A multisegment detunable HEM-mode dielectric resonator for increased patient comfort at 7 T
Rita Schmidt1, Wouter M. Teeuwisse1, and Andrew Webb1

1Radiology, Leiden University Medical Center, Leiden, Netherlands

Synopsis

At ultrahigh field (≥7T) cavity- and waveguide-based RF coils become possible to construct, and have the advantage that simple designs can be used. One such design uses materials with a high dielectric constant. So far only completely circular structures have been produced; to improve the usability of such a dielectric resonator, it is important to be able to design a splittable and easily detunable resonator. In this work, we tested designs that allow splitting of the cylinder to sections, while maintaining the circularly polarized RF field. In-vivo measurements of the knee at 7 Tesla showed the feasibility of this approach.

Introduction

At ultrahigh field (≥7T) cavity- and waveguide-based RF coils become possible to construct, and have the advantage that simple designs with intrinsically homogeneous modes can be used. One such design uses materials with a high dielectric constant: in these designs the lowest frequency modes are suitable for MRI1-5. So far only completely circular structures have been produced, and in order to improve the usability of such a dielectric resonator (DR) both in terms of patient comfort as well as being able to integrate receive coil arrays, it is important to be able to design a splittable and easily detunable resonator. However, splitting a DR breaks the symmetry and the possibility to create circularly polarized RF field distribution is lost. In this work, we performed electromagnetic (EM) simulations, and constructed and tested initial designs that allow splitting of the cylinder to two or more sections, while maintaining the circularly polarized RF field. In-vivo measurements of the knee at 7 Tesla showed the feasibility of this approach

Methods

3D EM simulations were performed using finite integration technique (FIT) software (CST Microwave Studio, Darmstadt, Germany) to investigate the properties of the DR and the dimensions of the additional copper connectors required to achieve a circularly polarized B1+ field in the homogenous HEM11 mode3. In these simulations an annular cylinder was cut into two and four sections: the dimensions were 121 mm outer diameter, 75 mm inner diameter, height 90 mm, with relative permittivity (εr) of water: a phantom of the same electrical properties as muscle was used. Based upon the results of the simulations, the dimensions of the copper connectors (providing a pathway for the otherwise disrupted electric field) were 5 mm diameter and 12 mm length (on each side); six such connectors were built on each side of the structure. In-vivo images of the knee of a volunteer were acquired on a Philips Achieva 7 T MRI system. Imaging parameters: gradient echo: FOV 26x24 cm2, spatial resolution 1.0 x 1.0 x 3.0 mm3, TR/TE 300/2.9 ms; three-point Dixon: FOV 26 x 18 cm2, spatial resolution 1.0 x 0.7 x 7.0 mm3, TE1 2.4 ms, ΔTE 0.33 ms, 3 echoes, TR 300 ms.

Results

Figure 1 shows electromagnetic simulations of the modal eigenvalues demonstrating that the symmetry of the HEM mode is broken in the disconnected two halves, and the corresponding two-fold broken symmetry for the disconnected four quarters. The symmetry of the HEM11 mode is recovered when the copper connectors with optimized dimensions are added for both the two- and four-section setups. Figure 2 shows the constructed two-half splittable DR. Water-tight copper connectors are implemented using copper coated neoprene foam to allow for a flexible contact when the two halves are pressed together. Network analyzer plots of the connected and disconnected halves were used to confirm the connections. Two critically-coupled impedance matching loops, placed at 90° with respect to one another, were used to couple energy into the frequency degenerate HEM modes. S11, S22 and S21 measurements all gave values less than -20 dB. Figure 3 shows the results from in-vivo scanning of the knee of a volunteer with the splittable design. Good fat/water separation is achieved in the Dixon images, allowing clear visualization of the cartilage layer.

Conclusions

A new design is demonstrated which can produce a homogeneous circularly polarized B1+ field from a splittable dielectric resonator. The two-section design is required for patient positioning and comfort. The four-section design can be used for situations in which active detuning is required. In this case a simple arrangement using PIN diodes allows connections to be made electronically. It should be noted that a four-section (or greater number) design is necessary since all modes are destroyed when the sections are disconnected, as shown in Figure 1. Although other methods of active detuning have been shown for DRs2, this one is much more effective and easier to implement.

Acknowledgements

No acknowledgement found.

References

[1] Wen H. et.al. J.Magn.Reson. 1996; 110: 117–123, [2] Haines K. et al. J.Magn.Reson, 2009; 200: 349-353. [3] Aussenhofer S.A., Webb A.G. Magn. Reson. Med. 2012; 68: 1325-1331, [4] Lu J,Zhang X, Rutt B. Proc. Intl. Soc. Mag. Reson. Med. 2013; 21: 4376. [5] Aussenhofer S.A., Webb A.G. Proc. Intl. Soc. Mag. Reson. Med. 2014; 22:1354.

Figures

Figure 1: Electromagnetic simulations of the magnetic field for five configurations: full cylinder, two-halves and four-quarters cylinder without connector, two-halves and four-quarters cylinder with connectors

Figure 2: The setup: a) the splittable two-halves dielectric resonator with two impedance matching loops mounted on the bottom half, b) end-plates of one half-annulus showing the copper connectors inner (top) and outer (bottom) sides

Figure 3: In vivo knee imaging: top row – gradient echo images, middle and bottom rows – water and fat images of the three-points Dixon method.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3511