Temperature measurements

Four fiber-optic temperature sensors (Neoptix Inc., Quebec, Canada) were placed at the periphery of the gelled-saline filled phantom with the presence of the artificial hip joints to certify that there was minimal heat loss to the environment during the measurements similar to the method as described previously¹⁵. We measured the initial temperatures of the sensors positioned inside the phantom and the time it took to reach equilibrium with the environment. We considered that thermal equilibrium has been reached when the difference between temperatures measured by the temperature sensors in the phantom and temperature inside the magnet bore was less than 0.05 °C.

Fifty-six fiber Bragg grating (FBG, SJ Photonics Co., South Korea, wavelength range from 1510 to1590 nm) temperature sensors (28 FBG sensors on each joint) were located on a couple of the artificial hip joints to measure the temperature variations of the artificial hip joints due to RF-induced heating (Fig. 6). The FBG temperature sensors were connected with an interrogator (Model: SJP-M-02-950, SJ Photonics Co. Ltd., South Korea), which was connected with the personal computer to read and record the wavelength with a frequency of 970 Hz around the hip joints during MRI scanning. Finally, the measured wavelength was converted to temperature.

Schematic diagram and photographs showing the locations of fiber-optic Bragg grating (FBG) temperature sensors around artificial hip joints.

These FBG-based temperature sensors (0.3 mm diameter) minimize perturbations of RF fields. The FBG temperature system has a temperature resolution of 0.01 °C. The FBG sensors were calibrated with traceability. On each MRI scanner, temperature before and after MRI scanning was recorded as the temperature varies, and the difference between the measured temperatures was obtained.