Kecheng Yuan

Rapid changes in gradient magnetic fields induce eddy currents, posing a considerable challenge to the accuracy and stability of magnetic resonance imaging (MRI) systems. Accurate measurement of eddy currents is crucial for the rapid and efficient design of eddy current compensation parameters. The “six-point sample method” provides a systematic way to measure and quantify gradient eddy currents while its coils have inherent limitations in channel isolation. The device proposed in this study leverages the shielding effect of coaxial cables to reduce mutual interference between coils. It utilizes coaxial cables to construct coil units and compares the channels crosstalk with traditional copper-strip coils. Following system calibration, eddy current compensation was implemented on the 1.5-T MRI scanner using the dedicated eddy current measurement device. Results revealed that −15 dB coupling was achieved at a distance of 72 mm for the coaxial-cable coils, which was more than half shorter than the 120 mm required for the copper-strip coils. In addition, the coaxial-cable coils exhibited better transmission characteristics. With a power crosstalk ratio above 30 dB between any two units, the channel isolation is remarkably higher than that of copper-strip coils, exceeding it by approximately 50%. The device was employed to measure the eddy currents and subsequently, based on the results, compensation parameters were rapidly iterated to achieve good eddy current correction.

This study aimed to improve the accuracy of Electrical Properties Tomography (EPT) by proposing a Fat-Water Quantification-based EPT (FW-EPT) using the Dixon technique, and provided a feasible approach for obtaining Electrical Properties (EPs) from current clinical routing modalities. Nine human liver-mimicking phantoms were built with varying fat-water content at 64 MHz. The EPs were measured using the open-ended coaxial probe method, and a fat-water signal was obtained through Dixon scanning. Subsequently, three sets of fitted models were established: F-EPs, considering only fat information; W-EPs, considering only water information; and FW-EPs, considering both fat and water information. To assess the accuracy of these models, FW-EPT experiments were conducted on two healthy subjects, and the results were evaluated using literature values as a reference benchmark. Experiments showed that the FW-EPs fitted model offered the best accuracy. Compared with the literature values, the average relative errors for human liver conductivity and relative permittivity at 1.5T MRI were lower than 2.89% and 5.37%, respectively. The scanning time for clinical human MR experiments was approximately 22 s. FW-EPT enabled faster, higher-resolution, and more precise imaging of EPs in human liver tissue. The findings of this study offered new insights for clinical EPT.

Purpose To develop an MR-conditional microwave needle that generates a spherical ablation zone and clear MRI visibility for MR-guided microwave ablation. Methods An MR-conditional microwave needle consisting of zirconia tip and TA18 titanium alloy tube was investigated. The numerical model was created to optimize the needle's geometry and analyze its performance. A geometrically optimized needle was produced using non-magnetic materials based on the electromagnetics simulation results. The needle's mechanical properties were tested per the Chinese pharmaceutical industry standard YY0899-2013. The MRI visibility performance and ablation characteristics of the needle was tested both in vitro (phantom) and in vivo (rabbit) at 1.5T. The RF-induced heating was evaluated in ex vivo porcine liver. Results The needle's mechanical properties met the specified requirements. The needle susceptibility artifact was clearly visible both in vitro and in vivo. The needle artifact diameter (A) was small in in vivo (A(shaft:) 4.96 +/- 0.18 mm for T1W-FLASH, 3.13 +/- 0.05 mm for T2-weighted fast spin-echo (T2W-FSE); A(tip:) 2.31 +/- 0.09 mm for T1W-FLASH, 2.29 +/- 0.08 mm for T2W-FSE; tip location error [TLE](:) -0.94 +/- 0.07 mm for T1W-FLASH, -1.10 +/- 0.09 mm for T2W-FSE). Ablation zones generated by the needle were nearly spherical with an elliptical aspect ratio ranging from 0.79 to 0.90 at 30 W, 50 W for 3, 5, 10 min duration ex vivo ablations and 0.86 at 30 W for 10 min duration in vivo ablations. Conclusion The designed MR-conditional microwave needle offers excellent mechanical properties, reliable MRI visibility, insignificant RF-induced heating, and a sufficiently spherical ablation zone. Further clinical development of MR-guided microwave ablation appears warranted.