a magnet keeper - shield assembly adapted to hold and store a permanent magnet used to generate a high gradient magnetic field . such a field may penetrate into deep targeted tumor sites in order to attract magnetically responsive micro - carriers . the magnet keeper - shield assembly includes a magnetically permeable keeper - shield with a bore dimensioned to hold the magnet . a screw driven actuator is used to push the magnet partially out of the keeper - shield . the actuator is assisted by several springs extending through the base of the keeper - shield .