Evaluation of RF heating on hip joint implant in phantom during MRI examinations

We evaluate radiofrequency (RF) heating of two kinds of hip joint implants of different sizes, shapes and materials. Temperature rises at various positions of each implant are measured and compared with a computer simulation based on electromagnetic-field analysis.

Two kinds of implants made of cobalt-chromium alloy and titanium alloy were embedded at a 2-cm depth of tissue-equivalent gel-phantom. The phantom was placed parallel to the static magnetic field of a 1.5 T MRI device. Scans were conducted at the specific absorption rate of 2.5 W/kg for 15 min, and temperatures were recorded with RF-transparent fiberoptic sensors. Temperatures of the implant surface were measured at 6 positions, from the tip to the head. Measured temperature rises were compared with the results of electromagnetic-field analysis.

The maximum temperature rise was observed at the tip of each implant, and it was 9.0 degrees C for the cobalt- chromium implant and 5.3 degrees C for the titanium implant. The simulated heating positions with electromagnetic-field analysis accorded with experimental results. However, a difference in temperature rise was seen with the titanium implant.

RF heating was confirmed to take place at both ends of the implants in spite of their different shapes. The maximum temperature rise was observed at the tip where there is large curvature. The value was found to depend on physical properties of the implant materials. The discrepancy between experimental and simulated temperature rises was presumed to be the result of an incomplete model for the titanium implant.