Evaluating the accuracy of the variable flip angle technique for temperature measurement near metallic implants

This study assessed the accuracy of a T1-based temperature measurement technique for metal-inserted materials using 3T magnetic resonance imaging (MRI). A variable flip angle (VFA) sequence with dual echoes was used to obtain images of a phantom and an ex vivo experiment. T1 values were calculated when the metal-inserted materials were subjected to heating and were acquired in the proximity of the metal implant of the phantom and ex vivo tissue. The values were converted into temperature measurements using a fiber-optic thermal sensor (FOS) that was placed within the metal substances. Additionally, proton resonance frequency shift (PRFS) was calculated from phase images acquired with the dual echoes, which allow for a comparison of temperature fidelity between the VFA and PRFS techniques. In the phantom experiment, the root mean square temperature error based on T1 values was approximately 0.12 ℃, which was comparable to that obtained from the FOS. In contrast, the PRFS demonstrated a substantial temperature measurement error of approximately 11.21 ℃, as determined by the root mean square calculation. A similar pattern was observed in the ex vivo experiment, where the T1-based temperature measurement error was minimal at approximately 0.30 ℃, while the PRFS-based temperature measurement error was considerably higher at around 5.44 ℃. These findings indicate that the VFA technique enables precise monitoring of temperature alterations in metal-inserted materials, engendering its incorporation into clinical MRI sequences for temperature assessment during magnetic resonance radiofrequency (RF) exposure.