A magnet structure for Nuclear Magnetic Resonance imaging apparatus, which magnet structure has at least two opposing magnetic pole pieces, which are located at a certain distance from each other and delimit an imaging region, which pole pieces are formed by at least one massive layer of a magnetically permeable material, and at least one layer of magnetically permeable material consisting of a pack of superimposed sheets (2, 2') or foils, electrically insulated from each other, each of which sheets has cuts (102, 102') arranged over the surface of the sheet (2, 2') in positions that are at least partly non coincident with the cuts (102, 102') of at least one, or both adjacent sheets, the magnetically permeable sheets or foils (2, 2') have a first face on one side of the said sheet and a second face on the opposite side of the sheet and the cuts (102, 102') have such a width and such a length and are so arranged on each sheet (2, 2'), that the cuts (102, 102') of a sheet or foil are offset and not coincident with respect to the cuts (102, 102') of the adjacent sheet or foil, as a result of said adjacent sheet being laid over the previous sheet in an overturned position, i.e. with the first face turned toward the first face of the first sheet or with the second face of said adjacent sheet turned toward the second face of the first sheet, and the said sheets having the same plan size, whereby they are aligned in the proper position by aligning the peripheral edges thereof and each sheet is ideally divided into two halves (202, 302) along an axis coincident with a sheet overturning axis, about which each successive sheet is overturned by 180° relative to the adjacent preceding sheet of the layer of sheets of each pole piece, there being provided an identical pattern of cuts (102, 102') for all sheets (2, 2'), which pattern differs in the arrangement and/or orientation and/or length and/or width of the cuts (102, 102') in the two halves (203, 302) of each sheet (2, 2') such that, when two sheets (2, 2') are superimposed in a mutually overturned or reversed condition, i.e. with the first faces or the second faces of said two superposed sheets (2, 2') in contact with each other, the cuts (102, 102') of a sheet are disposed in offset positions with respect to the cuts (102, 102') of the overlying sheet in both ideal halves (202, 302) of said adjacent sheets (2, 2'), characterized in that the patterns of cuts (102, 102') on the sheet (2, 2') have a repeated and/or orderly and/or recurring geometric cut arrangement, the cuts (102, 102') of the two ideal halves (202, 302) of the sheet (2, 2') being formed according to the same arrangement pattern which, in the second ideal half of the sheet (2, 2'), is offset to a certain extent in one or two directions subtending the plane formed by the sheet (2, 2') with respect to the periphery of the second ideal sheet half, relative to the position in the arrangement pattern of the first ideal half of the sheet (2, 2'), such that the cuts (102, 102') of two superposed sheets (2, 2'), adhering by their first faces and their second faces, coincide with cut-free portions and/or have a small number of intersection points between the cuts of the two superposed sheets (2, 2').