Brushes can often be used in conjunction with magnets to assist in the ferrous debris being removed from the pipe wall. The resulting ferrous debris can then be collected with the magnets mounted on the pig. Magnets can also be utilized to trip a magnetic non-intrusive pig signaller (CD52 Bandit). 

Detection devises are looking for a change in magnetic field to indicate to an operator when a pig has passed a critical section in the line or has arrived at the launcher/receiver.

NEODYMIUM IRON BORON MAGNETS Temperature Rating: 100°C / 212°F suggested maximum