Synopsis

Keywords: RF Arrays & Systems, RF Arrays & Systems

As the lower natural abundance of X-nuclei, it is important to enhance that the signal-to-noise ratio (SNR) of the X-nuclei is as high as possible by designing a well-designed multiple-tuned coils. In this study, a new triple-tuned RF coil system capable of ¹H / ¹⁹F / ²³Na imaging was proposed. The performance of the triple-tuned coil was evaluated compared to their counterpart single-tuned coils by the numerical electromagnetic simulation. Imaging tests at 3T MRI were performed on the phantom by using the triple-tuned RF coil.

Introduction

X-nuclei MRI are able to provide important cellular processes, morphological and metabolic information in tissues, which is significant for enabling both anatomical visualizations, as well as quantification of physiological processes^1,2. The main challenge with X-nuclei NMR signal detection is that the sensitivity and the concentration of X-nuclei is much less than that of ¹H. The signal-to-noise ratio (SNR) in X-nuclei MRI largely depends on the electromagnetic properties and performance of radio frequency (RF) coils. A well-designed multi-tuned RF coil is required with the ability of simultaneous provide the minimum coupling between channels and nuclei-of-interest as well as highest achievable SNR and homogenous B1+ fields at both ¹H and X-nuclei ^3-5.
In this study, we proposed a new triple-tuned four-channel RF coil array, which can operate at ¹H/¹⁹F/²³Na frequencies within the single physical loop structure. There is no need to consider interference between the ¹H and X-nucleus as the coil resonates at three frequencies generated in one structure achieved by triple-tuned circuits added in the excited ports. The feasibility of the proposed coil was analyzed by the numerical electromagnetic simulation in the scattering parameters and B1+ field distributions. The performance of the triple-tuned coil was compared to their counterpart single-tuned coils by analyzing the maximum, minimum, mean value of the B1+ field, as well as the homogeneity and standard deviation. The phantom experiments at 3T were shown the ability of the triple-tuned RF coil system MR imaging of ¹H / ¹⁹F / ²³Na / imaging.

Method

Modeling and simulations of the ¹H/¹⁹F/²³Na triple-tuned four-channel RF coil (Fig.1) and their counterpart single-tuned coils were performed using the software Computer Simulation Technology, (Darmstadt, Germany). The triple-tuned four loop coil was 16 cm in diameter. The size of each triple-tuned loop array was set 16mm×6mm with the 1mm copper width. The matching and tuning network of the triple-tuned resonating at ¹H/¹⁹F/²³Na as shown in Fig.1 (b) were added in the excited ports in a circuit co-simulation. Excitation signal sources with phase difference of 90 degrees were selected to generate an ideal circularly quadrature mode. The simulations were performed for a cylindrical phantom setup with size 12 cm in diameter and 300 mm length (conductivity permittivity). The performance the proposed triple-tuned coil was evaluated by S-parameters and B1+ field in the simulations. B1+ field maps of the central transverse plane were scaled to an accepted power of 1W. For the simulation results, the maximum, minimum, mean value of the B1+ strength, as well as the homogeneity and standard deviation were calculated and compared to their counterpart single-tuned coils. The homogeneity of the B1+ field in the region of interest (ROI) was calculated as follows：
B1+_homogeneity = (B1+_max-B1+_min) / (2×B1+_mean)
All MR measurements studies were carried out on a 3T MR system. A 12 cm diameter cylindrical phantom containing H₃PO₄ with a concentration of 85 %, 300 mmol/L NaCl and C₆F₆ solution with a concentration of 98%. The MR images of ¹H/¹⁹F/²³Na were acquired by using the self-made GRE sequence with scan parameters of TR/TE=500ms/5.4ms.FOV=200mm× 200mm, Thickness=20mm, Matrix=128×128. Average=4, Scan time= 3 min 40 s

Results

Fig.2 shows the S-parameters of the triple-tuned four-channel RF coil. The results displayed good tuning, matching and decoupling between the channels at ¹H / ¹⁹F / ²³Na frequencies. Fig.3 shows the B1+ field maps of the triple-tuned coil and their counterpart single-tuned coils. Table 1 shows simulated B1+ field data. At ¹H frequency, the triple-tuned coil provided 87.55% maximum of B1+ field strength generated by the single-tuned ¹H coil, it provided the same minimum and mean value as the single-tuned coil, to within 0.4%. It provided 99.55% homogeneity and 99. 59% standard deviation of B1+ field strength generated by the single-tuned ¹H coil. At ¹⁹F frequency, the triple-tuned RF coil provided 98.96 % maximum of B1+ field strength generated by the single-tuned ¹⁹F coil, it provided the same minimum and mean value as the single-tuned coil, to within 1.2 %. It provided 99.50 % homogeneity and 97.96 % standard deviation of the B1+ field strength generated by the single-tuned 19F coil. At ²³Na frequency, the triple-tuned RF coil provided 99.55 % maximum value compare with the single-tuned ²³Na coil, it provided the same minimum and mean of B1+ field as the single-tuned coil, to within 0.3%. It provided 98.29% homogeneity and 98.07% standard deviation of the B1+ field strength generated by the single-tuned ²³Na coil.
Fig. 4 shows the ¹H / ¹⁹F / ²³Na MR measured results obtained with the specific phantom which including the three nuclei signals simultaneously.

Discussion/Conclusion

In this study, a new triple-tuned four-channel RF coil array was proposed, which can operate at ¹H/¹⁹F/²³Na three frequencies within the single physical loop structure. The proposed coil can provide the almost same performance of B1+ field compared to their counterpart single-tuned coils proved by analyzing the maximum, minimum, mean value of the B1+ field, as well as the homogeneity and standard deviation. Simultaneous ¹H / ¹⁹F / ²³Na MR images at 3T are successfully acquired in phantom using the developed triple-nuclear RF coil with a self-made 3T multinuclear MR RF system. Future investigation will focus on the animal study.