Magnetic structure for MRI machines, which machine has a U-shaped or annular, that is O-shaped geometry having two opposite poles between which a magnetic field is generated and are borne at a predetermined distance one with respect to the other by a magnetic yoke having an inverted U shape or an annular one, that is an O shape, which poles generating the magnetic field and/or at least a part of which yoke delimit a cavity housing at least a part of the patient body, while inside the volume of said cavity a partial volume is generated wherein values of the magnetic field are such to guarantee the acquisition of MRI images having quality characteristics sufficient to be used like diagnostic images, so called imaging volume. The invention relates also to a machine for detecting MRI images particularly of the anatomical region of the hand or the foot or part of joints and which machine has a magnetic structure of the type described above.