Inspired by the device used to find lost coins in the sand, Johns Hopkins University biomedical engineering students have invented a small handheld metal detector to help doctors locate hidden orthopedic screws that need to be removed from patients' bodies. The device emits a tone that rises in pitch as the surgeon moves closer to the metal screw. It also serves as a surgical tool to guide the removal of the hardware.
The battery powered device blends electronic technology from traditional metal detectors with design features from modern medical instruments used in minimally invasive procedures. The device features two coils of wire, one in the search probe and the other in a control box. When electric current flows through the two coils, each produces an oscillating magnetic field. Circuitry in the control box then compares the frequency of the oscillations in the two coils and translates the difference in frequencies into an audible sound.
The search coil is located inside a non-metallic, needle-shaped portion of the probe, which sits inside a hollow tube. The two parts of the probe form a device that the doctor inserts near the site of a screw that needs removal. The probe is then inserted to help the doctor find the head of the screw. Its movements can be observed by the C-arm fluoroscope imaging equipment often used in these operations. When the screwhead is found, the coil detector is removed. The doctor inserts a screwdriver through the tube section to remove the screw.