A hybrid permanent magnet structure (71) for providing a uniform magnetic field within a region of interest while allowing access via open ends by a patient for NMR scanning and by a surgeon to the patient via a large opening (56) at the top. The magnet structure comprises nested inner and outer magnetic structures separated by ferromagnetic material and surrounded by a ferromagnetic yoke (70). The inner magnetic sections comprise end blocks of triangular cross section (64, 66), and the outer magnetic sections comprise end blocks of trapezoidal cross section (65, 67, 68). Field distortion compensating means is provided by a primary filter structure to the ferromagnetic material and by magnetic insertions or a secondary filter structure.