Properties of Laser-Modified Materials for Reduced Eddy-Current Heating In MRI Environments

Interactions of radio-frequency (RF) magnetic fields with implanted metallic medical devices pose potential safety risks in patients during magnetic resonance imaging (MRI). Thermal injuries at the metal-tissue interface caused by the RF field, e.g., at the tip of a cardiac pacemaker electrode, have been identified as detrimental to patients wearing such devices. Consequently, the noninvasive diagnostic advantages of MRI are inapplicable to many patients in clinical routines.

In this work, theoretical and experimental analyses are carried out for reducing MRI-induced heating by tailoring electromagnetic properties at the surface of the material. A laser surface modification technique is used to modify material properties. This technique is advantageous in that the implanted alloy is directly surface-treated to reduce the power deposited into it during an MRI sequence. Electromagnetic and thermal models are developed to determine the temperature distribution in a metallic implant during RF magnetic field pulses. The mathematical models consider the repetition rate and shape of the pulsed RF field (63.86 MHz) to predict the temperature rise with both conduction and convection heat transfer effects. The experimental effort is based on using a Nd:YAG laser to modify the property of flat thin foils of thicknesses up to 250 μm. The transmissive properties of both the laser-treated and as-received samples are measured at the above-mentioned frequency and their reflectances are determined from the experimental data. An increase in the reflectance of the laser-treated samples and a decrease in the transmissive response have immediate implications for reducing heating of materials during MRI.