An NMR imaging apparatus (10) having a desired imaging volume for imaging of biological tissue, said apparatus comprising bias means (20) for generating a bias field, means for generating gradient fields, and radio frequency means for applying a pulse of electromagnetic radiation to the biological tissue and for detecting the resultant signals emitted from said tissue; wherein said bias means comprises a plurality of dipole ring magnets, (20) each dipole ring magnet comprising a plurality of segments, (22) each segment comprising an oriented, anisotropic permanent magnet material (23) arranged in a ring so that there is a substantially continuous ring of permanent magnet material; wherein the inner radius of at least one dipole ring magnet is different from the inner radius of an adjacent dipole ring magnet.