Introduction

Simultaneously acquisition of MRI imaging of multiple nucleus can not only provide high-quality anatomical signals, but reflect the information of tumor molecules and ion metabolism, which is expected to employ to the early prediction and evaluation of therapeutic effect of tumor^1-3. The detection capability of extremely weak signals depends on the corresponding RF transceivers, which is the core component of RF signal acquisition. Significant SNR enhancement can be achieved with improved radiofrequency (RF) coils³.
A four-frequency ¹H / ¹⁹F / ²³Na/ ³¹P RF coil system was developed in our previous study⁴. Good tuning, matching and decoupling performance was obtained from the bench tests. In this study, we renew the structure of the RF coil with adding the triple tuned local ¹⁹F / ²³Na / ³¹P receive coil to improve the weak nuclei MR imaging quality. Furthermore, the phantom imaging studies were conducted at self-made 3T multinuclear MR system. The phantom experiments at 3T were shown the ability of the RF coil system MR imaging of ¹H / ¹⁹F / ²³Na / ³¹P imaging. The combination of triple tuned local ¹⁹F / ²³Na / 31P receive coil also indicated the improved SNR of corresponding nucleus.

Method

The structure of the quadruple-nuclear RF coil system was improved with adopted three-layer nested structure and was shown in Fig. 1. The outer and inner layer coils worked in ¹H/¹⁹F and ²³Na/³¹P resonated frequencies, respectively. The new triple-tuned local receive coils was added in the innermost layer to improve the ¹⁹F/²³Na/³¹P signal sensitivity. The circuit schematics of the coil elements are shown in Fig.1 (e), which contained a dual-tuned rectangle loop element for ¹⁹F/³¹P Imaging and a butterfly element for ²³Na imaging. The size of channels of ¹⁹F/³¹P were each 40 mm × 40 mm and of ²³Na were each 70 mm × 55 mm.
All on-system studies were scanned using 3T MRI system, the operative mode of the quadruple-nuclear RF coil was controlled by the multi-frequency and multichannel sequential control system. The solution composition of phantoms for multiple nucleus imaging were described as follows: phosphoric acid solution(H₃PO₄) with a concentration of 85 % was used for ³¹P imaging, the cylindrical phantom with 300 mmol/L NaCl solution used for ¹H imaging and ²³Na imaging, C₆F₆ solution with a concentration of 98 % were used as the ¹⁹F phantom. In addition, a specific phantom filled with three solution was customized to evaluate the quality of ¹H/¹⁹F/²³Na^/31P imaging simultaneously as shown in Fig.1 (f). The Multinuclear MR imaging was scanned by the self-made GRE（gradient echo）sequence⁵，which utilized the double gradient echo to achieve the simultaneous acquisition of ¹H / ¹⁹F / ²³Na/ ³¹P MR imaging. ³¹P signal was excited by the first echo and the ¹H、¹⁹F、²³ Na was excited by the second one simultaneously. The detailed information of RF pulse was shown in the Fig. 2.
The scan parameters were as follows parameters was used to obtain signals: TE/TR= 5.4/400 ms, Slice thickness=20 mm, Matrix =128×128, Average = 4, Scan time = 3 min 40 s. FOV of the ¹H acquisition was 200 mm × 200 mm. Based on the sequence, the corresponding FOV of other non-proton was determined by the frequencies. Complete image reconstruction with the independently developed spectrometer system. SNR distributions were calculated by dividing the signal value by the noise standard deviation for the quantitative evaluation of ¹H / ¹⁹F / ²³Na/ ³¹P imaging quality.

Result

Fig. 3 shows the ¹H / ¹⁹F / ²³Na/ ³¹P MR measured results obtained with the specific phantom which including the four nuclei signals simultaneously. Fig. 4 shows the MR images obtained with the individual test phantom. All the tests indicated the developed coil system can be used for multi-nuclei MR imaging. SNR comparisons between the approach with and without the local triple-tuned receive RF coils were also shown in Fig. 3 (c)-(d) and Fig. 4 (c)-(d). The position of the local ¹⁹F / ²³Na/ ³¹P receive RF coil was indicated by the white dotted-line, the SNR of the region indicated by the red arrow is significantly improved. Especially, in the individual phantom for ¹⁹F / ²³Na/ ³¹P imaging, the SNR of the ROIs in ¹⁹F / ²³Na/ ³¹P image showed in Fig. 4 (d) was all Increased more than 50 % by adding the local triple-tuned receive RF coils.

Discussion/conclusion

In this study, simultaneous ¹H/¹⁹F/²³Na/³¹P MR images at 3T are successfully acquired in phantom using the developed quadruple-nuclear RF coil with a self-made 3T multinuclear MR RF system. The results demonstrated the feasibility of synchronized ¹H/¹⁹F/²³Na/³¹P imaging. The improved SNR of ¹⁹F/²³Na/³¹P was also achieved by the imaging approach combined the quadruple-nuclear RF coils and triple tuned local receive coils. Future investigation will focus on the animal study, which aimed to obtain the multi-nuclei information of tumor simultaneously in vivo.

Acknowledgements

This work was supported in part NSFC under Grant No. 81627901; the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000); city grant JCYJ20170413161314734; Youth Innovation Promotion Association of CAS No. 2017415.