Introduction

Whole body MR imaging at ultra-high field remains challenging due to transmit B1(B1+) field inhomogeneity, as radiofrequency (RF) wavelength in human tissues is much shorter than the dimensions of human torso¹ and causing constructive/destructive interference patterns in resultant images. In order to mitigate the issue, RF shimming methods including static parallel transmission (static pTx, or static RF shimming) and dynamic parallel transmission (dynamic pTx, or dynamic RF shimming) have been proposed on the basis of multi-channel transmission hardware architecture.² Here we present the initial results of abdomen and pelvis imaging at 5T with static RF shimming, and demonstrate the capability of the 5T MR system for routine body imaging.

Methods

Experiments were performed on a whole body 5T MRI scanner (United Imaging Healthcare, Shanghai, China) as shown in Figure 1. The parallel RF transmission system consists of eight independent RF power amplifiers (RFPAs), each with a peak power of 8kW. The volume transmission coil used was an eight-channel loop array coil. For the initial testing, only the amplitude and phase of each channel were changed to optimize the target B1+ fields which is commonly used in 3T system as static RF shimming method. The static RF shimming procedure starts with a prescan calibration step with a B1+ mapping sequence. The total acquisition time for all eight channels in prescan takes less than 1 minute. Desired weights of each transmit channel are then calculated using magnitude least squares optimization and used for the later scan.³ The T2-weighted (T2W) fast spin echo (FSE) sequence with fat saturation (FOV: 380mm*300mm, Matrix: 336*225, TR: 4000ms, TE: 87.2ms, Slice Thickness: 6mm, Slice Number: 24) was selected as the demonstrating protocol for abdominal imaging (Figure 2). Another routine T2W FSE sequence (FOV: 350mm*350mm, Matrix: 416*312, TR: 4800ms, TE: 75.04ms, Slice Thickness: 3mm, Slice Number: 26) was chosen for image quality assessment of pelvic imaging (Figure 3).

Results

Figure 2 and Figure 3 demonstrate the B1+ profiles of both circular polarization (CP) mode and optimized transmitting mode used after static RF shimming process in abdomen and pelvis. The coefficient of variance (CV) reflects the degree of B1 homogeneity, and greater CV values correspond to greater inhomogeneity. Before static RF shimming, severe shading artifacts can be seen in T2W FSE images. After static RF shimming, B1+ uniformity within the abdomen and pelvis was improved to yield smaller CV values, and the resultant image became more uniform. The CV value decreased by nearly 16% compared with that of CP mode in abdomen, and by 33% in pelvis.

Conclusion

Preliminary results show great promise for routine whole body MR clinical imaging at 5T, an area where 7T has been struggling due to more severe B1+ inhomogeneity and SAR constraints. The static RF shimming procedure takes less than a minute and can be implemented as part of the prescan procedures. Further improvements, such as by dynamic RF shimming, will be investigated in future study.

Acknowledgements

This work is supported by National Key R&D Program of China N0.2017YFC0108800.