Slotted-tube-resonator design for whole-body MR imaging at 14T
Jérémie Daniel Clément1, Arthur Magill2, Hongxia Lei3, Özlem Ipek3, and Rolf Gruetter4,5,6

1CIBM-LIFMET, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 2Forschungszentrum Jülich, Jülich, Germany, 3CIBM-AIT, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 4LIFMET, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 5Department of Radiology, University of Geneva, Geneva, Switzerland, 6Department of Radiology, University of Lausanne, Lausanne, Switzerland


The purpose of the study was to build a slotted-tube resonator for whole-body MR imaging at 14T. Flip angle maps were computed to assess the transmit field distribution in a phantom. A longitudinal coverage of 8 cm and flip angle homogeneity are observed and spin-echo images were acquired.


Birdcage volume coils present limited longitudinal field-of-view (FOV) and demonstrate high transmit field at the center of a sample but lower transmit field close the borders at 14T [1]. The purpose of the study was to design a slotted-tube resonator to increase the longitudinal coverage and to improve the homogeneity of the transmit field.


A slotted tube resonator [2] (STR) was built with copper and two coaxial PVC tubes. Shielding was ensured by surrounding the outer tube (∅ = 100 mm) with copper and two parallel copper plates were placed on the inner tube (∅ = 56 mm) with an opening angle of 102° (Fig. 1A). The length of the parallel copper plates was chosen to excite the ¾-lambda mode (37.5 cm) in the STR and adjusted to tune it at 600 MHz. The feeding cable was connected between the two copper plates and the STR was matched to 50 Ohms (S11 = -28 dB). Common-modes on the coaxial cable were diminished with a balun. Finite difference time domain (FDTD) simulations were performed on Sim4Life 2.0 (ZMT, Switzerland) for the exact model, to assess the B1+ field distribution in the STR (Fig. 1C). In simulations, the STR was tuned to 600 MHz and matched to the input impedance of the source (S11 > -30dB). In-vitro measurements were performed on a 14T horizontal-bore MR scanner (Varian, Palo Alto, CA, USA) on a cylindrical phantom (∅ = 50 mm, length = 80 mm), filled with distilled water. It was placed at approximately 15 cm from the source, where the B1+-field is maximal (Fig. 1C). Flip angle maps were computed in transverse and coronal planes using two GRE images at different flip angles (60/120°, TR/TE = 20000/4.1 ms, 1 mm slice thickness, transverse: FOV = 35x35 mm2, 128×128 data matrix and coronal: FOV = 100×35 mm2, 512×128 data matrix) [3].


The simulated B1+ map is shown for the empty STR (Fig. 1C). A ¾ - lambda resonance is observed as it was expected from the dimensions of the tubes. The experimental flip angle maps are shown for the STR (Fig. 2C) with the phantom placed inside. The desired flip angle is achieved in coronal and transverse planes of the phantom. Moreover, the distribution pattern in the phantom is similar to the simulated one, without the phantom (Fig. 1C) suggesting that the resonant mode remains when the STR is loaded with the phantom. The measured spin-echo images are shown for the STR in coronal and transverse planes (Fig. 2A). Almost the whole phantom is visible. However, due to the limited linearity range of the gradients, distortions are visible at the edges of the image. Signal intensity profiles (Fig. 2B) demonstrate an 8 cm coverage in the coronal plane and almost 3 cm in the transverse plane.

Discussion and conclusion

In this work, we showed that the STR design is suitable for whole-body MR imaging at 14T. The FOV and flip angle homogeneity in longitudinal direction are large compared to a birdcage coil [1] and allow for whole-body imaging. However, with the STR, the position of the sample is not arbitrary as it has to be placed where the B1+-Field is maximal. A travelling-wave approach [4] appears to be an interesting outlook for further improvements. We conclude that the STR design offers an increased longitudinal coverage compared to a birdcage coil, and makes the whole-body MR imaging at 14T feasible.


This study was supported by Centre d’Imagerie BioMédicale (CIBM) of the UNIL, UNIGE, HUG, CHUV, EPFL and the Leenaards and Jeantet Foundations.


[1] A. Magill et al. A High-pass Birdcage Coil for Small Animal Imaging at 600MHz/14.1T. ISMRM Proceedings, 2628 (2012)

[2] H. Schneider et al. Slotted tube resonator: A new NMR probe head at high observing frequencies. Review of Scientific Instruments. vol. 48, 68(1977)

[3] R. Stollberger and P. Wach. Imaging of the Active B1 Field in Vivo. Magnetic Resonance in Medicine. vol. 35, Issue 2 (1996)

[4] D. Brunner et al. Travelling-wave nuclear magnetic resonance. Nature, vol. 457, pp. 994-998 (2009)


Figure 1. A) Axial dimensions of the STR and B) STR design. C) Longitudinal dimensions and simulated B1+ map, shown in the waveguide without the phantom, normalized to 1W delivered power.

Figure 2. A) Coronal and transverse spin-echo images of the phantom and B) Signal intensity profile along the extraction line shown in A) (dashed black line). C) Flip angle maps in coronal and transverse planes.

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