A new magnetic field mapping method in MRI is presented. This technique is ideal for severe inhomogeneities where plane warp cannot be ignored. The present study employs a ferromagnetic ball to create a perturbation within the imaged volume. The magnetic moment and position of the device are acquired experimentally with a new technique that excites magnetic equipotentials within a volume. A three dimensional perturbation field is then reconstructed from which an accurate field map is acquired for any slice within the volume. This method is compared with phase imaging, which is commonly used to map the magnetic field perturbation. Preliminary investigations show that this method is accurate and provides field maps that do not suffer from distortion in the slice select and read direction. This method can help in the correction of susceptibility artifacts by providing an accurate map of the perturbing field generated by magnetic markers on medical instruments.