Magnetic resonance imaging or MRI is an imaging modern technique to obtain detailed organs and tissues images throughout the body without the use of x-rays or the so-called ionizing radiation. MRI uses powerful external static magnetic fields along with RF waves that interact with hydrogen atoms to obtain powerful and clear images of the various organs and tissues. Unlike x-ray and CT scan, MRI produces no ionizing radiation; hence, there is no limit on the number of MRI scans a patient can have in a given year.
Birdcage Coil MRI design
INTERACTION WITH METALS
MRI produces no ionizing radiation. However, it has another big limitation. It interacts with metals in two distinct ways. The first is related to the external large static field which generates a magnetic force on ferromagnetic materials, such as iron, steel, nickel, etc. If the patient wears jewelry or has a medical implant such as a pacemaker that contain even some ferromagnetic impurities, a dangerous or even a life threating force can be produced. Obviously, the higher is the ferromagnetic concentration, the stronger is the magnetic force. The second interaction with the metals is caused by the varying RF waves used in the MRI machines. The waves induce a current in the metallic parts. The current definitely produces heat due to Joule effects which, in turn, causes burns around the metallic parts. The higher is the frequency of the radio waves and electric conductivity of the metallic parts, the higher is the current and, thus, the worse are the burns.
In recent years, MRI manufacturers and implant designers started to try to overcome these limitations. Electromagnetic simulation is instrumental to, virtually, explore innovative ideas to remedy those limitations and make MRI imaging more democratic. Regulatory bodies such as the FDA also require simulation results before passing to actual testing. Both EMWorks’ popular electromagnetic packages, EMS and HFWorks, can readily be used to virtually prototype MRI and implant designs.
RF-Heated Metallic Stent Implant inside MRI
EMS FOR STATIC FORCEEMS has been used by numerous MRI and implants companies to compute the magnetic force acting on the ferromagnetic parts due to the external magnetic field. In addition to the force, EMS also computes the magnetic field and flux density in the entire MRI machine.
Magnetic flux density around the MRI head obtained by EMS
HFWORKS FOR RF FIELDS
HFWorks is likewise widely used by MRI and implants designers to study the high frequency aspects of the MRI machines. It computes the electromagnetic fields and SAR in the entire machine. It is also equipped with a built-in thermal solver that computes the temperature in the entire machine.
Temperature and local SAR distributions around Medical Hip Implant
SUMMARY AND TAKEAWAYS
Is it safe to wear a metal an MRI scan? It is still not safe because of the force acting on ferromagnetic parts due to the powerful static magnetic field and the temperature induced by RF fields. These limitations will soon change thanks to the undergoing innovative research by MRI and implant manufacturers. Both EMWorks’ electromagnetic simulation products are in the heart of such research because they are widely used by the said manufacturers.