Relative Magnetic Force Measures and Their Potential Role in MRI Safety Practice

Background Magnetic field markings are occasionally used at MRI sites to provide visual feedback of magnetic field strength at locations within the MRI scan room for safety purposes. In addition to magnetic field line markings, relative magnetic force, or ratio of magnetic to gravitational forces on an object, may be considered a useful complementary metric to quantify the risk associated with bringing objects containing ferromagnetic material into the magnetic field. Purpose To develop and validate methods for deriving useful relative magnetic-force measures including a simple force index for application to MRI safety. Study Type Phantom. Phantom A special-purpose rig was built to experimentally measure relative magnetic forces on small ferromagnetic objects. Field Strength Ranging from 1.5T to 7T. Assessment Quantitative comparisons were made between theoretical and measured relative magnetic forces on six objects containing ferromagnetic material: a piece of iron, a paper clip, a Kelly clamp, nail clippers, a cell phone, and a small permanent magnet. Statistical Tests An analysis based on the Bland-Altman method was employed. Results After correction of the 1.5T data to account for assumed positioning errors of the test rig, limits of agreement between measured and estimated relative forces in the four MRI systems were +/- 0.16, where a relative force of 1.0 indicates that the magnetic force is equal to gravitation force. There was no significant bias in the data (P < = 0.05). Data Conclusion Accurate measures of relative magnetic forces on ferromagnetic objects can be derived for MRI safety purposes. Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2020;51:1260-1271.