INTRODUCTION

Ultra-high field (UHF) (≥ 7T) MRI has the potential for high signal-to-noise (SNR) ratio and enhanced tissue contrast¹. However, due to the decreased radiofrequency wavelength at ultra-high fields, substantial wave effects exist and this can lead to $$$B_{1}$$$-field inhomogeneity^{­2, 3}. Therefore, it becomes challenging to design body coils for imaging a large region of interest (ROI) such as the torso. Although using RF coils with large physical size would be ideal, such designs are challenging. Specifically, the current distribution along the coil surface presents weakened points owing to the current phase inversion, thereby creating local dark regions of the $$$B_{1}^{+}$$$ field. This unfavourable phenomenon becomes more severe when the electrical length of the coil is further extended at UHF frequencies. In this work, a segmented loop coil is presented using coupled line phase shifters^{4, 5}. This design aims to achieve a current distribution that is in-phase when the electrical length of the coil is larger than 1.5$$$\lambda$$$ at the operating frequency of 298MHz and consequently, improve the $$$B_{1}^{+}$$$ homogeneity over a large ROI.

METHODS

As shown in Fig.1 (a), the length and width of the proposed coil is 600mm and 200mm, respectively. The coil is composed of two concentric segmented line loops which are symmetrically arranged. Both loops are made of several segments with identical length Ls (160mm), except the first two sections that connect to the feeding port of the inner loop (with L₀ of 155mm). These two segmented line loops are separated with a distance of S (1mm) and were designed so the gaps between loop segments aligned with the centre of the other loop segments. The designed RF coil is mounted on a 0.5mm Rogers RO4003 substrate with dielectric constant of 3.55 and loss tangent of 0.0027. To demonstrate the performance of the design as a transmit RF coil for body imaging, full-wave simulations were conducted using CST Microwave Studio (Darmstadt, Germany). An 8-channel loop array was simulated by equally distributing the loops in a cylindrical contour with a diameter of 600mm (Fig.1 (b)). A homogeneous phantom was designed with width of 450mm, length of 800mm and height of 250mm, filling with body-like material ($$$ε_{r}$$$ = 35 and $$$\sigma$$$ = 0.4S/m). For comparison, a conventional loop coil with the same dimensions was also simulated. In all simulations, coil elements were tuned and matched for 7T imaging at 298MHz. The $$$B_{1}^{+}$$$ shimming process was performed with a custom algorithm run on MATLAB (MathWorks, MA, USA) by optimising the amplitude and phase of each channel.

RESULTS and DISCUSSION

Fig.2 compares the simulated current distributions of the conventional loop coil and the proposed coil. It can be observed that current on the conventional loop presents a phase inversion near the centre of the coil, hence causing significantly weakened current intensity. However, the proposed coil is capable of retaining an in-phase current, thereby a nearly uniform current distribution can be achieved throughout the coil surface. This current distribution can help to improve the $$$B_{1}^{+}$$$ homogeneity inside the subject. Fig. 3 shows the $$$B_{1}^{+}$$$ field distribution from a single coil after normalizing its intensity by 1W accepted power at the excitation port. It can be seen that for the conventional loop coil, $$$B_{1}^{+}$$$ dark bands are observed in central coronal plane along x-direction and central sagittal plane along y-direction (Fig.3 (c) and (e)), due to out-of-phase current inducing a null current intensity. On the other hand, $$$B_{1}^{+}$$$ fields generated by the proposed coil exhibits an improved distribution along the z-direction (Fig.3 (d) and (f)). Consequently, magnitude of the normalized $$$B_{1}^{+}$$$ field in the central axial plane from conventional loop coil is significantly smaller than that from the proposed coil (Fig.3 (a) and (b)). The $$$B_{1}^{+}$$$ shimming results by using 8-channel coils in axial and coronal planes are shown in Fig.4. In circular polarization (CP) mode, both the conventional and proposed coils generated inhomogeneous $$$B_{1}^{+}$$$ field distributions. However, the proposed coil array can illuminate the central part of the phantom and shows stronger $$$B_{1}^{+}$$$ intensity in the central axial plane (Fig.4 (a)). Fig.4 (b) shows the shimming results after applying the optimized amplitude and phase to each excitation port of the coil array. It can be clearly observed that a uniform $$$B_{1}^{+}$$$ field distribution in a large ROI can be obtained by using the proposed coil array, whereas the conventional loop coil array presents an undesirable $$$B_{1}^{+}$$$ field distribution with several dark regions in the same ROI. The quantitative comparison of the $$$B_{1}^{+}$$$ field inside the ROI was given in Table.1. By using the proposed coil, the $$$B_{1}^{+}$$$ uniformity was improved by approximately 46% and 48% in axial and coronal planes, respectively.

CONCLUSION

A novel design for a large RF body coil is presented, which has demonstrated the capability of producing an in-phase current distribution on the surface of a large loop coil. An 8-channel coil array has been simulated with a homogeneous torso phantom. Compared with the conventional loop array, significant improvements on both homogeneity and intensity of $$$B_{1}^{+}$$$ field can be observed.

Acknowledgements

No acknowledgement found.