Method and apparatus for exposing stimulable phosphor sheet

The resolution of a stimulable phosphor sheet is improved by applying a magnetic field to the stimulable phosphor sheet in the direction perpendicular to the surface plane thereof, e.g., by using a pair of magnets, while exposing the stimulable phosphor sheet to radiation from radioactive isotopes. The magnetic field may be applied by using a superconducting coil, instead of using the magnets. An electrical field may also be applied instead of or in addition to the magnetic field. As the travel paths of charged particles are wound and focused, expansion of the size of the beam spots (the areas whereon radiation emitted by the radioactive isotopes has been recorded) is restrained.