Introduction

Birdcage transmit coils are used in whole-body MR scanners for exciting nuclei of the patient for imaging. Although birdcage coils show a rapid field decay outside the field of view (FoV), the level of the radiated RF signal is still in a range that it can interact or disturb devices in the proximity of the scanner, e.g. injectors. We investigated a setup consisting of three birdcage coils that should reduce the radiated RF level while maintaining imaging performance.

Methods

The proposed 3 birdcage coil design has been modelled in CST Studio Suite (Version 2020.07, Dassault Systèmes, France) (see fig. 1). The inner birdcage (main birdcage) represents the TX coil for imaging and is located in the iso-center of the system. The auxiliary birdcages are placed in the same distance to the main birdcage on the bore tube of the system. The coils are tuned to $$$23.6\,{MHz}$$$ ($$$0.55\,T$$$). The simulation setup also included the gradient coil and the magnet, “open boundaries” were used to limit the simulation domain.
The main birdcage is driven in circular polarized excitation mode and the resulting $$$B1+$$$ field strength is scaled to a reference value of $$$B1+ = 28\mu T$$$. Magnitude and phase of the main H-field are measured with several field probes (FP) distributed within the scanner room. The measured field response was subsequently used to determine the drive signal V of the auxiliary birdcage resonators to generate a field for destructive superposition of the main field. The auxiliary birdcages are driven by two feeding ports each. Each port provides one degree of freedom whereby an exact superposition of one field component in one point is mathematically possible. Basically this function was optimized for V with least square methods:
$$minimize\left\{H_{main}+\overrightarrow{V}* H_{aux}\right\}_{FP}$$.
As important criteria, the quality of the field volume used for imaging must not be affected by the field of the auxiliary birdcages.

First the selection of different points and field components for optimization was analyzed concerning reduction of field strength and feedback to the imaging volume. The influence on the FoV, the s-parameters of all ports and the circularity of the field in the FoV were analyzed; the quality of the circular excitation was evaluated by the ratio of the left- and righthand rotating field components.
Secondly, the effects of reducing the degrees of freedom by connecting the auxiliary birdcages’ feeding ports were investigated to reduce costs for transmit electronics. Therefore, the cancellation was performed at a distance of $$$3\,m$$$ with all four auxiliary ports individually fed, with two ports of one birdcage connected by a $$$90\,^{\circ}$$$ hybrid coupler and with two ports of different birdcages connected via a $$$3\,dB$$$ power splitter.

Results

A cancellation of the x- and y-components of the main field in points along the z-axis of the bore tube proved to be suitable regarding a reduction of the field strength up to $$$60\,dB$$$.
Figure 2 shows the reduced magnetic field strength along the z-axis for a cancellation of the x- and y-component using optimization points at $$$1.5\,m$$$, $$$2\,m$$$ and $$$3\,m$$$ distance from the iso-center. As a reference the magnetic field strength of a MR system without additional birdcages is plotted. The optimizations can be recognized by a clear notch.
In all field reduction experiments the excitation performance within the imaging FoV could be maintained, i.e. $$$B1+$$$ strength and homogeneity: The feedback of the auxiliary birdcages on the FoV is dependent on the distance between the main and auxiliary birdcages. In order not to deteriorate the quality of circular polarization, a decoupling between the birdcages of at least $$$-18\,dB$$$ is necessary, which can be achieved by a distance of $$$25\,cm$$$ between neighboring birdcages.
Minimizing the degrees of freedom deteriorated the cancellation results significantly as shown by figure 3, where a notch does no longer occur and a reduction of $$$36\,dB$$$ for the $$$3\,dB$$$ power splitter and of $$$18\,dB$$$ for the $$$90\,^{\circ}$$$ coupler is obtained in $$$3\,m$$$ distance. The absolute amplitude and phase error of the feeding signals for the power splitter compared to the individual feed were $$$0.02\,\sqrt{W}$$$ and $$$1.6\,^{\circ}$$$, for the $$$90\,^{\circ}$$$ coupler the amplitude error was ten times larger respectively the phase error twice.

Discussion

Due to the design of the auxiliary coils similar to the antenna structure of the transmit coil, a similar field distribution is generated for destructive superposition. A cancellation in single points without deteriorating the quality of the magnetic field in the imaging volume could be shown.
A reduction of at least $$$20\,dB$$$ could be observed at distances from the iso-center greater than $$$2\,m$$$ after optimization in points further than $$$2\,m$$$ away from the iso-center using the proposed BC design.
The increased amplitude and phase error in case of interconnecting ports via a $$$90\,^{\circ}$$$ coupler results in a lower reduction of field strength.

Conclusion

A new body coil design was proposed that allows a significant reduction of the radiated field while maintaining imaging performance, but with high precision requirements for practical realization. The reduced emission can lower the high burden of MR immunity of 3^rd party devices in the proximity of the scanner.

Acknowledgements

No acknowledgement found.