Introduction

In MRI, shim coils can compensate for B0 variations to avoid geometric distortion and signal dropout. This is particularly important in echo-plane imaging (EPI), susceptibility weighted imaging (SWI) and T2* weighted imaging¹. However, in an integrated B0/Rx coil system², losses in SNR and reductions in transmit coil efficiency were observed due to the addition of B0-shim coils¹. In this work, the perturbations of the transmit field (B1⁺ field) and receive sensitivity were firstly investigated quantitatively by using a loop with different diameters, turns and distances from the sample. Based on the study, a 5-channel shim coil was constructed for small animal imaging at 3T and then the transmit efficiency of RF coil and SNR losses were evaluated through imaging experiments on a 3T MR scanner.

Methods

As a simplified model, a circular copper loop that connected to the shim power controller was placed on the surface of a 4 cm diameter phantom, as shown in Figure 1 a. The impacts of circle loops with different diameters (D=3 cm, 4 cm, 5 cm, 6 cm), turns (N=1, 2, 4, 8) and distances from the phantom (d=1 cm, 2 cm, 3 cm, 4 cm) on SNR and B1+-field were evaluated at a 3 T Siemens MRI system. A 2D GRE sequence (TR/TE=500 ms/10 ms, flip angle=60^o, FOV=100 mm, matrix size =64x64, slice thickness=5 mm, slices=1, bandwidth=390 Hz/pixel) was applied to acquired images for SNR calculation with equation: sqrt(2)·sgnal/σ (σ is the standard deviation of noise image). To assess the transmit field, the double-angles method³ was applied, of which images were acquired with a 2D GRE sequence (TR/TE=3000 ms/10 ms, flip angle=45^o/90^o). To diminish the impact of transmit field magnitude on SNR comparison, the normalized SNR⁴ was calculated to evaluate the receive sensitivity. However, multi-coil shimming is more efficient in practical applications than a single loop shimming. Hence, based on the above investigations, the interferences of a 5-channel shim coil that designed for small animal temperature mapping at 3T with RF coils were also studies. The coil elements with a 4 cm diameter were placed on a polycarbonate material former, which had 2 cm distances away from the phantom, as shown in Figure 1 b. Notably, the 5-channel shim coil was set with the optimized currents to shim B0-field corresponding with the actual application. Additionally, the maps of the shim coil with the transmit voltage set as the voltage loading only phantom was acquired.

Phantom results

Figure 2 illustrated the SNR maps, B1⁺ maps and normalized SNR maps of the different diameters loops (d=1 cm, N=1), from which can be known that as the larger is the diameter, the greater is the SNR loss and the receive sensitivity. The distributions of B1 field were less affected by the diameter of the loop. Similarly, the three maps of the loops with different turns (d=1 cm, D=4 cm), were demonstrated in Figure 3, of which distributions were more seriously damaged with more turns, especially the B1⁺ field distributions. Based on the study of different distances away from the phantom (D=4 cm, N=1), as shown in Figure 4, it can be obtained that the closer is distance, the more is the SNR loss. In the three parameters, the degradation of the SNR maps, B1⁺ maps and the receive sensitivity was the most susceptible to the number of loop turns with a 1 cm distance from the phantom. In the study of the 5-channel shim coil, the reduction of SNR, transmit efficiency and the received sensitivity within 3% can be neglected with the appropriate coil diameter, turn and distance away from the sample.

Discussion

The degradation of SNR, transmit efficiency and the receive sensitivity with more loop turns may be caused by the close distance from the sample, which can be reduced by increasing the distance between coil and phantom or decreasing the diameter of coil element. Most importantly, the B0-shim performance of the multi-coil should be focused since the interferences with RF coils can be neglected in an appropriate design.

Conclusion

Interferences of local B0-shim coils with RF coils were investigated experimentally, finding that a local shim coil with less number of turns, smaller coil loop sizes and farther distances to imaging samples would generate less interferences to RF coils. By using this conclusion as a design guideline, a 5-channel shim coil was designed and constructed. The impact of the shim coil on the RF coils for this specific coil system in the aspects of SNR, B1⁺ field and receive sensitivity was minimal and could be omitted in practice.

Acknowledgements

This work was supported in part by NSFC under Grant No. 61571433, 81527901, 81527901, 81327801 and 81627901, Guangdong Province grants 2014A030313691, 2015B020214006 and 2014B030301013. Youth Innovation Promotion Association of CAS No. 2017415, city grants KQJSCX20160301143250 and JCYJ20170413161314734 and a Pengcheng Scholar Award.