Cartridge-moveable shield

A cartridge is configured to be inserted into a medical instrument for treating patients with radioactive seeds. The cartridge includes a rotating internal barrel that is loaded with a plurality of seeds, where the internal barrel has a plurality of conduits for accepting the seeds. As the internal barrel rotates, a seed from one of the conduits is deposited into a shuttle, and then the shuttle is fully extended so as to provide the seed to the medical instrument. The shuttle is then retracted, and the barrel is rotated to the next conduit, so as to receive a seed from that next conduit. The cartridge includes a first shield and a second shield, each having slots where the slots of the first and second shields are aligned in a first verification or calibration mode, and where the slots are not aligned in a normal, seed depositing mode. The second shield also includes a plurality of numeric indicators that indicate a current number of seeds remaining in the cartridge.