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

Abstract

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.

Keywords: Metal implant; T1-based thermometry; magnetic resonance imaging (MRI); temperature mapping; variable flip angle (VFA).

Figure 1

Overview of the experimental setup and processing pipeline. A lumbar fusion metal is embedded in an agar phantom, surrounded by oil phantoms, and scanned with a fiber-optic thermal sensor inserted near the metal tip. After heating, the sample is imaged using the VFA sequence with three flip angles (FA =2°, 10°, and 20°) and dual echoes. The magnitude images from these FAs are used to calculate a T1 map and subsequently a temperature map. Additionally, a phase image (delta-TE) is computed using dual-echo images at FA 10°, which is converted into a temperature map via the PRFS method. The arrows and labels in the image indicate key components such as the sensor location, phantom materials, and signal pathways. delta-TE, phase difference between echoes; FA, flip angle; MR, magnetic resonance; PRFS, proton resonance frequency shift; rad, radian; TE, echo time; VFA, variable flip angle.

Figure 2

Accumulated temperature measurements of the phantom experiment between the VFA and PRFS. We observe that as the phantom is heated, the temperature pattern of the VFA (yellow dot box) around the FOS (red line) elevates substantially, whereas the PRFS temperature remains relatively consistent across all scan times. FOS, fiber-optic thermal sensor; PRFS, proton resonance frequency shift; VFA, variable flip angle.

Figure 3

Comparison of temperature profiles acquired using the VFA and PRFS methods against the FOS in the phantom experiment. The VFA-derived temperature closely follows the FOS measurements, resulting in a low RMSE of 0.12 ℃. In contrast, the PRFS-derived temperatures deviate from the FOS trend, leading to a larger RMSE of 11.21 ℃. FOS, fiber-optic thermal sensor; PRFS, proton resonance frequency shift; RMSE, root mean square error; VFA, variable flip angle.

Figure 4

Temperature measurements in the ex vivo experiment using a metallic implant. (A) The VFA-derived temperature profile closely aligns with the FOS measurements, while the PRFS shows a significant deviation, reflected in the higher RMSE. (B) Spatial temperature maps at different scan times (120 and 600 seconds) illustrate that the VFA method shows gradual temperature increases consistent with heating, whereas the PRFS method presents inconsistent patterns, particularly in the region marked by the yellow dotted box. FOS, fiber-optic thermal sensor; PRFS, proton resonance frequency shift; RMSE, root mean square error; VFA, variable flip angle.