Permanent magnet keeper-shield assembly

Strong magnets include electromagnets and permanent magnets. An electromagnet capable of producing a large magnetic field is cumbersome and requires a very large power supply. Permanent magnets exist of relatively compact size that are capable of producing large magnetic fields. However, permanent magnets cannot be turned off and since their polarity is not rapidly switching, they saturate surrounding permeable material and their emitted field cannot be attenuated. Consequently, permanent magnets are difficult to shield and those that produce large magnetic fields are difficult to handle and store in many settings for which such a magnet might otherwise be useful. 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.