of the remaining layers, such that the width of the gap between that layer and the exterior walls of the trailer at the lateral extremities of the shielding is greater than its width in a central area. The rigid frame maintains the gap and prevents contacting of the shielding and the trailer walls.